Changes in benthic and pelagic production interact with warming to drive responses to climate change in a temperate coastal ecosystem

Changing sea temperatures and primary productivity are rapidly altering marine ecosystems, but with considerable uncertainty in our understanding of the relative importance of these drivers and how their interactions may affect fisheries yield through complex food webs. Such outcomes are more difficult to predict for shallow coastal ecosystems than those in pelagic and shelf habitats, because coastal food webs are fuelled by a combination of separate pelagic and benthic energy pathways. Using long-term, empirical field data, we developed a novel multispecies size spectrum model for shallow coastal reefs. We include size-structured benthic and pelagic resources and trophic structures, allowing us to explore potential climate change scenarios that involve varying combinations of warming with changes in benthic and pelagic resources. Our model predicts that changes in resource levels will have much stronger impacts on fish biomass and yields than changes driven by physiological responses to temperature. Under increased plankton abundance, species in all trophic groups were predicted to increase in biomass, average size and yields. By contrast, changes in benthic resource produced variable responses across coastal trophic groups. Increased benthic resource led to increasing benthivorous and piscivorous fish biomass, yields and mean body sizes, but decreases in herbivores and planktivores. When resource changes were combined with warming seas, physiological responses generally decreased species biomass and yields. Our results suggest that the source, size and abundance of primary and secondary producers are critical to understanding impacts of warming seas on coastal fish communities. Understanding changes in benthic production and its implications for coastal fisheries requires urgent attention. Our modified size spectrum model provides a framework for further study of benthic and pelagic energy pathways that can be easily adapted to other ecosystems.