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

Abstract

Lake Sonachi, Kenya, is a small alkaline-saline crater lake that over the past 175 years has experienced considerable fluctuations in lake depth (Z max ϭ 3-18 m) and an alternation of meromictic and holomictic episodes lasting from a few years to several decades. Paleolimnological methods were used to reconstruct the long-term dynamics of algal and invertebrate communities in Lake Sonachi in relation to the historical evolution of their physical and chemical environment. Multivariate statistical analysis revealed only weak correlation between the stratigraphic distributions of fossil algal pigments, diatoms, and chironomid larvae in 210 Pb-dated sediment cores and the documented or reconstructed variation in lake depth, mixing regime, and surface-water conductivity. The eventful biological history of Lake Sonachi exemplifies the complexity of long-term community dynamics in tropical African soda lakes and reveals how phytoplankton community structure can exert direct control on benthic and planktonic invertebrate communities. The modest phytoplankton abundance and photosynthetic activity of Lake Sonachi when compared with other tropical African soda lakes represent recent lake conditions, resulting from a dramatic decline of filamentous cyanobacteria (e.g., Spirulina platensis) between the 1930s and 1970s and incomplete replacement by the small coccoid cyanobacteria (e.g., Synechococcus bacillaris), which are dominant today. This reduction in algal biomass favored benthic and planktonic invertebrates by reducing the prevalence of complete water-column anoxia associated with intense nighttime respiration of cyanobacterial blooms. Anoxia-intolerant halobiont chironomids expanded during an episode of low lake level (Z max Ͻ 4 m), holomixis, and high conductivity (Ͼ9,000 S cm Ϫ1

East African soil erosion recorded in a 300 year old coral colony from Kenya

“…These peaks most likely relate to droughtbreaking flood events [McCulloch et al, 2003], which occur after or within prolonged droughts when desiccated landscapes are prone to water-induced soil erosion. Ba/Ca peaks at 1794, 1822 -1826 fall within the well-documented Lapanarat-Mahlatule drought ($1750 to 1820-1830's), the latter Ba/Ca peaks match a major regional lake-filling phase dated to 1815 ± 8 in the central Kenya Rift Valley [Verschuren et al, 1999[Verschuren et al, , 2000. This is beyond the upper Sabaki river catchment but within the area of high correlation between OND precipitation and Sabaki river discharge (Figure 4a).…”

Aquatic biodiversity and saline lakes: Lake Bogoria National Reserve, Kenya

“…It is well understood that saline lakes have a limited species complement in macroorganisms in contrast to considerable biodiversity in micro-organisms (Beadle, 1981;Grant et al, 1990;Duckworth et al, 1996) and that production of the few species that dominate each trophic level is high (Vareschi, 1987). Attention has now focussed on their temporal dynamics (Verschuren et al, 1999), because of the fundamental scientific interest but increasingly because of the applied value in understanding human interference with their hydrological balance [e.g. Mono lake, California (Jellison et al, 2001) and Lake Nakuru, Kenya (Anon, 2003)].…”

Midges: Chironomidae and Related Diptera

“…Verschuren et al (1999), however, were able to describe their stratigraphy in an east African soda lake. Walker & Heiri (unpublished) have recently discovered head capsules (Fig.…”

“…Verschuren and co-workers have pioneered similar research in east Africa (Mees et al, 1991;Verschuren, 1994Verschuren, , 1997Verschuren et al, 1999Verschuren et al, , 2000aVerschuren et al, , 2000bDe Deyne, 1999). Surface samples collected from diverse habitats have clearly demonstrated the importance of salinity, water depth and specific benthic habitats to the midge fauna of Lake Naivasha, Kenya (De Deyne, 1999).…”