THE ENIGMATIC QUATERNARY SHELL BED DEPOSITS OF LAKE TANGANYIKA, TANZANIA: A STORY OF ANTHROPOGENIC OR ENVIRONMENTAL CHANGE?

Lake Tanganyika, the largest of the East African tropical Rift Lakes, is one of the most diverse lakes on Earth. In addition, it is a significant resource to a large and growing population in its four neighboring countries. Dwindling fish stocks and changes in the benthic organisms within nearshore lake environments as a result of anthropogenic land use change have put an emphasis on studying one of the most important ecological niches of the lake, the shell beds. Although the shell beds of Lake Tanganyika cover several hundred square kilometers of the shallow nearshore environment, it is still not known how the death assemblages formed and why there are few observed living mollusk populations on or adjacent to the shell beds. Two explanations that can account for why clear life assemblage analogues to the Tanganyika shell beds cannot be found today are 1) the shell beds are ancient deposits that reflect a natural, long term environmental change over longer timescales; or 2) the ecological communities that created the shell beds have experienced significant and rapid environmental perturbation caused by recent human-induced disturbance which has created a near shore ecosystem in which constituent shell bed mollusk species can no longer live.

The field work for this study took place in Buhingu, Tanzania on the eastern shore of Lake Tanganyika adjacent to the Mahale Mountains National Park. Data collection consisted of SCUBA sampling, ponar bottom sediment grab sampling, underwater photographic surveys, sidescan sonar imaging of the lake bottom, compilation of regional climate records, and acquisition of Landsat and proprietary satellite imagery. Initial observations and interpretations of sedimentological facies, progradation rates of three major river deltas, land use change based on satellite imagery, and benthic aquatic species abundance data suggest that a causal link may exist between recent anthropogenic land use change, the distribution and extent of muddy veneers covering modern shell beds, and abundance of benthic aquatic species which inhabit the shell beds. The discovery of a small population of live mollusks, within a river delta that drains from a relatively unaltered, pristine watershed in the Mahale Mountains National Park provides further evidence for these initial interpretations.