Decline in ocean net primary production coincident with the 2023 heat wave

Abstract Marine primary productivity is pivotal for transferring carbon dioxide from the atmosphere into the ocean. Global marine net primary production (NPP) rates derived from satellite-based estimates of primary producer biomass, sea surface temperature, and light availability are estimated to be ~4 Pmol C y-1 1–3. A major outstanding question is the sensitivity of NPP to future ocean warming, both regionally and globally. Here we used chlorophyll a data from the last 21 years to estimate NPP and determined that April 2023 global NPP - when sea surface temperatures (SSTs) reflected a substantial marine heat wave - dropped to 2.76 Pmol C y-1, or 22% below the time series mean. Using a spatially integrated approach, we show that the pelagic ocean experienced the most prominent decrease in NPP during peak SSTs in September 2023. The decrease in NPP was most pronounced in equatorial regions of the Atlantic and Pacific, as well as the northern Atlantic; all of these areas exhibited positive SST anomalies. Finally, we compared optimal temperatures (Topt) of phytoplankton photosynthesis to observed SSTs and show that during the NPP decline, in 65% of the surveyed area, the Topt was exceeded, suggesting a physiological mechanism for the observed decrease in NPP.