Primary producer dynamics associated with evaporative concentration in a shallow, equatorial soda lake (Lake Elmenteita, Kenya)

Abstract

Lake Elmenteita (0°27′ S, 36°15 ′E) lies on the floor of the rift valley at 1776 m above sea level in Kenya. As a consequence of lower than average rainfall, the mean depth decreased from 1.1 to 0.65 m during the study period (February 1973 to August 1974). The initiation of major biological changes coincided with a period of rapid evaporative concentration in 1973 (February to April) when the conductivity increased from 19.1 to 27.0 mmhmos cm-1. Spirulinaplatensis, Spirulina laxissima and Anabaenopsis arnoldii decreased in abundance precipitously in parallel with large declines in chlorophyll a concentration and phytoplankton photosynthetic rates. Once the overall abundance of phytoplankton had declined and the transparency had increased, primary productivity by benthic algae increased significantly. Paradiaptomus africanus, the only copepod living in the lake, was abundant in February and March 1973, but was gone by May. Eight hypotheses to explain these changes are evaluated and converge on the suggestion that a rate of change of salinity greater than 5 mmhmos cm-1 per month and a salinity exceeding 25 mmhmos cm-1 cannot be tolerated by P. africanus and adversely effects the nitrogen fixer, A. arnoldii. Furthermore, the loss of P. africanus and oxygenation of the sediments by benthic algae reduce the rate of recyling of nutrients which alters phytoplankton abundance and species composition.