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Spatial and temporal patterns of phytoplanktonabundance and composition in three ecological zonesin the Tanzanian waters of Lake VictoriaGW Ngupula a , ASE Mbonde b & CN Ezekiel aa Tanzania Fisheries Research Institute , PO Box 475, Mwanza, Tanzaniab Tanzania Fisheries Research Institute , PO Box 46, Sota, Shirati, TanzaniaPublished online: 20 Jul 2011.

To cite this article: GW Ngupula , ASE Mbonde & CN Ezekiel (2011) Spatial and temporal patterns of phytoplanktonabundance and composition in three ecological zones in the Tanzanian waters of Lake Victoria, African Journal of AquaticScience, 36:2, 197-206, DOI: 10.2989/16085914.2011.589118

To link to this article: http://dx.doi.org/10.2989/16085914.2011.589118

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African Journal of Aquatic Science 2011, 36(2): 197–206Printed in South Africa — All rights reserved

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AQUATIC SCIENCEISSN 1608–5914 EISSN 1727–9364doi: 10.2989/16085914.2011.589118

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The increase of unregulated activities, such as the release of raw sewage and urban/industrial effluents, and of agricul-tural activities in the catchment and around the shoreline of Lake Victoria contribute significantly to the already polluted and eutrophic state of its waters (Hecky 1993, World Bank 1996). These changes affect the structure of its communi-ties by altering species composition, distribution, and abundance patterns, and ultimately community structure, which in turn affect ecological functions (Wetzel and Likens 2000). An example is the reported changes in phytoplankton species composition from highly nutritious diatoms, the most important food item of Nile Tilapia (Oreochromis niloticus), to mainly less nutritious and undesirable Cyanobacteria (Kling et al. 2001, Mugidde 2001, Mbonde et al. 2004).

In the absence of proper management strategies, and with a rapidly increasing human population, plus the influence of global warming, these problems have continued to increase over time. The nearshore waters, where major rivers enter and which are in close proximity to densely populated areas, are the most highly impacted compared to the intermediate and deep offshore waters.

Patterns of abundance in algal biomass are important in understanding ecosystem productivity, especially when correlated with physico-chemical parameters. Despite many existing studies on the phytoplankton of Lake Victoria, few report on conditions in the Tanzanian waters of the lake,

and none correlate phytoplankton abundance with physico-chemical parameters in the different ecological zones of the lake. Such correlation can indicate ecosystem changes in the catchment and the associated impacts on the lake, information that is useful to managers.

In the present study it was assumed that water quality (i.e. its physico-chemical parameters) in the various ecolog-ical zones of the lake was highly variable, and influences the variability of phytoplankton abundance and composition. Spatial and temporal abundance patterns and composition of phytoplankton, as well as physico-chemical parameters, were studied in various ecological zones of the lake from September 2005 to October 2007, in order to establish the relationship between phytoplankton and water quality.

Materials and methods

Study area and sampling strategyPhytoplankton was sampled at 51 stations in the Tanzanian waters of Lake Victoria (Figure 1) between September 2005 and October 2007. The climate of the Lake Victoria basin is characterised by two distinct seasons: a dry season in July–September and a wet season in October–June with two rainfall maxima, short rains in October–December and long rains in March–May. February–March 2006 and 2007 were rainy periods during which the lake

Spatial and temporal patterns of phytoplankton abundance and composition in three ecological zones in the Tanzanian waters of Lake Victoria

GW Ngupula¹*, ASE Mbonde² and CN Ezekiel¹

¹ Tanzania Fisheries Research Institute, PO Box 475, Mwanza, Tanzania² Tanzania Fisheries Research Institute, PO Box 46, Sota, Shirati, Tanzania* Corresponding author, e-mail: ngupula@yahoo.co.uk

Received 25 April 2010, accepted 31 January 2011

Phytoplankton abundance and composition in relation to physico-chemical parameters were investigated from September 2005 to October 2007 at 51 stations of various depths in the nearshore, intermediate and deep offshore waters of Lake Victoria. Shallow nearshore waters had the highest abundance of phytoplankton (mean 2.12 × 108 ind. l–1), compared to the intermediate (1.96 × 108 ind. l–1) and deep offshore waters (7.59 × 107 ind. l–1). Bacillariophyta was the most abundant group (48.17% of total phytoplankton) and was uniformly distributed in all waters, followed by Cyanobacteria (33.33%), which decreased with distance offshore. Chlorophyta, the third highest in abundance (15.5%), increased with distance offshore. A total of 92 phytoplankton species were recorded. The most diverse phyla were Chlorophyta with 37 species, of which Staurastrum sp. and Siphonctosiphon polymorphus were the most abundant species, followed by Bacillariophyta with 25 species and Cyanobacteria with 21 species. In general, the phytoplankton community was dominated by the less nutritious species Nitzschia acicularis (Bacillariophyta), Lyngbya circumcreta and Microcystis flos aquae (Cyanobacteria). These findings contrast with those of most studies from before 1990, but agree with many thereafter. Eutrophication, affecting availability of nutrients etc., and pollution, affecting transparency and water pH, were the forces responsible for the observed spatial and temporal variability in phytoplankton abundance patterns and composition.

Keywords: Bacillariophyta, Chlorophyta, dominance, eutrophication, Lyngbya circumcreta, Microcystis flos aquae, Nitzschia acicularis

Introduction

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was stratified, July–September 2006 and 2007 were dry periods with mixing, and November–December 2005 and 2006 were short rains periods with the re-establishment of moderate stratification.

The Tanzanian waters of the lake were divided into three ecological zones: shallow nearshore, intermediate and deep offshore, each zone comprising two ‘depth strata’. In the shallow nearshore zone, depth stratum I comprised sampling stations that were <10 m deep and depth stratum II stations that were 10.1 to 20 m deep. This region was characterised by some macrophyte coverage and by river discharges, as well as by intense human influences including the release of raw sewage and, in the catchment, urban and industrial development, unplanned settlements and agriculture that accelerate phosphorus, nitrogen and sediment inflow into the lake.

In the intermediate zone, depth stratum III comprised stations 20.1 to 30 m deep, and depth stratum IV stations 30.1 to 40 m deep. It was felt that this zone possessed the characteristics of both shallow nearshore and deep offshore waters. In the deep offshore zone, depth stratum V comprised stations 40.1 to 50 m deep and depth stratum VI stations >50 m deep. These were mostly open offshore waters isolated from nearshore influences.

The term ‘depth stratum’ used in this study, meaning the horizontal categorisation of the lake waters according to maximum depth, is based on the Standard Operation Procedures (SOPs) developed by the Implementation of Fisheries Management Plan project (IFMP) for the harmoni-sation of Lake Victoria research in the three riparian countries bordering it: Tanzania, Kenya and Uganda.

Sampling strategy and analysis of samplesSamples for phytoplankton species diversity were obtained using a 10 μm mesh plankton net towed vertically three times from the bottom to the surface. A concentrated sample of 100 ml was preserved with Lugol’s solution and formaldehyde (0.7% and 2.5% final concentrations, respec-tively). In the laboratory, samples were examined at 400× magnification using an inverted microscope. Identifications of phytoplankton species were based on the keys of Van Meel (1954), Mossile (1984) and John et al. (2002). A de la Motte water sampler was used to collect samples for phytoplankton abundance and distribution, and for these a sample of 250 ml was fixed with 0.7% Lugol’s solution and 2.5% formaldehyde.

In the laboratory, samples for numerical abundance were allowed to settle for 48 h to a final volume of 20 ml. This was homogenised and 2 ml were analysed using an inverted microscope at 400× magnification. Phytoplankton species were counted as numbers of filaments, colonies or cells, depending on the species. At least 10 fields were counted from each sample.

Dissolved oxygen (DO), pH, temperature, and turbidity were measured using a Seabird profiler and water samples were taken for spectrophotometric measurement of chloro-phyll a and the following nutrients: NO3

– (cadmium reduction and diozoic complex method); NH4

+ (phenate method); soluble reactive silica (SRSi; heteropoly blue method); and total nitrogen (TN) and total phosphorous (TP) after persul-phate digestion, the latter using the ascorbic acid method. Nutrient measurements were made as outlined by APHA (1995) but, where necessary, the analytical protocol was

AFRICAUganda

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Figure 1: Distribution of sampling stations in the Tanzanian waters of Lake Victoria

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DEPTH STRATUM (m)

CyanophytaChlorophytaBacillariophytaEuglenophytaDinophytaChrysophyta

(a) Temporal variation

(b) Taxa, spatial abundance

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DATESep. 2005 Nov. 2005 Mar. 2006 Dec. 2006 Oct. 2007

WATER CATEGORYShallow nearshore waters Intermediate waters Deep offshore waters

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CyanophytaChlorophytaBacillariophytaEuglenophytaDinophytaChrysophyta

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Figure 2: Phytoplankton total abundance in the shallow nearshore, intermediate and deep offshore zones of Lake Victoria, its spatial variation among six depth strata and its temporal variation among five sampling periods between September 2005 and October 2007. Depth stratum I: <10 m deep; depth stratum II: 10.1–20 m deep; depth stratum III: 20.1–30 m deep; depth stratum IV: 30.1–40 m deep; depth stratum V: 40.1–50 m deep; depth stratum VI: >50.1 m deep. Error bars represent SD

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checked against the protocol recommended by Crul (1995). Secchi transparencies were measured using a black-and-white Secchi disc of 20 cm diameter.

The Kolmogorov-Smirnov test was used to test data for normal distribution (P > 0.05), thereafter the abundance data were transformed to a log (x + 1) transformation before application of statistical tests. One-way ANOVA was used to test the significance of spatial variations (i.e. differ-ences among phytoplankton means from the various depth strata) and of temporal variations (i.e. differences among phytoplankton means from the various sampling periods). Pearson product-moment correlations were used to test the significance of relationships between physico-chemical parameters and phytoplankton abundance. All tests were carried out using the SPSS version 18 statistical package. PRIMER 5 was used for calculating the Shannon-Wiener diversity, richness, and the equitability indices.

Results

The shallow nearshore waters had the highest abundance of phytoplankton (mean ± SD = 2.12 × 108 ± 7.01 × 107 ind. l–1), compared to that of intermediate (1.96 × 108 ± 1.14 × 108 ind. l–1) and deep offshore waters (7.59 × 107 ± 4.04 × 107 ind. l–1) (Figure 2d). Depth stratum II had the overall highest abundance (1.27 × 108 ± 5.40 × 107 ind. l–1) and depth stratum VI the lowest (2.89 × 107 ± 1.59 × 107 ind. l–1) (Figure 2c). The mean abundances in the depth strata, as well as the means of some phytoplankton taxa in different depth strata were significantly different (one-way ANOVA, P < 0.01) (Figure 2b). Total phytoplankton abundance was highest in October 2007 (6.69 × 108 ± 1.3 × 108 ind. l–1), high in March 2006 (6.01 × 108 ± 1.40 × 108 ind. l–1), and lowest in September 2005 (3.21 × 108 ± 7.34 × 107 ind. l–1) (Figure 2a). An one-way ANOVA test for temporal variations (means of each depth strata in five sampling periods) returned insignifi-cant results (P > 0.05).

Depth stratum I had the most diverse (Shannon’s H′ = 3.08) and species rich (richness index = 3.836) phytoplankton community, but with comparatively low equitability (0.722) (Table 1). Whereas the diversity index decreased with depth, the equitability index increased (Table 1).

With the exception of Secchi transparency and SRSi, the other physico-chemical parameters (DO, pH, N/P, and turbidity) decreased with depth (Figure 3). Dissolved oxygen was generally higher in the nearshore waters (6.91 mg O2 l−1 in depth stratum I to 7.07 mg O2 l−1 in depth stratum II), contrasting with the deep offshore waters which had the lowest (range: 5.94 mg O2 l−1 in depth stratum V to 5.35 mg O2 l−1 in depth stratum VI). There was little variation in pH with the waters varying from circumneutral (an average of 7.45 recorded in depth stratum VI) to slightly alkaline (an average of 8.66 recorded in depth stratum I). Secchi transparencies increased from that in shallow nearshore waters (1.26 m in depth stratum I to 1.6 m in depth stratum II) to deep offshore waters (3.90 m in depth stratum V to 3.7 m in depth stratum VI), while turbidity was higher in the shallow nearshore waters (4.86 NTU in stratum I to 2.57 NTU in depth stratum II) than in intermediate (1.10 NTU in depth stratum III to 2.80 NTU in depth stratum IV) and offshore waters (1.99 NTU in depth stratum V to 1.25 NTU in depth stratum VI).

The lowest values for TN/TP (4.93) were recorded in depth stratum II whilst the highest (10.31) were recorded in depth stratum III (Figure 3). The SRSi concentrations, which were generally low, varied between 0.50 mg l−1 in depth stratum II and 0.86 mg l−1 in depth stratum V (Figure 3). With the exception of NO3

– concentration, which increased with depth and towards offshore, the other physico-chemical parameters decreased with depth (Figure 4). The lowest and highest nitrate concentrations, 109.33 μg N l−1 and 199.25 μg N l−1, were recorded in depth strata I and VI, respectively. Water temperatures were somewhat higher in the shallower areas of the lake (24.54 °C in depth stratum I to 26 °C in depth stratum II) than in intermediate (24.1 °C in depth stratum III to 24.2 °C in depth stratum IV), and deep offshore waters (24.14 °C in depth stratum V to 24.35 °C in depth stratum VI).

High chlorophyll a concentrations occurred in the interme-diate waters, and the highest value (21.5 μg l−1) was recorded in depth stratum II, while the lowest value (4.55 μg l−1) was recorded in depth stratum VI (Figure 4). Variation in TP followed a more or less similar pattern to that of TN. However, the values for TP were lower in the nearshore waters (109.57 μg l−1 in depth stratum I to 163.30 μg l−1 in depth stratum II) and in the offshore waters (94.43 μg l−1 in depth stratum V to 95.75 μg l−1 in depth stratum VI) than those of TN, which varied between 1129.43 μg l−1 in depth stratum I and 554.40 μg l−1 in depth stratum VI.

The NH4+ concentrations varied inversely with NO3

–, with the highest concentration (77.05 μg N l−1) being recorded in depth stratum I and the lowest (43.25 N μg l−1) in depth stratum V (Figure 4). Pearson product-moment correlation for the test of significance of the relationship between phytoplankton abundance and physico-chemical parameters returned significant relationships for Secchi transparency (r = −0.49, P < 0.01), pH (r = 0.52, P < 0.01), DO (r = 0.65, P < 0.01), depth (r = −0.98, P < 0.01), and TP (r = 0.79, P < 0.05).

Bacillariophyta was the most abundant (48.17%) and widely distributed of all the phytoplankton groups recorded, followed by Cyanobacteria (33.33%) and Chlorophyta (15.5%) (Figure 5).

A total of 92 phytoplankton species was encountered during the study (Table 2), including 37 Chlorophyta, 25 Bacillariophyta and 21 Cyanobacteria species. In these groups the dominant species were Staurastrum sp. and Siphonctosiphon polymorphus (Chlorophyta), Nitzschia acicularis (Bacillariophyta), and Lyngbya circumcreta and Microcystis flos aquae (Cyanobacteria). Other groups present

Depth stratum Number of species

Diversity index (H′)

Richness index (d)

Equitability index (J)

I (<10 m) 71 3.08 3.836 0.722II (10.1–20 m) 63 2.88 3.323 0.696III (20.1–30 m) 60 2.84 3.211 0.692IV (30.1–40 m) 63 2.95 3.366 0.711V (40.1–50 m) 52 2.98 2.888 0.754VI (>50.1 m) 52 2.98 2.969 0.753

Table 1: Number of phytoplankton species, diversity index (H′), richness index (d), and evenness or equitability index (J) in six depth strata of the Tanzanian waters of Lake Victoria

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Figure 3: Dissolved oxygen (DO), pH, SRSi, Secchi transparency, N/P ratio, turbidity values (averaged over the water column), and phytoplankton abundance in various depth strata of Lake Victoria from September 2005 to October 2007. Depth stratum I: <10 m deep; depth stratum II: 10.1–20 m deep; depth stratum III: 20.1–30 m deep; depth stratum IV: 30.1–40 m deep; depth stratum V: 40.1–50 m deep; depth stratum VI: >50.1 m deep. Error bars represent SD

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Figure 4: Temperature, chlorophyll a, TN, TP, NO3–-N, and NH4

+-N values (averaged over the water column) in various depth strata of Lake Victoria from September 2005 to October 2007. Depth stratum I: < 10 m, depth stratum II: 10.1–20 m, depth stratum III: 20.1–30 m, depth stratum IV: 30.1–40 m, depth stratum V: 40.1–50 m, depth stratum VI: >50.1 m. Error bars represent SD

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were Euglenophyta (four species), Dinophyta (three species) and Chrysophyta (two species).

Discussion

The effects of environmental and other pressures are evident in Lake Victoria. Most changes in the lake’s ecosystem are related to management problems (failure to regulate human activities in the lake’s catchment and around its shoreline) and natural causes (a drop in lake level as a result of climatic variations and global warming impacts). For example, the observed declines in DO, pH, turbidity, chloro-phyll a, TN, TP and NH4

+ towards offshore locations are related to increased human impacts on shallow nearshore waters (Lung’aiya and Kenyanya 2001). Lake Victoria has deteriorated over the last decade, primarily due to unregu-lated human settlement, urban sewage discharges and unplanned agriculture and mining (Lung’aiya and Kenyanya 2001). The increase in these activities has contributed to rendering the lake environmentally unstable (World Bank 1996, Machiwa 2001).

The current study reports higher values of NO3– (199.25–

109.33 μg N l−1) than those reported previously in the Kenyan waters (128 μg N l−1; Lung’aiya and Kenyanya 2001; 8 μg N l−1, Melack 1976; 100 μg N l−1, Lung’ayia et al. 2000;

10–112 μg N l−1, Talling 1966) and the Tanzanian waters (109 μg N l−1, Kishe 2004). The main sources of dissolved inorganic nitrogen in Lake Victoria are rainfall, runoff from fertilised agricultural land, sewage outflows, remineralisation processes, and in situ N-fixation by Cyanobacteria (Hecky 1993, Lung’aiya and Kenyanya 2001, Kishe 2004).

Nitrogen is generally considered to be a limiting nutrient in Lake Victoria (Lung’aiya and Kenyanya 2001). The higher nitrate values recorded in this study could partly explain the high algal biomass, as indicated by chlorophyll a concentra-tions, and the phytoplankton blooms observed in the shallow nearshore waters during sampling. However, the chloro-phyll a values recorded in the inshore waters (21.5 μg l−1) and offshore waters (4.55 μg l−1) are lower than the value of 61 μg l−1 reported in the central Nyanza Gulf area (Lung’aiya and Kenyanya 2001).

According to Hecky (1993), sedimentation rate has increased from 57 g m−2 y−1 before 1960 to 90 g m−2 y−1 thereafter. The high turbidity in the shallow nearshore areas is mainly due to suspended sediments from inflowing rivers and increased algal biomass as a result of allochthonous nutrient inputs into the lake (Lung’aiya and Kenyanya 2001). Unregulated agricultural activities on the lake shore and in its wetlands have been suggested as the main cause of the high levels of phosphorus observed in the lake

47%

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<10 mI

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Chlorophyta

Cyanobacteria

Bacillariophyta

Euglenophyta

Dinophyta

Chrysophyta

Figure 5: Percentage contribution of phytoplankton taxa in six different depth strata in the Tanzanian waters of Lake Victoria from September 2005 to October 2007

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(Lung’aiya and Kenyanya 2001, Kishe 2004). However, atmospheric deposition has also been shown to be a signifi-cant source. According to Tamatamah et al. (2005), 13.5 × 103 kg of TP are deposited annually into the lake from the atmosphere, and the annual fluxes of phosphorus measured on the southern and western shores of Lake Victoria (1.8–2.7 kg ha–1 y–1) are near the upper range of similar fluxes measured in the tropics. Generally, environ-mental degradation, particularly through eutrophication and

pollution, is a threat to the structure of lake communities, especially invertebrate species composition, distribution, abundance, and community structure, which in turn affect the lake’s ecological functioning (Mavuti and Litterick 1991, Sarvala et al. 2003, Mbonde et al. 2004).

The phytoplankton community of Lake Victoria is still dominated by Bacillariophyta (Nitzschia acicularis) and Cyanobacteria (Microcystis and Anabaena species) as reported in previous studies (Lung’ayia et al. 2000, Kling

SpeciesEcological zone

SpeciesEcological zone

S I D S I DChlorophyta Cyanobacteria (cont.)

Ankistrodesmus falcatus 108 47 41 Lyngbya limnetica 47 95 0Closterium sp. 27 34 41 Merismopedia tenuissima 660 149 34Botryococcus braunii 98 257 81 Merismopedia punctata 85 14 27Closterium aciculare 7 7 7 Microcystis flos aquae 2 160 1 581 515Closterium acutum 20 0 0 Microcystis aeruginosa 572 318 146Coelastrum cambricum var. nasutum 41 20 41 Oocystis lacustris 27 27 27Coelastrum microporum 54 27 47 Oscillatoria sp. 14 0 0Coelastrum sp. 30 8 0 Spirulina sp. 61 14 7Cosmarium sp. 74 34 14 Romeria sp. 487 244 203Pediastrum duplex 176 27 34 Stichococcus minutissimus 0 7 0Pediastrum duplex var. cohaerens 0 59 8 BacillariophytaPediastrum simplex 118 14 41 Aulacoseira nyassensis 264 91 14Pediastrum simplex var. sturmii 7 54 0 Cyclotella sp. 1 053 1 246 494Pediastrum tetras 190 894 132 Cymbella sp. 34 0 0Scenedesmus bijugatus 0 14 0 Chodatella subsalsa 20 14 14Scenedesmus acuminatus 34 42 27 Diatoma sp. 7 0 0Scenedesmus perforatus 41 416 135 Fragilaria sp. 20 0 7Scenedesmus armatus 20 0 0 Navicula sp. 670 322 135Scenedesmus dimorphus 27 0 7 Nitzschia sp. 108 108 14Scenedesmus quadricauda 156 68 14 Nitzschia acicularis var. major 41 291 0Scenedesmus sp. 251 108 81 Nitzschia asterionelloides 148 271 76Scenedesmus arcuatus 34 0 0 Nitzschia acicularis 6 230 6 215 2 140Selenastrum gracile 7 0 0 Nitzschia nyassensis 335 146 81Sphinctosiphon polymorphus 305 183 203 Nitzschia lacustris 728 982 190Staurastrum anatinum 61 406 203 Rhizosolenia victoriae 122 95 34Staurastrum gracile var. nyassae 47 247 68 Rhapolodia sp. 14 0 0Staurastrum gracile var. protractum 20 203 102 Stephanodiscus sp. 7 27 27Staurastrum paradoxum 7 54 54 Stephanodiscus astraea 27 7 41Staurastrum leptocladum 0 7 20 Surirella constricta var. africana 7 0 7Staurastrum limneticum 14 41 74 Surirella fullebornii 14 7 0Staurastrum sp. 95 102 25 Surirella cuspidata 74 0 0Staurastrum longiradiatum 0 0 7 Surirella nyassae 0 7 0Staurastrum cuspidatum 7 27 34 Surirella sp. 71 20 0Staurastrum tetracerum 0 14 0 Synedra cunningtonii 498 359 135Tetraedron trigonum 98 102 14 Synedra acus 7 0 20Tetraedron haustatum 7 0 0 Synedra ulna 41 20 14Tetraedron caudatum 0 7 0 Euglenophyta

Cyanobacteria Phacus sp. 14 0 0Anabaena flos aquae 1 354 775 281 Euglena sp. 7 0 0Anabaena circinalis 7 0 20 Trachelomonas sp. 74 95 14Anabaena spiroides var. crassa 0 14 7 Trachelomonas volvocina 190 68 7Anabaenopsis tanganyikae 0 1 1 DinophytaAphanocapsa sp. 0 135 0 Glenodinium sp. 464 501 291Anabaena sp. 14 0 0 Ceratium sp. 0 0 7Chroococcus dispersus 176 7 20 Peridinium sp. 61 27 14Kirchneriella contorta 98 51 14 ChrysophytaLyngbya circumcreta 1 225 1 110 542 Mallomonas sp. 210 196 54Lyngbya sp. 47 0 7 Dinobryon sp. 7 0 0Lyngbya contorta 196 190 81

Table 2: Average abundances (ind. l–1) of phytoplankton species in the shallow nearshore (S), intermediate (I) and deep offshore (D) zones of the Tanzanian waters of Lake Victoria in September 2005, March 2006, December 2006 and October 2007. All values have been divided by 104

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et al. 2001, Mugidde 2001, Mbonde et al. 2004). The dominance of these taxa is an indication of poor water quality caused by eutrophication, i.e. increased nitrogen and phosphorus (Kling et al. 2001). Eutrophic conditions favour a decrease in the diversity of phytoplankton assemblages (Moss 1998) while leading to dominance of a few large, colony-forming species of Cyanobacteria such as Microcystis and Anabaena (Wetzel 1983). Phytoplankton taxonomic composition, size distribution, the organisation of cells into filaments or colonies, and the possession of flagellae or buoyancy vesicles are related to factors such as nutrient availability (phosphate and nitrate), degree of stratification, and zooplankton grazing (Moss 1998, Sarvala et al. 2003).

Generally, compared to the situation in the 1990s, the lake’s phytoplankton composition has changed consider-ably. Talling (1965) compared phytoplankton composition between different areas and seasons in Lake Victoria and found neither a dominant taxon nor a dominant species. In contrast, the current study shows the dominance of Nitzschia acicularis, Lyngbya circumcreta and Microcystis flos aquae.

Slightly fewer (92) phytoplankton species were recorded in the present study compared to that by Mbonde et al. (2004), who reported 113 species during the dry and wet seasons of 2002. Comparing their data to the study by Cocquyt et al. (1993), Mbonde et al. (2004) noticed that some species were declining while others were new discoveries. The observed decrease in number of species can be attributed to a number of factors, among which increased pollution and eutrophication are important (Mavuti and Litterick 1991, Lung’aiya and Kenyanya 2001). Currently, Lake Victoria’s water quality is very variable, especially when comparing inshore versus offshore water, as observed in this study.

The main concern regarding the release of excess nutrients into Lake Victoria, especially phosphorus and nitrogen, is the threat of eutrophication (Hecky 1993, Lung’aiya and Kenyanya 2001). Eutrophication affects the ecology and productivity of an entire ecosystem through changing competitive interactions and trophic regimes (Mavuti and Litterick 1991, Hecky 1993). Because of eutrophication, in Lake Victoria there is excessive growth of phytoplankton and/or macrophytes that impair water quality by causing high turbidity, low dissolved oxygen, and the production of toxic gases such as hydrogen sulphide (Sarvala et al. 2003, Kishe 2004). The changes in water quality have over time led to the proliferation of the most resistant/adapted species of phytoplankton (Nitzschia acicularis, Microcystis and Anabaena species) and macrophytes at the expense of the more sensitive ones (diatoms and chlorophyta, e.g. Pediastrum tetras) (Mavuti and Litterick 1991, Mugide 2001, Sarvala et al. 2003).

Phytoplankton blooms, which were frequent in the shallow nearshore waters, especially in bays, may not be a direct threat to humans, but such conditions may kill aquatic organisms, especially during the night when oxygen is depleted. High concentrations of some highly toxic species can produce nerve and liver toxins capable of killing mammals that drink the water (Moss 1998, Sekadende et al. 2005). However, the current occurrence of high densities of Pediastrum tetras (Chlorophyta), which was not recorded here previously due to severe environmental degradation, may be a sign of environmental recovery in some areas.

The decrease in phytoplankton abundance towards offshore stations noted in this study was previously noted in the Ugandan waters of the lake, where abundance was higher in Pilkington Bay compared to Napoleon Gulf and to open waters (Ramlal et al. 2001). We attribute this offshore decrease to light limitation towards the deeper waters, as well as to a decrease in nutrients. Shallow nearshore waters had higher values for most nutrients than the far offshore waters.

Conclusions

The current eutrophic state of much of Lake Victoria impacts on the phytoplankton species composition by favouring the dominance of a few species. This could impact the herbivore Oreochromis niloticus, which depends on nutritious diatoms as its main food item (Njiru et al. 2004). Different ecological zones in the lake are impacted differently by developments in the catchment and along the lake’s shoreline, with the shallow nearshore waters being the most highly impacted. Despite the current high densities of Nitzschia species and Microcystis sp., the reappearance of chlorophytes and the increase in abundance and diversity of bacillariophytes implies that recovery to a healthy and inherently stable ecosystem is possible in Lake Victoria, provided significant efforts are made to manage and regulate activities around the main lake and in its catchment. Further studies on the phytoplankton of Lake Victoria are needed to understand their contribution to the ecology and production of the lake’s fisheries.

Acknowledgements — The authors are grateful to the Implementation of Fisheries Management Plan (IFMP) project for sponsoring this work, and are also grateful for the support extended by TAFIRI staff during data collection. We specifically thank Ismael Kimirei, TAFIRI, for reviewing an early version of this manuscript.

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