Assessment of Plankton Size Structure From CMIP6 Earth System Models With a Novel Pelagic Size Structure Database

Abstract Plankton community structure influences biogeochemical and ecosystem processes, such as sequestration of atmospheric CO 2 , carbon export to the ocean floor, and the productivity of higher trophic levels. One means of analyzing community structure is through the distribution of biovolume across size classes (the size spectrum), since size is a proxy for plankton functional traits. To understand how climate forcing affects plankton communities, we assessed the size spectra in the historical simulations of seven Earth System Models (ESMs) included in the 6th Coupled Model Intercomparison Project and analyzed projected changes under a high emissions scenario (SSP5‐8.5). We compared historical estimates with the Pelagic Size Structure database (PSSdb), a novel size structure dataset from imaging systems. The median slope from models ranged from −1.66 to −1.07, with shallower slopes from this range approximating both the theoretical expectation and PSSdb observations (−1.05), with variations around the median representing differences in the total biovolume distribution across plankton functional groups. Consistent with the observations, most ESMs show steeper slopes and lower biovolume in oligotrophic subtropical gyres compared with productive ocean regions. Historical versus climate change simulations reveal increases in slope and biovolume at high latitudes, associated with greater biomass and productivity, and decreases at lower latitudes, consistent with nutrient limitation from stronger stratification. We emphasize the need for expanded observational data. Despite ESMs not being designed to simulate size, the plankton size spectra from models provide insights on large‐scale ecological and biogeochemical processes, and how climate change could affect these dynamics in the future.