DOC from BioSCape

The biodiverse and rapidly changing Greater Cape Floristic Region (GCFR) of southern Africa is outlined by coastal bays that receive dissolved organic matter (DOM) from rivers draining complex catchments composed of natural, agricultural, and urban land classes. As part of NASA's BioSCape field campaign (October-November 2023), we characterized the optical properties of three GCFR coastal bays (St. Helena, Walker, and Algoa) and four inland systems (Rietvlei wetland, Zeekoevlei lake, Theewaterskloof dam, and Klein River Estuary) in relation to carbon cycling. Measurements of the optical properties of colored DOM (CDOM), including absorption at 300 nm (ag300), spectral slope (S275-295), and fluorescence, highlighted the bio-optical complexity associated with terrestrial influences from contrasting watersheds, urban disturbances, biological activity, and rapid transformations along this dynamic coastline. CDOM in the coastal bays was characterized by an order of magnitude lower ag300 (0.5 – 2.8 m-1) and considerably higher S275-295 (0.020 – 0.031 nm-1) compared to upstream waters (25 – 71 m-1 and 0.012 – 0.019 nm-1, respectively), suggesting intense biological production and/or photochemical degradation. Coastal DOM was mostly (>75%) composed of protein-like fluorescent products, indicative of bacterial utilization, while inland DOM was mostly composed of humic and highly aromatic materials. Satellite retrievals, using OLCI and MSI imagery, captured the disproportionate yet ephemeral impact of extreme events – intense riverine discharge following extensive periods of drought – on coastal CDOM plumes, and revealed that seasonal hydrological cycles, catchment biodiversity, and human activity are the primary drivers of biogeochemical variability along this globally significant, coastal biodiversity hotspot.