Fluxes and behaviors of groundwater-borne nutrients in the ocean

Abstract

Submarine groundwater discharge (SGD) into coastal waters is an important pathway for transporting nutrients to the ocean. Although the fluxes of SGD and associated nutrient discharge to the ocean at local and regional scales have been well studied, detailed estimates and models of SGD-related nutrient inputs to coastal waters on a global scale are few in number. Therefore, in this study, the global magnitude of SGD and its associated nutrient fluxes into the global ocean were estimated using a radium isotope (228Ra). The re-estimated magnitude of global SGD flux was approximately 1–1.5 times the river discharge and SGD-derived nutrient fluxes were comparable to the river-driven fluxes to the global ocean. These results imply that SGD is a critical source of nutrients to the ocean and therefore plays a critical role in marine productivity.

Nutrient fluxes through SGD can be calculated by multiplying the endmember concentrations of nutrients in groundwater by the SGD flux. However, groundwater nutrient concentrations are altered through biogeochemical reactions in the subterranean estuary (STE), where mixing between groundwater and aquifer solids occurs, prior to entering the ocean. Furthermore, the adsorption and desorption behaviors of silicon (Si) and phosphorus (P) in the STE have not been investigated although these processes influence Si and P fluxes through SGD. Based on laboratory experiments, rapid desorption of P (5–20 μmol/L) occurred from the sediment columns when Si was 40−90% removed in the initial stage within the first 24 hours. These results suggest that Si adsorption can result in significant P desorption from sediments in the STE into seeping groundwater.

Nutrient inputs via SGD play a significant role in nutrient cycling and primary productivity in the coastal ocean. This study based on seasonal sampling campaigns shows that SGD plays a critical but different role in nutrient budgets and stoichiometry in coastal waters off a volcanic island depending on open-ocean nutrient conditions. When bay seawater was influenced by the N-limited Tsushima Current, SGD was the major source of DIN in N-limited bay waters. SGD was also the absolute source of DIP in P-depleted bay waters that were influenced by P-limited Changjiang River diluted water (CDW). In addition, excessive inputs of nutrients from SGD resulted in an almost complete transfer of SGD-derived nutrients to the open ocean during the season in which a large flux of SGD occurs.