| The shallow back reef pools on Ofu Island, American Samoa, are highly-fluctuating environments where seawater temperature and irradiance often reach levels thought to cause bleaching of reef-building corals, yet bleaching is rare in the pools. I hypothesized that environmental fluctuations associated with the pools, and biological mechanisms associated with back reef host corals and their algal endosymbionts, both reduce the effects of elevated seawater temperatures and high irradiance levels. I tested the hypotheses with a series of water table and field experiments. Water table experiments using Porites lobata and P. cylindrica suggested that intermittent flow associated with semi-diurnal tides, and low irradiances caused by turbidity or shading, reduce photoinhibition and bleaching of back reef corals during warming events. Reciprocal transplant experiments (RTEs) of the corals P. lobata and Pocillopora eydouxi between the highly-fluctuating back reef pools and a nearby environmentally consistent forereef site demonstrated that back reef colonies of both species survived better and grew more than their forereef counterparts regardless of transplant site. For P. lobata, growth was more affected by source population than transplant site, suggesting effects of either developmental phenotypic plasticity or genetic polymorphism. For P. eydouxi, growth was only affected by transplant site, demonstrating effects of physiological phenotypic plasticity. In addition, a survey of zooxanthella genotypes of P. lobata, P. eydouxi, Galaxea fascicularis, and Acropora gemmifera colonies at the back reef and forereef sites used for the RTEs showed a higher proportion of heat-resistant Clade D genotypes in back reef colonies of all four coral species except P. lobata. In conclusion, these experiments demonstrated that environmental characteristics and biological mechanisms associated with fluctuating reef environments and their corals maintain the resilience of coral populations to disturbances for three reasons. First, potentially stressful conditions (elevated seawater temperatures and high irradiance levels) are moderated by environmental characteristics such as semi-diurnally intermittent flow. Second, exposure to these conditions likely maintains selection for mechanisms of acclimatization (phenotypic plasticity) or adaptation (genetic polymorphism) within host coral populations. Third, exposure of coral colonies to diurnally and seasonally fluctuating conditions maintains zooxanthella genotype diversity in some host coral species. |
