The impact of a marine heatwave on the productivity and carbon budget of a NW Mediterranean seaweed forest

Marine forests support coastal biodiversity and ecosystem functioning. Nonetheless, how their productivity and carbon uptake might be affected by extreme events, such as marine heatwaves (MHWs), is yet to be explored. We experimentally evaluated the changes in oxygen and carbon budgets of the benthic community formed by the fucoid Ericaria brachycarpa induced by the exposure to a MHW. Rocks colonized by E. brachycarpa and associated macroalgal and invertebrate assemblages were collected at Capraia Island (NW Mediterranean) and put into six 500 L tanks at 23 ◦C. After 10 days of acclimation, the seawater temperature in three randomly chosen tanks was gradually elevated to 30.5 ◦C and maintained for 5 days, to simulate a MHW predicted by the end of the century under the RCP 8.5 scenario. Oxygen and carbon metabolic rates of the whole community were evaluated under light and dark conditions, using transparent and black incubation chambers, respectively. The exposure to the MHW caused a reduction in Net Community Productivity (NCP) and increased Community Respiration (CR). There was a trend for MHW to enhance total DIC release through the reduction of calcification and the increase of respiration rates, thus shifting the community metabolism to net heterotrophic. Lower net productivity and carbon uptake suggest that the role of these forests in sustaining coastal food webs and mitigating CO2 emissions could be reduced under future climates. These results have implications for devising climate-proof strategies of conservation and restoration of macroalgal forests.