Conference Agenda

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.

The 2016 El Niño is the strongest warm ENSO phase of the 21st century recorded to date, leading to the development of severe temperature anomalies - marine heatwaves (MHWs) - across the equatorial Pacific Ocean. Here we analyze biogeochemical model output and machine learning (ML)-based reconstructions of Argo oxygen and backscattering measurements to demonstrate the impact of the 2016 El Niño in weakening oceanic carbon export - the transfer of biogenic carbon from the surface ocean to depth. We show that equatorial Pacific anomalies in modeled carbon flux, and ML-based optical particle backscatter and ecosystem respiration, display interannual oscillations linked to ENSO cycles, with an acute reduction in export (- 50 %), respiration (- 30 %) and backscatter (- 20% ), during the 2016 El Niño. This plunge in export production is attributed to a large decrease in chlorophyll biomass observed from space, and modeled ecological changes in phytoplankton community composition.

The Southern Ocean plays a vital role in global CO₂ uptake, but the magnitude and even the sign of the flux remain uncertain. Physical mechanisms for carbon uptake are emphasized, with the role of biology in surface ocean carbon uptake potentially underestimated in coastal high latitude systems, and the influence of phytoplankton phenology is underexplored. This study focuses on the West Antarctic Peninsula, a case study region experiencing rapid climate change, to examine shifts in seasonal surface ocean carbon dioxide uptake. We used 20 years of in situ air‐sea CO₂ flux data from research vessels and satellite‐derived Chlorophyll‐a data from OC-CCI as a proxy for phytoplankton biomass. OC-CCI was used for the Chlorophyll-a record because the OC-CCI atmospheric correction approach (POLYMER) results in more representative spatial distributions of coastal phytoplankton biomass than other approaches due to adjacency effects along bright icy coastlines in polar regions. We observed that the seasonal cycles of both air‐sea CO₂ flux and Chlorophyll‐a concentration intensified poleward. The amplitude of the seasonal cycle of the non‐thermal component of surface ocean pCO₂ increased with increasing latitude, while the amplitude of the thermal component remained relatively stable. These results suggest that pronounced biological uptake occurs over the shelf in austral summer despite reduced CO₂ solubility in warmer waters, which typically limits carbon uptake through physical processes. Chlorophyll-a concentrations and air-sea CO₂ fluxes were tightly coupled across all years, especially when phytoplankton biomass was high. These results suggest that an ocean-color-based air-sea CO₂ flux algorithm may be developed to estimate surface ocean carbon uptake from space.

A major challenge in understanding the oceanic carbon cycle is estimating the sinking flux of organic carbon exiting the sunlit surface ocean, i.e., carbon export. Existing algorithms derive carbon export from satellite ocean color, but often display poor accuracy. One reason is that they neglect offsets created temporally by the lag between production and export, which combined with horizontal advection can result in a spatial offset of hundreds of kilometers in dynamic regions such as Eastern Boundary Upwelling Systems. Here we use a Lagrangian “growth-advection” (GA) satellite-derived product, where plankton succession and export are mapped onto surface oceanic circulation following coastal upwelling, thus explicitly representing these offsets. We show that the GA product succeeds in representing export measured off the California coast, despite relying exclusively on satellite winds and currents (no ocean color). In situ export is best represented by a combination of GA export, proportional to modeled zooplankton, and export derived from ocean color, related to local phytoplankton. Both products also correlate with a long-term time series of abyssal carbon flux. However, their spatial and temporal patterns are very different, underscoring the need to better constrain, and take into account, zooplankton contribution to export and its offset from primary production. These results provide insights on export spatiotemporal patterns and a path toward improving satellite-derived carbon export in the California Current and beyond.