Emergent temperature sensitivity dominates phytoplankton growth and dampens net primary production seasonal variations on the Northeast US Shelf

Phytoplankton growth responds to temperature change through both thermal-trait-mediated effects on physiology and community composition and nutrient effects associated with stratification. The conventional view suggests that dominant nutrient effects lead to decreased productivity under warming. Here we integrate field data on the Northeast US Shelf with numerical models to constrain these two effects and identify the drivers of phytoplankton growth rate and net primary production variations in surface waters. Both model and data agree that thermal-trait-mediated effects outweigh nutrient effects, resulting in higher growth rates with increased temperature. Net primary production remains stable from winter to summer due to the compensatory effect of phytoplankton biomass reduction, offset by an elevated phytoplankton growth rate. Our findings provide quantitative evidence showing pronounced thermal-trait-mediated effects and reveal the importance of phytoplankton growth rate in modulating net primary production seasonality, offering valuable insights into the warming effects on phytoplankton dynamics across various spatiotemporal scales.