Marine heatwaves modulate food webs and carbon transport processes.

Marine heatwave (MHW) impacts on ecosystem functions and services remain poorly constrained due to limited time-resolved datasets integrating physical, chemical, and biological parameters at relevant scales. Here we show that combining over a decade of autonomous Biogeochemical (BGC)-Argo float measurements with water-column plankton community profiles reveals the impacts of MHWs on particulate organic carbon (POC) production, transformation, and transport in the northeastern subarctic Pacific Ocean. POC concentrations are exceptionally high during the 2015 and 2019 MHWs, linked to detritus enrichment and shifts in plankton community structure. Instead of being rapidly exported to depth, particles <100 µm accumulate in mesopelagic waters, where slow remineralization over the year reduces deep particle flux and carbon sequestration potential. This enhancement is absent in the 2014 and 2020 MHWs, underscoring variability in ecosystem responses to extreme events. These findings highlight the need for sustained, multi-platform observations to assess and predict carbon-cycle responses to thermal extremes.