Ecological Responses of Phytoplankton to a Submesoscale Eddy in the Agulhas Current System

Submesoscale eddies strongly regulate upper-ocean physical–biological interactions, yet their ecological impacts remain poorly observed because conventional satellite measurements lack sufficient spatial resolution. Here, we combine Surface Water and Ocean Topography (SWOT) sea surface height observations with multi-resolution VIIRS ocean-colour data to investigate phytoplankton responses to a submesoscale eddy in the Agulhas Current system off South Africa. SWOT resolves a compact cyclonic eddy with a 15 cm sea surface height anomaly and a radius of only 15 km. In contrast, the eddy is almost completely absent from the conventional DUACS altimetry product. Near-simultaneous 750 m VIIRS observations reveal localized enhancements of chlorophyll a (Chla) and particulate organic carbon (POC) that coincide with the eddy. The 750 m observations also captured higher local extremes, with Chla exceeding 0.60 mg m−3 and POC exceeding 120 mg m−3. These biological features become progressively weaker in the 4 km and 9 km products, indicating that coarse-resolution observations may smooth fine-scale Chla and POC variability. Hydrographic observations reveal a cold-core structure and the upward displacement of dense subsurface water. These observations suggest that submesoscale upwelling and frontal convergence jointly promote localized Chla and POC enhancements. Our results reveal an association between submesoscale physical variability and localized biological variability. They highlight the potential of integrating SWOT with high-resolution ocean-colour observations to investigate fine-scale ecosystem dynamics and associated carbon variability.