Contrasting metabolic responses to nutrient enrichment and thermal stress in Hawaiian corals

Abstract

Local-scale environmental factors on coral reefs, such as nutrient input, have varying effects on coral physiology and survivorship dependent upon sources, concentrations, and composition of enrichment. Anthropogenic inputs of excess nutrients, including effluent from sewage outfalls, promote algal dominance and are associated with coral reef decline. In contrast, natural nutrient enrichment from seabird guano has been argued to enhance coral growth rates and recovery from heat-induced coral bleaching. This study aimed to compare the effects of anthropogenic (primary-treated sewage effluent), seabird-derived (guano), and mineral (inorganic nitrogen salt) nutrient sources crossed with two weeks of 3ºC heat stress on the metabolism of two dominant coral species in Hawai‘i: Montipora capitata and Porites compressa. In M. capitata, guano enrichment increased productivity (net photosynthetic production, NPP, by 31% and gross photosynthetic production, GPP, by 26%) relative to unenriched controls with no changes to respiration rates under ambient temperature conditions. While heating depressed NPP by 48 and 66% for the inorganic nutrient-enriched and unenriched control M. capitata fragments, respectively, no equivalent thermal stress response was detected in effluent- and guano-enriched corals, suggesting these treatments dampened thermal stress. Four weeks after the heating event there was no detectable residual effect of prior heating on M. capitata metabolic rates: while the effluent- and guano-enriched corals maintained elevated photosynthetic rates, heated corals from each nutrient treatment had similar photosynthetic and respiration rates to their respective corals maintained at ambient temperatures. In contrast to M. capitata, P. compressa was initially unaffected by nutrient treatments alone, but was highly sensitive to heat stress, exhibiting 90-150% reductions in production and respiration rates across all nutrient treatments that lingered (3-39%) weeks after heating ended. During and following the heat stress, the nutrient enrichments generally manifested as decreased productivity relative to the controls, with no nutrient affects among thermally stressed corals, suggesting nutrient enrichment did not facilitate thermal recovery in Porites compressa. Our results highlight contrasting strategies of coral genera in utilizing organic and inorganic enrichment source compounds to influence primary production rates. The effects of guano, effluent, and inorganic nutrient sources in situ should be further contrasted before applying enrichment as a conservation method to promote thermal resilience