FRESHWATER MUSSEL SHELLS (CHAMBARDIA WISSMANNI) AS ARCHIVES OF PAST AFRICAN RIVER BIOGEOCHEMISTRY

Freshwater mussel shells have the potential to record environmental conditions in rivers around the world. This study uses water chemistry data collected fortnightly over one year (March 2010-2011) from the Oubangui River at Bangui (Central African Republic) to complete the first ever calibration study of an African freshwater mussel species (Chambardia wissmanni). Shell δ¹⁸O values correlate with modeled δ¹⁸O_s suggesting shells are precipitated in isotopic equilibrium. However, growth ceases or slows for > 5 months during the rising limb of the hydrograph which coincides with high total suspended sediment (TSM) load. δ¹⁸O_w values are largely controlled by precipitation and evaporation, so are related to discharge (Q). Thus, if there is a strong correlation between δ¹⁸O_w values and Q, δ¹⁸O_s values should track Q. A linear relationship proved more accurate than a logarithmic relationship, but still overestimated both peak Q (calculated ≈ 11,000 m³s^-1, actual ≈ 8,500 m³s^-1) and base flow (calculated ≈ 2,600 m³s^-1, actual ≈ 750 m³s^-1). δ¹³C_s values were more negative compared to expected shell values calculated using published fractionation factors between aragonite and HCO₃^-, indicating metabolic carbon (C_M) is incorporated in the shells, as has been noted in many bivalve species. A mixing model using the end-member δ¹³C_s values of HCO₃^- and soft tissues (as a proxy of C_M) suggests that yearly C_M incorporation varies from 0 to 29 % with high values corresponding to high TSM during the rising limb of the hydrograph. This supports that C. wissmanni filtration is impaired by high discharge, increased turbidity and high TSM, since these shells incorporate more C_M during this time. Living and archived C. wissmanni shells have the potential to provide a high-resolution record of past African river biogeochemistry and to detect large scale changes in discharge, which may be applicable to other tropical rivers.