The ecology of Lake Nakuru : VI. Synopsis of production and energy flow.

Abstract

The major pathways of energy flow in Lake Nakuru (East Africa) are presented. The trophic structure of this equatorial alkaline-saline lake shows no predictable long term continuity. During the five years of this study it had a bloom of Spirulina platensis that persisted at least two years, it had periods with low algal densities and in addition it had various transitional phases with dramatic fluctuations of species composition and density.The Spirulina platensis bloom is characterized by a rich and almost unialgal bloom of the cyanophyte Spirulina platensis minor, with a mean biomass of 3,500 kJ m-3 (20 kJ ≈ 1 g dry weight). Net photosynthetic rates were very high at depths with optimal light conditions (230 kJ m-3 h-1), but algal self-shading made integrated rates modest (45 kJ m-3 24 h-1) relative to the high biomass. Of the eight primary consumers only five species contributed significantly to the consumer biomass of 220 kJ m-3: the flamingo Phoeniconaias minor, the cichlid fish Sarotherodon alcalicus grahami, the copepod Lovenula africana, the dipteran larva Leptochironomus deribae, and the rotifer Brachionus dimidiatus. Consumption rates were ≈50% of net photosynthetic rates, production rates ≈10%. Secondary consumers (90% being the pelican Pelecanus onocrotalus and the Greater Flamingo Phoenicopterus ruber) had a biomass of about 6.8 kJ m-3. Pelicans consumed almost the whole fish production (7.5 metric tons wet weight/day).At low algal densities the lake had a more diverse algal population but a reduced mean biomass of 1,500 kJ m-3 and mean net photosynthetic rates of 12 kJ m-3 24 h-1. Primary consumer species diversity and biomass were also reduced. Consumption rates sometimes exceeded primary production rates. Rotifers probably contributed ≈50% to total consumption and ≈75% to total secondary production but the estimates of their role is speculative as the relative contributions of algae, bacteria and detritus to rotifer consumption are not known. Transitional phases are characterized by rapidly changing abiotic and biotic conditions with algal breakdowns and sudden population peaks at all levels. Rotifers dominated secondary consumers, they contributed 25% to the total biomass of 380 kJ m-3, 90% to the total consumption rate of 290 kJ m-3d-1 and 95% to the total production of 41 kJ m-3d-1.The discussion focusses on problems of measuring primary production in alkaline-saline lakes, and the control of producer and consumer densities. The difficulty in assessing the importance of bacteria and rotifers is emphasized. Also questions of ecological stability and efficiency are addressed. Finally, some recommendations for conservational policy are included.