Award: OCE-2347307

Climate change is having widespread impacts on Earth's ecosystems both through direct and indirect mechanisms. Increasing temperature is the most well-known of these. The summer of 2023 was the hottest in recorded history in south Florida, heating the surrounding shallow seas to unprecidented highs. Along with temperature stress, rising seawater temperatures also drive already limited seawater oxygen availability to critically low levels. However, the Gill Oxygen Limitation Theory (GOLT) predicts that the negative effects of low oxygen should be greatest on large water-breathing animals becaue of the inverse relationship between respiratory tissue surface area and body volume. We thus capitalized on this historic heatwave event to test its effect on the oldest multicelluar animal - sponges - in the context of the GOLT. Through benthic surveys and mesocosm experiments, the project furthered our understanding of the implications of the GOLT)as applied to non-gilled organisms and the potential effects of climate change on coastal sponge communities driven by changes in temperature and oxygen. Our mesocosm studies revealed that the filtration efficiency of sponges with high surface area to tissue volume ratios are less affected by elevated temperatures and dissolved oxygen regimes. Field surveys of sponge community structure before and after the extreme heat event of 2023 indicate that inshore sponge communities were substantially changed by the event. Moreover, the inshore sponges most effected by the heatwave were, as predicted by GOLT, the largest individuals of spherical taxa with the lowest surface area to volume ratios. In contrast, sponge communities on deeper, offshore reefs did not change appreciably after the heatwave. We hypothesize that those populations were more resilient because they exist in deeper, cooler waters and because most reef sponge taxa have prostrate or tubular morphologies that afford them high surface-to-volume ratios. GIS-based models incorporating our mesocosm and field survey data predict that the filtration of bacterioplankton bythe inshore sponge community was reduced by approximately 30% after the 2023 heatwave. Last Modified: 01/02/2025 Submitted by: MarkJButler