Marine ecosystems with elevated temperature and terrestrial input support simplified food webs

Tropical coastal ecosystems are increasingly threatened by rising seawater temperatures and terrestrial nutrient input, yet the combined impact of these stressors on food web complexity remains poorly understood. Using marine lakes in Raja Ampat, Indonesia, as natural analogs for varying environmental conditions, we investigated how temperature and terrestrial influence shape food web structure and carbon source utilization. We applied an approach at 2 scales, combining whole food web stable isotope analysis in 2 contrasting lakes—one with elevated temperature and high terrestrial input, the other with ocean-like conditions—with a broader survey of 16 additional lakes using the filter-feeding mussel Brachidontes as a proxy for baseline carbon sources. Biodiversity, food web complexity, and functional redundancy were reduced in the marine lake with higher temperatures and greater terrestrial input. Bulk stable carbon and nitrogen isotope values from Brachidontes from 16 different marine lakes revealed a shift in the main food source underlying the food web from marine to terrestrial organic matter, with a shift from particulate organic matter to the use of sponge-derived material (reworked/recycled organic material) in environments with high temperatures and high terrestrial inputs. The use of Brachidontes as an integrative indicator of basal carbon sources offers a valuable tool for monitoring and managing the ecological impacts of environmental change in tropical coastal systems. Extrapolating our findings to coastal marine systems, our results suggest a potential shift toward simplified, less resilient trophic structures in ecosystems with combined thermal and terrestrial stressors.