Nutrient Cycling and Water Pollution in Lake ZapotlÆn, Mexico€¦ · Nutrient Cycling and Water...
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Nutrient Cycling and Water Pollution in Lake ZapotlÆn, Mexico By Tracie A Greenberg A thesis submitted in conformity with the requirements for the degree of Master of Science Graduate Department of Geography University of Toronto ' Copyright by Tracie A. Greenberg (2009)
Transcript of Nutrient Cycling and Water Pollution in Lake ZapotlÆn, Mexico€¦ · Nutrient Cycling and Water...
Nutrient Cycling and Water Pollution in Lake Zapotlán, Mexico
By
Tracie A Greenberg
A thesis submitted in conformity with the requirements for the degree of Master of Science Graduate Department of Geography
University of Toronto
© Copyright by Tracie A. Greenberg (2009)
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Abstract
Nutrient Cycling and Water Pollution in Lake Zapotlán, Mexico
Master of Science, 2009 Tracie A. Greenberg
Department of Geography, University of Toronto Lake Zapotlán is a small (1100 ha) endorheic lake in western Mexico that is internationally
recognized by RAMSAR. It receives point source pollution from partially treated sewage from
two surrounding cities, as well as non-point sources, including urban runoff, agricultural runoff,
erosion and consequent deposition of sediment as a result of deforestation surrounding the Lake.
The purpose of this study was to determine the severity of pollution in the Lake through
measurement of nutrient and bacteria levels and assess for potential human health and ecological
risks in Lake Zapotlán. Results found that nutrient levels have increased since 1994 and that they
are high enough to cause eutrophication problems. Partially treated wastewater contributes over
30 tonnes of phosphorus to the Lake each year. E. coli levels were extremely high and could
pose a health risk to those participating in recreational activities on the Lake.
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Acknowledgements I would first like to thank my supervisor, Dr. Harvey Shear, for accepting me as one of his first graduate students. His wealth of knowledge and experience in water quality was extremely helpful for my graduate work. I would not have had such an interesting thesis topic if it were not for Harvey�s valuable international networking skills acquired through both his personal time and his Canadian Federal Government career. Harvey�s guidance was always there when needed and I appreciate the advice he gave me throughout my entire graduate degree. A huge thank you goes to Dr. Brian Branfireun for your ongoing support and guidance during the many trips to Ciudad Guzman and throughout my entire graduate degree. Brian, despite your extremely busy schedule, you were always available to assist me with any questions I had and with my field work in Mexico. I appreciate the advice you offered and will take your academic ethics with me throughout my life. Thank you so much to Dr. Nathan Basiliko for letting me use his laboratory and for his constant assistance and advice on sample analysis techniques. I would not have been able to complete my masters without your tremendous generosity and kindness. Your positive energy on science will always encourage me in the future. Also, thank you for offering Cori�s assistance in the lab. Cori Armes, I appreciate all the time you put aside to assist me on the Lachet. Your constant dedication in helping me through my lab work will always be remembered. We now have a close bond over our feelings on that instrument Evan Malczyk, thank you for your constant assistance and comfort while in Mexico. You were a huge help to me during our field sampling and encouraged me to make the most out of any situation. I will always have the unpleasant memories of �Station 13�, but only you could make me smile and laugh when thinking back to this wastewater sampling site. A huge thank you also goes out to Dr. Gonzalo Rocha Chavez, Dr. Michel Para, and Dr. Jose de Anda. Without the help from all of you, I could not have completed my graduate work. Gonzalo, your constant dedication to assist me with everything during my stay in Ciudad Guzman will never be forgotten. Michel, your constant energy and wealth of acquaintances in Mexico have made my trips to Ciudad Guzman unforgettable. Jose, thank you for your assistance in planning and carrying out the field work in Ciudad Guzman and for your generosity in organising the October sample analysis at CIATEJ. Most importantly, to all three of you, thank you for making Ciudad Guzman feel like home through your hospitality and kindness. Finally, thank you to my family and friends for your constant support and encouragement during the last 2 years. A special thank you goes out to my office mates, Geoff, Carolyn, Cori, and Evan for taking the ease off any stress I had and making me laugh during the worse times. You have all encouraged me to continue being positive and to never give up.
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Table of Contents Chapter 1: Introduction ...............................................................................................................1
1.1 Site Description.................................................................................................................3 1.2 Brief Chapter Overview.....................................................................................................4
Chapter 2: Study Site Review......................................................................................................6 2.1 Activities Affecting Water Quality in Lake Zapotlán .........................................................8 2.2 Factors affecting nutrients in Lake Zapotlán ......................................................................9
2.21 Typha latifolia and Eichhornia crassipes ......................................................................9 2.22 Fishing Industry.........................................................................................................10
2.3 Bacteria in Lake Zapotlán................................................................................................11 2.4 Purpose of Study .............................................................................................................13 2.5 Research Questions .........................................................................................................13
Chapter 3: Methods...................................................................................................................14 3.1 Sample Collection ...........................................................................................................14 3.2 Nutrient Sampling ...........................................................................................................14
3.21 Sample Collection in October, 2007...........................................................................14 3.22 Sample Analysis in October, 2007 .............................................................................16 3.23 Sample Collection in February and July, 2008 ...........................................................17 3.24 Sample Analysis in February, 2008 and July, 2008 ....................................................20
3.3 Escherichia coli Sampling ...............................................................................................21 3.31 Sample Collection for October, 2007, February, 2008, and July, 2008 .......................21 3.32 Sample Analysis for October, 2007, February, 2008, and July, 2008..........................22
Chapter 4: Nutrient Trends in Lake Zapotlán.............................................................................23 4.1 Nutrient Trends in Lake Zapotlán over 13 years...............................................................23
4.12 Results.......................................................................................................................23 4.13 Nitrogen Comparison of 1994, 2003 & 2007..............................................................24 4.14 Phosphorus Comparison of 1994, 2003, & 2007 ........................................................25 4.15 Discussion: Nitrogen Trends over 13 Years ...............................................................26 4.16 Discussion: Phosphorus Trends over 13 years ............................................................27
4.2 Seasonal Comparison of Nutrients in Lake Zapotlán........................................................27 4.21Seasonal Physical and Chemical Parameters ...............................................................27 4.22 Seasonal Nitrogen Comparison for October, 2007, February, 2008, and July, 2008....28 4.23 Phosphorus Seasonal Comparison from October, 2007 to July, 2008 .........................30 4.24 Discussion: Seasonal Nitrogen Patterns......................................................................34 4.25 Discussion: Seasonal Phosphorus Trends ...................................................................35
4.3 Overall Discussion on Nutrient Status in Lake Zapotlán ..................................................38 Chapter 5: Preliminary Phosphorus Balance in Lake Zapotlán...................................................41
5.1 Phosphorus Inputs ...........................................................................................................41 5.11Wastewater Effluent ...................................................................................................41 5.12 Runoff .......................................................................................................................43
5.2 Phosphorus Stored within the Lake..................................................................................44 5.21 Phosphorus in Lake Water .........................................................................................44 5.22 Phosphorus in Sediments ...........................................................................................44 5.23 Phosphorus in Water Hyacinth...................................................................................45 5.24 Phosphorus in Tule ....................................................................................................45
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5.3 Phosphorus Outputs.........................................................................................................46 5.4 Discussion of Storage within Lake Zapotlán: Sources and Sinks......................................47 5.5 Discussion on the Overall Phosphorus Status of the Lake ................................................49
Chapter 6: Escherichia coli Levels in Lake Zapotlán.................................................................51 6.1 Results of E.coli Levels in October 2007, February, 2008 and July, 2008 ........................51 6.2 Possible sources of E.coli in Lake Zapotlán .....................................................................52 6.3 The effect of Carbon on E. coli levels in Lake Zapotlán...................................................55
Chapter 7: Summary and Conclusions .......................................................................................57 7.1 Seasonal and Historical Nutrient Trends in Lake Zapotlán...............................................57 7.2 Phosphorus Balance.........................................................................................................58 7.3 Bacteria Levels in Lake Zapotlán.....................................................................................58 7.4 Conclusions.....................................................................................................................59 7.5 Future Work and Recommendations ................................................................................60
8.0 References...........................................................................................................................62
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List of Tables Table 4.1: Table 4.2: Table 4.3: Table 4.4 Table 4.5: Table 4.6: Table 5.1: Table 6.1:
Basic physical and chemical parameters for some representative Lake stations Basic physical and chemical parameters averaged over a year in Lake Zapotlán Percent (%) of Orthophosphate to Phosphorus in Lake Zapotlán, February 2008 Percent (%) of Orthophosphate to Phosphorus in Lake Zapotlán, July 2008 Total Phosphorus per gram of Sediment in Lake Zapotlán Relating the Trophic State to the State of a Water Body Confidence Interval of Mass Balance Calculation Dissolved Organic Carbon levels in Lake Zapotlán , October 2007, February, 2008, and July 2008
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28
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34 39 42 55
List of Figures Figure 1.1: Figure 2.1: Figure 2.2: Figure 2.3: Figure 2.4: Figure 3.1: Figure 3.2: Figure 3.3: Figure 4.1: Figure 4.2: Figure 4.3: Figure 4.4: Figure 4.5: Figure 4.6: Figure 4.7: Figure 4.8: Figure 4.9: Figure 5.1:
Geographical Location of Lake Zapotlán Basin Average Precipitation per Year in Lake Zapotlán, 1995-2007 Percentage of Precipitation per month over a Year in Lake Zapotlán, averaged from 1995-2007 Surface area of Lake Zapotlán covered with aquatic plants Fishery Harvest in Lake Zapotlán, 2001-2008 Sampling Stations in Lake Zapotlán, Jalisco, Mexico, October, 2007 Photos of sampling the surrounding wastewater discharge channels at station 13 in a) October, 2007 and b) February, 2008 Sampling Stations on Lake Zapotlán, Jalisco, Mexico, February, 2008 and July 2008 Comparison of Total Nitrogen Levels for October of 1994, 2003, and 2007 Comparison of Total Phosphorus in October of 1994, 2003, and 2007 Total Nitrogen in Lake Zapotlán for October 2007, February, 2008, and July, 2008 Total Nitrogen in Lake Zapotlán Discharge Channels for October, 2007, February, 2008, and July, 2008 Total Phosphorus in Lake Stations for October 2007, February, 2008, and July, 2008 Total Phosphorus in Discharge Channel Stations for October 2007, February, 2008 and July 2008 Example of the role of wetlands in reducing Total N concentrations (mg/L) in the Lake, July 2008 The role of wetlands in reducing total P (mg/L) in the Lake, July 2008 Comparison of Total Phosphorus Levels in Hamilton Harbour, Canada to those in Lake Zapotlán, Mexico Arroyo locations for the July, 2008 precipitation sampling event
3 7 7
9 10 16 17
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Figure 5.2: Figure 5.3: Figure 6.1: Figure 6.2:
Preliminary Phosphorus Mass Balance in Lake Zapotlán Comparison of water clarity in a) 2001 and b) 2007 Escherichia coli levels in CFU per 100ml in Lake Zapotlán Water Hyacinth (Eichhornia crassipes) Surface Cover in the a) San Sebastian del Sur wastewater effluent canal and b) Ciudad Guzman wastewater effluent canal.
47 49 51 54
Disclaimer The research presented in this study was conceived of and executed by the author with guidance from Dr. Harvey Shear. Water and sediment sample collection and data analysis were completed solely be the author. Additional sediment and fish data analysis were completed by the Centro de Investigación y Asisténcia en Tecnología y Diseño del Estado de Jalisco (CIATEJ) in Guadalajara and the University of Guadalajara, Centro Universitario del Sur (CUSUR). Additional data were provided by the following individuals: Jose Guadalupe Michel Para, Gonzalo Rocha Chavez, Jose de Anda Sanchez, and Evan Malczyk.
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Chapter 1: Introduction Freshwater is a renewable but limited resource essential to all ecosystems on Earth. It is an
important source for irrigation, drinking water, sanitation, hydroelectric generation, and for many
human activities worldwide. Although, freshwater lakes only comprise a small percentage
(0.009%) of the available surface freshwater in the hydrologic cycle, they are key ecological
components (Reddy, 2005).
Currently, the world�s lakes are in crisis as a result of increased anthropogenic disturbance
(Jorgensen & Vollenweider, 1994; Kalff, 2002). Increased pressure through irrigation and
drinking water extraction, nutrient input, and contamination are contributing to the impairment of
natural water quality (Hamilton, 2005; Reddy, 2005). The water is often degraded so much that it
becomes of minimal use or unusable in some regions (Reddy, 2005). Many lakes worldwide are
becoming eutrophic because of nutrient runoff from increasing populations surrounding water
bodies (Prepas & Charette, 2003).
The United States Geological Survey (USGS), (2007), defines eutrophication as �a process
whereby water bodies, such as lakes, estuaries, or slow-moving streams receive excess nutrients
that stimulate excessive plant growth�. Eutrophication was originally identified as a natural
process of lake aging which takes thousands of years, whereby some lakes eventually become
wetlands (Lindeman, 1942). Naturally, lakes were found to enter phases where the water would
become eutrophic depending on biogeochemical changes in their catchments (Williams &
Hecky, 2005). Today, many lakes are eutrophic because of increased runoff of nutrients from
surrounding anthropogenic activities.
Anthropogenic eutrophication is one of the major factors leading to degraded water quality
worldwide (Kira, 1997; Williams & Hecky, 2005). In the past, there was considerable scientific
debate about the cause of eutrophication. Nitrogen, carbon, and phosphorus were all suspected of
causing eutrophication, until a study by Schindler & Fee in the late 1960s proved that
phosphorus was the cause (Schindler and Fee, 1974). The project completed at the Experimental
Lakes Area in Northwestern Ontario ruled out nitrogen and carbon as contributors to
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eutrophication because these elements are abundant in the atmosphere and are not easily
controlled. Soluble Reactive Phosphorus (SRP), or Orthophosphate, the inorganic form of
phosphorus, is the form that is available for uptake by algae resulting in it often being the
determining factor for algal growth in a water body (Kalff, 2002). Through whole lake
enrichment experiments, phosphorus proved to be the nutrient causing eutrophication (Schindler
and Fee, 1974).
Human activities, mainly through the use of phosphorus-containing agricultural fertilizers, and
the discharge of sewage containing biologically available phosphorus, have altered the nutrient
status of many of the Earth�s water bodies (Shear & de Anda, 2005). In North America,
phosphorus input to lakes from human waste, is very low because of regulations on phosphates
in detergents and because of the limitation of phosphorus content in sewage treatment effluent
(Kaushik, 2005).
Tropical and subtropical lakes are more susceptible to eutrophication than are temperate lakes
because of the high pressure on these freshwater resources for domestic, industrial and waste
disposal purposes (Hamilton, 2005; Everard et al., 2005; Venugopalan et al., 2005). There are
two defined seasons in subtropical lakes, wet and dry, which can change nutrient inputs and
consequently levels of eutrophication throughout the year (Reddy, 2005). Phosphorus loadings
to a lake in the wet season generally derive from the use of detergents, human waste, fertilizers,
and the use of livestock manure for agriculture. Loadings to a lake during the dry season
predominantly come from the wastewater treatment plants (Ecobichon, 2001; Kaushik, 2005).
High precipitation rates in the wet season result in an increase in nutrient runoff to surrounding
water bodies in the subtropics (Venugopalan et al., 2005). Untreated wastewater is generally a
large contributor of nutrients to lakes for both seasons in the subtropics because of insufficient
funding and resources to treat the wastewater before it enters the lake. Sub/tropical lakes are also
exposed to higher concentrations of nutrients from runoff of agricultural chemical fertilizers and
detergents, waste that would otherwise be banned or severely regulated in developed countries
(Ecobichon, 2001). A high rate of logging (sometimes illegal) in subtropical regions also causes
increased topsoil erosion resulting in an increase in runoff discharging into surrounding water
bodies (Venugopalan et al., 2005).
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Compared to temperate lakes, minimal research has been carried out on eutrophication in tropical
lakes, and even less research has been completed on subtropical lakes (Hamilton, 2005). The
research in this thesis attempts to fill this void by examining nutrient problems in a subtropical
freshwater lake in western Mexico.
1.1 Site Description
Lake Zapotlán is a subtropical lake located in the south of Jalisco State, Mexico, between 19°34�
and 19°53� north latitude and 103°24� and 103°38� west longitude (Figure 1). Lake Zapotlán was
listed as a RAMSAR site in 2005 in recognition of its important migratory waterfowl and the
wetlands surrounding the Lake (RAMSAR, 2007). The second largest city in Jalisco State,
Ciudad Guzman, with a population of about 96 000 inhabitants and an annual growth rate of
about 2.1%, is in the Lake Zapotlán Basin (CEASJ, 2008). Lake Zapotlán is also the future site
for water sports for the 2011 Pan American games (Quadro de Medalhas, 2009).
Figure 1.1: Geographical Location of the Lake Zapotlán Basin
There has been a marked degradation of the water quality in Lake Zapotlán during the last few
decades driven by non-point and point source inputs (Oritz-Jimenez, 2006b). Discharge from
essentially untreated municipal sewage from the nearby towns, construction of municipal roads
crossing the Lake, deforestation, and runoff from numerous farms are some potential sources for
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the degraded water quality (Oritz-Jimenez, 2006). These processes have caused an increase in
nutrients in the Lake, resulting in an abundance of rooted and floating aquatic vegetation known
as cattail (Typha latifolia) called Tule by the locals, and water hyacinth (Eichhornia crassipes),
called lirio by the locals. There have also been frequent blooms of green algae, and possibly
cyanobacteria (Microcystis aeruginosa), a species that may cause health and odour problems (de
Anda, 2007). Esherichia coli (E. coli) has also been found in the past (Ortiz-Jimenez, 2006) at
levels significantly higher than those that would be permitted in Canadian water bodies.
Past research on Lake Zapotlán includes a limnological survey of the Lake in 1994 (Universidad
de Guadalajara, 1994); a nutrient/food chain model in 2003 (Oritz-Jimenez et al., 2006a); an
estimation of trophic states using a General Purpose Simulation System (GPSS) in 2003 (Oritz-
Jimenez et al., 2006b); a hydrological balance of Lake Zapotlán in 2005 (Oritz-Jimenez et al.,
2005); a heat balance and water-nutrient chain interactions assessment in 2006 (Oritz-Jimenez &
de Anda, 2007); and a sustainable indicator identification study in 2002-2006 (Shear & de Anda,
2009). A comparison of past nutrient or E. coli levels in the Lake has never been determined,
nor has a phosphorus mass balance of the Lake. The purpose of this thesis is to fill all three of
these information gaps. A nutrient concentration comparison over a decade, and then over the
wet and dry seasons in one year was completed, followed by a phosphorus mass balance of the
Lake. A determination of bacteria levels was also completed to assist in assessing the water
quality in Lake Zapotlán.
1.2 Brief Chapter Overview The next chapter (2) presents an overview of the study area, including hydrological, geographical
and geological information about the basin. The main uses of the Lake are described in this
chapter, followed by a discussion of the possible components that could be controlling the
nutrient status in the Lake. A review of existing knowledge on possible Escherichia coli (E.
coli) sources, as well as human and ecological health problems associated with the bacterium is
discussed, followed by a statement on the purpose of this research.
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Chapter 3 discusses the data collection used to assess the water quality in Lake Zapotlán for both
nutrient and bacteria concentrations. Data collection methods and analytical equipment used to
determine nutrient and E. coli concentrations are discussed in detail.
Nutrient trends in Lake Zapotlán are compared over 13 years (1994, 2003 and 2007) as well as
over one hydrologic cycle in chapter 4. Previous nutrient data from studies in 1994 (Universidad
de Guadalajara, 1994) and 2003 (Oritz-Jimenez et al., 2006) are compared to those values
determined during research in 2007. An overall assessment of the water quality in Lake
Zapotlán is discussed in this chapter.
Chapter 5 provides a preliminary total phosphorus (total P) balance of the Lake. Phosphorus
inputs are discussed, followed by an analysis of potential phosphorus storage in the Lake, and
then a discussion of possible phosphorus outputs. The chapter concludes with a discussion of the
phosphorus dynamics in the Lake.
Chapter 6 provides an analysis of bacteria levels in Lake Zapotlán. A spatial examination of
Escherichia coli (E. coli) concentrations in the Lake are presented, followed by a discussion on
the possible sources of E. coli and the factors within the Lake that are affecting the bacteria
levels.
The final chapter (7) provides a summary of the results from this research, followed by some
ideas for future work on Lake Zapotlán. The chapter concludes with recommendations for use by
the locals and for the health of the participants in the upcoming Pan American games, to improve
water quality in Lake Zapotlán in the near future.
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Chapter 2: Study Site Review Lake Zapotlán is a small (1100 ha), shallow, endorheic freshwater lake with only a few
intermittent streams flowing into it. It is the main water reservoir for a group of integrated
neighbouring closed basins such as Lake Sayula, Lake San Marcos, and Lake Atotonilco (Oritz-
Jimenez et al., 2006). The topography varies from an elevation of 1490 masl at the surface of the
Lake, to 3900 masl at the El Aguila Peak located in the volcano Nevado de Colima near the Lake
(Oritz-Jimenez et al., 2005). The Mexican National Water Commission classifies the quality of
groundwater in this watershed as suitable for drinking (CNA, 2004). However, due to frequent
earthquakes in the region from volcanic and tectonic origin, the sewage system of Ciudad
Guzman has been affected by leaks that allow untreated flow to enter the aquifers, contaminating
them by infiltration (Oritz-Jimenez et al., 2005). Some traces of methane have been found in
some wells used for irrigation and water supply for the urban areas, resulting in some health
problems such as oxygen deprivation (asphyxiation) among the population (Universidad de
Guadalajara, 1999).
Precipitation, groundwater and runoff are the main natural contributors of water to the Lake
(Oritz-Jimenez et al., 2005). The only permanent water source entering the Lake is sewage
effluent from Ciudad Guzman and San Sebastian del Sur (Oritz-Jimenez et al., 2005). The main
outflows from the Lake are water extractions for irrigation (37.4%), evaportranspiration (33.6%),
and evaporation (29%) (CEASJ, 2003). Due to the high permeability of the soil (INEGI, 2001)
there are very few surface streams. The basin has about 1576 ha of irrigated land, of which 700
ha are irrigated with lake water, and the remainder with groundwater (CEASJ, 2003). The main
water uses in the basin are for irrigation and livestock (90%), industrial use (7%), and urban use
(3%) (INEGI, 2001). The water column is considered completely mixed due to permanent
winds, the shallow nature of the Lake, and the short water residence time of about seven months
(Oritz-Jimenez et al., 2005).
Lake Zapotlán has undergone significant hydrological changes in the past, including a decrease
in volume, as a result of the climate and its location in a closed basin (Oritz-Jimenez et al.,
2005). The total precipitation varies slightly year-to-year, but averages about 723mm per year
from 1995 � 2007 (figure 2.1). Over 80% of precipitation occurs during the summer months
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(June-September) and less than 6% of precipitation during the winter months (November-
February) (figure 2.2).
0.0100.0200.0300.0400.0500.0600.0700.0800.0900.0
1000.0
95 96 97 98 99 00 01 02 03 04 05 06 07
Year
Annu
al P
recip
itatio
n (m
m/y
ear)
Figure 2.1: Average Precipitation per Year in Lake Zapotlán, 1995-2007
*dataset is incomplete for 2005 Source of Data: CEASJ, 2008 (unpublished) & Fundación Produce, 2008
Figure 2.2: Percentage of Precipitation per month over a Year in Lake Zapotlán, averaged from 1995-2007
Source of Data: CEASJ, 2008 (unpublished) & Fundación Produce, 2008
The mean annual evaporation in the basin is about 611mm, with maximum evaporation up to
252mm per month during the spring (March � May), and minimum evaporation of about 88mm
in December (Oritz-Jimenez et al., 2005). The climate is warm with an average annual
temperature of 19.6°C, with a variation of ±5.9°C (Oritz-Jimenez et al., 2005). The minimum
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temperatures are in typically in January and the maximum temperatures are in July (INEGI,
2001).
2.1 Activities Affecting Water Quality in Lake Zapotlán
Anthropogenic activities in the basin are putting the Lake at risk through nutrient enrichment.
The primary sources of nutrients to Lake Zapotlán are from the discharge of municipal
wastewater, runoff from agricultural activities, and urban runoff from Ciudad Guzman and San
Sebastian del Sur (Oritz-Jimenez et al., 2005). On average, only about 48% of wastewater is
treated before it enters Lake Zapotlán (CEASJ, 2008). Insufficient funds to repair aging
equipment and to pay employees may have resulted in the discharge of untreated urban
wastewater. In addition, pesticide use has increased in the past decade to accommodate the
increasing population through increased crop growth (Rocha Chavez, 2008, personal
communication).
The poor harvesting practices of the Atenquique paper mill have contributed to water pollution in
the Lake (Shear & de Anda, 2009). Over exploitation of the surrounding forests has also caused
habitat alteration through changes in land use and loss of biodiversity. Deforested areas that are
now used for agriculture and cattle grazing have resulted in nutrient runoff and have accelerated
the erosion process in the basin, increasing solids transport to the Lake (Shear and de Anda,
2009). The uncontrolled deforestation in addition to the erodible features of the soil, the
substitution of agriculture from forest in the lower part of the basin, and the increase in urban
area in Ciudad Guzman have all contributed to a high accumulation of sediments in the Lake
(Oritz-Jimenez et al., 2005). The discharges draining to the basin carry about 550 000 tonnes of
sediment per year to the Lake (UdG, 2002), resulting in an increase of solids, and possibly
nutrients and contaminants flowing into the Lake.
The construction of municipal roads crossing the Lake (see Figure 1) has segmented it into three
independent water bodies causing stagnant zones in the Lake (Oritz-Jimenez et al., 2005).
Combined, these anthropogenic activities are resulting in excess of nutrients in the Lake. Lake
Zapotlán has had poor water quality in the last few decades. In recent years, blooms of
cyanobacteria (Microcystis aeruginosa), have been found, likely the result of excess nutrient
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input from the surrounding land uses (Ortiz-Jimenez, 2005). This cyanobacterium species could
cause taste and odour problems as well as a potential risk to human health (Shear & de Anda,
2005).
2.2 Factors affecting nutrients in Lake Zapotlán
2.21 Typha latifolia and Eichhornia crassipes
The cattail (Typha latifolia), called �tule� by the local people, and water hyacinth (Eichhornia
crassipes) called �lirio� are known to take advantage of the nutrient rich and stagnant waters in
Lake Zapotlán (INEGI, 2000). Both plants are able to successfully colonize new habitats and
form dense mats along the shorelines, reducing access to the Lake for fishermen and for
irrigation (Greenfield et al., 2007; Williams & Hecky, 2005; Rommens, et al., 2003). Water
hyacinth in particular has the potential to double in biomass in a matter of days and is able to
migrate easily due to its free-floating vegetative form (Williams & Hecky, 2005).
Eutrophication of lakes from anthropogenic sources can amplify the problem of these nuisance
floating plants.
In Lake Zapotlán, these two species had grown uncontrollably and covered almost 70% of the
surface of the Lake by 1993 before action was taken in 1995 by the municipality to eliminate the
E. crassipes (Figure 2.3). A reduction in weed abundance through mechanical destruction of the
plants every few years has now decreased the surface cover in the Lake to about 30% of the
littoral zone (Oritz-Jimenez et al., 2005).
0
10
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1982
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Year
Surf
ace
Are
a
Typha latifolia
Eichhornia crassipes
Plant Control by Harvesting
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Figure 2.3: Surface area of Lake Zapotlán covered with aquatic plants Source: Ortiz-Jimenez, 2005 Mechanical shredding of additional E. crassipes has reduced its coverage in the Lake for now,
but if excessive loading of nutrients to the Lake continues, more severe measures to decrease
nutrient loadings may be necessary. Prolonged anthropogenic nutrient inputs will not only
maintain the presence of these nuisance plants, but will have the potential to bring about a return
to its nuisance status (Williams & Hecky, 2005). Commercial fishing, waterfowl habitat,
recreational activities, and the aesthetics of the Lake can all be affected by these plants (Shear &
de Anda, 2005).
2.22 Fishing Industry
The main economic activity of Lake Zapotlán is the fishery. It is based on two introduced
species, both highly valued for commercial harvest- the common carp (Cyprinus carpio)
accounting for 68% of the catch and tilapia (Oreochromis aureus) accounting for 32% (Shear &
de Anda, 2009). Fishery production tripled in 2005 (Figure 2.4) after an all time low in
production occurred in 2001. In 2005, over 65 tonnes of tilapia were stocked by the local
government through subsidies for the local fishing industry (Rocha Chavez, 2008, personal
communication).
Figure 2.4: Fishery Harvest in Lake Zapotlán, 2001-2008 Source: Para, 2008 & Gomez, 2009 (personal communication) The increase in fish production in a lake may have a direct impact on the biomass of algae in a
lake (MOE, 2004). In return, the increase in nutrients in a lake can impact the water chemistry in
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lake, altering fish habitat (Kalff, 2002). In most lakes, an increase in nutrients can result in
excessive algal growth, which can then affect the amount of light penetration and dissolved
oxygen levels in the lake, possibly posing a threat to fish habitat. Examining the relationship
between nutrient enrichment and the fishery production in Lake Zapotlán is necessary to
understand the impact of cultural eutrophication in the Lake.
2.3 Bacteria in Lake Zapotlán
Esherichia coli (E. coli) is a common bacterium that lives in human and animal intestines. There
are several strains of E .coli which can be present in large numbers causing illness through
ingestion, particularly in highly contaminated waters (Edge et al., 2001; Noble et al., 2003;
LeJeune & Wetzel, 2007). Examples of infections which can be acquired from recreational
activities in polluted water are: conjunctivitis (eye), ear infections, nose infections, throat
infections, or more serious infections such as dysentery and gastrointestinal illnesses (Wieske
and Penna, 2002; Schiff et al., 2003).
Two different bacterial indicators are commonly used to test water for bacteria levels. They are
total coliform and fecal coliform (comprised mostly of E. coli) (Jin et al., 2004). Most often, both
total coliform and fecal coliform are measured in Colony Forming Units (CFU) per 100ml
(Gibbons, 1994). Total coliform bacteria tests are the most commonly used indicator because
total coliforms include bacteria found in human and animal waste (fecal coliforms) as well as
bacteria that occur naturally in water and soil (Noble et al., 2003). Total coliform (TC) counts
give a general indication of the sanitary condition of the water. Fecal coliforms are the group of
coliform bacteria present specifically in the gut and feces of humans and animals and comprise
mostly of E. coli (Noble et al., 2003). The presence of fecal coliform in water may indicate
recent contamination by human sewage or animal droppings which could also contain other
bacteria, viruses or disease causing organisms. It is the best-suited contamination indicator
because high concentrations of this organism typically indicate a high risk for the potential of
human illness (Noble et al., 2003; Jin et al., 2004, and Englebert et al., 2008).
There are several point and non-point sources that are responsible for contaminating water with
bacteria. The main sources of contamination include wastewater effluent, bird feces, and runoff
12
from storm drains and agricultural fields (Haack et al., 2003; Noble et al., 2003 and Englebert et
al., 2008). Partially treated wastewater effluent is a prime source of bacterial contamination in
water bodies worldwide (Haack et al., 2003). The amount of untreated wastewater effluent
entering subtropical water bodies is significant, compared to most temperate lakes, due to
insufficient economic resources (Kaushik, 2005). Many subtropical countries may not have the
funding to treat their wastewater properly. Lake Zapotlán still receives over 52% of the
untreated wastewater effluent and 48% of the treated effluent from Ciudad Guzman and San
Sebastian del Sur (CEASJ, 2008).
Several recent studies in North America have found that the elevated levels of E. coli on Great
Lakes beaches are caused by the increasing bird populations (Fogarty et al., 2003; Charlton &
Milne, 2004; Edge & Hill, 2007). Since Lake Zapotlán is recognized as a RAMSAR site for its
importance in migratory bird populations (RAMSAR, 2007), elevated E. coli levels in the Lake
could also be a result of bird droppings from the large bird population.
Large watersheds may contain urban, suburban and rural areas that contribute bacterial
contamination to the stream network. An increase in precipitation in the wet season potentially
causes elevated E. coli levels adjacent to water bodies from storm water overflow, urban runoff
and agricultural runoff (Sanders et al., 2005; Haack et al., 2003; Olyphant et al., 2003; Schiff et
al., 2003; Wieske & Penna, 2002;). Manure from the surrounding agricultural land could be a
potential E. coli contributor through runoff during precipitation events (Panti et al, 1984).
During these events, the contaminated runoff water can reach lakes and rivers, causing a serious
threat to human health (Olyphant et al., 2003).
Bacteria levels in Lake Zapotlán have been elevated in the past (Oritz-Jimenez, 2006). A study
on Lake Zapotlán in 2005 (Oritz-Jimenez) demonstrated high levels of E. coli in the southern
part of the Lake, closest to the area where the wastewater effluent form Ciudad Guzman enters
the Lake. The bacteria levels in the entire Lake are unknown so far. An overall assessment of
bacteria levels in Lake Zapotlán would be useful in determining the potential risk to human
health through both recreational activities and through possible contamination from handling
fish.
13
2.4 Purpose of Study
There is a need for nutrient and bacterial levels to be assessed in Lake Zapotlán. Determining
the nutrient concentration trends and balance of any lake or reservoir also constitutes an
important tool in understanding its chemical and biological processes (Shear & de Anda, 2005).
Water quality has evidently decreased in the past decade due to anthropogenic sources such as
increased wastewater input, increased urban runoff, and increased fertilizer use on adjacent
farms. These activities have the potential to affect the ecology of the Lake as well as create
potential human health problems associated with high bacteria levels in the Lake.
2.5 Research Questions
A comparison of past nutrient levels in the Lake has never been undertaken, nor has a
phosphorus mass balance of the Lake or a bacteriological study of the entire Lake. The purpose
of this research is to:
1) identify the nutrient trends in Lake Zapotlán over the last 13 years ;
2) identify the nutrient trends in the Lake during the wet and dry seasons to assess for differences
between the seasons, and to determine the source and patterns of nutrients in the Lake;
3) complete a preliminary phosphorus balance in the Lake to determine possible sources of
nutrients to the Lake; and
4) to assess the bacteria levels in the Lake to determine potential human health risks and to
determine the recreational water quality in Lake Zapotlán.
This information will be useful in identifying the quality of the water to assess risks in irrigation,
fishing, and recreational use in the town.
14
Chapter 3: Methods
The methods involved in sample collection to assess the water quality in Lake Zapotlán for both
nutrient and bacteria concentrations are described below.
3.1 Sample Collection
Water samples were collected on 3 occasions over a period of one year. The first sampling
period was from October 4-8, 2007 at the end of the rainy season; the second from February 18-
21, 2008 during the dry season; and the third from July 1-3, 2008 during the rainy season.
During each sampling period, all station locations were determined using a GPS for
reproducibility in future site visits. Dissolved oxygen, air temperature, water temperature,
conductivity, pH, total suspended solids, and turbidity measurements were taken at each station.
The number of samples differed during the three sampling periods, as well as the method of
nutrient analysis. These differences are described in the sections below.
3.2 Nutrient Sampling
3.21 Sample Collection in October, 2007
Twenty-six water samples and eight sediment samples were collected from twelve stations on the
surface and in surrounding wastewater discharge channels of Lake Zapotlán from October 4-8,
2007, during the end of the rainy season. Ten fish samples were also collected from local
fishermen during this sampling period. Of the twenty-six water samples collected, thirteen were
acidified and thirteen were not acidified to prepare for the difference in analytical procedures,
depending on the auto-analyser available for use in the Mexican Federal Laboratory as described
in the analysis section below. The twenty-six water samples were collected to analyse for total
phosphorus (total P), total nitrogen (total N), nitrate (NO3), and dissolved organic carbon (DOC)
according to the United States Environmental Protection Agency (USEPA) water quality
guidelines (USEPA, 2007). The sediment samples were collected to analyse for total P and total
N and the fish samples were collected to analyse for total P, total N, and bioavailable
phosphorus.
For water, seven stations were sampled from a small fishing boat within 50m - 300m of the
shoreline (stations 2, 3, 5, 6, 7, 8, and 10), and one from the deepest point in the Lake (station 4),
15
where both a surface and a 3m depth sample were taken (Figure 3.1). The Lake depth ranged
from 1.4m to 4.1m, making surface grab sampling a sufficient method for Lake Zapotlán. It was
not possible to get closer to the shore due to extensive mats of �tule� (Typha latifolia). Four
discharge channels were sampled by driving to the site (stations 1, 9, 11, and 13). Three stations
(1, 11 and 13) consisted of untreated sewage from the local wastewater treatment plant
(discharge channels), and one was a natural wetland (station 9) (Figure 3.1). A 4L jug was used
to collect surface grab water samples. A Kemmerer bottle was used to collect depth samples at
station 4. All water samples that were to be acidified (total P) were done so with 2 ml/L
concentrated sulphuric acid (H2SO4), then stored in a cooler at 4o C (US EPA, 2007). All non -
acidified samples (ortho P, total N, DOC) were placed in a freezer at -20 o C until further
analysis.
Sediment samples were collected at all stations on the Lake (2, 3, 4, 5, 6, 7, 8, 10) using an
Ekman Dredge. Approximately 10g of wet sediment was placed into Ziploc bags, labelled for
further analysis, and then stored in a 4o C cooler until sampling was completed for the day. They
were then stored in a freezer at -20 o C until laboratory analysis. The fresh fish that were
collected from the local fishermen were also put into a freezer immediately until analysis took
place at the laboratory.
All water, sediment, and fish samples to be analysed for total P, total N and NO3 were taken to a
federal laboratory in Guadalajara, Mexico (Centro de Investigación y Asisténcia en Tecnología y
Diseño del Estado de Jalisco (CIATEJ) for analysis. Water samples that were to be analysed for
DOC were returned to the University of Toronto Mississauga (UTM) in Ontario, Canada.
16
Figure 3.1: Sampling Stations in Lake Zapotlán, Jalisco, Mexico, October, 2007
Source: Google Earth, 2009
3.22 Sample Analysis in October, 2007
Half (2L) of each 4L jug of water sample was filtered using 0.45µm cellulose membrane filters
using a vacuum pump. The water samples were analysed for total P and particulate P at CIATEJ
using the Mexican National Standards (Secretaría de Medio Ambiente y Recursos Naturales,
2007). Total P was measured in the unacidified samples that were analysed within 48 hours.
Total P and total N in sediment samples, as well as total P, total N and bioavailable P in fish
samples were all analysed using these same Mexican National Standard Procedures at CIATEJ,
NOM-AA-26-1980 for total N and NOM-AA-50-1978 for total P (Instituto Nacional de
Ecologia, 2005).
Water samples were analysed at UTM for DOC and total N with a Lachat IL 550 TOC TN auto-
analyzer after an adjustment to pH 2 using method 10-140-39-1-C (Latchat Instruments,
Milwaukee, WI, USA).
17
3.23 Sample Collection in February and July, 2008
Fifty-six water samples and eight sediment samples were collected from fourteen stations on the
surface and in surrounding discharge channels and wetlands of Lake Zapotlán from February 18-
21, 2008 during the dry season and from July 1-3, 2008 during the wet season. Twenty-eight
water samples were collected to analyse for total P, of which fourteen were filtered in situ and
fourteen were left unfiltered. The next twenty-eight water samples were collected to analyse for
total N, orthophosphate (PO4), and DOC, of which fourteen were filtered in situ and fourteen
were unfiltered. The eight sediment samples were collected to analyse for total P, PO4, and total
N.
Eight stations were sampled at the surface from a small fishing boat on the Lake (stations 2, 3, 4,
5, 8, 10, 12, and 14) [Figure 3.3]. Six surrounding wastewater discharge channels were sampled
by driving to the site (stations 1, 9, 11, 13, 15 and 16) (Figure 3.2). Four stations (1, 11, 13, and
15) consisted of untreated sewage from two local wastewater treatment plants, one station (16)
consisted of treated wastewater, and one station (9) was a natural wetland (Figure 3.3).
a) b) Figure 3.2: Photos of sampling the surrounding wastewater discharge channels at station 13 in a) October, 2007 and b) February, 2008 Although depth samples were taken at station 4 in the October, 2007 sampling period, only
surface samples were taken during the February, 2008 and July, 2008 sampling periods. Analysis
of the October, 2007 depth sampling at station 4 determined that nutrient concentrations were
very similar throughout the water column due to constant mixing in the shallow Lake.
18
Some sampling station locations differed slightly in February, 2008 and July, 2008 from those
stations sampled in October, 2007. Station 16 was only sampled in July, 2008, the wet season,
because this discharge channel did not have any flowing water during the dry season (February,
2008).
Station 13 is the main Ciudad Guzman wastewater discharge channel and could be a main
contributor of nutrients to the Lake. For this reason, water samples from station 13 were
collected twice in July, 2008; one sample was collected on the first day, July 1, 2008 of sampling
and then one on the last day, July 3, 2009 of sampling to confirm consistency in the nutrient
concentrations in the wastewater input stream. Results showed that the nutrient concentration in
both water samples from station 13 were very similar; therefore, station 13 was only sampled
once in July, 2008.
Station 15 was sampled in February, 2008, but was not sampled in July, 2008. After further
research on the discharge channels, it was found that station 15 was actually upstream of the
wastewater discharge channel that had already been sampled at station 13. Because station 15
was located upstream of station 13, it was only necessary to collect the water samples from
station 13, which is closer to the Lake and is more representative of nutrient concentrations
entering Lake Zapotlán.
Two nearshore Lake stations (station 6 and 7) from the October, 2007 sampling period were
omitted during the February, 2008 and July, 2008 sampling because analysis from the October,
2007 samples concluded that stations 14 and 2 (which are close to stations 6 and 7) represented
these two stations sufficiently. Station 5 was moved slightly to the South-West in February, 2008
and July, 2008 to represent the centre of the Lake better.
Total P water sampling was conducted by dipping a 30ml glass vile into the water, rinsing the
sample bottle three times, and then filling it to the top, then acidifying the total P samples with
either 0.3ml of 25% HCL acid in February, 2008 or 0.3ml of 30% H2SO4 in July, 2008 according
to the National Laboratory for Environmental Testing (NLET) schedule of services standards
(Environment Canada, 2008). All acidified samples (pH <2), were then stored in a cooler at 4o C
19
(US EPA, 2007). HCL was the only preservative available during the February sampling period.
This is not an ideal preservative, but it provides the sample with a pH <2 and is sufficient for the
analysis to be completed (Carlson & Simpson, 1996). All filtered samples were collected by
filling a clean syringe with the station water and then filtering the water through a Millipore
Swinnex filter holder with a 0.45µm glass fibre filter inside (Basiliko, 2008). According to
Carlson and Simpson, (1996), cellulose membrane filters are ideal for filtering most particulates
for soluble reactive phosphorus (SRP), but glass fiber filters are effective and more consistent to
use for several analytical tests (total P, total N, and DOC). For this reason, glass fiber filters
were used for both the February, 2008 and July, 2008 sampling periods. All non-acidified
samples (ortho P, total N, and nitrate) were kept in a cooler in the field, and then transferred to a
freezer until further analysis.
In July, 2008 an additional thirteen filtered and thirteen unfiltered replicate water samples were
taken to cross reference with those samples analysed at the University of Toronto Mississauga
laboratory due to analytical delays from the February, 2008 sample analysis. These samples
were collected in 100ml glass jars and then preserved with 1ml 30% H2SO4 to analyse for total P
at the National Laboratory for Environmental Testing (NLET). This laboratory is fully
accredited through the Canadian Association for Environmental Analytical Laboratories
(CAEAL) and was used to ensure current analysis at the University of Toronto was accurate.
The replicate samples were also collected and sent to NLET to ensure samples were processed in
a timely manner in case there were time delays due to difficulties with the auto-analyser at UTM,
as there had been for the February sample analysis.
Sediment samples were collected at all stations in the open Lake (2, 3, 4, 5, 6, 7, 8, 10) using an
Ekman Dredge. Approximately 10g of wet sediment was placed into Ziploc bags, then labeled
and placed in a freezer for further analysis at the laboratory.
20
Figure 3.3: Sampling Stations on Lake Zapotlán, Jalisco, Mexico, February, 2008 and July 2008
Source: Google Earth, 2009
Additional water samples were collected in July, 2008 from the arroyos (intermittent streams)
surrounding the Lake. These runoff samples were collected from three (3) arroyos at 20 min
intervals during a precipitation event in July, 2008 to determine the average concentration of
total P from urban and agricultural runoff into Lake Zapotlán. There are many intermittent
arroyos surrounding Lake Zapotlán, but only three were chosen for sampling (figure 5.1) because
of the high probability of water in them reaching the Lake, and because of time constraints
during event sampling.
3.24 Sample Analysis in February, 2008 and July, 2008
To determine PO4 in the sediment, as well as in filtered and unfiltered water samples, 8ml of
sample was syringed out of the original sample container and placed in a 10ml vile to be auto-
analysed using Quik-Chem method 10-115-01-1-A on a Latchat FIA+8000 series continuous
flow auto-analyzer (Lachat Instruments, Milwaukee, WI, USA).
21
A sub-sample of each extract was oxidized in Potassium persulfate to determine total P in
solution (Williams et al., 1995). To measure total P in the water samples, 2.5ml of sample was
mixed with 5ml K2SO4, and then sealed in a container, shaken for one hour at 200 rpm on an
oscillating shaker, autoclaved for one hour, settled for 24 hours, and then syringed out for
analysis (Basiliko et al., 2006). Total P was auto-analysed using Quik-Chem method 10-115-01-
1-A on a Latchat FIA+8000 series continuous flow auto-analyzer (Lachat Instruments,
Milwaukee, WI, USA).
To determine total P in the sediment, approximately 5g of wet sediment was weighed out in a tin
dish, then sealed in a container with 0.5 M potassium sulfate (K2SO4) solution (3g NaOH and
13.8g potassium persulfate), shaken for one hour at 200rpm on an oscillating shaker, autoclaved
for one hour, settled for 24 hours and then syringed out for analysis according to Basiliko et al.
(2006). Total P in the sediment was analysed using Quik Chem Method 10-115-01-1-A (Latchat
Instruments, Milwaukee, WI, USA).
Water samples analysed for DOC and total N were poured into 10ml glass tubes and analysed
using the Lachat IL 550 TOC TN auto analyzer after a pH adjustment to 2 using method 10-140-
39-1-C (Latchat Instruments, Milwaukee, WI, USA). After analysis, it was found that the DOC
samples that had been acidified in the field had caused the humic fraction of the DOC to
precipitate. This altered the DOC values and therefore, they could not be used for analysis in
this thesis. Evan Malczyk, another MSc student at the University of Mississauga collected water
samples for DOC at the same stations in February and July, 2008, so these samples were used for
analysis in this thesis instead.
3.3 Escherichia coli Sampling
3.31 Sample Collection for October, 2007, February, 2008, and July, 2008
Escherichia coli (E.coli) samples were collected on the Lake and in surrounding discharge
channels at the same time as water and sediment were collected during all three sampling
periods. Nine E.coli samples were collected from stations 2, 3, 4, 5, 6, 7, 8, 10, and 13 in
October, 2007 (Figure 3.3), and ten E.coli samples were collected from stations 1, 2, 3, 4, 5, 8,
10, 12, 13 and 14 in both February and July, 2008 (Figure 3.3). The number of samples and
22
stations changed in February and July from those stations sampled in October to coincide with
the additional stations sampled during these latter sampling periods as described previously.
Samples for bacteriological analysis were collected in 500ml sterile bottles provided by the
University of Guadalajara, Centro Universitario del Sur (CUSUR) campus. The sample bottles
were kept closed until they were filled. The bottle was held near the base, filled to the top at a
depth of 15 to 30cm below the surface facing towards the current, then capped immediately
(Great Lakes Information Network, 2006). Immediately after the samples were collected, they
were stored in a cooler with ice to maintain a temperature below 10o C, and then brought to the
CIATEJ or CUSUR bacteriological laboratory for analysis.
3.32 Sample Analysis for October, 2007, February, 2008, and July, 2008
In October, E.coli samples were brought to CIATEJ and were analysed for the total colony
(fecal) forming units per 100 millilitres (CFU/100ml) using the Mexican National Standards
(Secretaría de Medio Ambiente y Recursos Naturales, 2007).
In February 2008 and July 2008, the E.coli samples were brought immediately to CUSUR to
analyse for total colony forming units per 100 millilitres (CFU/100ml) using the Microbiológico
de Agua, Laboratorio de Microbiología standards at CUSUR (Sepulveda, 2008).
23
Chapter 4: Nutrient Trends in Lake Zapotlán
*This chapter uses content from Water Pollution IX, Greenberg, T., Shear, H., de Anda Sanchez, J. and Ortiz-Jimenez, M-A, Preliminary Analysis of Water Pollution in a Small Lake in Western Mexico, pp13-21, 2008, ISBN, with permission from WIT Press, Southampton, UK. 4.1 Nutrient Trends in Lake Zapotlán over 13 years Data from past nutrient studies on Lake Zapotlán, including the limnological survey by the
University of Guadalajara (1994), and the nutrient/food chain model in 2003 (Oritz-Jimenez et
al., 2006a) were used to compare nutrient concentrations with those completed for this thesis in
2007. The nutrient studies in 1994 and 2003 both analysed nutrients in October, providing a
consistent timeframe to compare nutrient concentrations over the past 13 years in 1994, 2003,
and 2007. Only those stations that were coincident with each other for all three years were
selected for analysis. Analytical methods for 1994 are based on National Institute of Ecology
standards (National Institute of Ecology, 2005). Analyses of the 2003 and 2007 samples are
based on the standards used at the CIATEJ laboratory (Secretaría de Medio Ambiente y Recursos
Naturales, 2007). A comparison of past nutrient levels in the Lake had never been determined.
This section will compare nutrient levels in 1994, 2003 and 2007 as well as over one year (in the
wet and dry seasons) to determine the trends in nutrient levels and seasonal patterns in the Lake.
It will also discuss the work necessary to complete a phosphorus mass balance of the Lake.
4.12 Results Basic physical and chemical parameters for some representative Lake stations from October
2007 are presented in Table 4.1.
24
Table 4.1: Basic physical and chemical parameters for some representative Lake stations Reprinted from: Water Pollution IX, Greenberg, T., Shear, H., de Anda Sanchez, J. and Ortiz-Jimenez, M-A, Preliminary Analysis of Water Pollution in a Small Lake in Western Mexico, pp13-21, 2008, ISBN, with permission from WIT Press, Southampton , UK
Parameter Units Sta 2 Sta 3 Sta 4 Sta 4 Sta 5 Sta 8 Sta 10 Date yyyy-
mm-dd
2007-10-06
2007-10-05
2007-10-05
2007-10-07
2007-10-05
2007-10-06
2007-10-05
Lake Depth m 3.37 3.7 4.1 4.1 3.1 1.5 1.8 Sample Depth m 0.5 0.5 0.5 3.5 0.5 0.5 0.5 Water Temperature
°C 26.6 27.0 25.7 25.7 26.4 24.3 26.9
Conductivity uS/cm 679 690 683 683 688 686 681 Dissolved Oxygen
mg/l 10.53 9.81 8.76 8.76 9.34 6.25 8.83
pH 8.83 8.73 8.86 8.86 8.97 8.97 8.97 Secchi Depth cm 31 35 28 28 27 28 26 Total Dissolved Solids
mg/l 762 768 763 763 767 768 760
It is apparent from these data that the Lake is horizontally and vertically well mixed with regard
to temperature. Variations in dissolved oxygen, pH and turbidity correlate with the location of
stations near stands of tule or in very shallow water. At the deepest station (4), a depth profile
shows that the Lake is also well mixed vertically.
4.13 Nitrogen Comparison of 1994, 2003 & 2007
The results for total nitrogen (total N) concentrations for October of 1994, 2003, and 2007 in the
Lake are shown in Figure 4.1. Nitrogen concentrations were similar for 1994 and 2003 (p>0.05),
but both are consistently lower than in 2007 (p<0.05). The 1994 total N values averaged 1.4
mg/l ± 0.7. The average total N for 2003 was 1.4 mg/l ± 0.08, and concentrations were similar at
all stations. In 2007, however, average total N values were 3.7 mg/l ±0.3.
25
Figure 4.1: Comparison of Total Nitrogen Levels for October of 1994, 2003, and 2007. Reprinted from: Water Pollution IX, Greenberg, T., Shear, H., de Anda Sanchez, J. and Ortiz-Jimenez, M-A, Preliminary Analysis of Water Pollution in a Small Lake in Western Mexico, pp13-21, 2008, ISBN, with permission from WIT Press, Southampton , UK
4.14 Phosphorus Comparison of 1994, 2003, & 2007
The data for total phosphorus in the Lake showed some horizontal variability in concentrations
for all three years analyzed (figure 4.2). Furthermore, a comparison of the average phosphorus
concentrations in October 1994, 2003, and 2007 showed a significant difference (p<0.05)
between concentrations in 2003 and those in 1994 and 2007. The 1994 phosphorus
concentrations averaged 0.11 mg/l ± 0.06; the 2003 levels averaged ten times higher at 1.15 mg/l
± 0.32; and the 2007 values averaged 0.23 mg/l ± 0.06. Phosphorus in 2003 was highest at all
stations, except station 12, where total P was highest in 2007.
26
Figure 4.2: Comparison of Total Phosphorus in October of 1994, 2003, and 2007
Reprinted from: Water Pollution IX, Greenberg, T., Shear, H., de Anda Sanchez, J. and Ortiz-Jimenez, M-A, Preliminary Analysis of Water Pollution in a Small Lake in Western Mexico, pp13-21, 2008, ISBN, with permission from WIT Press, Southampton , UK 4.15 Discussion: Nitrogen Trends over 13 Years
The total N concentrations have shown an increase in the Lake since 1994. At these
concentrations, total nitrogen was at or below the standards for US states (EPA, 2003) in 1994
and 2003, but exceeded most standards (> 1.0 mg/l) in 2007. The 2.1% growth in population in
Ciudad Guzman (Oritz-Jimenez, 2006) may have contributed to increased nitrogen
concentrations through the flow of partially treated wastewater into Lake Zapotlán in 2007.
Increased fertilizer use (Rocha Chavez, 2008) on the adjacent farmland would also have
increased nitrogen concentrations in the Lake after precipitation events through runoff. The high
total N concentration in station 9 could be attributed to the cattle grazing directly in the wetland
during most of the year (Rocha Chavez, 2008). Also, high levels of fish stocking in 2005 could
have resulted in high nitrogen concentrations through an increase in fish waste excretion. In
addition, the high total N concentrations in the sediment (1600 to 2900 ppm in 2007), indicates
that it could be a potential source of nitrogen release to the water column.
This preliminary analysis of historical and recently collected data shows that there is an
increasing trend in total nitrogen in Lake Zapotlán. Total N concentrations presently exceed US
EPA standards (EPA, 2003).
27
4.16 Discussion: Phosphorus Trends over 13 years A possible explanation for the large difference in phosphorus concentrations between 2003 and
2007 is that the wastewater treatment plants were not working properly in 2003, resulting in a
large amount of phosphorus entering the Lake (CEASJ, 2007). This could also explain the
higher total P levels in 2007 at station 12. The Ciudad Guzman wastewater treatment plant was
not working properly during this sampling period (Rocha Chavez, 2008) and the flow from the
plant which enters the Lake near station 12 could have resulted in high levels of phosphorus
entering the Lake at this point.
Runoff could potentially be contributing to the high phosphorus concentrations in the Lake as
well. Ortiz - Jiménez et al. (2006b) indicated that 53.3% of the annual inflow to Lake Zapotlán
comes from runoff, whereas sewage discharge accounts for 11.38% of the annual inflow to the
Lake. Furthermore they point out that average surficial runoff to Lake Zapotlán from 1982-2003
was 18.9 Mm3. In 1994, however, that runoff was just 7 Mm3, whereas in 2003 it was 17.5 Mm3
(Ortiz-Jiménez et al., 2005). This would have resulted in far less total P entering the Lake from
surficial runoff in 1994 than in 2003, possibly explaining the lower phosphorus concentrations in
1994. In 2007, precipitation levels were 26% lower than in 2003 (Figure 2.1), so the runoff
would generally be about 26% less than in 2003, or 12.4 Mm3. This indicates that runoff in 2003
was somewhat higher than in either 1994 or 2007, and this could account for some of the
elevated P concentrations seen in the Lake in 2003.
4.2 Seasonal Comparison of Nutrients in Lake Zapotlán
4.21 Seasonal Physical and Chemical Parameters It is apparent from the seasonal data (Table 4.2) that Lake Zapotlán is horizontally and vertically
well mixed. The shallowness of the Lake and the near constant wind allows sunlight and
nutrients to reach the bottom, enabling photosynthesis to occur throughout the water column.
The high temperatures, high dissolved oxygen levels, and high pH likely enable rapid biological
productivity and growth in the Lake. Nutrients show similar patterns to the physical and
chemical parameters over the wet and dry seasons (i.e. conductivity, dissolved oxygen, and TDS
all vary depending on season).
28
Table 4.2: Basic physical and chemical parameters averaged over a year in Lake Zapotlán
4.22 Seasonal Nitrogen Comparison for October, 2007, February, 2008, and July, 2008
Total Nitrogen in Lake Zapotlán averaged 1.4 mg/L ±0.42 over the period of study. The highest
total N concentration was found in October (3.79 mg/L), at the end of the wet season, and the
lowest total N concentration was found in February (0.93 mg/L), during the dry season (Figure
4.3). Total N concentrations in all seasons are significantly different from one another (p<0.05).
Oct-07 Feb-08 Jul-08
Mean Max Min Mean Max Min
Mean Max Min Lake Temperature (°C) 26.2 28.2 24.3 19.0 20.7 16.2 24.2 26.5 21.2 Air Temp.(°C) 24.3 26.0 28.2 21.4 23.4 18.0 27.1 31.7 23.5 Dissolved Oxygen (mg/L) 8.3 9.8 8.7 7.6 10.4 4.1 N/A N/A N/A pH 8.9 9.0 0.3 8.5 8.9 8.0 8.7 9.1 7.7 Depth (m) 2.4 4.1 1.4 1.8 4.3 0.5 2.8 4.4 1.9 Conductivity (S/cm) 684 690 679 1072 1533 975 944 1923 317 Total Dissolved Solids (mg/L) N/A N/A N/A 0.7 0.6 0.6 0.6 0.7 0.2
29
Figure 4.3: Total Nitrogen in Lake Zapotlán for October 2007, February, 2008, and July, 2008
Stations 2 and 3 had the highest total N concentrations in October, 2007 at 2.56 mg/L and 3.79
mg/L respectively. Lowest total N concentrations were at stations 2 (0.85 mg/L), 3 (0.85 mg/L)
and 4 (0.83 mg/L), although most total N concentrations were constant throughout the Lake in
February.
Total N in the surrounding wastewater discharge channels was highest in February at station 13
(74.5 mg/L), the Ciudad Guzman wastewater effluent channel (Figure 4.4). Lowest total N
concentrations were found at station 9 (2.6mg/L), the wetland, during all three seasons. Total N
concentrations were also high at station 11 in February (19.7mg/L).
30
Figure 4.4: Total Nitrogen in Lake Zapotlán Discharge Channels for October, 2007, February, 2008, and July, 2008
Total N in sediment ranged from 1600 mg/kg at station 8 (closest to the wetland without
wastewater discharge) to 2900 mg/kg at station 2 (closest to the wetland with constant
wastewater discharge). The mean total N in the sediment was 2275 mg/kg ±570mg.
4.23 Phosphorus Seasonal Comparison from October, 2007 to July, 2008 Phosphorus levels in Lake Zapotlán over one year in October 2007, February 2008, and July
2008 averaged 0.25 mg/L ± 0.13 (Figure 4.5). Results indicate that total P is significantly higher
in October (wet season) than in February (dry season) (p<0.05), but generally the same in
October and July (wet seasons) (p >0.05), as well as in February and July (wet and dry seasons)
(p>0.05) in 2007.
31
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
1 2 3 4 5 6 7 8Stations
Tota
l Pho
spho
rus
(mg/
L)
October
February
July
Figure 4.5: Total Phosphorus in Lake Stations for October 2007, February, 2008, and July, 2008
Station 12 had the highest total P levels in both February 2008 and July 2008 at 0.47mg/L and
1.88mg/L respectively. Stations 2 and 4 had the lowest total P levels in February at 0.07mg/L
and 0.09mg/L respectively. Station 12 is closest to the Ciudad Guzman wastewater effluent
outlet, whereas stations 2 and 4 are more central in the Lake; however, station 5, in the centre of
the Lake had a higher total P level than stations 2 and 4, at 0.16mg/L in February. The July
samples were collected after a major rainfall. The wastewater effluent may have created a plume
in July, 2008, causing the higher total P levels in the centre of the Lake during the wet season. It
is highly likely that excessive untreated wastewater was bypassed during this rain event and
contributed to the high phosphorus concentrations near the Ciudad Guzman wastewater effluent
outfall.
Total P concentrations in the wastewater effluent discharge channels entering Lake Zapotlán
complement the high total P concentrations at station 12 (Figure 4.6). Station 13, which is the
32
wastewater effluent channel that flows directly into Lake Zapotlán at station 12, had the highest
total P concentrations (9mg/L � 17.2mg/L) out of all the discharge channels flowing into the
Lake for all three seasons. The highest total P concentrations at station 13 were in October, 2007
(17.2mg/L), followed by February, 2008 (16.8mg/L), and July (9mg/L). This is opposite to the
seasonal pattern for station 12 (outfall for station 13), where the lowest total P concentrations
were found in October, 2007 and highest total P concentrations were found in July, 2008.
0
2
4
6
8
10
12
14
16
18
20
1 9 11 13Station
Tota
l Pho
spho
rus
(mg/
L)
October
February
July
Figure 4.6: Total Phosphorus in Discharge Channels for October 2007, February, 2008 and July 2008
Station 1 also had the second highest total P concentrations (6.0mg/L � 7.72 mg/L). This station
is in the wetland adjacent to the Ciudad Guzman wastewater effluent canal (Station 13); the
wetland may be acting as a filter for a portion of the effluent before it enters the Lake. Station 11
is closest to the San Sebastian del Sur wastewater effluent canal and had high total P
concentrations in February, 2008 (8.09mg/L). The San Sebastian del Sur wastewater treatment
plant was not working during this sampling period (Rocha Chavez, 2008, personal
communication). Station 9, a wetland site, had the lowest total P concentrations (0.1-0.98mg/L).
It is also the only discharge channel station that is does not have wastewater effluent in it.
33
A comparison of orthophosphate concentrations in Lake Zapotlán in February and July (Tables
4.3 & 4.4) demonstrate that the percent of orthophosphate (of total P) in the Lake is higher in
July, the wet season, than in February, the dry season. February has the highest percent of
orthophosphate concentrations at station 8 (86.6%), close to a large wetland, and station 14
(80.3%), closest to the San Sebastian del Sur wastewater effluent outflow. The lowest percent of
orthophosphate in February are found at stations 4 (23.2%), 3 (25.9%) and 5 (27.3%). Station 4
is the deepest spot in the Lake, station 3 is closest to a wetland, and station 5 is in the centre of
the Lake.
Table 4.3: Percent (%) of Orthophosphate to Phosphorus in Lake Zapotlán, February 2008
% orthophosphate to Phosphorus in February 2008
Station 2 3 4 5 8 10 12 14 MeanTotal Orthophosphate 0.0354 0.0504 0.0209 0.0434 0.237 0.153 0.208 0.144 0.112 Total Phosphorus 0.075 0.194 0.090 0.159 0.274 0.210 0.471 0.179 0.207 % Orthophosphate 47.2% 25.9% 23.2% 27.3% 86.6% 72.9% 44.2% 80.3% 50.9% Table 4.4 Percent (%) of Orthophosphate to Phosphorus in Lake Zapotlán, July 2008
% orthophosphate to Phosphorus in July 2008
Station 2 3 4 5 8 10 12 14 MeanTotal Orthophosphate 0.195 0.132 0.188 0.228 0.241 0.192 0.774 0.322 0.284 Total Phosphorus 0.293 0.277 0.269 0.336 0.377 0.231 1.88 0.323 0.498 % Orthophosphate 66.6% 47.7% 69.9% 67.9% 63.9% 83.1% 41.2% 99.7% 67.5% The highest percent of orthophosphate in July is found at stations 14 (99.7%) (closest to San
Sebastian del Sur wastewater effluent outfall), and station 10 (83.1%) (closest to station 14). The
lowest percent of orthophosphate concentrations are found at station 12 (41.2%), closest to the
Ciudad Guzman wastewater effluent outflow, and station 3 (47.7%), near centre of Lake.
Total P concentrations in the sediment averaged from 0.055mg P/g dry weight in February, the
dry season, and 0.029mgP/g dry weight in July, the wet season (Table 4.3). Highest total P
34
concentrations were found at station 12 in both the wet and dry seasons (0.062 mg P/ g dry
weight and 0.24 mg P/ g dry weight, respectively). This is the station closest to the Ciudad
Guzman wastewater effluent discharge outfall. Lowest total P concentrations were found at
station 2 (0.0061 mg P/g dry weight), the station closest to a large area of wetlands.
Station February July 2 0.0063 0.0059 3 0.0640 0.0402 4 0.0519 0.0490 5 0.0043 0.0376 8 0.0789 0.0174
10 0.0381 0.0135 12 0.2437 0.0619 14 0.0096 0.0059
Min. 0.0043 0.0054 Max. 0.2437 0.0619 Mean 0.0557 0.0298
Table 4.5: Total Phosphorus per gram of Sediment in Lake ZapotlánZapotlán
4.24 Discussion: Seasonal Nitrogen Patterns
Seasonal total N concentrations in Lake Zapotlán demonstrate higher values in October, 2007,
the end of the wet season. Stations 2 and 3 are situated closest to the wetlands surrounding the
Lake and have the highest total N concentrations in October. It is possible that the wetlands
were contributing to higher total N concentrations through decomposition of the aquatic plants
found near these stations at this time of the year (Cronk & Fennessy, 2001).
Although stations 11 and 13, the wastewater discharge channels for San Sebastian del Sur and
Ciudad Guzman, show high total N levels in February, the outfall stations closest to these
channels (stations 14 and 12) do not reflect these values. Stations 12 and 14 actually show lower
values in February, than in July when total N levels in the discharge channels is lower. The
aquatic plants in the discharge channels could be utilizing some of the nitrogen before it is able
to enter the Lake.
The increase in precipitation in July, could be contributing to an increase in total N through
runoff from both agricultural fertilizer and manure (station 8 -1.53 mg/L) as well as from
wastewater effluent runoff (station 14 -1.53mg/L; station 12 - 1.77mg/L). Figure 4.7 shows the
35
July data indicating that the total N concentrations are highest at those stations located close to
the wastewater effluent channels, but significantly less than in the channels themselves. Similar
reductions of total N from the effluent channel to the Lake can be seen in figures 4.3 and 4.4 The
wetlands on the southern and eastern part of the Lake clearly have a role in reducing these high
total N concentrations.
1.531.43
1.40
1.36
1.53
1.29
1.29
1.77
1.43 1.53
1.36
1.40
1.53
1.77
1.29
1.29
44.055.71
Figure 4.7: Example of the role of wetlands in reducing Total N concentrations (mg/L) in the Lake, July 2008 *Bars on diagram represent total N levels: the taller and wider the bar, the more total N present. Values on bars are in mg/L. The average total N concentration in Lake Zapotlán of 1.4 mg/L ±0.42 results in it being
classified as slightly eutrophic according to standards established in many US states (total N >
1mg/L) (EPA, 2003). However, in comparison to other shallow lakes worldwide, such as Lake
Okeechobee, USA (1.64mg/L ± 0.44), or Lake Taihu, China (2.34 mg/L ± 1.10), Lake Zapotlán
has low nitrogen concentrations (James et al., 2009).
4.25 Discussion: Seasonal Phosphorus Trends Results for total P show higher concentrations during the rainy seasons in the Lake (October and
July), and lower total P concentrations during the dry season (February). In the wet season, there
is an increase in precipitation, resulting in an increased flow in bypassed wastewater as well as
36
runoff flowing into the Lake from agricultural fields. Station 12 is closest to the Ciudad Guzman
sewage outflow and it is likely that partial or untreated sewage water was contributing to the high
total P concentrations at this station. Through personal observation while sampling over the
year, July, 2008 was the only sampling period where precipitation occurred, and the only
sampling period where there was actual flowing water in all of the discharge channels
surrounding the Lake.
This seasonal pattern observed could be a possible explanation for the seasonal total P levels in
stations 13 (wastewater canal) and 12 (within Lake station closest to wastewater outflow from
13). The high total P concentrations at station 12 in July, 2008 could be caused by increased
runoff and wastewater bypass from increased precipitation during the wet season, and the
effluent entering the Lake with little retention by the wetlands. Total P concentrations in station
13 (the main wastewater channel), which flows into the Lake near station 12, were lowest during
the wet season, likely due to dilution with rainwater (Figure 4.8). During the dry season, in
February and part of October, the water with the high total P concentration in station 13 is either
diverted through the wetlands and filtered before it enters the water, or it is used for irrigation
and does not enter the Lake. This may explain the lower total P concentrations in the Lake
compared to the wastewater channels (Figures 4.5 and 4.6) in the dry season. It should be noted
that even in July, during periods of high flow, the wetlands still act as a nutrient �filter� and
37
reduce the concentrations of total P entering the Lake (Figure 4.8).
0.240.17 0.19
0.21
0.19
0.24
0.27
1.68
8.11
7.44
Figure 4.8: The role of wetlands in reducing total P (mg/L) in the Lake, July 2008 *Bars on diagram represent total P levels: the taller and wider the bar, the more total P present. Values on bars are in mg/L. In comparison to other fresh water bodies, Lake Zapotlán has extremely high total P
concentrations (Reddy, 2005). Figure 4.9 shows an evaluation of the total P levels in Hamilton
Harbour, Ontario, Canada compared to total P values in Lake Zapotlán, Mexico.
38
86 87 88 89 90 91 92 93 94 95 96 97 98 99 00020406080
100120140160
Tota
l Ph
osph
orus
(ppb
)
Year
250Average Total P in Lake Zapotlan (0.25mg/L)
Figure 4.9: Comparison of Total Phosphorus Levels in Hamilton Harbour, Canada to those in Lake Zapotlán, Mexico Source: Bay Area Restoration Council, 2007
Total P levels in Hamilton Harbour over the past decade are considered high at 34µg per L (Bay
Area Restoration Council, 2007). Lake Zapotlán total P concentrations (0.25mg/L or 250 ppb
(µg/l)) are extremely high compared to those in Hamilton Harbour. Total P concentrations > 0.1
mg/L, or 100µg/L are considered dangerous to an aquatic system (EPA, 2003).
4.3 Overall Discussion on Nutrient Status in Lake Zapotlán
The trophic state of a lake can generally be found through calculating the Trophic State Index
(TSI) (Carlson, 1977) using three variables: chlorophyll, Secchi depth, and total phosphorus.
The secchi depth and total phosphorus values are available for Lake Zapotlán, but chlorophyll
values could not be collected for this research. Determining the chlorophyll concentrations for
this thesis would have been very beneficial to determine the overall water quality; however, the
necessary instrumentation was not available on site during this research. An estimate of the TSI
of Lake Zapotlán can be classified based on Figure 4.6 taken from Carlson & Simpson, 1996.
39
Table 4.6: Relating the Trophic State to the State of a Water Body Source: Carlson & Simpson, 1996
TSI Chl-a(ug/L) SD (m) TP (ug/L) Attributes <30 <0.95 >8 <6 Oligotrophy: Clear
water, oxygen throughout the year in the hypolimnion
30-40 0.95-2.6 4-8 6-12 Hypolimnia of shallower lakes may become anoxic
40-50 2.6-7.3 2-4 12-24 Mesotrophy: Water moderately clear; increasing probability of hypolimnetic anoxia during summer
50-60 7.3-20 1-2 24-48 Eutrophy: Anoxic hypolimnia, macrophyte problems possible
60-70 20-56 0.5-1 48-96 Blue-green algae dominate, algal scums and macrophyte problems
70-80 56-155 0.25-0.5 96-192 Hypereutrophy: (light limited productivity). Dense algae and macrophytes
>80 >155 <0.25 192-384 Algal scums, few macrophytes
The average secchi depth in Lake Zapotlán is 2.8m, falling in the range of Mesotrophic and the
total phosphorus in the Lake is 0.25mg/L or 250µg/L, falling in the range of over-hypereutrophy.
If these two components were averaged, then the Lake would be classified as eutrophic. The
EPA would also classify this Lake as eutrophic based on the fact that the nutrient concentrations
exceed the standards (total P concentrations > 0.1 mg /l; total N concentrations >1.0 mg/L)
(EPA, 2003). However, Lake Zapotlán does not demonstrate all of the qualities of a eutrophic
lake as listed above (table 4.6). During the period of study over one year, algae blooms were not
observed and the Lake was far from anoxic, but there were macrophyte problems with the Tule
and Lirio. Therefore, it was not relevant to classify the Lake based on the TSI index. The
nutrient concentrations in the Lake are extremely high, yet the Lake does not exhibit algae
blooms. Possible reasons for this abnormal lake behavior are difficult to understand and require
further research on the within Lake biogeochemical processes.
40
41
Chapter 5: Preliminary Phosphorus Balance in Lake Zapotlán
This chapter first discusses a general phosphorus mass balance for Lake Zapotlán, followed by a
discussion on the processes occurring within the Lake.
5.1 Phosphorus Inputs
Wastewater effluent and urban runoff are predicted to be the main nutrient input sources to Lake
Zapotlán (Oritz-Jimenez et al., 2005). There are several sources that could be contributing
nutrients to the Lake; however, most of the arroyos (intermittent streams) dry up during the
months of October to May (Rocha Chavez, 2008, personal communication). Through
observation during sampling periods, it was evident that even during a large precipitation event,
the water in most arroyos did not reach the Lake.
5.11Wastewater Effluent
During both the wet and dry seasons, there was a continuous flow of water entering the Lake
through the wastewater discharge channels from Ciudad Guzman and San Sebastian del Sur.
The amount of untreated wastewater effluent entering Lake Zapotlán from these wastewater
treatment plants (WWTPs) is considerable as shown below. Estimates of municipal untreated
sewage volumes into Lake Zapotlán were taken from data provided by the Jalisco State Water
Commission (CEASJ, 2008).
In Ciudad Guzman, treated water volumes for domestic and industrial use were estimated at
26,773,320 L/day (309.9 L/sec) based upon an average treatment volume of 280 L/person/day
with a population of 95,619 in 2007 (CEASJ, 2008; Malczyk, 2009, personal communication).
Since the treated volume of wastewater is about 70% of the total water used (309.9 L/sec), the
production of wastewater per person is about 217 L/sec. In 2007 the wastewater treatment plants
(WWTPs) in Ciudad Guzman were treating on average 147.3 L/sec. Estimated untreated
wastewater inputs from Ciudad Guzman are 70 L/sec. Therefore, an estimated 6 050 000L, or
6050 m3 of untreated wastewater enters the Lake from Ciudad Guzman per day, or 2.2Mm3 per
year.
42
The San Sebastian del Sur wastewater treatment plant was not in operation during the period of
study. Approximately 11 L/sec of wastewater flows directly to the Lake from this community,
meaning about 950 400 L/day, or 940.4 m3 of untreated wastewater enter Lake Zapotlán per day,
or 0.34Mm3 per year (CEASJ, 2008; Malczyk, 2009, personal communication).
Estimates of the amount of total phosphorus in these wastewater inputs were taken from the
Comisión Estatal del Agua (CEASJ, 2008). The average total P concentration over the three
sampling periods in October, 2007, February, 2008, and July, 2008 in the Ciudad Guzman
wastewater canals (combined) was about 11.32mg/L in the untreated and about 2.0mg/L in the
treated wastewater. This results in about 25 tonnes of phosphorus per year from the untreated
wastewater effluent and 4.9 tonnes/year from the treated wastewater effluent; therefore the
wastewater from both Ciudad Guzman WWTPs contributes about 30 tonnes of Phosphorus to
Lake Zapotlán per year.
The average total P concentration in the San Sebastian del Sur wastewater canal was about
2.77mg/L in the untreated canal (there is no treated wastewater in this canal). This results in an
estimate of about 0.96 tonnes/year of total P from this source. Based on the confidence in this
calculation, 0 meaning no confidence to 5 meaning full confidence (table 5.1), this calculation
would receive a confidence of 4 because of the estimated runoff.
Combined, both the Ciudad Guzman wastewater effluent (30 tonnes/year), and the San Sebastian
del Sur wastewater effluent (almost 1 tonne per year) contribute about 31 tonnes of Phosphorus
per year to Lake Zapotlán.
Table 5.1: Confidence Interval of Mass Balance Calculation Confidence Interval Explanation
0 No Confidence 1 Almost no Confidence 2 Lack of Confidence 3 Moderate Confidence 4 Confident 5 Full Confidence
43
5.12 Runoff
Runoff samples were collected from three (3) arroyos at 20 minute intervals during a
precipitation event in July, 2008 to determine the average concentration of total P from urban
and agricultural runoff into Lake Zapotlán.
Figure 5.1: Arroyo locations for the July, 2008 precipitation sampling event
Source: Google Earth, 2009
Arroyo 3 had the highest phosphorus concentrations averaging 1.2 mg/L; arroyo 1 had the
second highest concentration at 0.82 mg P/L; and arroyo 2 has the lowest concentrations at 0.31
mg P/L. Overall, the average total phosphorus concentration in runoff to the Lake was
0.77mg/L. Since the estimated runoff to Lake Zapotlán is 18.91Mm3/year (Ortiz-Jimenez,
2005), the estimated amount of phosphorus entering the Lake from runoff is 13.4 tonnes/year.
This calculation would receive a confidence of 3.5 since only three arroyos were sampled during
this precipitation event.
Combined urban runoff and wastewater effluent, both treated and untreated, provide
approximately 44.4 tonnes of Phosphorus per year to Lake Zapotlán.
44
5.2 Phosphorus Stored within the Lake
5.21 Phosphorus in Lake Water
Total Phosphorus concentrations in Lake Zapotlán over the three sampling periods in October,
2007, February, 2008, and July, 2008 averaged 0.25 mg/L (Figure 4.5). The volume of the Lake
is about 19.612 Mm3 (Ortiz-Jimenez, 2005), so the water in Lake Zapotlán contains about 4.9
tonnes of Phosphorus. This calculation has a confidence interval of 4 due to the estimate of the
Lake volume.
5.22 Phosphorus in Fish The average weight of the fish in the Lake is about 230g (Malczyk, 2009, personal
communication) and about 301 tonnes of fish are harvested each year. This indicates that the
fishermen are extracting about as many fish as they are stocking each year. Each fish has about
2.1mg of total phosphorus per fish, which means that about 0.63 tonnes of phosphorus is held
in the fish in the Lake.
5.22 Phosphorus in Sediments
The mean total P concentration per gram of sediment in Lake Zapotlán is about 0.04 mg TP/g
(Table 4.5). The actual sedimentation rate for Lake Zapotlán is unknown. The sedimentation
rate of Lake Chapala, a lake close to Lake Zapotlán, is about 0.35cm/year (Fernex et al., 2001; de
Anda et al., 2004). Because of high sediment deposition from runoff into Lake Zapotlán (Rocha
Chavez, 2008), the sedimentation rate in Lake Zapotlán is likely higher than the sedimentation
rate for Lake Chapala. Nevertheless, it is estimated that the top 1cm of sediment in Lake
Zapotlán likely reflects the years sampled during the period of study (2007-2008).
Based on very high dissolved organic carbon levels in Lake Zapotlán (mean= 48.5 mg/L), and
through observation of the sediment porosity and water content, the bulk density of the sediment
in Lake Zapotlán was estimated to be between 0.11 � 0.35 g/cc (Avnimelech et al., 2001; van
Dokkum et al., 1998).
The Lake is 1100 hectares (Oritz-Jimenez, 2005) or 11 000 000m2 in area and the average total P
concentration per gram dry weight of sediment sampled is 0.04 mg TP/g (Table 4.5). The bulk
density is estimated to be between 0.11 -0.35 g dry weight/cc, so the total phosphorus in the
45
surface sediments ranges from about 0.48 to 1.54 tonnes. This calculation has a confidence
interval of 2.5 because of the estimated range of bulk density.
5.23 Phosphorus in Water Hyacinth Water hyacinth (Eichhornia crassipes) could be a major sink for the phosphorus entering the
Lake. Total P in water hyacinth was not measured in Lake Zapotlán, but a study by Rommens et
al., (2003) on the uptake of phosphorus in water hyacinth in a similar subtropical eutrophic lake,
Lake Chivero in Zimbabwe, estimated that the total amount of phosphorus that was taken up in
840,000 m2 of E. crassipes each day was 50kg. Since Lake Zapotlán (area 11 x 106 m2) is
presently covered with about 2% E. crassipes (220,000 m2) (Figure 2.3), then the total amount of
phosphorus can be estimated at 13kg each day or 4745 kg (4.75 tonnes) of total phosphorus over
one year, with a confidence interval of 3, depending on the area of coverage and total phosphorus
in the plants. If shredding has occurred in a particular year, the total phosphorus in these plants
could account for an even larger sink of phosphorus in Lake Zapotlán.
5.24 Phosphorus in Tule Tule (Typha latifolia) is another aquatic plant that covers about 27% of Lake Zapotlán and could
be a potential sink of phosphorus as well. No measurements of phosphorus in the Tule were
taken, but a value can be estimated from the literature. Takashi estimated that the total Typha
latifolia dry weight biomass below the water line was about 2.39 kg/m2 (Takashi et al., 2005). If
these plants cover 27% of the 1100 hectares of Lake Zapotlán, or 297 hectares (2 970 000m2)
and have an estimated 2mg of phosphorus per gram of dry weight (Cronk & Fennessy, 2001),
then the Typha latifolia could account for 14.2 tonnes of Phosphorus in the Lake each year
below the waterline. The confidence interval is 4 because of tule area coverage estimates.
Total P concentrations in tule above the water line were not measured. A study by Maddison &
Mander (2005) estimated total P to be between 3000 � 3500mg TP/kg in tule leaves above the
water line. The above ground biomass of tule in their study varied from 0.32kg to 3.02kg dry
weight /m2 (Maddison & Mander, 2005). If tule covers 27% of the 1100 hectare Lake (297
hectares), and the average total P in the leaves is 3250 mg TP/kg, then the above water portion of
tule contains an estimated 3.09 - 29.1 tonnes of total P. The confidence interval in these tule
46
calculations is 2.5 due to the large range in dry weight estimates of the plants above the water
line.
The Typha latifolia and Eichhornia crassipes in Lake Zapotlán could potentially be the largest
sink of phosphorus in the Lake. Combined, the Lake water (4.9 tonnes), fish (0.63 tonnes),
sediments (0.48 � 1.54 tonnes), and aquatic plants (4.8 tonnes water hyacinth + 17.1 to 43.3
tonnes Tule = 21. 9-48.1 tonnes) store an estimated 27.9 - 55.1 tonnes of total Phosphorus.
5.3 Phosphorus Outputs
The main outflows from Lake Zapotlán are water extraction for irrigation (37.4%),
evapotranspiration (33.6%), and evaporation (29%) (CEASJ, 2003). A potential output of total P
is through fish harvesting. The fishermen harvest about 301 tonnes per year (Para, 200 & Gomez,
2009, personal communication) and they are extracting about as many fish as they are stocking.
The fish in Lake Zapotlán only had an average of 2.1mg TP/g dry weight of fish. It is likely that
fish are harvested at a young age (Rocha Chavez, 2008, personal communication). With a
harvest of about 301 tonnes per year, and an average of 2.1 mg TP/g of dry weight of fish, only
about 0.63 tonnes of total P is removed through fish harvest in Lake Zapotlán. The confidence
interval for this calculation is 3 because of the variability in fish harvest per year.
Another possible output source of total P from Lake Zapotlán is through the water used for
irrigation on the surrounding agricultural land. If 700 ha of the 1658 ha of land are irrigated
(CEASJ, 2008) with Lake water (0.25mgP/L), and the average water extraction estimate is
8.5Mm3 (8 500 000 000 L) (Oritz-Jimenez, 2005), then 0.89 tonnes/year of Phosphorus could be
removed from the Lake for irrigation. The confidence interval in this calculation is 3.5 because
of variability in the actual water used for irrigation.
Typha latifolia (tule) is harvested from Lake Zapotlán and could be a potential output source of
Phosphorus as well. The actual amount of tule that is harvested is unknown, but if one assumes a
harvest of 100 tonnes per year, the amount of P removed would be 325 kg, based on the
calculation in the storage section above. The confidence in this calculation is low at 2.5 because
of the variability in the actual amount of tule harvested.
47
Combined, the fish (0.63 tonnes), irrigation water (0.89 tonnes) and Typha latifolia harvesting
(0.325 tonnes) could remove an estimated 1.85 tonnes of total Phosphorus per year in Lake
Zapotlán (Figure 5.2).
Fish Harvest= 0.63 tonnes/year
Tule Extract ion= ~ ~ 0.33 tonnes/year
Wastewater Effluent= ~ 31 tonnes/year
Aquatic Plants= ~ 22.1-48.1 tonnes
Seasonal Runoff (urban, agricultural)= ~ 13.4 tonnes/year Irrigation=
~ 0.89tonnes/year
Lake Zapotlan
+
TP INPUTS: ~ 44.4 Tonnes/Year
Storage of TP in Lake: ~27.9 � 55.1 Tonnes
TP OUTPUTS: 1.85 Tonnes/Year
Sediments= 0.5-1.5 tonnes
Lake Water =4.9 tonnes
Fish= 0.63 tonnes
Figure: 5.2: Preliminary Phosphorus Mass Balance in Lake Zapotlán
5.4 Discussion of Storage within Lake Zapotlán: Sources and Sinks
Overall, there is an input of approximately 44.4 tonnes of phosphorus per year (mostly from
untreated wastewater effluent), of which about 27-55 tonnes are accounted for within the Lake
(water, aquatic plants, fish and sediment). Only about 1.85 tonnes of phosphorus per year are
removed from the Lake, which means that the aquatic plants must account for most of the
phosphorus storage within the Lake. Due to unreliable data sources and numerous estimations
for most data used in the calculations, the confidence interval for the entire preliminary
phosphorus balance calculations is a 3.
The extensive areas of the cattail (Typha latifolia) and water hyacinth (Eichhornia crassipes) in
Lake Zapotlán can be attributed to the high phosphorus concentrations in the Lake. As the
volume of sewage effluent discharge in a lake increases, the surface area covered by cattail
48
(Typha latifolia) and water hyacinth (Eichhornia crassipes) increases (Greenfield et al., 2007;
Rommens et al., 2003). From 1982-1994, these plants grew uncontrollably and had managed to
cover almost 70% of the surface of the Lake before action was taken in 1995 by the municipality
to reduce the plant cover to 30% (Oritz-Jimenez, 2005) (Figure 2.3). These plants are known to
have an impact on commercial fishing, waterfowl habitat, recreational activities, in addition to
being noxious (Gottschall et al., 2007). They absorb and immobilize a large quantity of nutrients
directly from the water column (Klumpp et al., 2002). Total P concentrations in Lake Zapotlán
absorbed an estimate of between 22 -48 tonnes of phosphorus per year. It is likely that these
plants absorb the higher end of this estimate, or else there would be a lot of phosphorus
unaccounted for within the Lake.
The municipalities in the Lake Zapotlán basin shred the water hyacinth and leave debris in the
water column, resulting in a large transfer of nutrients to the water column (James et al., 2002).
Although this method has lower control costs (Stewart & McFarland, 2000), it would be more
effective to harvest the plants and take them directly out of the water column to reduce the
transfer of nutrients back to the Lake. It is very likely that Lake Zapotlán experiences eutrophic
conditions after the water hyacinth are shredded every few years. Monitoring the change in
phosphorus in the Lake after shredding takes place would be useful to examine.
Sediments accounted for a small fraction of total P (0.5-1.54 tonnes), indicating that they could
be a potential source of phosphorus under reducing conditions. Table 4.2 indicated that the Lake
is under an oxidizing environment and it is unlikely that the sediments are releasing phosphorus
to the water column. It is possible that there are reducing conditions in the surrounding wetlands;
however, the redox potential was not measured during this research and it was not possible to
determine this.
The extraction of fish is minimal in terms of the output of phosphorus in Lake Zapotlán, but the
fishery may have an effect on water clarity within the Lake through algae ingestion by fish. In
2007/08, during the period of study, the Lake did not have the dense algae biomass that occurred
in 2001. Observations through photographs and personal communication (Shear, 2008; Rocha
Chavez, 2008) on Lake Zapotlán in 2001 reveal an abundance of algae with extremely low water
49
clarity. In 2001, when fish harvests (and stocks) were very low, at about 100 tons per year (see
Figure 2.4), water clarity appeared to be very low with abundant algae present (Figure 5.3a).
2001 2007
Figure 5.3: Comparison of water clarity in 2001 and 2007
After fish stocking resumed in 2005, increasing the fish stock by 4 times the number in 2001 (as
estimated from the harvest), water clarity in Lake Zapotlán appeared to be significantly improved
(Figure 5.3 b). This is only a casual observation since no data were collected in 2001. It is
unknown if the fish stocking has had a significant effect on the phosphorus concentrations in
Lake Zapotlán. It is evident, however, that based on total N concentrations in the Lake in 2007
compared to 2003 (see Figure 4.2), that the fish may have contributed to increased total N
concentrations through waste excretion. Total N concentrations increased from 1.4mg/L in 2003
to 3.4mg/L in 2007. The number of increased fish stocked resulted in increased total N through
increasing excretions; it is very likely that the fish may have contributed to the water clarity
through increased algae ingestion in Lake Zapotlán.
5.5 Discussion on the Overall Phosphorus Status of the Lake
It is evident that wastewater is a major source of phosphorus to Lake Zapotlán. The bulrush
cotton tail (Typha latifolia) and water hyacinth (Eichhornia crassipes) are likely large
phosphorus sinks, but Eichhornia crassipes may also act as a source after shredding takes place
on the Lake. Typically in tropical shallow lakes, the internal recycling of phosphorus from
50
sediments represents a significant long-term input (Shear & de Anda, 2005). However, the
oxidizing conditions in Lake Zapotlán make it unlikely that the sediments are releasing
phosphorus to the water column, although this was not measured directly. The abundant fish in
the Lake could be ingesting and controlling the excess algae in the Lake. If government funded
fish stocking is discontinued, the Lake may revert to the conditions observed in 2001.
51
Chapter 6: Escherichia coli Levels in Lake Zapotlán
6.1 Results of E.coli Levels in October 2007, February, 2008 and July, 2008
Levels of E.coli at ten stations in Lake Zapotlán vary significantly, but were all consistently high
over the three sampling periods in October, 2007, February, 2008 and July 2008 (Figure 6.1).
Levels of E.coli in Lake Zapotlán were divided into four (4) categories to provide an overall
assessment of fecal coliform contamination, measured in colony forming units (CFU) per 100ml
in the Lake.
10
13
1
12
82
35
4
14
Legend
CFU per 100ml
< 1000
1000-4999
5000-9999
>10 000
Station #
Figure 6.1: Escherichia coli levels in CFU per 100ml in Lake Zapotlán
Overall, E.coli levels in the Lake ranged from 350 to over 130 000 CFU per 100 ml. The
wastewater discharge channel stations (1 and 13) had the highest levels of E.coli, with counts
over 16 000 and 130 000 CFU per 100ml respectively, where as the Lake stations had lower
levels of E.coli. The centre of the Lake (station 5) had the lowest E.coli level, averaging 350
CFU per 100ml. The Lake stations in the south-east portion of the Lake closest to the Ciudad
Guzman wastewater effluent outflow (stations 2 & 12) had lower E.coli levels than other Lake
stations in the north-west portion of the Lake closest to the San Sebastian del Sur wastewater
effluent (stations 10 & 14).
5
52
In comparison to 2006 (Oritz-Jimenez), E.coli levels in Lake Zapotlán have increased and spread
throughout the Lake. In 2006, high bacteria levels were only found in the southern portion of the
Lake, near the Ciudad Guzman wastewater effluent outflow. Now, bacteria levels are extremely
high throughout the entire Lake.
E.coli levels in Lake Zapotlán are extremely high compared to E.coli levels in North American
lakes. In the United States, beaches are posted as unsafe for recreation when E.coli levels are
above 235 E.coli per 100ml (USEPA, 2008); Canada has more stringent standards with beach
postings when levels reach 100 E.coli per 100 ml (Health Canada, 1999). Beaches along the
Great Lakes in Canada and the United States (US) were open for recreational activities more
than 58% of the season in 2007 (Greenberg et al., 2009). If Lake Zapotlán were to follow the
Canadian or US standards, the Lake would be posted as unsafe for recreational activities at all
times. In Lake Zapotlán, even the area in mid-Lake (station 5) with the lowest E.coli levels at
350 CFU per 100 ml would be considered dangerous to human health by North American
standards. The extremely high pathogenic bacteria levels in Lake Zapotlán pose severe risk of
illness to human health.
6.2 Possible sources of E.coli in Lake Zapotlán
It is unknown what is specifically causing the elevated E.coli levels in Lake Zapotlán, but the
untreated wastewater effluent flowing into the Lake could be a major source. Higher E.coli
levels in the Lake were found at those stations situated near a sewage outfall (stations 1 and 13).
Station 13 would be expected to have high bacteria levels, since the samples collected from this
station consisted of both treated and untreated sewage; however, station 1 only had treated
sewage in it, so one would have expected much lower E.coli levels. Improper functioning of the
wastewater treatment plant may account for the elevated levels of E.coli. Another factor may be
the lack of disinfection of the final treated wastewater effluent.
The two stations (10 and 14) closest to the San Sebastian del Sur wastewater effluent channel
show higher levels of E.coli than those stations (2 and 12) closest to the Ciudad Guzman
wastewater effluent. Considering that the highest bacteria levels found were in the Ciudad
Guzman wastewater effluent canals, it is unusual that the stations closest to these outflows do not
53
have higher E.coli levels than those stations in proximity to the San Sebastian del Sur effluent
canals. The wetlands surrounding the south-eastern part of the Lake could be reducing the
bacteria levels at those stations, acting as a filter for the effluent before it enters the Lake.
Natural and constructed wetlands have been used for the purification of wastewater in many
parts of the world (Boutilier et al., 2008; Meuleman et al., 2003; Song et al., 2008; Vymazal,
2005). The wetlands closest to the Ciudad Guzman wastewater effluent outflow are considerably
larger in area compared to the wetlands located closest to the San Sebastian del Sur wastewater
effluent flow. Based on the different E.coli levels in Lake Zapotlán , the wetlands may be the
reason for lower E.coli levels in the Lake at the stations closest to the Ciudad Guzman
wastewater effluent outflow.
The abundance of water hyacinth in the San Sebastian del Sur wastewater canal may be another
reason for higher E.coli levels in the northern part of the Lake. MacIntyre et al. (2006) found that
E.coli levels declined significantly when floating plants were removed. Plants naturally cover
the water column and provide a favourable attachment sites for E.coli. Their work suggested that
after the floating plants were removed, an open space was created, allowing penetration of
natural UV radiation causing decreased E.coli levels (MacIntyre et al., 2006). Figure 6.2
demonstrates the difference in surface cover in a) The San Sebastian del Sur wastewater effluent
canal and b) the Ciudad Guzman wastewater effluent canal. Extensive mats of water hyacinth
(Eichhornia crassipes) covered most of the surface of the San Sebastian del Sur wastewater
effluent canal (closest to stations 10 and 14); whereas the Ciudad Guzman wastewater effluent
canal (closest to stations 2 and 12) was generally open without the growth of plants.
54
a) b) Figure 6.2: Water Hyacinth (Eichhornia crassipes) Surface Cover in the a) San Sebastian del Sur wastewater effluent canal and b) Ciudad Guzman wastewater effluent canal.
Although station 8 (on the western side of the Lake) is adjacent to a wetland, levels of E.coli
were higher than those stations closest to the wetlands on the south-east side of the Lake (stations
2 and 12). Through personal observation during sampling, numerous livestock, mainly cattle,
were seen wading along the shoreline as well as in the wetland. It is likely that E.coli is being
transported to the Lake through runoff of cattle excrement (LeJeune &Wetzel, 2007; Miller &
Beasley, 2008). Manure produced from cattle grazing on the surrounding shoreline as well as
through excretion directly into the wetland could be causing the higher E.coli levels near this
wetland (station 8).
Several studies investigating E.coli sources in Canada have shown that the elevated levels of
E.coli on Great Lakes beaches are caused by the increasing bird populations (Edge & Hill, 2007;
Charlton & Milne, 2004; Fogarty et al., 2003). Bird droppings might be a large contributor to
the elevated levels of Escherichia coli in recreational areas. There is a possibility that the large
population of migratory waterfowl is also contributing to the high E.coli levels in Lake Zapotlán.
A polymerase chain reaction (PCR) analysis is required to determine the specific source of
E.coli. Unfortunately, a PCR analysis was not possible for this research due to time and funding
constraints.
55
6.3 The effect of Carbon on E. coli levels in Lake Zapotlán
Although, multiple factors affect microbial growth in water, organic matter provides a carbon
and energy source essential to the growth of bacteria, including coliforms (Bouteleux et al.,
2005; LeChevallier et al., 1990; LeChevallier et al., 1991; Mathieu et al., 1992; Pedley et al.,
2004; Yeh et al., 1998). Algae blooms have the capability of modifying the quality of organic
matter in water and have been suggested as a catalyst for coliform growth (Boualam et al., 2002;
LeChevallier et al., 1990; Lowther &Moser, 1984). Algae provide a nutritional source for
bacterial growth through secretion of organic compounds (through algal cell lysis), releasing
large quantities of dissolved organic matter (DOM), including dissolved organic carbon
(vanLoon & Duffy, 2005). The algae in the Lake as well as the detritus from the aquatic plants,
tule and water hyacinth, likely provide a significant source of DOC (Table 6.1).
Table 6.1: Dissolved Organic Carbon levels in Lake Zapotlán , October 2007, February, 2008, and July 2008 Source: Malczyk, 2009
Station Units Sampling Period Mean October, 2008 February, 2008 July, 2008 1 mg/L 20.2 68.2 62.2 36.792 mg/L n/a 46.7 50.9 48.8 3 mg/L n/a 35.5 52.1 43.8 4 mg/L 26.3 54.6 38.5 39.8 5 mg/L 35.6 55.6 51.8 35.2 8 mg/L n/a 60.8 44.9 40.1
10 mg/L 54.9 57.3 57.7 56.6 11 mg/L 14 58.9 110.6 61.2 12 mg/L n/a 57.5 52.4 35.9
Min 14 46.7 14.4 35.2 Max 54.9 68.2 110.6 61.2
Mean 31.41 55.45 58.74 48.54
Table 6.1 presents the amount of dissolved organic carbon in Lake Zapotlán over the year of
study. Since most analytical methods for measuring organic matter in water actually determine
the carbon content (van Loon & Duffy, 2005), this measure is used as a determinant of organic
matter in Lake Zapotlán. Dissolved organic carbon (DOC) levels in Lake Zapotlán averaged
48.54 mg/L. Both Lake and wastewater discharge channel stations have the highest DOC
concentrations in July, the wet season. This is the time of year when high levels of nutrient input
through anthropogenic sources enters the Lake through runoff, thus increasing algae production.
56
Lake et al. (2001) found that there is a strong link between the end of an algal bloom and the
presence of coliforms in an aquatic system. Ortiz- Jimenez, (2006) concurred with this through
evidence of a positive correlation between E.coli levels and total P and N levels in Lake Zapotlán
in 2005 (Oritz-Jimenez, 2006).
In addition to the nutrients, the high numbers of tilapia and carp in the Lake contribute to high
DOC levels in the Lake. As production and feeding rates increase, organic matter rises through
fish excrement, which further increases carbon content in the water column (Lim & Webster,
2006). These high values of DOC (Sobek et al., 2007; Lake et al., 2001; Reddy & Vijayjumar,
2005), generally found throughout the entire year in the Lake present an ideal habitat for
bacterial growth in Lake Zapotlán.
It is important to decrease E.coli levels in Lake Zapotlán to reduce the health risk for those in
contact with the bacteria in the water. Those who use the Lake for recreational purposes such as
rowing, canoeing are at risk of illness (Edge et al., 2001; LeJeune & Wetzel, 2007; Noble et al.,
2002). Infections can be acquired from recreational contact in polluted water including
conjunctivitis (eye), ear infections, nose infections, throat infections, or more serious infections
such as diphtheria, dysentery, and gastrointestinal illnesses (Schiff et al., 2003; Wieske and
Penna, 2002). The fishermen also have similar health risks through contact with fish exposed to
sewage derived pathogens. Research has shown that the presence of bacteria on fish skin is
related to the microbial communities within the water (Fattal et al., 1992; El-Shenawy & El-
Samura, 1994). A higher density of fecal coliforms is more likely observed on fish exposed to
untreated sewage (Loomer et al., 2008). The fish in Lake Zapotlán are exposed to untreated
wastewater throughout the year, creating an increased risk of illness to the fishermen.
The extremely high E.coli levels in Lake Zapotlán pose a health risk to the local residents from
Ciudad Guzman who use the Lake for recreational purposes, as well as for fishing. E.coli levels
in Lake Zapotlán must be significantly decreased to reduce health risks associated with high
bacteria levels.
57
7.0 Summary and Conclusions
Lake Zapotlán has been shown to have very high nutrient and E. coli concentrations.
Anthropogenic activities surrounding the Lake have degraded the water quality significantly.
The activity of most concern to the Lake is the constant input of partially treated wastewater
effluent which causes a significant amount of nutrients and bacteria to enter the Lake. Runoff
from increasing fertilizer use on agricultural farms, the felling of forests, and most importantly,
the increase in urban runoff may pose an ongoing stress to the Lake as well. Within Lake
processes such as water hyacinth, the bulrush cotton tail, increased fish stocking, and slightly
increased wastewater treatment have all proven to be affecting the water quality in Lake
Zapotlán.
7.1 Seasonal and Historical Nutrient Trends in Lake Zapotlán
The preliminary analysis of historical and recently collected data shows that there are increasing
trends in both total phosphorus and total nitrogen in Lake Zapotlán. Nitrogen levels in the Lake
have increased threefold from 1994 to 2007 possibly due to increased fish excretion. Although
total P levels decreased in 2007 from the high levels in 2003, there is still an overall increase in
total P levels in the Lake since 1994. The slight improvement in the treatment of wastewater
entering the Lake may have been the cause for improved nutrient levels in the Lake. Seasonal
patterns of total N and total P indicate that the runoff from the surrounding land use does have an
impact on the nutrient levels in the Lake. Higher nutrient levels were found in the Lake during
the rainy season compared to the dry season.
Lake Zapotlán presently has total P concentrations averaging 0.25mg/L that would result in it
being classified as eutrophic according to the Carlson Trophic State Index (TSI) (Carlson, 1977)
and the US EPA standards (> 0.1 mg /l) (EPA, 2003). However, the Lake does not exhibit all the
signs of eutrophy according to the TSI, and further research on the biogeochemical processes in
the Lake need to be examined.
Given that the growth in human population is 2.1% (Oritz-Jimenez, 2006), and that the
wastewater treatment plants operate at very low efficiency (CEASJ, 2008), the loading of
58
phosphorus, and hence the concentrations of phosphorus in the Lake, are likely to continue to
increase.
7.2 Phosphorus Balance
Lake Zapotlán receives a significant amount of phosphorus from both the Ciudad Guzman and
San Sebastian del Sur wastewater effluents. The wastewater effluent channels as well as the
urban and agricultural runoff channels contribute an estimated 30 tons of phosphorus to the Lake
each year. Since Lake Zapotlán is endorheic and has no outlet sources, it is assumed that the
aquatic plants, Typha latifolia and Eichhornia crassipes, are preventing the Lake from becoming
severely eutrophic.
The cattail and water hyacinth may have the largest influence on nutrient levels in the Lake by
acting as a sink for the excessive phosphorus inputs. It is unlikely that sediments are a
contributing source of phosphorus since the Lake has oxidizing conditions throughout the year.
The number of fish that have been stocked in the Lake since 2005 may have been one cause for
the decreased number of algae in the Lake through the ingestion of algae by fish. The
mechanical shredding of the water hyacinth could be another cause for the varying nutrient levels
in the Lake. This management technique, which occurs every few years, results in a large
transfer of nutrients to the water column (James et al., 2002). Currently, the shredding process is
efficient in reducing the surface cover of the plants in the Lake, but if excessive loading of
nutrients to the Lake continues, more significant measures are necessary. Prolonged
anthropogenic nutrient inputs will not only maintain the presence of these water nuisance plants,
but it will have the potential to return to its nuisance status (Williams & Hecky, 2005).
Commercial fishing, waterfowl habitat, recreational activities, and the aesthetics of the Lake will
all be affected.
7.3 Bacteria Levels in Lake Zapotlán
Bacteria levels in Lake Zapotlán are extremely high because of untreated wastewater effluent
input from the surrounding wastewater effluent canals, and possibly waterfowl excretion. The
high Dissolved Organic Carbon concentrations in the Lake promote the growth of coliform
bacteria.
59
The extremely high bacteria levels in Lake Zapotlán pose a health risk to the local people who
use the Lake for recreational purposes as well as for fishing. An even greater threat from the
high E.coli levels would be to the athletes participating in the 2011 Pan American games. E.coli
levels in Lake Zapotlán must be significantly reduced to improve health risks associated with
high bacteria levels.
7.4 Conclusions Any further degradation and habitat fragmentation through human interference may negatively
affect the Lake ecosystem in the near future. The increase in nutrients, mainly from the untreated
wastewater entering the Lake, will likely cause an increase in water hyacinth, cattail and algae
production in the Lake. This could then reduce oxygen levels, causing an unsuitable habitat for
both the fish and migratory waterfowl populations. The unsuitable conditions for the fish would
reduce production levels, further affecting the fishing industry in Lake Zapotlán. The degraded
water quality conditions also pose a threat to the recreational users and to the overall aesthetics
of the Lake, reducing the possibility of tourism.
The rapidly expanding population surrounding Lake Zapotlán has the potential to severely
threaten the biodiversity and sustainability of Lake Zapotlán. Wastewater treatment from Ciudad
Guzman and San Sebastian del Sur must improve to reduce nutrient and bacteria levels. It is
only a matter of time before the Lake becomes an entire wetland. This will limit the fishery
production, tule harvesting, recreational activities, and it will discourage the potential for tourism
in the area.
Lake Zapotlán requires an improvement in water quality in the near future so that the
surrounding populations can use it as a site for productive fishing, tule harvesting, recreational
activities, and so that it can continue to be an important migratory waterfowl site. Most
importantly, an improvement in water quality must be implemented before the health of the local
recreational users on the Lake becomes a potentially significant risk.
60
7.5 Future Work and Recommendations
Future research on Lake Zapotlán that would be beneficial in determining the overall water
quality include obtaining proper runoff and wastewater data for the Lake, and completing a PCR
analysis to determine the sources of coliforms. Although, results show that wastewater is a large
component to the bacterial contamination in the Lake, the abundant number of waterfowl in the
Lake could also be contributing to the extremely high E.coli levels. To effectively determine the
source of E.coli to the Lake, a PCR analysis should be carried out.
To develop a robust phosphorus balance in the Lake, proper runoff and wastewater data are
needed. During this study, the actual amount of treated wastewater was unknown at all three
wastewater treatment plants. Also, the value for the runoff into Lake Zapotlán from the
surrounding land uses was only an estimate in the phosphorus balance as well. Identifying the
actual percent of wastewater treatment at these plants, in addition to knowing the actual nutrient
runoff value from the wastewater discharge channels and the arroyos surrounding the Lake,
would assist in determining a more precise phosphorus balance. It would also be beneficial to
complete a chlorophyll-a analysis to assess the algal biomass. The role of excessive nutrients in
the Lake on the production of algae blooms for only certain years should also be examined to
determine the biochemical processes that are controlling the periodic algae blooms in the Lake.
The role of sediments as a source or sink for nutrients should be examined by measuring the
redox potential in the Lake. With precisely known wastewater and runoff quantities, algal
biomass and redox potential, the eutrophication and the biogeochemical Lake processes that
control nutrients in the Lake could be better assessed and used to effectively determine an entire
nutrient mass balance in the Lake.
Further work on Lake Zapotlán should determine the main sources and sinks of phosphorus in
the Lake basin. Since it is likely that Lake Zapotlán could have significant eutrophic conditions
after the water hyacinth are shredded every few years, monitoring the change in phosphorus in
the Lake after shredding takes place would be beneficial to examine in the future. It will also
provide a better understanding of the concentration of nutrients stored in the aquatic plants.
61
Lake Zapotlán is home to many migratory birds including cormorants (Phalacrocorax niger).
These birds are well known guano producers who use their guano for nest making; this guano
adds nutrients to a lake. A study by Venugopalan et al. (2005) on the impact of nutrients on a
tropical freshwater lake in Lake Kokilamedu, India confirmed that guano is rich in nitrogen (total
nitrogen 13.8%) and phosphorus (orthophosphate 3.0mg P/g dry weight) (Venugopalan et al.,
2005). The large input of bird droppings could result in increased nutrient levels in Lake
Zapotlán. To effectively quantify the nutrient concentration in waterfowl guano and feces and the
impact this has on a lake, a large number of samples would be needed. This would require a lot
of time and funding at the field site, which was not possible for this research. It could be useful
for future nutrient research on the Lake though.
Although the specific phosphorus controlling element is unknown in Lake Zapotlán, it is
important to reduce the phosphorus inputs significantly. If nutrient input through wastewater
effluent is not reduced in the near future, it is very likely that the water quality problems will
become more severe in the near future affecting the economic, ecological, and recreational
activities on Lake Zapotlán.
62
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