| Summary: | Alkalinity on coral reefs can vary widely over tidal and diel cycles and elucidating the drivers of these variations is important to ocean acidification research. Porewater advection through carbonate sands can be both a source and sink of total alkalinity (TA) over the course of a diel cycle. Flux rates from insitu advective chambers (Heron Island) were as high as 8.84 mmol m-2 d-1, and rates increased with advection. Fluxes of CO2 associated with respiration mostly drive the increase in dissolution with advection, decreasing any buffering capacity of TA released into coral lagoons. Data from Rarotonga (Cook Islands) indicates that submarine groundwater discharge (SGD) can also be an important source of TA to coral lagoons, with flux rates as high as 1,080 mmol m-2 d-1. The influence of SGD and porewater advection varied over the course of diel and tidal cycles, with SGD fluxes accounting for 27% to 97% of the two fluxes. The buffering capacity of any TA released from SGD is also influenced by concurrent fluxes of CO2 from groundwater. In order to properly measure any changes in community calcification on coral reefs based on changes in TA, all drivers of TA variability need to be recognized.
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