Environmental factors controlling the distributions of benthic invertebrates on rocky shores of Lake Malawi, Africa

An airlift was used to collect 170 quantitative samples of benthic macroinvertebrates from the rocky nearshore zone of islands in southern Lake Malawi from August 1998 to Febuary 1999. Total invertebrate densities were very low (< 6,000 ind/M-2), and were dominated by Ostracoda, Hydracarina, and Chironomidae. Most of the insects were lotic forms, including hydropsychid caddisflies, heptageneid mayflies, and dryopoid beetles. Sampling sites differed in degree of exposure to wave-action, substratum texture, depth (1.5 to 5 m), and abundance of fish, and the density and diversity of the benthic fauna varied significantly with each of these factors. Both numbers of taxa and densities of invertebrates were greater at exposed than sheltered sites, and decreased with increasing depth. Gravel substratum harbored more taxa and higher densities of macroinvertebrates than did cobble or bedrock. The only seasonal change observed was a decrease in the abundance of animals during the rainy season at a sheltered site where sediment was deposited from a large river. There was a weak inverse relationship between the densities of fish and larger invertebrates on cobble and bedrock substrata. Bedrock surfaces covered by fish-exclusion cages supported five-fold more invertebrates than open surfaces after 2 months. Overall, the benthic fauna of rocky shores in southern Lake Malawi is a taxonomically rich community similar in composition and diversity to those reported from other Great Lakes worldwide. Continuous, intense fish predation appears to be responsible for low invertebrate densities and probably influences invertebrate species diversity.