Effects of elevated seawater temperature and nitrate enrichment on the branching coral Porites cylindrica in the absence of particulate food

Compounded disturbances are becoming increasingly common on coral reefs. Impacts of global warming, which is generally perceived as the most serious threat to coral reefs today, often coincide with various common anthropogenic disturbances, such as pollution and overharvesting. To better evaluate the impact of global warming, interactions between elevated temperature and background disturbance should be investigated. In this study, the physiological response of the reef-building coral Porites cylindrica was investigated, when exposed to enrichment of dissolved inorganic nitrate (+ 15 muM for 14 days) and elevated seawater temperature ( + 2 C for 48 h), in the absence of particulate food. It was shown that P. cvlindrica was able to tolerate the temperature exposure without losing symbiotic microalgae or chlorophyll pigments, although the photosynthetic capacity was affected. Nitrate enrichment significantly reduced primary production rate, although zooxanthella population density and chlorophyll concentrations were not affected. The combination of elevated temperature and nitrate enrichment produced an even more pronounced reduction of the production rate. Since coral respiration rate remained unaffected by the treatments, this implies that the corals were unable to acclimate to impaired photosynthesis. However, there was no indication of subsequently reduced tissue growth or increased host catabolism based on tissue biomass measurements. On the other hand, all corals, including controls, lost tissue biomass during the exposure, suggesting a dependence on particulate food. Our results imply that corals on nutrient-exposed reefs may be more stressed during periods of elevated temperature, compared to corals in more pristine areas.