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Nutrient control of phytoplankton production in Lake Naivasha, Kenya
Lake Naivasha, Kenya, 2002
Lake Naivasha, a shallow tropical lake in Kenya's Rift Valley, has an unstable water column and is moderately eutrophic. Nutrient (bottom-up) control of primary production is more important than grazing (top-down) control. Experimental nutrient enrichment was used to investigate bottom-up control in more detail. Minor nutrients were not found to be limiting, whilst nitrogen was more limiting than phosphorus with an algal preference for ammonium over nitrate. Sediments form a phosphorus sink but there is hypolimnetic release from the one area showing regular temporary stratification. This indicates that the rate of primary production in the water column could double if conditions change to allow lake-wide nutrient release from sediments. Both external and recycled nutrient regeneration are important.
Trophic state and nutrient limitation in Lake Baringo, Kenya
African Journal of Aquatic Science, 2018
The trophic state of Lake Baringo and factors that could be limiting the development of algal biomass in it were investigated during one wet/dry hydrological cycle in 2014-2015. Water samples were analysed for dissolved inorganic nutrients, including NH + 4 , NO 2 − and NO 3 − , total phosphorus and Chlorophyll a. Light attenuation was estimated using Secchi depth. The trophic state was determined using Carlson trophic state indices (CTSI). Deviations in CTSI, nutrient ratios and ambient nutrient concentrations were used to identify factors limiting phytoplankton growth. The mean values measured for Secchi depth, nitrate, total phosphorus and Chlorophyll a showed significant seasonal variation (p < 0.05). Based on the Carlson trophic state index, the results show that Lake Baringo is eutrophic. However, the lake is also experiencing phosphorus limitation and poor light penetration, because of high turbidity, which is more pronounced during the wet season.
Physical and biogeochemical limits to internal nutrient loading of meromictic Lake Kivu
Limnology and Oceanography, 2009
Lake Kivu is one of the large African Rift lakes situated between the Democratic Republic of the Congo and Rwanda. In its permanently stratified hypolimnion, unusually high methane concentrations have increased further in recent decades. Because methanogenesis is, in part, dependent on supply of organic material from the photic zone, it is necessary to quantify upward nutrient fluxes from the saline, nutrient-rich deep waters. These upward fluxes are mainly driven by advection caused by subaquatic springs. Biogenic calcite precipitation drives surface-water depletion and deep-water enrichment of Ca 2+ , Sr 2+ , and Ba 2+ . Methane is mainly oxidized aerobically at the redox interface at 60 m, with a small contribution of anaerobic methane oxidation. A subaquatic spring that sustains the major chemocline at 250 m depth was depleted of N, P, and CH 4 , and concentrations of major ions were slightly lower than in the lake water of the same depth. Enrichment of the deep waters with nutrients and CH 4 are driven by mineralization of settling organic material, whereas SiO 2 is influenced by uptake and mineralization of diatoms and inputs through subaquatic springs. Dissolved inorganic phosphorus and Si fluxes supplied by internal loading through upwelling were found to be lower than the estimations for Lakes Malawi and Tanganyika. In contrast, N flux was within the lower range for Lake Malawi, whereas it was assumed to be totally lost by denitrification in Lake Tanganyika. In Lake Kivu, nutrient uptake by primary production is three times higher than nutrient upward fluxes.
Nutrient relationships in shallow water in an African Lake, Lake Naivasha
Oecologia, 1981
In the littoral zone of a shallow, tropical lake (Lake Naivasha, Kenya), average nutrient composition of emergent macrophytes along a permanent transect (0–2m depth) on a dry weight basis was: P 0.23%; N 0.96%; and S 0.11%. In the hydrosoil the average composition was much lower, sediments were: P 0.03%; N 0.24%; and S 0.05%. The water depth varied, with lake edge being exposed during the annual drawdown for a part of the year and subsequently being inundated. Water quality varied considerably during the year (temperature 19–28°C; pH 7.0–8.0; conductivity 282–975 μ Scm-1). Of the three nutrients in the water of the littoral zone, N had the highest mean concentration (4.25 mg·1-1) while P was intermediate (1.90) and sulphur had the least (0.99). The distribution of nutrients followed a decreasing gradient from shore to open water. High levels of nutrients were recorded in September following the inundation of drawdown soil and plant material. The large stock of nutrients generated in the littoral zone helps to replenish nutrients in the open lake where low concentrations are typical.
Phosphorus inputs to Lake Naivasha, Kenya, from its catchment and the trophic state of the lake
Lake Naivasha, Kenya, 2002
The main river supplying Lake Naivasha, Kenya, the Malewa, drains a catchment given over to largely subsistence cultivation and animal husbandry. The lake itself is the focus for an intensive horticultural industry based upon irrigation from the lake. The Malewa, however, is relatively independent of the impact of industry, and so its contribution to eutrophication of the lake was evaluated. Two periods of study, a very wet-dry and a 'normal' wetdry season showed that the river contribution of phosphorus led to a total phosphorus loading of 1.4 g m −2 lake surface ann −1 in the very wet period compared to 0.2 in the 'normal'. Chlorophyll 'a' in the open water of the lake was significantly related to soluble reactive phosphorus. The lake is now eutrophic by normal limnological criteria.
This study, carried out between November 2003 and February 2005, aimed to investigate the temporal trends of conductivity, ions, nutrient concentrations and phytoplankton biomass expressed as chlorophyll a in the Kenyan Rift Valley saline-alkaline lakes namely Nakuru, Bogoria and Elmentaita. The influence of environmental variables on phytoplankton biomass has always been of much interest in understanding phytoplankton dynamics. Being shallow and endorheic, these lakes’ chemical, physical and biological properties were found to be strongly influenced by the hydrologic cycle within their catchment area. The lakes are characterised by high nutrient concentrations but with low Ntot: Ptot ratios. Significant differences between surface and near-bottom samples for water temperature, chlorophyll a and some nutrients were found in these lakes. A stepwise Discriminant Analysis with lakes as defined groups resulted in a significant model with SRP, nitrate-N, conductivity and light supply being of major importance. A significant correlation between specific conductivity and total alkalinity (Kendalls τ = 0.85, n = 132) was calculated. While L. Bogoria showed the least temporal variation in conductivity-(65-73 mS cm−1), larger variations were observed in L. Elmentaita (21-77 mS cm−1). Na+ and K+ form the main cations with Cl−, HCO3− and CO32 being the major anions in all the three lakes. Flouride was detected in high quantities (mean values L. Bogoria 72 meq 1−1, L. Nakuru 17 meq 1−1 and L. Elmentaita 71 meq 1−1). A PCA followed by multiple regression analysis with chlorophyll a as dependent variable showed that nitrate-N, conductivity, phosphorus and light supply were the key variables influencing algal biomass in these lakes.
Aquatic Ecology, 2007
Primary production rates, chlorophyll and phytoplankton biovolume were measured monthly from April 2003 to November 2004 in Lake Tana, a large tropical lake in the highlands of Ethiopia. The lake is characterised by low nutrient concentrations, and a low water transparency due to high silt load of the inflowing rivers during the rainy seasons (May-November) and daily resuspension of sediments in the inshore zone. The mean chlorophyll-a concentrations varied seasonally and ranged from 2.6 mg m -3 to 8.5 mg m -3 (mean: 4.5 mg m -3 ) in the offshore zone. Primary production was measured using the light-dark bottles technique. We incubated only at three depths, i.e. 0.6, 1.2 and 1.8 m. Therefore, we may have missed a substantial part of the depth production profile and probably also frequently missed P max . Gross primary production in the openwater averaged 2.43 g O 2 m -2 d -1 and ranged between 0.03 g O 2 m -2 d -1 and 10.2 g O 2 m -2 d -1 ; production was significantly higher in the inshore zone. The highest production rates were observed in the post-rainy season (Oct-Nov), which coincided with a bloom of Microcystis and higher chlorophyll levels. This seasonal high production is probably caused by a relatively high nutrient availability in combination with favourable light conditions. The gross primary production rates of L. Tana are among the lowest compared with other tropical lakes. This will be partly the result of our underestimation of gross primary production by often missing P max . Another cause is the oligotrophic nature of the lake in combination with its relatively low water transparency. The gross primary production per unit chlorophyll in the openwater zone was in the same range as in 30 other tropical lakes and reservoirs. The higher primary production in the inshore zone is probably the result of the daily water column mixing (Z mix ‡ Z t ) in this area, enhancing nutrient recycling. A large proportion of the annual primary production is realised in one of the four seasons only. This productive post-rainy season is relatively short (2 months) and therefore efficiency of transfer of matter between the first and second trophic level of the Lake ecosystem will be poor.
Hydrobiologia, 2009
Increasing degradation of the water quality, caused by overuse and salinization, leads to considerable changes of the phytoplankton composition in Kenyan Rift Valley lakes. Exemplarily, the phytoplankton communities and biomasses of deteriorating freshwater Lake Naivasha and salinizing Lake Oloidien were studied between 2001 and 2005, accompanied by physico-chemical measurements (pH, total phosphorus and nitrogen, alkalinity, conductivity). Over the last three decades, the ecology of these two water basins has been subjected to dramatic changes, caused by excessive use of water and catchment area by man. In L. Naivasha a shift in the dominance of coccoid cyanobacteria towards dominance of Chlorophyceae (Botryococcus terribilis) was observed. Lake Oloidien exhibited a shift in the dominance of coccoid Chlorophyceae towards dominance of cyanobacteria (Arthrospira fusiformis, Anabaenopsis elenkinii). Phytoplankton findings and chemical data demonstrate that L. Naivasha has developed towards a eutrophic freshwater lake while L. Oloidien has progressed towards a hypereutrophic alkaline-saline lake.