Does the decline of gastropods in deep water herald ecosystem change in Lakes Malawi and Tanganyika?

1. Ancient, deep lakes have traditionally been considered as stable, ecological islands, well buffered from environmental change because of their great depth. However, they are not immune to anthropogenic and climatic stress. Ecosystems of the permanently stratified warm Lakes Malawi and Tanganyika in the Great East African Rift are particularly delicate. Their stratification regime has historically limited the distribution of benthic biota to a ‘bathtub ring of biodiversity’, namely substrata in the upper, oxygenated water layer. 2. We use historical data on the endemic deep‐water molluscs of these lakes to assess present‐day stress on their benthic ecosystems. During the 20th century, these molluscs have probably decreased in abundance and migrated to shallower water. 3. These apparent trends have a significance beyond species‐based conservation, foremost because deep‐water organisms heavily rely on the position and temporal stability of the oxycline and therefore provide an early warning of large‐scale changes in the distribution of dissolved oxygen. Oxygen demands have increased in the East African Great Lakes over the last century whereas ventilation of deep water has remained the same or declined. 4. The combination of these factors is resulting in a narrowing of the ring of biodiversity and a changed nutrient flux through this ring. Reduction in the habitat available inevitably puts biota at risk, whereas changes in nutrient flux may cause shifts in the entire ecosystem or the collapse of parts of it. 5. Considering the socioeconomic value of these lakes and the potentially grave implications for their faunal biodiversity and entire ecosystems, existing evidence of faunal decline, especially in taxa that depend strongly on the stratification regime, is of great concern. Moreover, because the factors responsible are widespread and include surface‐water warming, increased run‐off and eutrophication, respiration stress may also develop in other tropical and subtropical lakes.