General nutrient distribution in the water columnof Northern Lake Tanganyika

David Nahimana,1,2 Natacha Brion,1,∗ Willy Baeyens,1 andGaspard Ntakimazi2

1Vrije Universiteit Brussel, WE-ANCH, Pleinlaan, 2, 1050 Brussels, Belgium2Universite du Burundi, Faculte des Sciences, B.P. 2700 Bujumbura, Burundi

∗Corresponding author: nnbrion@vub.ac.be

A one-year’s sampling programme of ammonium, nitrate and nitrite, and silica was performed at astation located in the pelagic area of northern Lake Tanganyika, about 4 km offshore in the vicinity ofBujumbura city. Sampling was generally conducted twice a month and water samples were collected at10 m intervals between 0 and 100 m depth and analyzed spectrophotometrically for dissolved inorganicnutrients. Physico-chemical parameters were determined during field work with specific probes, and furtherwork was done in the laboratory for dissolved oxygen, which was measured using the Winkler method.Meteorological data allowed for the distinction of four contrasting seasonal situations: 1) September toDecember 2004 with low rainfall and decreasing wind speed, 2) December 2004 to April 2005 with lowwind speed and high rainfall, 3) April to June 2005 with sporadic rainfall and slightly increasing windspeed, and 4) June to August 2005, a typical dry season with no rainfall and high wind speed. Low tohigh temperature stratification periods were recorded and seemed to be linked to the hydrodynamics of thelake. Below the mixed layer the water is generally anoxic, but in July a deepening of the oxygenated mixedlayer down to the bottom was recorded. Within the photic zone, phytoplankton activity implied most of theyear undetectable levels of nitrate and nitrite. However, strongly fluctuating and surprisingly high levelsof ammonium and the lowest concentrations for silicates were observed. While ammonium accumulatesin the dark anoxic waters, nitrate and nitrite peak levels are mostly observed between 30 and 70 m depthdue to nitrification processes. Below the photic depth and down to the bottom, silicates increased due tobiogenic silica dissolution. Our data mostly corroborate earlier works and confirm the strong coupling ofmeteorological data and water column properties in northern Lake Tanganyika.

Keywords: Seasons, hydrodynamics, photosynthesis, respiration

Introduction

Climate and related changes in environmentalvariables (Plisnier, 2000), together with their impacton Lake Tanganyika productivity have been recentlyreported (Verburg et al., 2003; O’Reilly et al., 2003).However, the effect of climate change as reason forthe productivity decrease is differently appraised(O’Reilly et al., 2003; Sarvala et al., 2006). Despitethe oligotrophic features of Lake Tanganyika, previ-

ous investigators were surprised by its high fish pro-ductivity which implied comparison to marine envi-ronments. In spite of that, the northern part of LakeTanganyika has been suggested to sustain the highestprimary production of the lake (Sarvala et al., 1999)probably because of increased nutrient input fromthe Rusizi River (Langenberg et al., 2003). The latterauthors suggested that clarifying nutrient cycles isan a priori requirement to understanding the produc-tion and ecosystem functioning in Lake Tanganyika.

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Aquatic Ecosystem Health & Management, 11(1):8–15, 2008. Copyright C© 2008 AEHMS. ISSN: 1463-4988 print / 1539-4077 onlineDOI: 10.1080/14634980701878116

Nahimana et al. / Aquatic Ecosystem Health and Management 11 (2008) 8–15 9

As a contribution in that way, Brion et al. (2006)showed that external inputs of DIN (dissolved in-organic nitrogen) could only sustain 25% of the Ndemand of phytoplankton, suggesting that internalN sources, linked to water movements includinginternal waves and diffusion processes, could be im-portant in sustaining primary production in the sur-face waters. The main hydrological characteristics ofnorthern Lake Tanganyika were studied by Coulterand Spigel (1991) and recently by Plisnier et al.(1999), Chitamwebwa (1999), Naithani et al. (2002,2003), and Naithani and Deleersnijder (2004). Morerecently, Bergamino et al. (2007) reported satelliteimagery evidence of higher chlorophyll a concen-tration values of the Bujumbura basin compared tothe rest of the northern Lake Tanganyika.

The Lake Tanganyika area is known to have twomain climate features: 8 months for the wet sea-son (September to April) and 4 months for the dryseason (May to August). In general, from Januaryto April (rainy season), the water column is stableand highly stratified with surface waters depleted innutrients and anoxic nutrient rich deep waters. Dur-ing May to September (dry season), strong windsinduce the deepening of the nutrient-depleted epil-imnion in the northern part of the lake, where the wa-ter column remains stratified. Plisnier et al. (1999)reported that in Lake Tanganyika an internal seichecontinues to oscillate with decreasing amplitude forat least one year after the trade winds cease and thisinduces periodic water quality parameters variation.In northern Lake Tanganyika, around October, theinternal waves and the thermocline oscillations re-sult in nutrient pulses to the surface layers and deepwaters move back towards the surface as a secondaryupwelling period. The northern end of Lake Tan-ganyika is generally believed to respond strongly tothese various hydrodynamic events (Coulter, 1963;Langenberg, 1996). However, the available data areold, too scarce and fragmentary to comprehend theimportance of water mixing in introducing nutrient-rich hypolimnetic water into the epilimnion and af-fecting productivity in the northern part of the lake(Langenberg et al., 2003).

In this paper the general inorganic nutrient dis-tribution in the water column of northern Lake Tan-ganyika is described and compared to earlier works.Vertical and seasonal variations in the photic depthare presented and discussed in the framework of thehydrodynamic events of the lake. Phosphate has notbeen measured during this research, but literaturedata are referred to herein.

Material and methods

Between September 2004 and August 2005 andgenerally twice a month, water samples were col-lected from an inflatable motorboat at a station lo-cated more than 4 km offshore Bujumbura (29◦17′00E–3◦28′00 S). Water samples were collected be-tween 0 and 100 m at 10 m intervals using a 2 LNiskin bottle attached to a graduated cable for sub-surface samples. The depth of sampling was referredto as the vertical position of the cable, but when thecable was slanting due to weather conditions, thereal depth was the corresponding depth of samplingdivided by cosinus α (where α is the angle formed bycable from the vertical position). Water transparencywas determined using a Secchi disc and temperatureby using a probe. The photic depth, i.e. the depthwhere 1% of surface light penetrates, was estimatedto twice the Secchi disk depth (Parsons and Taka-hashi, 1973).

Additionally, a Winkler bottle was filled imme-diately with collected water and spiked with man-ganese sulfate solution, sodium hydroxide mixedwith potassium iodide solution and sulfuric acid forthe analysis of dissolved oxygen (Winkler method).Remaining water was kept in acid-rinsed plastic bot-tles, and cooled with ice until further treatment in thelaboratory. For the analysis of nutrients, water wasfiltered on pre-combusted (8 h at 450◦C) glass-fibreGF/F filters (Whatman) the same day. Filtrates werekept in the fridge till the following day, when nutri-ent analyses were performed. Oxygen titrations weredone the same day as the nutrient analysis. Oxygensaturation (%) values were calculated from the dis-solved oxygen (mg/l) concentration with a classicalmethod by taking into account the water tempera-ture, the altitude and pressure of the sampling site.

Ammonium concentrations were measured withthe indophenol blue complexion colorimetricmethod according to Koroleff (1969). Nitrate is firstreduced to nitrite on a cadmium-copper column ac-cording to Gardner et al. (1976) and nitrite is anal-ysed with the sulphanilamide colorimetric methodaccording to Bendschneider and Robinson (1952).The absorbance is read respectively at 630 nm forammonium and 540 nm for nitrate and nitrite usinga 5 cm cell pathway with a spectrophotometer (Uni-cam Heλios Gamma and Delta). The concentrationof silica was determined according to Rainwater andThatcher (1960). The silicon in solution as silicicacid (H4SiO4) or silicate (SiO2−

3 ) reacts with acidicammonium molybdate to form the molybdate blue

10 Nahimana et al. / Aquatic Ecosystem Health and Management 11 (2008) 8–15

color that is analysed with a spectrophotometer at700 nm using a 5 cm cell. In these conditions, de-tection limits for NH+

4 , NO−2 , NO−

3 and Silicate are0.3, 0.05, 0.05 and 2µM respectively. Standard de-viation on replicate measurements was less than 5%.Phosphate data were obtained from Plisnier (1996).

Meteorological data as daily average of mini-mum and maximum air temperature, rainfall andwind velocity were obtained from IGEBU, InstitutGeographique du Burundi.

Results and discussion

Meteorological data are presented in Figure 1 andallow the distinction of basically 4 different seasonalsituations. The period from September to December2004 was a dry period with low rainfall and high butdecreasing wind speed, typical for the end of the dryseason. From December 2004 to April 2005 was arainy period with low wind speed, typical for therainy season. During the period from April to June2005 sporadic rain was observed, but with slightlyincreasing wind speed, showing a transition situationbetween rainy and dry season. From June to Augustwas a dry period with increasing wind speed, typi-cal of the dry season. These 4 contrasting trends de-

picted by our meteorological data allow us to studythe distribution of the inorganic nutrients in order toshow implicitly the link between the weather and thechemical composition of the water column. Indeed,Langenberg et al. (2002) found a strong couplingbetween meteorological and water properties.

Vertical variations

The influence of hydrodynamics on the nutri-ent dynamics of the water column of northernLake Tanganyika was assessed by the investigationand discussion of vertical variations. Vertical varia-tions of temperature, oxygen, nitrate+nitrite (NOx),ammonium and silicate are presented by show-ing an example for each of the four typical sea-sonal situations as depicted in Figure 2. The re-sults in this paper corroborate earlier ones (Heckyet al., 1991; Langenberg et al., 2002, 2003), butextend over a whole year while previous investi-gations dealt with short periods of time. Thermalstratification of the water column was observedfrom 28 September to 18 May, and it may be ob-served that the depth of the mixed surface layer in-creased from 28 September (<10 m deep) to 18May (60 m deep). For the dry season situation of

Figure 1. Seasonal variation of rainfall (mm/week−1) and wind speed (m/s−1) in Bujumbura (data from IGEBU).

Nahimana et al. / Aquatic Ecosystem Health and Management 11 (2008) 8–15 11

Fig

ure

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tion

sof

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,oxy

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nitr

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trat

e(N

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12 Nahimana et al. / Aquatic Ecosystem Health and Management 11 (2008) 8–15

11 July, the temperature was roughly similar for thewhole water column despite the slight decreasingtrend linked to the higher evaporation cooling of thesurface water. Dissolved oxygen concentration pro-files were similar to temperature profiles with oxy-gen rich mixed surface waters and oxygen depletedbottom waters during the stratification periods, andoxygenated waters over the entire water column inJuly 2005 (dry period). During the wet season andat the transition between wet and dry seasons (31January and 18 May), NOx profiles are typical of astable, stratified water column as described in Brionet al. (2006). The epilimnetic waters at 30 to 50 mdepth are depleted in NOx due to phytoplankton ac-tivity. Below the photic zone and before the watersbecome anoxic (between 30–50 to 70 m) NOx ac-cumulates probably because of nitrification. Finallywhen approaching anoxic bottom waters, NOx de-creases probably due to consumption by denitrifi-cation. For the dry season situations (28 Septemberand 11 July) the whole water column was depletedin NOx. In July, this showed again, the extensionof surface waters down to the bottom of the studiedwater column but in September, as the water columnshowed a very narrow mixed layer (<10 m), the pres-ence of a very small sub-photic NOx peak is proba-bly linked to the secondary upwelling. Ammoniumprofiles show much more variability than expectedprobably because of the very dynamic nature of thisnutrient. In September 2004, surface waters are de-pleted in NH+

4 and maximum NH+4 concentrations

are recorded at 10 m depth. NH+4 then decreases

to a minimum at 50 m, in parallel with the oxygendecrease, probably because of consumption by phy-toplanktonic organisms. When oxygen levels are attheir lowest, NH+

4 accumulates as is classically ob-served in dark anoxic waters. The same general trendis observed on 31 January but with an extreme largescatter in measurements, and this time with impor-tant nitrification as a NOx peak is observed between30 and 70 m depth. During the transition period of18 May, the profiles are as classically described (seeBrion et al., 2006): NH+

4 is depleted in the euphoticzone due to phytoplankton consumption and belowdue to nitrification (NOx peak). In the dark anoxicwaters, NH+

4 accumulates as described previously.Finally in July, the whole water column is depleted inNH+

4 showing again the extension of surface watersdown to the bottom. Silicate profiles show that withrainy seasons, mixed surface waters had the lowestconcentrations. Below the photic depth and downto the bottom, silicates increased as a result of bio-

genic silica dissolution. At the end of the rainy sea-son, the surface waters with lower silicates deepenprogressively in dry season and a constant concen-tration from surface to bottom is seen again in July.Finally, referring to Plisnier (1996, Figure A.2.4),phosphate measured off-Bujumbura during August1994-July 1995 period showed an increasing trendfrom lowest concentration values within the photiclayer to higher concentration values (5 to 10 times)at the bottom of the 100 m water column. As mostof the silicates and phosphates are supplied to epil-imnion by vertical loading processes (Hecky et al.,1991), the two nutrients show similar trends withinthe surface layer (Edmond et al., 1993). Howeverthe concentration levels must be different because,to our knowledge and contrary to phosphorus sug-gested to be sometimes limiting (Jarvinen et al.,1999), silicates never limit productivity in LakeTanganyika.

Seasonal variations in the euphoticwater layer

The euphotic zone corresponds to the water layerwhere algae are active. The photic depth (Figure3) fluctuates between 16 and 27 m (90% of thedata, n = 18) with a median at 21 m. This value islower than the 40 m yearly average euphotic depth ofLangenberg et al. (2002) but close to the one calcu-lated from Plisnier et al. (1999) Secchi depth data forthe same research area in 1993–1994 (transparancyof 8.7 m to imply 17.4 m photic depth). Averagetemperature, oxygen saturation and nutrient concen-trations in the photic layer were calculated for eachof the 18 vertical profiles and seasonal variations arediscussed (Figure 3).

The period from September to December 2004was a dry period with low rainfall, high but decreas-ing wind speed, and lowest temperatures due to evap-orative cooling of surface waters by the wind, typicalfor the end of the dry season. There has been reportedthat the thermocline is the critical barrier to the up-ward diffusion of nutrients into the epilimnion in thewet season (Hecky et al., 1991) and the thermoclinenever breaks down in the northern Lake Tanganyika.The diffusive fluxes of N and P to the epilimnion ofLake Tanganyika are in Redfield proportions (Heckyand Kilham, 1988; Edmond et al., 1993). At the endof that dry period, silicates were lowest, NOx werehighest with a decreasing trend, and ammonium washigh as well. High N-nutrients for the dry periodwere previously reported by Brion et al. (2006) and

Nahimana et al. / Aquatic Ecosystem Health and Management 11 (2008) 8–15 13

Figure 3. Seasonal variations of photic layer depth, average photic layer temperature, oxygen saturation (%), ammonium (NH4),nitrate + nitrate (NOx), and silicates (Si).

Langenberg et al. (2003) probably mainly linked toatmospheric dry deposition (Brion et al., 2006) andwater circulation from the South. At the same time,silicates were lowest and dissolved oxygen was over-saturated and this clearly shows the presence of aphytoplankton bloom, taking advantage of the highnutrient supply to grow. The low silicate concentra-tions also show the importance of diatoms and/orchrysophytes in this bloom. Indeed, diatoms andchrysophytes are the most important algal group thatinfluence the silica concentrations in the northernLake Tanganyika during the mixing period (Heckyand Kling, 1981, 1987). Additionaly, cyanobacteria

blooms have been reported to occur around October–November period (Salonen et al., 1999; Hecky andKling, 1981) when the fixation of nitrogen is re-quired to sustain productivity. From December toApril, rainfall is the highest, wind speed the lowestand surface water temperatures increased as typicalfor the rainy season. Dissolved oxygen levels arestill over-saturated, NOx becomes undetectable andammonium dropped down to low levels, but sili-cates increased. This was still showing the presenceof a phytoplankton bloom but this time, diatoms(or chrysophytes) are not active anymore in spiteof the still available Si and NH+

4 , and are probably

14 Nahimana et al. / Aquatic Ecosystem Health and Management 11 (2008) 8–15

replaced by other more competitive phytoplanktoncommunities most likely cyanobacteria (Hecky andKling, 1981, 1987). Indeed, the high water tem-peratures and the high water transparency favourcyanobacterial bloom (Talling, 1987) that was oftenfound as an important component of the phytoplank-ton assemblage of Lake Tanganyika (Hecky, 1991;Salonen et al., 1999). Accordingly, Hecky et al.,(1991) noted that the wet season features imposea strong seasonal pattern on phytoplankton abun-dance and composition.

From April to June, rainfall is still important,wind speed starts to increase, and water tempera-tures are generally decreasing, which is indicativefor the transition from the rainy to the dry season.This time dissolved oxygen concentration becomesunder-saturated showing the dominance of respira-tion processes in the water column. NOx are still ex-hausted, ammonium dropped down to undetectablelevels and silicates decreased slightly. This situa-tion probably showed the end of the phytoplanktonblooming period with a dominance of heterotrophicprocesses. In addition, specifically for April, thepeak of NH+

4 concentration observed is probably dueto wind stress which could have induced the mixingof sub-euphotic waters with the surface layer waters.In fact, the drop in dissolved oxygen concentrationsand temperature linked to the peak of the wind veloc-ity supports this possibility as an effect of reducingcompounds brought in contact with oxidative wa-ters similar to what was noted by Langenberg et al.(2003).

Finally from June to August we normally reachtypical dry season conditions. NOx and NH+

4 are stillexhausted, silicates are high and oxygen concentra-tions are still under-saturated showing dominance ofheterotrophic respiration processes. Clearly, nutrientdepleted waters entrained northward by high windstress are dominating the water column of the north-ern Lake Tanganyika and phytoplankton is unable togrow. These observations corroborate the findings ofLangenberg et al. (2002) who reported a strong cou-pling of meteorological and water properties. How-ever, our dissolved inorganic nitrogen concentrationdata seemed to be slightly different from data of Lan-genberg et al. (2003) who recorded during the dryseason and on average twice the concentration of dis-solved inorganic nitrogen and phosphate within thephotic layer compared to wet season. As mentionedbefore, our nitrogen nutrient concentration valueswere generally undetectable during the dry seasonand there seemed to be less phytoplanktonic activity.

Conclusions

This work showed the basic features of nutrientvariability in northern Lake Tanganyika. The moni-toring programme over a period of one year showedthe succession of four contrasting seasonal situa-tions: 1) September to December 2004 (dry season):stratified water column, high NOx and NH+

4 con-centrations in the euphotic layer and active diatom-based primary production; 2) End-December 2004to April 2005 (rainy season): slightly stratified wa-ter column, no NOx and high but decreasing NH+

4and probably cyanobacteria based primary produc-tion; 3) End-April to June 2005 (a Rainy-Dry tran-sition period): stratification with a deep mixed sur-face layer, no NOx and low NH+

4 availability anddominant respiration processes; 4) July to August2005 (dry season): no stratification, no NOx and noNH+

4 , and still dominant respiration processes. Infact, ammonium is mostly detectable in the photiclayer in spite of undetectable levels reported in pre-vious studies. Other features reported herein mostlycorroborate previous works and confirm the strongcoupling between meteorological data and watercolumn properties in northern Lake Tanganyika.

Acknowledgments

The authors are indebted to IGEBU for the me-teorological data and to the Belgian Technical Co-operation for the financial support of this research.

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