Size-specific effects of increased sediment loads on gastropod communities in Lake Tanganyika, Africa

Abstract

The remarkable biodiversity of the littoral zone of Lake Tanganyika appears to be at risk through increasing sediment input caused by anthropogenic pressures. An in-situ field experiment was done to investigate the effects of increased sediment loads on the size-structure of gastropod communities on a rocky shore site in the lake. Gastropods were removed prior to the addition of sediment, and subsequent further removals were done after seven days and six months. Relative to controls, mean size of individuals of both Lavigeria grandis (Smith) and Lavigeria sp. Q decreased following the addition of sediment, while that of Lavigeria sp. P increased. A subsequent laboratory experiment found that survivorship of larger individuals of Lavigeria grandis was less than smaller individuals in the sediment-impacted treatment. It is hypothesised that decreased algal availability on sediment-impacted rock surface increased starvation of larger snails. Alterations to the size-structure of Lavigeria sp. P populations appear related to intraspecific differences in competitive ability. These results highlight the complex and unpredictable effects of increases in sediment loads on aquatic ecosystems and have implications for the conservation of littoral communities in sediment-impacted areas.