SUBMARINE GROUNDWATER DISCHARGE AND BIOLOGICALLY DRIVEN VARIABILITY IN MARINE CARBONATE CHEMISTRY IN MAUNALUA BAY, OAHU, HAWAII

We examined spatial and temporal variations in carbonate chemistry in the waters overlying two contrasting Hawaiian coral reefs, Black Point and Wailupe, located in Maunalua Bay on the southeastern coast of Oahu. These two locations, which are characterized by localized regions of submarine groundwater discharge, were sampled across a salinity gradient for basic water quality parameters, inorganic nutrients, DIC, TA, and δ¹³C-DIC values every three to six hours over a 24-hour sampling period. Data acquired from these field campaigns were used to evaluate spatial and temporal changes in marine carbonate chemistry resulting from both physical mixing between groundwater and marine water as well as biological activity. Groundwater derived DIC and TA comprised large fractions (up to 30%) of in situ coastal water DIC and TA reservoirs, and observed changes in coastal DIC and TA reservoirs due to groundwater discharge variability were frequently greater than changes due to biological processes (up to 17%). Failure to quantify groundwater derived constituent fractions, when they are present, will bias interpretations of reef biogeochemistry toward over-attribution of changes in carbonate chemistry to biological processes.