Dissolved Organic Matter Dynamics in South African Nearshore Waters and Freshwater Systems: Linkages to Changing Human Activity, Episodic Events, and Biodiversity

Abstract 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 the Klein River estuary) in relation to DOM biogeochemistry and carbon cycling. Measurements of the optical properties of colored DOM (CDOM), including absorption at 300 nm ( a g 300), spectral slope ( S 275–295 ), and fluorescence, highlighted the bio‐optical complexity associated with terrestrial influences, urban disturbances, intense biological activity, and rapid transformations along this dynamic coastline. CDOM in the coastal bays was characterized by an order of magnitude lower a g 300 (0.5–2.8 m −1 ) and considerably higher S 275–295 (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 compounds indicative of primary production, whereas inland DOM was mostly composed of terrigenous humic materials and had higher overall fluorescence signal. Satellite retrievals, using Sentinel‐3 OLCI and Sentinel‐2 MSI imagery, captured the relative influence of different rivers on coastal DOM dynamics and revealed that episodic events—extended periods of drought punctuated by heavy precipitation—are the primary drivers of biogeochemical variability along this globally significant, coastal biodiversity hotspot.