Conspecific interactions between corals mediate the effect of submarine groundwater discharge on coral physiology

Land-based inputs, such as runoff, rivers, and submarine groundwater, can alter biologic processes on coral reefs. While the abiotic factors associated with land-based inputs have strong effects on corals, corals are also affected by biotic interactions, including other neighboring corals. The biologic responses of corals to changing environmental conditions and their neighbors are likely interactive; however, few studies address both biotic and abiotic interactions in concert. In a manipulative field experiment, we tested how the natural environmental gradient created by submarine groundwater discharge (SGD) affected holobiont and symbiont metabolic rates and endosymbiont physiology of Porites rus. We further tested how the effect of SGD on the coral was mediated by intra and interspecific interactions. SGD is a natural land-sea connection that delivers nutrients, inorganic carbon, and other solutes to coastal ecosystems worldwide. Our results show that a natural gradient of nutrient enrichment and pH variability as a result of acute SGD exposure generally benefited P. rus, increasing gross photosynthesis, respiration, endosymbiont densities, and chlorophyll a content. Conspecifics in direct contact with the a neighboring coral, however, altered the relationship between coral physiology and SGD, lowering the photosynthetic and respiration rates from expected values when the coral had no neighbor. We show that the response of corals to environmental change is dependent on the types of nearby neighbor corals and how neighbors alter the chemical or physical environment around the coral. Our study underscores the importance of considering biotic interactions when predicting the physiologic responses of corals to the environment.