SEDIMENTATION IMPACTS ON BIODIVERSITY OF A UNIQUE BIOGENIC SUBSTRATE, THE GASTROPOD SHELL BEDS OF LAKE TANGANYIKA, EAST AFRICA

Shell beds in Lake Tanganyika (LT), East Africa, are a unique biogenic habitat, forming a distinctive ecosystem for endemic taxa such as crabs, sponges, fish and other snails. Shell beds form large littoral carpets up to 15x2 km, but <30 cm thick, often isolated among soft substrates. Previous studies of LT rocky substrates have shown blankets of anthropogenic sediment impact diversity and ecosystem function. Are shell bed habitats similarly threatened? We document gastropod abundance and diversity across a large shell bed that spans a low-gradient sandy shoreline north of Mahale National Park, Tanzania. This 15km shell bed is differentially impacted by deltaic anthropogenic sediment from two moderate-size rivers. At the north the Lagosa River forms a large, prograding, muddy delta; in the south the Katumbi River forms a smaller, sandy delta. Separate bulk sediment analyses show the northern study sites contain surficial mud content of 25-75% whereas the southern sites contain less than 25% mud.

Divers collected bulk surface shell samples from 50cm² quadrats along 9 transects, spaced ~ 1km, at 9, 12, 15, 20m depths. Shells were cleaned, dried, whole snail shells were separated from shell fragments and bivalves, and all snail specimens identified to genus. Samples were archived to voucher IDs and allow future fine-scaled analyses.

The shell beds are comprised primarily of the large endemic viviparid gastropod Neothauma tanganyicense. At 15m depth at all sites Lavigeria species were also common and increase in relative abundance where Neothauma was less abundant. Diversity was highest in the low-sed southern transects with 10 sympatric taxa, including many small (< 1cm) taxa (e.g., Syrnolopsis, Reymondia, Anceya). The high-sed northern sites showed both a reduction in abundance of Neothauma and lower diversity overall (4-5 genera), however included rare small gastropods, such as Mysorelloides.

Our results suggest that sedimentation is a real threat for gastropods living on shell bed substrates, as it is for rocky substrates. The modern composition and volume of sediment delivered to offshore deltas is heavily influenced by differences in land-use within the watersheds. This is further evidence that human terrestrial activity upstream is impacting diversity in many unexpected LT littoral habitats.