1Institute for Biodiversity & Ecosystem Dynamics, University of Amsterdam, The Netherlands. E-mail: michel@science.uva.nl
2Department of Biology, Cornell University. New York. E-mail: pbm3@cornell.edu
3Virginia Institute for Marine Sciences, P.O. Box 1346, Gloucester Point, Virginia 23062-1346, U.S.A. E-mail: kfrance@vims.edu

Direct and indirect effects of sedimentation on gastropods in the rocky littoral zone of Lake Tanganyika

Endemic gastropods are the dominant benthic macro-invertebrates on the faunally diverse, rocky substrates of Lake Tanganyika. As grazing herbivores, they form a critical focal group for monitoring the effects of increased anthropogenic sedimentation in Lake Tanganyika. Furthermore, since gastropod shells are a common component in geologic cores, the will be useful in examining paleoecological changes through time.

Habitat disruption has many effects on the resident fauna, including changes in diversity, abundance, life history characters, parasites and predators. We assessed these factors for gastropods in a paired sample design of sites that were differentially impacted by increased sedimentation from erosion in surrounding watersheds. Two sites near Kigoma Tanzania (Hilltop and Jakobsen's) and two sites near Gombe Stream National Park (Mwamgongo and Mitumba) were surveyed at four different depths (1, 2, 5, and 10 m). These sites correspond to sample areas for other projects on benthic productivity (C. O'Reilly) and ostracod temporal diversity (S. Alin), which together comprise a broad scale ecological profile. We collected data on gastropod species richness, evenness, sex ratios, reproductive status, parasitization and predation rates in 5 replicated quadrat samples at each depth (total 80 quadrats).

Sedimentation had no significant effect on snail species richness or abundance. The evenness of community composition was lower at sedimented sites, particularly below 1 m. Overall variation between and within sites, , and within depths was higher at sedimented sites than undisturbed ones. Frequencies of parasitism and shell scarring were lower at sedimented sites, and generally increased with depth. Lavigeria nassa adult size at reproduction was smaller at sedimented sites, suggesting that snail nutrition is affected by sediments.

Our results demonstrate the importance of spatial replication and detailed response measures in conservation surveys. Distributions of snails in Lake Tanganyika are patchy, varying not only among sites but also among replicates and depths within sites. This variation makes it difficult to generalise from studies conducted at small spatial scales. Many of the most easily measured responses to disturbance (e.g. diversity, abundance) were unaffected by sedimentation in our survey, whereas more time-consuming analyses (evenness, parasitism, predation, size at reproduction) revealed strong patterns. The differences in parasitism and predation reflect the influence of indirect effects of sedimentation on snails, mediated by sediment impacts on predatory and parasitic taxa.