The pulsating soft coral Xenia umbellata shows high resistance to warming when nitrate concentrations are low

Abstract

The resistance of hard corals to warming can be negatively affected by nitrate eutrophication, but related knowledge for soft corals is scarce. We thus investigated the ecophysiological response of the pulsating soft coral Xenia umbellata to different levels of nitrate eutrophication (control = 0.6, medium = 6, high = 37 μM nitrate) in a laboratory experiment, with additional warming (27.7 to 32.8 °C) from days 17 to 37. High nitrate eutrophication enhanced cellular chlorophyll a content of Symbiodiniaceae by 168%, while it reduced gross photosynthesis by 56%. After additional warming, polyp pulsation rate was reduced by 100% in both nitrate eutrophication treatments, and additional polyp loss of 7% d−1 and total fragment mortality of 26% was observed in the high nitrate eutrophication treatment. Warming alone did not affect any of the investigated response parameters. These results suggest that X. umbellata exhibits resistance to warming, which may facilitate ecological dominance over some hard corals as ocean temperatures warm, though a clear negative physiological response occurs when combined with nitrate eutrophication. This study thus confirms the importance of investigating combinations of global and local factors to understand and manage changing coral reefs.