Effects of depth, salinity, and substrate on the invertebrate community of a fluctuating tropical lake

Salinity is generally considered to be the dominant environmental factor regulating aquatic community structure in hydrologically closed lakes and wetlands, but it is not well known whether community response to long-term trends in hydrological balance is driven primarily by the direct physiological effect of salinity stress or by the habitat restructuring that accompanies changes in lake level and salinity. Attempts to separate the effects of various environmental factors on invertebrate populations in shallow fluctuating lakes through field study are hampered by the typically large temporal and spatial variation in species abundances and the long time scale of climate-driven habitat restructuring relative to the period of study. We used paleolimnological techniques to document long-term dynamics of the benthic invertebrate community inhabiting a shallow fluctuating lake in Kenya where during the period similar to 1870-1991 lake depth fluctuated between 4 and 19 m, and lakewater conductivity between similar to 250 and 14000 mu S/cm. Analyses of sediment texture, plant macrofossils, and fossil diatom assemblages in a (210)Pb-dated sediment core were combined with historical lake-level data to reconstruct continuous records of past changes in salinity and the distribution of various types of benthic habitat. Fossil invertebrate faunas recovered from the same core comprised 58 species of Ostracoda, chydorid Cladocera, and Chironomidae, with 26 common species together accounting for >95% of the recovered fossils. Uni- and multivariate statistical analyses of the faunal and environmental data revealed distinct species-specific responses to lake level, salinity, and papyrus-swamp development. Redundancy analysis and variation partitioning showed that salinity and swamp development together explained 51% of the observed historical variation in benthic community composition, and that their effects were independent, In the univariate. analyses, six species (23%) showed a strong response to salinity. Nine species (35%) responded strongly to swamp development, which determines availability of submerged vegetated substrates and turbulence-free, littoral mud bottoms. Five species (19%) responded primarily to lake level, which affects the general distribution of sand and mud bottoms and the intensity of subaqueous spring discharge. The remaining six species (23%) showed no distinct pattern or correlation with the selected environmental factors. These results suggest that a significant portion of the documented correlation between salinity and invertebrate community structure along the full gradient of inland aquatic ecosystems may be an indirect effect of broad but diffuse relationships between salinity and the distribution of various types of benthic microhabitat. Decade-scale environmental change experienced by individual closed-basin lakes tends to remain limited to a relatively narrow portion of this gradient, where covariance between salinity and other ecological determinants may be weak. Therefore, local populations of benthic invertebrates will be regulated by the fluctuating availability of specific habitat and associated resources rather than by the limit of their osmoregulating capacity