Evaluation of Submarine Groundwater Discharge as a Coastal Nutrient Source and Its Role in Coastal Groundwater Quality and Quantity

Globally, submarine groundwater discharge (SGD) is responsible for 3–4 times the water discharge delivered to the oceans by rivers. Moreover, nutrient concentrations in SGD are usually elevated in comparison to river fluxes. Here we review the major advances in the field of SGD studies and related nutrient fluxes to the coastal ocean. To demonstrate the significance of SGD as terrestrial nutrient pathway we compare stream and submarine groundwater discharge rates in a watershed on the windward side of Oahu, one of the major islands of the Hawaii archipelago. Our analysis of Kaneohe Bay, which hosts the largest coral reefs on the island revealed that SGD in the form of total (fresh+brackish) groundwater discharge was 2–4 times larger than surface inputs. Corresponding DIN and silicate fluxes were also dominated by SGD, while DIP was delivered mostly via streams. We quantified bulk nutrient uptake in coastal waters and also demonstrated that nutrients were quickly removed from the bay due to fast coastal flushing rates. This study demonstrates the need to understand SGD-derived nutrient fluxes in order to evaluate land-based coastal nutrient and pollution sources.