Long-term dynamics of algal and invertebrate communities in a small, fluctuating tropical soda lake

Lake Sonachi, Kenya, is a small alkaline-saline crater lake that over the p ast 175 years has experienced considerable fluctuations in lake depth (Z(ma x) - 3-18 m) and an alternation of meromictic and holomictic episodes lasti ng from. a few years to several decades. Paleolimnological methods were use d to reconstruct the long-term dynamics of algal and invertebrate communiti es in Lake Sonachi in relation to the historic evolution of their physical and chemical environment. Multivariate statistical analysis revealed only w eak correlation between the stratigraphic distributions of fossil algal pig ments, diatoms, and chironomid larvae in Pb-210-dated sediment cores and th e documented or reconstructed variation in lake depth, mixing regime, and s urface-water conductivity. The eventful biological history of Lake Sonachi exemplifies the complexity of long-term community dynamics in tropical Afri can soda lakes and reveals how phytoplankton community structure can exert direct control on benthic and planktonic invertebrate communities. The mode st phytoplankton abundance and photosynthetic activity of Lake Sonachi when compared with other tropical African soda lakes represent recent lake cond itions, resulting from a dramatic decline of filamentous cyanobacteria (e.g ., Spirulina platensis) between the 1930s and 1970s and incomplete replacem ent by the small coccoid cyanobacteria (e.g., Synechococcus bacillaris), wh ich are dominant today. This reduction in algal biomass favored benthic and plank-tonic invertebrates by reducing the prevalence of complete water-col umn anoxia associated with intense nighttime respiration of cyanobacterial blooms. Anoxia-intolerant halobiont chironomids expanded during an episode of low lake level (Z(max) < 4 m), holomixis, and high conductivity (>9,000 mu S cm(-1)) in the late 1940s and 1950s, whereas they had failed to do so under similar conditions in the late 19th century when algal abundance was high. Planktonic cladocerans developed high population densities only durin g two short meromictic phases in the 1970s when conductivity was modest (3, 000-6,000 mu S cm(-1)) and algal abundance was low. The decline of filament ous cyanobacteria also increased water-column transparency but apparently d id not increase benthic diatom abundance.