Warming simplifies marine ecological networks through losses in trophic and non-trophic interactions

Ocean warming is anticipated to push many marine species beyond their physiological limits, leading to shifts in community composition and ecological interactions. Coastal upwelling can provide temporary relief from heat stress but may also introduce challenges due to temporal hypoxia and acidification. As environmental extremes become more frequent and prolonged, understanding their impact on coastal ecosystems is critical. To examine how warming and upwelling influence trophic and non-trophic network complexity, we quantified trophic group biomasses and ecological network properties in benthic mesocosm communities. Over a 4.5 mo summer period, seawater temperatures were experimentally manipulated, ranging from ambient conditions to +5°C above ambient, and further crossed with transient upwelling events. The most striking result was that total biomass was about 50% lower in the warmest treatments compared to ambient conditions. Fucus spp. biomass was 50-76% lower in warmer treatments and entirely absent at +5°C. As a foundational species, Fucus loss led to a ~39% reduction in non-trophic interactions. Under extreme warming, predatory sea stars also disappeared. These shifts at both the base and top of the food web were associated with reduced connectance, omnivory, and generality in warmer treatments, as well as reductions in ecosystem functioning, such as community biomass and carnivorous fluxes, suggesting food web simplification with increasing temperature. By integrating experimental data with ecological network modelling, this study provides a holistic view of how coastal communities may respond to future climate change, highlighting the risks of coastal biodiversity loss on ecosystem functioning.