Altered nutrient cycling functionality in seagrass meadows under a simulated future marine heatwave event

Seagrasses are important contributors to environmental nutrient cycling in marine ecosystems and can improve water quality by absorbing excess nitrogen (N). However, these ecosystems are vulnerable to human-mediated pressures, including marine heatwaves (MHWs), particularly those of longer duration. We performed an experiment simulating a 30-d, +5°C intensity MHW to examine the effects on maximum potential N cycling and transformation rates in different system compartments associated with the tropical seagrass Halophila ovalis. Under the MHW, the seagrass exhibited higher ammonium assimilation rates in the leaves, increased respiration rates, and highly variable survival and growth than those at ambient temperature. Contrary to expectations, sediment denitrification rates were lowered under the MHW, reflecting the loss of microbial functions and therefore signifying reduced N removal benefits. Moreover, the lowered seawater total alkalinity under the MHW suggests that the habitat would provide less ocean acidification buffer under future climate change. However, cycling rates in vegetated and unvegetated sediments were not significantly different, showing that the seagrass does not strongly influence the sediment N cycling capacities at this time scale. Our study demonstrates that future MHWs may alter nutrient cycling rates across ecosystem compartments in seagrass meadows, potentially leading to reductions in ecosystem function and services.