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

Abstract

To better predict how ocean acidification will affect coral reefs, it is important to understand how biogeochemical cycles on reefs alter carbonate chemistry over various temporal and spatial scales. This study quantifies the contribution of fresh groundwater discharge (as traced by radon) and shallow porewater exchange (as quantified from advective chamber incubations) to total alkalinity (TA) dynamics on a fringing coral reef lagoon along the southern Pacific island of Rarotonga over a tidal and diel cycle. Benthic alkalinity fluxes were affected by the advective circulation of water through permeable sediments, with net daily flux rates of carbonate alkalinity ranging from −1.55 to 7.76 mmol m<sup>−2</sup> d<sup>−1</sup>, depending on the advection rate. Submarine groundwater discharge (SGD) was a source of TA to the lagoon, with the highest flux rates measured at low tide, and an average daily TA flux of 1080 mmol m<sup>−2</sup> d<sup>−1</sup>. Both sources of TA were important on a reef wide basis, although SGD acted solely as a delivery mechanism of TA to the lagoon, while porewater advection was either a sink or source of TA dependant on the time of day. On a daily basis, groundwater can contribute approximately 70% to 80% of the TA taken up by corals within the lagoon. This study describes overlooked sources of TA to coral reef ecosystems that can potentially alter water-column carbonate chemistry. We suggest that porewater and groundwater fluxes of TA should be taken into account in ocean acidification models in order to properly address changing carbonate chemistry within coral reef ecosystems

Nutrient subsidies restructure coral reef dissolved carbon fluxes via biogeochemical cascades

“…S1-S2). Prior studies have found elevated TA in SGD 15,28 and it has been hypothesized that this elevated TA may help buffer coral reefs from low pH waters in the SGD and possibly even from ocean acidification 28,38,39 . Conversely, at the low alkalinity site (Cabral), there was no relationship between pH and NEC (Fig.…”

“…Submarine groundwater discharge (SGD) is a natural land-sea connection that is found worldwide 25,26 and is a template to explore how allochthonous inputs affect interactions between biological and geochemical processes. SGD has a complex mixture of components and is a major source of new solutes that can influence coastal salinity, carbonate dynamics, nutrient fluxes, and the composition and concentration of dissolved organic matter [27][28][29] . SGDdriven solutes have been linked to changes in algal species richness 30,31 , coral and macroalgal growth 32,33 , bioerosion 32,34 , and net ecosystem production and calcification 15,35 on coral reefs.…”

“…Differences in the chemical composition of SGD can also have differential impacts on local biology. For example, in locations where total alkalinity (TA) in SGD is higher than the receiving waters 28 , SGD may buffer corals from low pH 28,38,39 , while locations where TA in SGD is lower than the receiving waters may compound the effects of low pH on calcifiers. Climate change and increasing coastal population densities will alter the flux and composition of SGD 40,41 and our ability to protect coastal ecosystem services is directly impacted by our understanding of how SGD affects biological and geochemical processes.…”