Benthic diatom distribution and grazing by Sarotherodon mossambicus in Lake Sibaya, South Africa

SUMMARY. Distributions of benthic diatoms and benthic floc in Lake Sibaya were examined to determine if preferential grazing by juvenile Sarotherodon mossambicus on benthic detrital aggregate in shallow water maximized diatom concentrations in the diet of the fish. Between 0.1 and 12.0 m depth, the range of diatom densities was 1–60 × 10 4 cells cm −2 with high but extremely variable densities above 3 m and consistently low densities at greater depths. Light intensity and burial by wave action appear to be primary factors affecting diatom abundance, but the effect of unidentified factors causing local patchiness is also evident. Substrate stabilization by diatoms was observed between 0.3 and 1.8 m depth. Weight of benthic detrital aggregate generally increased with depth such that diatom concentrations were variable above 3 m but were extremely low in benthic detrital aggregate at greater depths. Comparison of diatom concentrations in fish stomachs and benthic detrital aggregate shows that within the shallow waters, fish do not selectively graze those areas richest in diatoms.