Groundwater and porewater as major sources of alkalinity to a fringing coral reef lagoon (Muri Lagoon, Cook Islands)

[摘要] To better predict how ocean acidification will affect coral reefs, it isimportant to understand how biogeochemical cycles on reefs alter carbonatechemistry over various temporal and spatial scales. This study quantifies thecontribution of shallow porewater exchange (as quantified from advectivechamber incubations) and fresh groundwater discharge (as traced by²²²Rn) to total alkalinity (TA) dynamics on a fringing coral reef lagoonalong the southern Pacific island of Rarotonga over a tidal and diel cycle.Benthic alkalinity fluxes were affected by the advective circulation of waterthrough permeable sediments, with net daily flux rates of carbonatealkalinity ranging from −1.55 to 7.76 mmol m⁻² d⁻¹, dependingon the advection rate. Submarine groundwater discharge (SGD) was a source ofTA to the lagoon, with the highest flux rates measured at low tide, and anaverage daily TA flux of 1080 mmol m⁻² d⁻¹ at the sampling site.Both sources of TA were important on a reef-wide basis, although SGD actedsolely as a delivery mechanism of TA to the lagoon, while porewater advectionwas either a sink or source of TA dependent on the time of day. This studydescribes overlooked sources of TA to coral reef ecosystems that canpotentially alter water column carbonate chemistry. We suggest that porewaterand groundwater fluxes of TA should be taken into account in oceanacidification models in order to properly address changing carbonatechemistry within coral reef ecosystems.

[学科分类] 地球化学与岩石