Standing crop densities and distribution of Spirulina and benthic diatoms in East African alkaline saline lakes

Abstract

SUMMARY. The diversity of Eastern Rift Valley alkaline saline lakes is illustrated and their temporal fluctuations in ionic concentration are emphasized. The standing crop densities of phytoplanktonic blue‐green algae of the genus Spirulina and benthic diatoms were measured. Spirulina abundance was measured as dry mass density and the validity of the gravimatric estimates was checked by chlorophyll analysis and microscopic counting. Benthic diatom standing crops were assessed by chlorophyll determinations. Spirulina density is extremely variable in time and may fluctuate over more than an order of magnitude. High density blooms of Spirulina are not a common characteristic feature of these lakes. The causes of changes in Spirulina standing crop and the associated alterations in the structure of the primary producer community are discussed and three qualitative states which may occur are described. Similar standing crops of benthic diatoms (mean 45 mg m −2 of chlorophyll‐α) were found at all lakes wherever the water depth was shallow enough to allow light penetration to the sediment surface.