Environmental forcing
For the environmental forcing, we used the Sea Surface Temperature and Mixed Layer Depth (ECCO Consortium et al., 2021) daily mean of 10 years (2000- 2010) of an offshore region of the Mid-Atlantic Bight, a representative North Atlantic temperate ecosystem (Figures 1, SM1, and Supplementary Materials).
We ran the model for 50 years to allow the community to reach a steady-state condition (hereafter pre-heatwave conditions). Our focus is solely on examining the temperature effects on plankton communities of a specific water mass. Thus, we didn’t change the mixed layer depth (MLD) and nutrient input, acknowledging that this simplification reduces the complexity typically found in natural ecosystems. For the marine heatwave scenarios, we exposed the steady-state community to a 4 ˚C temperature anomaly lasting for one season (three months) for either winter (December- February), spring (March- May), summer (June- August), or autumn (September- November). Observed heatwaves range in duration from days to months, with no universally applicable average duration due to variations influenced by factors like event intensity, geographic location, and specific oceanographic conditions . For example, the oceanic waters of the Northeastern United States Continental Shelf have experienced long seasonal heatwaves up to 3 ˚C since 2012 . In this context, we have opted to investigate the impact of an extended and intense heatwave on plankton communities, portraying it as an extreme scenario that may become increasingly common within ecosystems due to ongoing climate change. After the heatwave event, we ran the model for another 19 years with the pre-heatwave SST to examine the period needed for the community to readjust to the pre-heatwave condition. To isolate the processes responsible for the observed changes, we ran two additional simulations. In the first, heatwaves solely affect protists’ rates, while in the second, only copepods are sensitive to the elevated temperature. These experiments enabled us to examine whether changes in communities are driven by direct temperature effects, indirect temperature effects, or a combination of both.