The Freshwater Oyster Etheria elliptica as a Tool to Reconstruct Climate Variability across the African Continent

Abstract:

The bivalve Etheria elliptica occurs abundantly in (sub)tropical African river basins. We investigate its potential use for the reconstruction of ambient water chemistry and climate by means of stable oxygen isotope ratios in specimens from the Congo river (Kisangani), the Oubangui river (Bangui) and the Victoria Nile (Jinja). Unlike other common African bivalve species, E. elliptica contains distinct organic-rich growth increments, previously suggested to correlate with lunar periodicity. However, cavities in the shell complicate age reading and little is known about the exact timing and continuity of these growth increments. We set up a comparative study between different techniques to visualize and enhance growth features, and find that staining with Mutvei’s solution and confocal fluorescence microscopy perform equally well. Despite the presence of cavities, growth lines can generally be followed from umbo to shell margin. Moreover, preliminary δ¹⁸O results of two micro-sampled specimens from the Oubangui river show that 12-13 growth lines occur within one year of growth. This corroborates that these increments can be used as temporal anchor points, providing a moon-monthly time frame for sequential microchemistry. In two Congo river specimens, δ¹⁸O_shell values vary between -1.9 and -3.8 ‰ (VPDB), in line with a predicted range of -2.1 to -4.1 ‰ based on fortnightly δ¹⁸O_water and T monitoring. Reconstructed intra-annual δ¹⁸O_water variability from δ¹⁸O_shell values and observed T correlates with discharge, reflecting rainfall and runoff variability in the upstream catchment area. In two Victoria Nile specimens, collected 20 km downstream from Lake Victoria, δ¹⁸O_shell values are high and relatively constant, varying between +1.8 and +3.2 ‰. Enrichment of ¹⁸O_shell is consistent with isotopically heavy rainfall signatures and elevated surface evaporation in Lake Victoria. These first results suggest that E. elliptica is well-suited for the reconstruction of African climate and hydrology at a continental scale.