Coral reef benthic community structure is associated with the spatiotemporal dynamics of submarine groundwater discharge chemistry

Submarine groundwater discharge (SGD) is an important transporter of solutes and fresh water in coastal systems worldwide. In high island systems with a mixed semidiurnal tidal cycle driving SGD, coastal biogeochemistry is temporally and spatially variable. Past studies have shown that SGD covaries with the local species composition, diversity, and richness of biological communities on a scale of meters. Empirical orthogonal function analyses (EOF)-a method analogous to principal components analysis which finds spatial patterns of variability and their time variation period-were used to define both the spatial and temporal variation in SGD using spatially resolved time series of salinity. The first two EOFs represented variability at the tidal 12-h period and the daily 24-h period, respectively, and were responsible for more than 50% of the SGD-derived salinity variability. We used the first two EOFs to explore spatiotemporally explicit patterns in SGD variability and their relationships with benthic community structure in reef systems. Distance-based linear models found significant relationships between multivariate community structure and variability in SGD at different periods. Taxa-specific logistic regressions showed that zoanthids and turf are more likely to be present in areas with high tidally driven SGD variability, while the inverse relationship is true for the invasive rhodophyteAcanthophora spicifera, calcifying macroalgae, the native rhodophytePterocladiellasp., the cyanobacteriaLyngbyasp., and the invasive chlorophyteAvrainvillea amadelpha. These results show that benthic communities vary with respect to SGD derived salinity at the scale of hundreds of meters resulting in spatially heterogeneous biotic patches.