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Coral reefs experience numerous natural and anthropogenic environmental gradients that alter biophysical conditions and affect biodiversity. While many studies have focused on drivers of reef biodiversity using traditional diversity metrics (e.g., species richness, diversity, evenness), less is known about how environmental variability may influence functional diversity. In this study, we tested the impact of submarine groundwater discharge (SGD) on taxonomic and functional diversity metrics in Mo‘orea, French Polynesia. SGD is the expulsion of terrestrial fresh or recirculated seawater into marine environments and is associated with reduced temperatures, pH, and salinity and elevated nutrient levels. Using a regression approach along the SGD gradient, we found that taxon and functional-entity richness displayed unimodal relationships to SGD parameters, primarily nitrate + nitrite and phosphate variability, with peak richness at moderate SGD for stony coral and the full benthic community. Macroalgae showed this unimodal pattern for functional-entity but not taxonomic richness. Functional community composition (presence and abundance of functional entities) increased along the gradient, while taxonomic composition showed a nonlinear relationship to SGD-related parameters. SGD is a common feature of many coastal ecosystems globally and therefore may be more important to structuring benthic functional diversity than previously thought. Further, studying community shifts through a functional-trait lens may provide important insights into the roles of community functions on ecosystem processes and stability, leading to improved management strategies. 

Coral reefs experience numerous environmental gradients affecting organismal physiology and species biodiversity, which ultimately impact community metabolism. This study shows that submarine groundwater discharge (SGD), a common natural environmental gradient in coastal ecosystems associated with decreasing temperatures, salinity and pH with increasing nutrients, has both direct and indirect effects on coral reef community metabolism by altering individual growth rates and community composition. Our data revealed that SGD exposure hindered the growth of two algae,Halimeda opuntiaandValonia fastigiata,by 67 and 200%, respectively, and one coral,Porites rus,by 20%. Community metabolic rates showed altered community production, respiration and calcification between naturally high and low exposure areas mostly due to differences in community identity (i.e. species composition), rather than a direct effect of SGD on physiology. Production and calcification were 1.5 and 6.5 times lower in assemblages representing high SGD communities regardless of environment. However, the compounding effect of community identity and SGD exposure on respiration resulted in the low SGD community exhibiting the highest respiration rates under higher SGD exposure. By demonstrating SGD’s role in altering community composition and metabolism, this research highlights the critical need to consider compounding environmental gradients (i.e. nutrients, salinity and temperature) in the broader context of ecosystem functions.