Effects of Submarine Groundwater Discharge on Coral Accretion and Bioerosion on Two Shallow Reef Flats

Abstract

Porites lobata nubbins and bioerosion blocks were deployed across two groundwater gradients in Maunalua Bay to measure coral accretion and bioerosion rates at different levels of submarine groundwater discharge (SGD). Coral accretion was approximated using buoyant weights, photos and linear extension; bioerosion rates were quantified using μCT scans. Chemical parameters, temperature and water motion were measured at each location. Coral accretion showed a modal response to SGD, and percent change in buoyant weights and nubbin surface area negatively responded to pH variation. SGD negatively affected coral survivorship indicating that at high SGD, salinity stress kills corals, but at mid-­‐levels nutrients increase accretion. SGD positively affected bioerosion; this increase was attributed to nutrients, but bioerosion was less sensitive to salinity stress. Coral accretion rates were two orders of magnitude higher than bioerosion rates; however, low coral cover indicates that the actual amount accreted is likely lower than suggested by rates alone.