Scale-dependent upwelling dynamics on coral reefs: what satellites don’t see.

Coral reefs are increasingly characterized by satellite-derived sea surface temperature (SST), and chlorophyll-a as a proxy for resource availability. However, these surface metrics often obscure depth-restricted processes that shape reef ecosystems. Internal wave-driven upwelling delivers cool, nutrient rich water to shallow reefs, but typically leaves no surface signal. Here, we quantified spatial and temporal variability in internal wave-driven upwelling across the Chagos Archipelago, central Indian Ocean, using high-frequency (10 s) in situ temperature records spanning a shallow depth gradient (10–25 m). Using unsupervised clustering, we identify three distinct subsurface thermal regimes differing in cold-pulse frequency, duration, and magnitude. The strongest upwelling regime was also the most spatially heterogeneous, demonstrating that upwelling operates as a gradient rather than a binary state. Geographic distance poorly predicted oceanographic similarity: sites 10 km apart on the same atoll were as or more dissimilar than those 170 km apart. Satellite SST tracked mean daily temperature well across depth but failed to resolve sub-daily variability driven by internal waves. Satellite chlorophyll-a was spatially uniform and did not track site-level differences in cold pulse delivery or estimated subsurface nitrate supply. Our findings reveal a hidden layer of environmental heterogeneity on coral reefs that is unresolved by widely used satellite products. Surface metrics of thermal stress may misrepresent reef conditions and obscure thermal refugia, while chlorophyll-a misses subsurface gradients in nutrient supply that fuel reef ecosystems. Without integrating in situ measurements with remote sensing, the biophysical connections determining which reefs persist – and where – will remain hidden.

Recommended citation: Danielle L. Spring, Michael D. Fox, J. A. Mattias Green, Robin Guillaume-Castel, Ronan C. Roche, Laura E. Richardson, Gage Mele, Eesaa Harris, John R. Turner and Gareth J. Williams "Scale-dependent upwelling dynamics on coral reefs: what satellites don’t see." (submitted)