Effects of submarine groundwater discharge on coral accretion and bioerosion on two shallow reef flats

Abstract

AbstractSubmarine groundwater discharge (SGD) is an important source of nutrients to many coastal reefs, yet there is little information on how SGD impacts key coral reef processes. Here, we investigated the effect of SGD on coral growth and bioerosion rates from Porites lobata nubbins and blocks of calcium carbonate (CaCO3) on two reef flats in Maunalua Bay, O'ahu. Over a 6‐month (coral nubbins) and yearlong (CaCO3 blocks) deployment period, we combined multiple metrics of coral growth (buoyant weight, surface area, and linear extension) and bioerosion with a suite of co‐measured physicochemical parameters that are indicators for SGD (carbonate chemistry, dissolved inorganic nutrients, temperature, salinity, and water motion). All coral growth metrics showed a modal response to SGD, and the percent change in buoyant weights and nubbin surface area were negatively related to pH variation. SGD negatively affected coral survival, indicating that at high levels of SGD, salinity stress could be killing corals, but at mid‐levels SGD‐associated nutrients could be increasing growth rates. SGD had a positive effect on bioerosion, most likely due to the positive effect of increased nutrients on bioeroding organisms. Further, coral accretion rates were two orders of magnitude higher than bioerosion rates; however, given the low coral cover on these reef flats, the total carbonate accreted by corals is much lower than suggested by rates alone. These results indicate that corals can thrive on SGD‐impacted reefs if isolated from secondary stressors, so active management to reduce macroalgae and sedimentation could allow coral recovery in Maunalua Bay.