Conservation implications of the provenance of modern sediment on a shell-rich platform of Lake Tanganyika (Kigoma, TZ)

Abstract

Shell-rich substrate (shell beds) in the littoral reaches of Lake Tanganyika, Africa, form a unique substrate that host a number of endemic organisms, including sponges and shell-dwelling cichlid fish. The predominant gastropod, Neothauma tanganyensis, whose shells comprise the majority of the shell beds, are themselves endemic to the lake. Sedimentologic and observational data suggest that one of these shell beds, located between the towns of Kigoma and Ujiji, Tanzania, on the western side of central Lake Tanganyika, is impacted by modern sedimentation. In the northern portion of the study discrete patches of the shell beds between ~6–30 m water depth are blanketed by 20 to >50 wt% mud; in these areas there are few to no sponges nor shell-dwelling cichlid fish (as evidenced by lack of occurrences of fish nests with shells). However, in the southern part of the study area, within the same depth range, the shell beds contain less mud, and sponges and fish nests with shells are more common. Mineralogic and geochemical data suggest the patterns and composition of the sediment are consistent with two sources. The sediment accumulating on the shell beds in the northern portion of the study area matches the chemistry of the weathered Precambrian metasedimentary rocks in hillslopes adjacent to the lake, within a region undergoing severe land-use changes. The sediment accumulating along the southern portion of the study site, particularly in deeper water, matches the chemistry of sediment from the Luiche River, which forms a delta at the southern end of the study site. This delta has prograded up to 1 km within the last 40 years; however, in this setting the bulk of deltaic sediment is shunted into deeper water and so appears to represent a less severe threat to the shallow-water shell-bed habitat than the local hillslopes. This is one of a few detailed studies of the sedimentology of a shell-bed habitat in Lake Tanganyika and the first to tie the sediment to its source. This has important implications for conservation of these habitats as methods and practices of conservations vary based on the types of disturbance. The study also has implications for biologists interested in community structure as certain species of cichlids appear to be efficient engineers of the substrate, forming shelly nests, but in areas of high mud content, are absent.