Marine heatwaves deeply alter marine food web structure and function

Abstract. Marine heatwaves (MHWs) have become longer, more frequent and more intense in recent decades. MHWs have caused large-scale ecological impacts. This includes coral bleaching; mass mortality of seagrass, fishes and invertebrates; and shifts in the abundance and distribution of marine species. However, the implications of these MHW-induced impacts on marine species for the structure and functioning of marine food webs are not clearly understood. In this study, we use the EcoTroph-Dyn ecosystem modelling approach to examine the impacts of MHWs occurring during the year's warmest month on the trophodynamics of marine ecosystems. EcoTroph-Dyn represents marine ecosystem dynamics at a spatial resolution of 1° longitude by 1° latitude and a temporal resolution of 15 d. We applied the model to simulate changes in trophodynamic processes, energy transfer and ecosystem biomass using daily temperature and monthly net primary production (NPP) data that were derived from satellite observations from 1998 to 2021. We compared and contrasted the simulated changes in biomass by trophic level, with results generated from temperature and NPP time series that had been filtered to remove MHWs. Our results show a significant decline in biomass between 1998 and 2021, specifically caused by MHWs. For example, in the northeastern Pacific Ocean, our model simulated an MHW-induced decline in biomass of 8.7 % ± 1.0 (standard error) from 2013 to 2016. Overall, MHW-induced biomass declines are more pronounced in the Northern Hemisphere and Pacific Ocean. Moreover, the MHW-induced declines in high trophic-level biomass were larger than in lower trophic levels and lasted longer post-MHW. Finally, this study highlights the need to integrate MHWs into modelling the effects of climate change on marine ecosystems. It shows that the EcoTroph approach, and especially its new dynamic version, provides a framework for understanding more comprehensively the implications of climate change for marine ecosystem structure and functioning.