Marine heatwaves are transforming Western mediterranean marine ecosystems.

We investigated ecosystem impacts of marine heatwaves (MHWs) in the Western Mediterranean Sea using a spatially explicit Ecopath with Ecosim (EwE) food web modelling framework. This model is forced with temperature time series, including or excluding MHWs, and simulates biomass dynamics through trophic interactions and energy flows. Our simulations suggest that declining biomass trends observed over the past decades in the Western Mediterranean, driven by long-term ocean warming and fishing pressure, have been exacerbated by the occurrence of MHWs. A north–south dipole in biomass rate of change emerged in response to MHWs: while the northern region displayed positive or neutral responses due to MHWs, the south, particularly the Alboran and Algerian seas, experienced negative impacts. Benthic producers and commercially important species (pelagic and demersal fish and invertebrates) were particularly affected, leading to catch reductions exceeding 10%. We identify previously unreported vulnerable groups and regions, supporting the use of ecosystem models in guiding adaptive management for the future. The increase in intensity and extension of MHWs with time suggests that larger effects may be expected in the future.