Nutrient inputs from submarine groundwater discharge on the Santiago reef flat, Bolinao, Northwestern Philippines

Submarine groundwater discharge (SGD) on the reef flat of Bolinao, Pangasinan (Philippines) was mapped using electrical resistivity, 222Rn, and nutrient concentration measurements. Nitrate levels as high as 126 μM, or 1-2 orders of magnitude higher than ambient concentrations, were measured in some areas of the reef flat. Nutrient fluxes were higher during the wet season (May-October) than the dry season (November-April). Dissolved inorganic nitrogen (DIN = NO3 + NO2 + NH4) and soluble reactive phosphorus (SRP) fluxes during the wet season were 4.4 and 0.2 mmoles m−2 d−1, respectively. With the increase population size and anthropogenic activities in Bolinao, an enhancement of SGD-derived nitrogen levels is likely. This could lead to eutrophic conditions in the otherwise oligotrophic waters surrounding the Santiago reef flat.

(a) Location of Santiago reef flat and the nearby Santiago and Silaki Islands. (b) Sources of nutrients in the Santiago reef flat. (c) Higher nutrient concentrations from SGD were measured during the wet season.Highlights► Submarine groundwater discharge may be a significant nutrient source at the Santiago reef flat. ► In some areas, NO3 levels were 1-2 orders of magnitude higher than ambient concentrations. ► Nutrient fluxes were higher in the wet (May-October) than the dry season (November-April). ► Increased coastal populations/activities will enhance SGD-derived N levels on the reef flat. ► Higher nutrients and changes in nutrient ratios could compromise coral reef restoration efforts.