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.

Both global and local factors affect coral reefs worldwide, sometimes simultaneously. An interplay of these factors can lead to phase shifts from hard coral dominance to algae or other invertebrates, particularly soft corals. However, most studies have targeted the effects of single factors, leaving pronounced knowledge gaps regarding the effects of combined factors on soft corals. Here, we investigated the single and combined effects of phosphate enrichment (1, 2, and 8 μM) and seawater temperature increase (26 to 32 °C) on the soft coral Xenia umbellata by quantifying oxygen fluxes, protein content, and stable isotope signatures in a 5-week laboratory experiment. Findings revealed no significant effects of temperature increase, phosphate enrichment, and the combination of both factors on oxygen fluxes. However, regardless of the phosphate treatment, total protein content and carbon stable isotope ratios decreased significantly by 62% and 7% under temperature increase, respectively, suggesting an increased assimilation of their energy reserves. Therefore, we hypothesize that heterotrophic feeding may be important for X. umbellata to sustain their energy reserves under temperature increase, highlighting the advantages of a mixotrophic strategy. Overall, X. umbellata shows a high tolerance towards changes in global and local factors, which may explain their competitive advantage observed at many Indo-Pacific reef locations.

Hard corals are in decline as a result of the simultaneous occurrence of global (e.g., ocean warming) and local (e.g., inorganic eutrophication) factors, facilitating phase shifts towards soft coral dominated reefs. Yet, related knowledge about soft coral responses to anthropogenic factors remains scarce. We thus investigated the ecophysiological response of the pulsating soft coral Xenia umbellata to individual and combined effects of phosphate enrichment (1, 2, and 8 μM) and ocean warming (26 to 32°C) over 35 days. Throughout the experiment, we assessed pulsation, mortality, Symbiodiniaceae density, and cellular chlorophyll a content. Simulated ocean warming up to 30°C led to a significant increase in polyp pulsation and by the end of the experiment to a significant increase in Symbiodiniaceae density, whereas cellular chlorophyll a content significantly decreased with warming, regardless of the phosphate treatment. The combination of phosphate enrichment and simulated ocean warming increased pulsation significantly by 41 – 44%. Warming alone and phosphate enrichment alone did not affect any of the investigated response parameters. Overall, X. umbellata displayed a high resilience towards ocean warming with no mortality in all treatments. Phosphate enrichment enabled soft corals to significantly increase their pulsation under increasing temperatures which may enhance their resilience towards ocean warming. This, in turn, could further facilitate their dominance over hard corals on future reefs.

Biological imaging is an essential tool to visualise and obtain reference data. In this context, the programme ImageJ has been widely used in many disciplines to determine the surface areas of planar biological samples in marine and aquatic experimental biology. Despite its range of advantages, ImageJ is relatively time-consuming, because of the need to manually select the target areas for quantification. Hence, we here evaluated the freeware programme Photopea as a potential alternative by comparing the accuracy and time required for the surface area quantification of exemplary algae compared with established ImageJ analysis. Our results show that Photopea is equally accurate as ImageJ, but 45% more time efficient. This time efficiency originates from using colour contrast that reduces the time needed to analyse each picture. Photopea thus offers an accurate, rapid, and cost-free tool to easily obtain reference data from field and laboratory experiments. This tool is particularly useful for experiments with an extensive sample size of specimens and thus can increase the power of study results.

Marine habitats experience transitions in the benthic communities from invertebrate- or seagrass-dominance to alternative states of algal dominance. Classical Mediterranean biodiversity hotspots are declining, and potentially replaced and overgrown by algal assemblages. Here we provide the first photographic proof of Mediterranean algal mats that overgrow seagrass meadows and hard-bottom habitats.