Abstract Summary
Under predicted future ocean conditions, elevated seawater temperatures and ocean acidification (OA) will affect coral simultaneously. Many studies have shown that these conditions can be detrimental to the physiology of both the coral host and its endosymbiotic algae. Moderate nutrient additions, however, may offer some physiological benefits and aid in mitigating elevated temperature and OA stress in corals. We predict that moderate increases in nutrient concentrations will reduce the negative effects of elevated temperature and pCO2 on coral physiology, but that these effects will be species specific. If true, some coastal marine environments may provide refuge to corals in the future. To investigate this, fragments of the two Indo-Pacific corals Acropora millepora and Turbinaria reniformis were grown for 33 days under 8 treatments of a fully factorial experimental design including two seawater temperatures (26.5°C, and 31.5°C), two pCO2 levels (401 μatm, and 760 μatm), and two nutrient levels (low nutrients at 0.4 μmol/L NO3/NO2 and 0.2 μmol/L PO4-3, and moderate nutrients at 3.5 μmol/L NO3/NO2 and 0.3 μmol/L PO4-3). Preliminary results suggest that T. reniformis is resilient under all temperature and pCO2 conditions, with no additional effect of the moderate nutrient addition observed on either the host or the endosymbiont physiology. In contrast, moderate nutrients stimulated the productivity of A. millepora endosymbionts when exposed to the dual stress of elevated temperature and pCO2, but at a cost to host calcification. In addition, isotopic evidence shows that inorganic nitrogen additions are incorporated and recycled between the endosymbiont and the host, and that the N incorporation diminishes under temperature and/or pCO2 stress. Overall, these results suggest that coastal environments with moderate nutrient additions may provide a refuge to the more susceptible A. millepora under predicted future ocean conditions and that the more tolerant T. reniformis is already adapted to future ocean conditions. Therefore, it is important to consider nutrient composition of reefs when developing conservation strategies for Acropora corals.
