The role of submarine groundwater in shaping nearshore coral physiology

Abstract

Although coral reefs evolved in oligotrophic waters, corals are now distributed across a variety of biogeochemical environments that shape their physiology. Allochthonous sources of nutrients – such as coastal runoff, rivers, and submarine groundwater discharge (SGD) – increased nutrient availability and altered carbonate dynamics. To investigate the influence of SGD on coral physiology, we conducted a seven-week field experiment along an SGD gradient in Moorea, French Polynesia. This gradient was characterized by tidally-regulated pulses of cooler, less saline water with lower pH, higher alkalinity, and elevated macronutrient concentrations (phosphate, N+N, and silicate). We evaluated the physiological responses of two common fringing reef corals, Porites rus and Pocillopora acuta, by examining changes in the holobiont (percent change in buoyant weight), endosymbiont photopigmentation (total chlorophyll content), and nutrient partitioning (δ15NH-S and δ13CH-S) when exposed to SGD influence for 7-8 weeks across 20 stations on the exposed reef. In P. acuta, total chlorophyll content and δ15NH-S both displayed a significant relationship with SGD influence, highlighting the sensitivity of the symbiont physiology to SGD. There were no effects of SGD on any of the measured parameters for P. rus.Isotope analyses showed P. rus and P. acuta display different feeding strategies: P. acuta relied more on heterotrophy (67% overlap in isotopic niche between the host and symbiont) while P. rus relied more on autotrophy (93% overlap in isotopic niche between the host and symbiont). δ15N values of zooplankton and particulate organic matter within and outside the seep indicated heterotrophic contributions to corals were predominantly derived from non-seep sources outside the seep. These results indicate that the complex biogeochemical inputs from SGD drive contrasting physiological responses within coral holobionts. Our findings reveal that submarine groundwater discharge, an often-overlooked coastal input, can drive differential physiological responses that may ultimately influence species composition and trophic dynamics on coral reefs.