Post on 08-Jul-2020
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Atmospheric Deposition of Phosphorus to Freshwater Lakes
Jennifer Boehme1, Rachel Schulhauser
1, Raj Bejankiwar
1
1 Great Lakes Regional Office, International Joint Commission, 100 Ouellette Avenue, Windsor,
ON N9A 6T3
This document is a working draft prepared for the IJC’s Lake Erie Ecosystem Priority (LEEP) by the GLRO Staff and is not for citation. Views expressed are solely those of the authors. See the draft LEEP report for findings and recommendations from the IJC.
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T
CONTENTS
Abstract ...................................................................................................................................... 3
Introduction ............................................................................................................................... 3
Methodology ............................................................................................................................... 6
Phosphorus Production ............................................................................................................. 6
Sources of Atmospheric Phosphorus ....................................................................................... 7
Deposition Variability ............................................................................................................... 9
Dominance of Deposition Pathways – Wet or Dry? ............................................................. 11
Implications for Lake Erie ...................................................................................................... 13
Conclusion ................................................................................................................................ 16
References ................................................................................................................................. 20
Table 1. The annual percentage (%) of a lake’s total phosphorus load as atmospheric deposition
....................................................................................................................................................... 12
Figure 1. Average estimated phosphorus loadings (MTA) for Lake Erie (1996-2002); data from
Dolan and McGunagle, 2005. Data includes estimates for contributions from Lake Huron and
unmonitored areas. Direct sources are point sources that discharge into Lakes Erie or St. Clair,
the Detroit or St. Clair Rivers or unmonitored areas. ................................................................... 18
Figure 2. Total phosphorus loadings for Lake Erie, 1967-2007. Includes point and non-point
source loads from the United States and Canada. Figure reproduced from Baker, 2011, data from
David Rockwell, (U.S. EPA and David Dolan (Univ. of Wisconsin, Green Bay) ....................... 19
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ABSTRACT
This paper is a critical review of the contribution of atmospheric phosphorus deposition
to the total phosphorus loadings to lake ecosystems, with a special focus on the relevance of this
source to Lake Erie and the Laurentian Great Lakes. It suggests that atmospheric phosphorus is
not negligible to total phosphorus loadings to lakes of varying surface areas, and may contribute
a significant portion of soluble reactive phosphorus to lake ecosystems. The relative proportions
of dry and wet phosphorus deposition are poorly characterized for many lakes, including Lake
Erie, and both pools demonstrate appreciable, though location-specific, contributions of soluble
reactive phosphorus to respective lake systems. No general trends in lake geographic
distribution or seasonality can be determined from the literature for atmospheric phosphorus
deposition in the lakes examined for this review. This high variability between lake systems,
along with the potential significance of atmospheric phosphorus deposition, provides support for
further study of Lake Erie and the Great Lakes to understand the phosphorus cycle for this large
freshwater system and its potential effect on nutrient modeling for the region.
INTRODUCTION
The global freshwater supply is approximately 2.5% of the world’s total water, with 0.1
million km3
of available freshwater in the form of lakes and marshlands (Worldwater.org). The
Laurentian Great Lakes contain approximately 18% of the world’s freshwater (Environment
Canada, 2009) and encompasses more than 765,000 km2
between 8 US states and the Canadian
province of Ontario with 10% and 30% of the American and Canadian populations residing in
the basin, respectively (Danz et al., 2007). In addition to their use as a drinking water source,
lakes and marshlands support biodiversity, wildlife habitat and other ecosystem services that
enhance human enjoyment and advance commercial activities. Globally, freshwater lake
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systems are impacted by the delivery of essential nutrients such as phosphorus via natural
pathways often influenced by anthropogenic activities, giving rise to unintended effects on
freshwater ecosystems.
Phosphorus is a limiting nutrient in aquatic ecosystems (Blake & Downing, 2009; Reutter
et al., 2011), and excesses of phosphorus from natural and anthropogenic activities have been
attributed as the principal driver of eutrophication in lakes, causing a diverse range of associated
water-related problems including hypoxia, harmful algal blooms, loss of biodiversity and other
adverse effects (Schindler, 1971; Pollman et al., 2002; Paytan & McLaughlin, 2011; Rossignol et
al., 2011).
One lake historically associated with eutrophication is Lake Erie in the North American
Great Lakes. Lake Erie is the twelfth-largest lake in the world with an area of 35,696 km2
(Herdendorf, 1983) and provides drinking water for over 11 million Americans and Canadians
located with its basin (North et al., 2007). In the 1960s and 70s, excessive phosphorus loading to
Lake Erie was a main driver of the lake’s eutrophication (Baker, 2011) which coincided with
high growth rates and nuisance accumulations of algae, particularly cladophora and
cyanobacteria (Herdendorf, 1983; Higgins et al., 2005; Han et al., 2010). The onset of hypoxia
in Lake Erie also has a long-recognized link to total phosphorus concentrations and loadings
from external sources (Burns & Ross, 1972; El-Shaarawi, 1987).
Lake Erie is situated within some of the most urbanized areas of the Great Lakes basin.
Phosphorus deposition rates to Lake Erie may be higher than to the upper Great Lakes due to
increased impacts of population and industrialization which add additional sources of
phosphorus to the lake (Shaw et al., 1989; Danz et al., 2007). In 1978, an established total
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phosphorus target load of 11,000 tonnes was set for Lake Erie under the Great Lakes Water
Quality Agreement (Fraser, 1987; Baker, 2011) in order to combat eutrophication, algae blooms
and other related water quality issues. There was a 56% decline in the total phosphorus loadings
to Lake Erie from 1968 to 1982 (Fraser, 1987), which improved the eutrophic conditions of the
lake. However, levels of phosphorus in Lake Erie have been rapidly rising in recent years (Baker,
2011), and have been tied to increased algae biomass during the summer months (Conroy et al.,
2005). The connection between algae growth rates and phosphorus sources in Lake Erie has
shifted, and clearer understanding of this shift is needed to provide for effective long term
management of eutrophication in all lake ecosystems.
Atmospheric deposition is one natural nutrient transportation pathway that provides a
non-controllable source, can prove difficult to quantify and measure and has a high potential for
human impact. Atmospheric sources can provide significant contributions of nutrients to
freshwater lake systems (Chen & Fontaine, 1997; Anderson & Downing, 2006) although this is
one of the least understood mechanisms of nutrient delivery. Phosphorus can find its way from
the airshed into lake ecosystems via inputs from 1) rain or snowfall, known as wet deposition;
and (2) gaseous and particulate wind transported particles, or dry deposition (Anderson &
Downing, 2006; Zhai et al., 2009). Wet and dry nutrient loadings to aquatic ecosystems,
particularly phosphorus, have increased over the years as a direct result of anthropogenic
activities (Herut et al., 1999; Zhai et al., 2009). Thus, it is essential to examine the atmospheric
deposition pathways of phosphorus in order to understand the host of water quality issues that
arise in aquatic systems due to phosphorus enrichment.
6
METHODOLOGY
This study comprised a review of available literature on atmospheric phosphorus deposition in
fresh and saltwater environments to obtain a complete description of the sources and transfer
mechanisms. Lakes of all sizes were included though relevance to the Great Lake Basin,
especially Lake Erie, was a primary consideration for this study. Research examining both wet
and dry deposition was rare as direct measurements of wet deposition are especially difficult to
collect and prone to method contamination. In limited cases studies identified levels of soluble
reactive phosphorus within the wet or dry fractions of atmospheric phosphorus delivered to
aquatic systems, but comprehensive studies on this question were not available for the Great
Lakes Basin. The resulting synthesis was undertaken to better describe current understanding of
atmospheric phosphorus inputs to Lake Erie and its potential impact the occurrence of harmful
algal blooms within the Great Lakes.
PHOSPHORUS PRODUCTION
The global biogeochemical cycle of phosphorus predominantly involves flows between
sources of biomass, land and water, with the atmospheric contribution being smallest and
consisting of phosphorus affixed to particulate material (Pierrou, 1976). The flow of phosphorus
between sources has been greatly accelerated by human activities, with food production and
consumption requirements estimated to comprise 90% of the global phosphorus demands
(Cordell et al., 2009), specifically for its demand in fertilizers. Phosphorus has many applications
outside of agriculture, including but not limited to detergents, steel production, water softening,
military applications, and ethanol fuel production.
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Phosphorus for human activities is presently sourced from mined phosphate rock.
Reserves are restricted to a small handful of countries, with the largest reserves in the United
States, China and Morocco (Cordell et al., 2009). The US is estimated to have approximately 25
years of phosphate from remaining domestic reserves, even though it was once the world’s
largest producer, consumer, importer and exporter (Cordell et al., 2009). Morocco and its near
monopoly on the Western Sahara’s phosphate supply represents more than a third of the global
supply of phosphate (Cordell et al., 2009).
Studies have suggested that the current reserves of phosphate are expected to be depleted
within the next 50-100 years, (Cordell et al., 2009; Paytan & McLaughlin, 2011), and yet food
production rates are expected to increase in response to a larger global population. The
phosphate that is extracted will be of lower grade then previously extracted and it will be
expensive to extract, process, and transport (Cordell et al., 2009). Mining activities increase the
physical transport of phosphate rock and its exposure to the natural aeolian (wind) erosional
processes that play an important role in nutrient dispersal within ecosystems. A portion of
phosphate that is extracted can become mobile within the atmosphere in the form of dust.
SOURCES OF ATMOSPHERIC PHOSPHORUS
As a medium for atmospheric transport, dust can travel great distances from its source
location depending on its particle size and eventually can be deposited into water bodies. In
addition to the example of phosphate rock particulates, numerous alternative natural and
anthropogenic sources of phosphorus are available to wind transport, and can contribute to
eutrophication and algae blooms in lakes from atmospheric deposition. Natural sources include a
combination of ocean aerosols (Ahn & James, 2001; J. Luo et al., 2011), vegetation detritus
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(Dolislager et al., 2012), biomass burning due to lightning (Grimshaw & Dolske, 2002), pollen
(Winter et al., 2002; Report on the Phosphorus Loads to Lake Simcoe 2004-2007, 2009; Blake &
Downing, 2009), insects (Tsukuda et al., 2004), volcanic activity (Tsukuda et al. 2006.pdf, n.d.;
Winter et al., 2002), guano (Tsukuda et al., 2004, 2006), erosive processes providing dust
aerosols (Herut et al., 1999), and microbial reduction of sewage sludge landfill and compost
heaps (Winter et al., 2002). Anthropogenic sources include coal combustion(Winter et al., 2002;
Hartmann et al., 2008; Report on the Phosphorus Loads to Lake Simcoe 2004-2007, 2009),
biomass burning (Morales et al., 2001; Winter et al., 2002; Zhang et al., 2002; Tamatamah et al.,
2005; Tsukuda et al., 2006; Blake & Downing, 2009; Dolislager et al., 2012), aeolian re-
suspension of phosphorus from disturbed soils including quarries, deforestation, agricultural
fields, soil tilling and ploughing, and unpaved roads (Herut et al., 1999; Morales et al., 2001;
Grimshaw & Dolske, 2002; Winter et al., 2002; Koelliker et al., 2004; Tsukuda et al., 2006;
Report on the Phosphorus Loads to Lake Simcoe 2004-2007, 2009; Blake & Downing, 2009;
Brown et al., 2011; Dolislager et al., 2012), urban inputs primarily associated with automobile
combustion emissions exhaust (Connor et al., 1992; L. Luo et al., 2007; Dolislager et al., 2012)
and industrial activities such as fertilizer production and coal combustion
(Grimshaw & Dolske, 2002; Koelliker et al., 2004; Tsukuda et al., 2006; Brown et al., 2011; L
uo et al., 2011). It is recognized that the largest source of atmospheric phosphorus is dust
aerosols followed by anthropogenic activities such as fuel combustion that release phosphorus
(Hargan et al., 2011). Phosphorus sources local to Lake Erie could also include sewage sludge
and compost heaps in the Lake Erie basin.
It is important to note that dust aerosols can originate within a local lake, watershed or
ecosystem. Generally, phosphorus originating from outside the local area of interest is attributed
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to true atmospheric deposition while sources from within an area of interest can be viewed as
contamination or local recycling rather than true deposition (Ahn & James, 2001). The
importance of these local vs. external sources to atmospheric deposition to lakes is dependent on
air circulation patterns between the sources relative to the lake of interest. It has been suggested
through several lake-wide studies that local emissions can have a larger impact on the lake than
long-range transported materials (Tsukuda et al., 2004; Tamatamah et al., 2005; Dolislager et al.,
2012). Differentiating local recycling from true deposition from external sources is important for
understanding the atmospheric contribution to a region’s phosphorus cycle. However, local
recycling is important from a management standpoint in that it can provide a transfer mechanism
between the airshed and watershed that significantly impacts the phosphorus loadings for lakes.
Atmospheric phosphorus deposition to Lake Erie can be attributed to a variety of natural
and anthropogenic sources from local and external locations. However, unique tracer methods
for known or suspected sources are required to assign an exact origin of atmospheric phosphorus
(Grimshaw & Dolske, 2002; Dolislager et al., 2012), and such methods for geographic
fingerprinting can be extremely difficult to develop.
DEPOSITION VARIABILITY
Weather conditions are a primary driver mobilizing phosphorus within the atmosphere
and can provide significant fluctuations in observed phosphorus levels, similar to observations
for dissolved phosphorus associated with river discharge, and anthropogenic activities also play a
major role (Baker, 2011). Significant regional variability for particulate phosphorus
concentration has been observed due to local particulate sources ( Tsukuda et al., 2004), and
high annual and seasonal variability accompanied by inconsistent long-term trends have been
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observed ( Ahn & James, 2001). Several global atmospheric deposition studies have found no
consistent seasonal trend for phosphorus deposition in Iowa ( Anderson & Downing, 2006), New
Jersey ( Koelliker et al., 2004), Florida ( Pollman et al., 2002), Lake Michigan (Miller et al.,
2000), and Lake Taihu in China(L. Luo et al., 2007).
However, other studies have found evidence for patterns in phosphorus deposition.
Seasonal activities related to agriculture were found to disturb soil and release phosphorus to the
air shed that was consequently available for deposition ( Anderson & Downing, 2006). A
maximum loading of phosphorus to lakes in the summer and spring periods and lowest rates in
the winter season were found at several locations around the globe, including Lake Simcoe in
Canada ( Brown et al., 2011), South Central Ontario, Canada(Hargan et al., 2011), Lake
Michigan in the United States( Eisenreich et al., 1977), Lake Victoria in East Africa (
Tamatamah et al., 2005), Japan ( T. Kunimatsu & Sudo, 2006), and Lake Taihu in China(Li et
al., 2011). Thus human activities can add detectable variability to particulate mobilization within
an airshed, which overcomes randomness observed elsewhere and is expressed within the
atmospheric phosphorus signal as anthropogenically-induced seasonal variability.
While agricultural activities have been identified here, and mining sites mentioned
previously, the EPA has identified unpaved roads, aggregate storage piles, and heavy
construction as additional potential sources of particulate matter of no greater than 10
micrometers in aerodynamic diameter, in the form of what it terms “fugitive” dust, so named due
to it being discharged into the atmosphere outside of a controlled flow stream ( US EPA, 2000).
Dust from such sites of elevated human activity could also act as distributed sources for
atmospheric particulate phosphorus.
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DOMINANCE OF DEPOSITION PATHWAYS – WET OR DRY?
Atmospheric deposition of phosphorus is known to be mainly derived from particulate
and aerosol materials found in dry deposition ( Zhai et al., 2009) with lesser inputs of
phosphorus identified in wet deposition such as rainwater ( Migon & Sandroni, 1999). The
fraction of particulate phosphorus dissolved in rainwater has been estimated as high as 13% (
Ridame & Guieu, 2002). As there is no natural gaseous form of phosphorus, it has no
atmospheric component in its global biogeochemical cycle.
Atmospheric deposition can contribute significantly to a lake’s total phosphorus load
(Table 1). A combination of wet and dry deposition to a lake is described by its total bulk
loading rate. However, a phosphorus deposition study performed in South Florida found that wet
and dry deposition are two independent processes with low correlation ( Ahn & James, 2001).
Thus, it is critical to determine whether wet or dry deposition is the most relevant for evaluating
a lake’s total phosphorus load.
Concentrations of P by dry deposition has been reported as 4 to 10 times greater than wet
deposition (Hicks et al., 1993; Ahn & James, 2001), attributed to the origin of phosphorus in
soils and its low incorporation into rainfall( Anderson & Downing, 2006). The deposition
pathway of phosphorus also determines the type of phosphorus that is deposited. Previous studies
have indicated that soluble (or bioavailable) fractions of phosphates on particulates will vary
based on the particulates’ source. For instance, soil-derived dust originating in the Sahara was
found to contain less soluble, bioavailable phosphorus relative to particulates of anthropogenic
origin( Migon & Sandroni, 1999). Additionally, the ions with which phosphates on mineral
particulates such as soils tend to associate (calcium, magnesium, aluminum and iron) or bind
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Table 1. The annual percentage (%) of a lake’s total phosphorus load as atmospheric deposition
Location
Study
Area
(km2)
Lake
Surface
Area (km2)
Study Duration
% of total
phosphorus
load as
atmospheric
deposition
Reference
Three Gorges
Reservoir, China 11531 1080 1995-2007 21.1 (Ma et al., 2011)
Lake Tahoe,
California and
Nevada, USA
812 512 1998-2001 25 (Zhang et al., 2002)
Silver Lake,
Wisconsin, USA 18 1.4 2004-2008 22
(Robertson et al.,
2009)
Lake Victoria, East
Africa 33700 68800 1999-2000 55
(Tamatamah et al.,
2005)
Lake Biwa, Japan 3144 675 1992-1993 8 (T. Kunimatsu &
Sudo, 2006)
Lake Simcoe, Canada 350 722 2004-2007 25-50 (Brown et al., 2011)
Lake of the Woods,
Canada and USA 14860 4349
1970-2005
13
(Hargan et al., 2011) Rainy River
Catchment, Canada
and USA
54887 75
Lake Michigan 117800 58000 1976 15.7 (Eisenreich et al.,
1977)
Lake Ontario 64030 18960 1972-1973 10 (Casey and Salbach,
1974)
Lake Huron 134100 59600 1973-1975 12.1 (IJC, 1976)
Lake Superior 127000 82100 1973-1975 19.3 (IJC, 1976)
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with (iron oxides) are only weakly soluble in water( Guieu, 2002). Indeed, particles from
anthropogenic sources have been found to release a greater proportion of their soluble
phosphorus into water more quickly than crustal sources( Mackey et al., 2012). For land use
areas devoted to agriculture, there is a greater percentage of soluble reactive phosphorus (SRP)
(Anderson & Downing, 2006) present in dry deposition. Specific atmospheric particulate
sources for Lake Erie and their relative contribution to total SRP content have not been studied in
detail; however, the land surrounding this lake is at once the most highly urbanized and highly
farmed of any Great Lake. A statistical study examining relative impacts of anthropogenic-
induced stress in the Great Lakes characterized certain Lake Erie watersheds is subject to
elevated levels of stress related to atmospheric deposition, based on 11 atmospheric variables as
measured by the National Atmospheric Deposition program ( Danz et al., 2007).
While dry atmospheric deposition is proportionally higher than wet deposition,
researchers have found that wet deposition accounts for the majority of the bulk loading rate
(Chen et al., 1985; Yang et al., 1996; Izquierdo et al., 2012). Phosphorus in rainwater is found
predominantly in its soluble or bioavailable form ( Ridame & Guieu, 2002) implying that the
relative magnitude of phosphorus provided by wet deposition can be a substantial fraction of
atmospheric phosphorus. While the details of pathways appear variable, the importance of
atmospheric deposition in delivering phosphorus to aquatic systems, and the resulting quality
impairments, is increasingly recognized ( Winter et al., 2002).
IMPLICATIONS FOR LAKE ERIE
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The atmospheric phosphorus contribution estimates for Lake Erie have been determined
by various long term studies, and have been generally attributed limited importance, which
reflects the assumptions of the broader community.
Recent results from Lake Simcoe in Ontario indicate there is reason to revisit such
assumptions, given the shifts in population and land usage surrounding the Lake Simcoe
tributaries and basin over time. As part of the Great Lakes Basin, Lake Simcoe is one of the
largest lakes in Canada outside the Laurentian Great Lakes. While its volume is much smaller
compared to Lake Erie, it is similar to Lake Erie in the proximity of its watershed to both major
urban and farming centers, and as such can be considered a bellweather for the Great Lakes
system. Early estimates of atmospheric phosphorus contributions to Lake Simcoe phosphorus
loadings were estimated as fraction of total phosphorus multiplied by regional precipitation, and
therefore encompassed only wet deposition (Winter et al., 2002). More recent direct monitoring
of atmospheric phosphorus allow for loadings to be calculated from measurements of bulk
deposition of total P, without distinguishing wet from dry atmospheric phosphorus (Winter et al.,
2007). Bulk deposition atmospheric phosphorus is thought to contribute a significant 25-50% of
the total phosphorus loadings to Lake Simcoe (Table 1).
The available information for Lake Erie indicates that atmospheric phosphorus is a small
but not inconsequential contribution to total phosphorus in the system. Previous efforts to
quantify regional phosphorus loading into Lake Erie (Figure 1) framed the question in terms of
monitoring programs characterizing total phosphorus rather than specifically targeting the
soluble reactive phosphorus fraction, and found that atmospheric deposition contributed higher
levels of total phosphorus than direct industrial discharge for 1996-2002 (Dolan & McGunagle,
15
2005). Summaries over time from the Heidelberg Project and combined data from the United
States and Canada indicate that annual loads for the atmospheric fractions of total phosphorus
have been reasonably consistent and were considered a minor component for the years data are
available between 1974 and 2007 (Figure 2). However, unlike the most recent measurements of
bulk deposition for Lake Simcoe, these estimates were developed based on reported precipitation
in the area and therefore represent only a fraction of bulk phosphorus deposition, namely wet
deposition of total phosphorus. Using the same methodologies, the most recent estimates for the
percent contribution of atmospheric wet deposition to total phosphorus to Lake Erie ranged
between < 10% for 1996-2002 (Dolan & McGunagle, 2005) and 7.1% estimated for 2002-2006
(Baker, 2011). While previous work indicates this approach would characterize a significant
fraction of dissolved reactive phosphorus (from atmospheric wet deposition), these estimates
omit dry deposition from reported values and thus do not encompass the total soluble reactive
fraction of atmospheric phosphorus.
Looking towards the future, the impacts of climate change on the water quality and
quantity of lakes and regional weather patterns are expected to have the potential to alter nutrient
transport within the Great Lakes region. Lakes have been termed sentinels for climate change
due to their relatively fast response to changes in surrounding terrestrial environment and
atmosphere (Williamson et al., 2008; Adrian et al., 2009). In the Midwest, storms of greater
frequency and intensity are generally expected to bring more extreme precipitation, followed by
longer periods between storms without rain. (Parry et al., 2007; McBean & Motiee, 2008; Karl
et al., 2009) Increased evaporation during warmer summers is also expected, leading to more
frequent regional water shortages or another extreme event, drought. Climate change is predicted
to shift both lake temperature and the overall level of water within lakes (Koonce & Czapla,
16
1996), and large increases in phosphorus inputs to lakes are also expected due to increased
storm events, largely in the form of surface runoff (Jeppesen et al., 2009). However, the impact
of more frequent, higher intensity storms with the likelihood of more frequent drought events on
future phosphorus loadings via atmospheric deposition is poorly understood. For instance, the
USGS SPARROW model does not include contributions from atmospheric phosphorus as a
nutrient term for base calculations and nutrient model projections, and the recent report on
another large regional watershed, the Chesapeake Bay (Pyke, 2008), did not describe loadings of
atmospheric phosphorus.
The potential impacts of climate shifts on phosphorus inputs to Lake Erie should be
considered in light of current observations and model predictions of phytoplankton bloom
dynamics in lakes. North temperate lakes have experienced a warming trend resulting in the
advancement of spring plankton bloom timing. This has been attributed to early lake
stratification or the relaxation of turbulent vertical mixing of the upper water-column depth, but
regardless of its cause, this trend effectively extends the length of time for favorable growth
conditions for all algal species (Shimoda et al., 2011), and increases the potential for lake algal
overgrowth in the presence of excess nutrients .
CONCLUSION
External to lakes themselves, results from some climate models predict a shift in future
precipitation patterns in the Great Lake region to include less snowfall and more heavy episodic
rain events. Intense storm events coupled with effects of human activity have the potential to
increase atmospheric phosphorus input into regional lakes; however, climate impacts on nutrient
budgets require more research. For example, for atmospheric contributions of phosphorus, a
17
better understanding of the sources of particulates to the Great Lakes and their role in nutrient
budgets for the lakes is needed.
While inputs from atmospheric phosphorus are low, they are not negligible in Lake Erie.
The delivery pathways and constituents of atmospheric phosphorus deposition may have special
relevance for supporting primary production in oligotrophic interior portions of the other Great
Lakes, while the contribution of atmospheric phosphorus to Lake Erie’s total phosphorus load is
currently understood to be relatively minor. The sources of atmospheric phosphorus in the Great
Lakes are not currently well understood, with the fraction from wet deposition more thoroughly
monitored and reported than from dry deposition. The question surrounding levels of soluble
reactive phosphorus within both wet and dry fractions is also currently unresolved. Given the
importance of developing effective management frameworks for phosphorus inputs to the Great
Lakes that reflect the current state of human impacts on nutrient cycling, phosphorus budgets and
nutrient models should be examined across the region for improvement, especially as such
models are modified to examine regional effects of climate.
18
Figure 1. Average estimated phosphorus loadings (MTA) for Lake Erie (1996-2002); data from Dolan and
McGunagle, 2005. Data includes estimates for contributions from Lake Huron and unmonitored areas.
Direct sources are point sources that discharge into Lakes Erie or St. Clair, the Detroit or St. Clair Rivers or
unmonitored areas.
5128
2162
605
55
1438
1080
Monitored Tributary
Unmonitored Area
Atmospheric
Direct Industrial
Direct Municipal
Lake Huron
Average estimated phosphorus loadings (MTA) for Lake Erie
(1996-2002)
19
Figure 2. Total phosphorus loadings for Lake Erie, 1967-2007. Includes point and non-point source loads
from the United States and Canada. Figure reproduced from Baker, 2011, data from David Rockwell, (U.S.
EPA and David Dolan (Univ. of Wisconsin, Green Bay)
20
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