Unexpected Ecological Vulnerability in Tropical Pacific Heatwave Refuge.

The equatorial western-central Pacific (WCP), characterized by low marine heatwave (MHW) occurrence and weak MHW intensity, is generally considered a refuge from MHWs. However, by integrating satellite observations with in situ measurements from BGC-Argo floats and TAO/TRITON buoys, this study reveals that the WCP exhibits the strongest phytoplankton response to MHWs in the global ocean, with near-surface chlorophyll concentration (CHL) declining by up to 50% per unit MHW intensity. This extreme response primarily arises from a concurrent reduction in both horizontal and vertical nutrient supply during MHWs. Eastward current anomaly inhibits zonal advection of nutrient-rich water from the east, while enhanced salinity stratification and shoaled mixed layer hamper vertical nutrient supply. Contrary to the near-surface, subsurface phytoplankton increase substantially during MHWs, mainly resulting from the improvement in nutrient and light conditions at depths below the shoaled mixed layer and near the base of the deepened euphotic layer. As the WCP hosts the world's most productive tuna fisheries and intense subsurface MHWs, our findings reveal a previously unrecognized ecological vulnerability with important implications for fishery management and carbon cycle in this vital marine system.