THE EFFECTS OF ENHANCED FEEDING ON THE ENDOSYMBIOTIC ALGAL CELL DENSITY OF BLEACHED MONTIPORA CAPITATA CORALS IN AN ACIDIFIED OCEAN
Rising seawater temperature and ocean acidification (OA) threaten the survival of coral reefs. In 2014 and 2015, repeat bleaching events in Hawaii provided an opportunity to experimentally test the following hypotheses on the coral species Montipora capitata: 1) OA slows recovery of algal endosymbiont density following bleaching, 2) Feeding on zooplankton enhances recovery of algal endosymbiont density following bleaching, and 3) Coral provenance may affect coral recovery rates from bleaching. Specifically, corals that are acclimated to the warmer and more acidic baseline conditions of Kāne‘ohe Bay will recover faster from bleaching than those acclimated to cooler and less acidic conditions from Waimānalo Bay. To test the hypotheses, we reared bleached and non-bleached ramets of M. capitata corals collected following a natural bleaching event in 2014 from Kāne‘ohe Bay and Waimānalo Bay in outdoor flow-through tanks under the following four conditions: 1) ambient seawater pH and not fed, 2) ambient seawater pH and fed, 3) acidified seawater and not fed, and 4) acidified seawater and fed. Endosymbiotic algal densities were quantified after one month, seven months, and 12 months in the experiment. Results indicated that feeding may be critical for the long-term recovery of the endosymbiotic algae of M. capitata at both sites, with corals in Kāne‘ohe Bay needing more zooplankton than what was provided in the experimental feeding. This research shows how feeding on zooplankton may reduce the impact of bleaching on corals in an increasingly acidified ocean.