Drivers of Phytoplankton Communities Across Thermal Regimes and Marine Heatwaves in the U.S. Northeast Shelf Journal of Geophysical Research: Oceans

Phytoplankton are a diverse group of photosynthetic microorganisms critical to global biogeochemical processes. Abundance, distribution, and size class structure of phytoplankton are controlled in part by environmental factors, driven by the ocean's physical and biogeochemical processes, which are impacted by climate change. How these forcing factors vary across thermal regimes in the U.S. Northeast Shelf Ecosystem (NES), a rapidly warming region where multiple marine heatwaves have occurred, is unknown. Environmental predictors of phytoplankton abundance and size class fraction variability were characterized using multivariate modeling via partial least squares regression (PLSR) and predictor significance determined via variable influence on projection (VIP) scores. Variability in size class abundance in the northern NES is primarily linked to the nutricline, while in the southern NES, it is more strongly associated with water column stability (sea surface temperature and salinity). Temporally, the cooler regime (approx. 1997–2010) is the most different from all other time periods and each marine heatwave results in specific differences for parameters from other heating conditions. Furthermore, chlorophyll and microplankton exhibit similar VIP scores to nano‐ and picoplankton, but inverse PLSR coefficients. Microplankton are less suited to warmer conditions and are more strongly associated with salinity and nutrient variability, which are coupled with reduced vertical mixing in the water column. Phytoplankton variability associated with changing oceanographic conditions will impact regional biogeochemical processes with consequences to the marine carbon cycle and food web.