Impacts of anthropogenic sedimentation on shell-bed habitats in Lake Tanganyika, Africa

Abstract

Lake Tanganyika, in central Africa, contains a diverse and endemic fauna under threat from global climate change, overfishing, and nearshore sediment pollution. Previous studies of sediment pollution focused justifiably on impacts along rocky shorelines where diversity is high, but Lake Tanganyika also contains widespread shelly accumulations (shell beds) unprecedented in the modern East African lakes, but where impacts are less constrained. Here we integrate multiple datasets from three sites along the Tanzanian shoreline to explore how variation in sedimentation rates and sediment quality impacts shell-bed substrate and diversity and abundance of ostracodes and sponges across sites that exhibit varying watershed characteristics. Taphonomic overprinting of the shells are similar over the three sites, suggesting lake-wide processes control their accumulation. However, shell bed distribution and sediment volume and compositions vary. There are also differences in the abundance of studied taxa. Where organic matter is diluted by clastic mud, ostracodes are less abundant and less diverse. Where sediment is pervasive and shell density is low, fewer sponges occur. Using the fallout radionuclide ²¹⁰Pb, the two sites with discontinuous shell beds show sedimentation rates at least twice as high as the site where shell beds are more continuous. These differences are likely related to modest differences in watershed morphology, urbanization, and land cover. Our study suggests that modern sediment pollution creates sediment blankets that cover extant shell beds and likely reduce live populations of the snails that contribute to the accumulations. This has important conservation implications as planning must focus on large watersheds where agriculture and urbanization tend to be higher.