Shell beds in modern lake or marine environments consist of hard parts derived from successive populations of mollusks and other organisms that accumulate over time. Shell beds are key windows on past environments in fossil outcrops, provide measures of diversity and change in modern environments, and are critical habitats for living systems. A better understanding of the origin and variability of the shell beds is important for understanding how ecosystems become structured as well as how speciation occurs as well as providing a means to predicting the distribution and make up of ancient shell bed deposits, which are common in the strata of old rift basins.  

In Lake Tanganyika, Africa, up to 30% of the shallow water bottom consists of densely packed shelly substrates dominated by shells of a single gastropod species (Neothauma tanganyicense). The shell beds of Lake Tanganyika hosts a number of species that utilize the shell beds as a substrate or for protection, e.g. sponges, fish, other mollusks, and crabs. The lake lies in the geologically active East African Rift and is a "natural laboratory" for evolutionary and ecological studies because much of the diverse fauna is endemic (lives only in that lake) and employ unique behaviors. Overall, the biological treasure in Lake Tanganyika is under threat from expanding human population as climate change, overfishing and land-use changes affect the lake's biology. Scientific studies have previously focused on pelagic fisheries because these are critical to the immediate health and wealth of the four riparian countries. However, the shell beds have received less attention. Thus a better understanding of the variability of the shell beds will allow for determinations of anthropogenic impacts.

This work focused on three modern shell beds along the eastern (Tanzanian) shore of Lake Tanganyika. These sites varied in the level of onshore (watershed level) land-use change from heavily impacted to lightly impacted. Through underwater sampling (SCUBA), grab sampling and sediment core sampling by ship, and geophysical data collection, we developed a variety of datasets that allowed us to:  1) develop a model for the formation of the shell beds in Lake Tanganyika; 2) assess environmental change that may have impacted the production; 3) determine the time averaging of the shell bed deposits; and 4) evaluate the mismatch between live and dead assemblages of mollusks and ostracodes across the shell beds.

The work was conducted by a collaborative team of geologists, paleontologists and biologists from the US and Tanzania. In the course of the work, The PIs and three graduate students went to the field areas to collect data and then used the data for their graduate research. Further, eleven undergraduates were active in parts of the research and most presented their research at undergraduate research symposia. Results of the work to date are published in seven peer-reviewed journals; three more manuscripts are in preparation. We have also presented results at various national and international meetings and several public venues.

The main results can be summarized as follows: 

1) Our calculations of sedimentation rates derived from age dating of the sediment cores suggests that there is a direct relationship between increasing offshore sedimentation rates and increasing level of onshore land use change. Prior to these historical, anthropogenic changes, the core data provides evidence for changes in climate that impacted the lake over the last ~1800 years, including creating environments favorable and then unfavorable for shell production.

2) The age of the shells span almost 3000 years, meaning these surficial deposits have existed for millennia. However there are spatial differences in the distribution of the ages of the shells;  younger shells generally occur in shallower water and older shells occur in deeper water. This suggests that changes in lake level, and other climatic controls, influence the accumulation and production of the shells. Further, the data shows that young to modern shells are rare within the shell bed deposits at the two sites that have experienced significant land-use change. However, live snails and young shells do occur within shell deposits at the site that has experienced only light land use change. We suggest this is a severe conservation issue as it implies production of new shells is at risk.

3) land use change has dramatically impacted the continuity and sediment coverage on the shell beds and this directly relates to differences in the abundance of live Neothauma as well as differences in the abundance and diversity of other organisms living on the shell beds. Regions where land use change is severe, shell beds are completely blanketed by mud and there are dramatic reductions in abundances and diversity of most groups.

Last Modified: 12/20/2019
Modified by: Michael J Soreghan