Seasonal Upwelling Shapes Coral Reef Community Structure and Photophysiology on the Pacific Coast of Costa Rica

Reef-building corals form the calcium-carbonate frameworks that underpin tropical coral reefs, yet global coral cover has declined by ~50% in recent decades, due to marine heatwaves and other stressors. Identifying refugia environments, such as upwelling systems, that buffer stress, promote recovery, and enhance resilience by promoting physiological plasticity that supports thermotolerance is therefore critical. Here, we compared benthic community composition, coral percent cover, and photo-physiology between an upwelling location in the Gulf of Papagayo and a non-upwelling location in Sámara on the Pacific coast of Costa Rica. Waters in Papagayo were cooler, more acidic, and richer in chlorophyll a. Reefs at this location exhibited higher crustose coralline algae, higher sea urchin cover, and lower macroalgae cover, compared to Sámara. Papagayo also showed higher stony coral cover, driven by Pocillopora spp., while Sámara was dominated by massive, heat-tolerant Porites spp.. When significant, photophysiological measurements showed 9.7 - 44.5% higher photosynthetic efficiency (Fv'/Fm') in Papagayo corals and 19.94 - 42.75 % higher maximum photosynthetic rates (P_max) in Sámara corals. These results highlight how contrasting environmental regimes within a relatively small geographic area can shape distinct coral community compositions and photophysiological strategies, with implications for identifying areas of reef persistence or refugia.