The ecology of Lake Nakuru (Kenya) : III. Abiotic factors and primary production.

Abiotic factors, standing crop and photosynthetic production were studied in the equatorial alkaline-saline closed-basin Lake Nakuru (cond. 10,000-160,000 μS). Meteorological conditions and abiotic factors offer suppositions for a high primary productivity: mean solar radiation is 450-550 kerg·cm-2·s-1, with little seasonal variation, regular winds circulate the lake every day and nutrient concentrations are usually high (>100 μg P-PO4·l-1). Oxygen concentrations near sediments were <1 gO2·m-3 for at least 6 h·d-1 in 1972/73, resulting in a release of ∼45 mg P-PO4·m-2·d-1. Attenuation coefficients vary from 3.6-16.5 according to algal densities and mean depth from 0-400 cm. Algal biomass was ∼200 g·m-3 (d.w.) in 1972/73, due to a lasting Spirulina platensis bloom (98.5% of algal biomass). In 1974 algal biomass suddenly dropped to 50 g·m-3 (d.w.). Spirulina and several consumer organisms almost vanished, but coccoid cyanobacteria, Anabaenopsis and diatoms increased. Several causes for this change in ecosystem structure are discussed. The use of the light/dark bottle method to measure photosynthetic production in eutrophic alkaline lakes is discussed and relevant experiments were done. Oxygen tensions of 2-35 gO2·m-3 do not influence primary production rates. Net photosynthetic rates (mgO2·m-3·h-1; photosynthetic quotient=1.18) reached 12-17.7 in 1972/73 and 2-3 in 1974, but vertically integrated rates were only 1-1.4 in 1972/73 and 0.8 in 1974, and daily net photosynthetic rates (gO2·m-3·24 h-1) 3.5 in 1972/73 and 1 in 1974. 50% of areal rates were produced within the 10 most productive cm of the depth profile. The disproportion between high algal standing crops and relatively low production rates is due to self-shading of the algae, reducing the euphotic zone to 35 cm in 1972/73 and 77 cm in 1974. Efficiency of light utilization is 0.4-2%, varying with time of day and phytoplankton density. In situ efficiencies show an inverse relationship to light intensities. Photosynthetic rates of L. Nakuru remain within the range of other African lakes (0.1-3 gO2·m-2·h-1). The relation of O2 produced/Chl a of the euphotic zone is 50% lower then in tropical African freshwater lakes and conforms to lakes of temperate regions.