Nutrient inputs to the coastal ocean through submarine groundwater discharge: controls and potential impact

Abstract

Nutrient input through submarine groundwater discharge (SGD) rivals river inputs in certain regions and may play a significant role in nutrient cycling and primary productivity in the coastal ocean. In this paper, we review the key factors determining the fluxes of nitrogen (N) and phosphorus (P) associated with SGD and present a compilation of measured rates. We show that, in particular, the water residence time and the redox conditions in coastal aquifers and sediments determine fluxes and ratios of N and P in SGD. In many coastal groundwater systems, and especially in contaminated aquifers, N/P ratios exceed those in river water and are higher than the Redfield ratio. Thus, anthropogenically driven increases in SGD of nutrients have the potential to drive the N-limited coastal primary production to P-limitation. River input of N and P to the coastal ocean has doubled over the past 50 yr. Results of a dynamic biogeochemical model for the C, N and P cycles of the global proximal coastal ocean (which includes large bays, the open water part of estuaries, deltas, inland seas and salt marshes), suggest that this has led to a factor 2 increase in primary production and biomass and a decline in water column N/P ratios, i.e. the system has become more N-limiting. With the same model, we show that an increase of SGD-N fluxes to , 0.7-1.1 Tmol yr 21 (with a SGD N/P ratio of 100; equal to ,45 -70% of pre-human riverine N-inputs) is required to drive the coastal ocean to P-limitation within the next 50 yr. q

Coupled radon, methane and nitrate sensors for large-scale assessment of groundwater discharge and non-point source pollution to coastal waters

“…Expanding 21 residential and commercial near-shore development is leading to increased nutrient inputs 22 to groundwater that eventually migrate into to coastal waters. Several-decades long 23 research shows that nitrogen inputs via non-point sources over large coastline areas cause 24 decline of ecological health and may support harmful algal blooms (Valiela et al, 1990; 25 1992; Slomp and Van Cappellen, 2004; Lee and Kim, 2007;Umezawa et al, 2008). 26…”

pH-Dependent iron oxide precipitation in a subterranean estuary

“…Submarine groundwater discharge (SGD) provides an important transport pathway for land-derived dissolved nutrients to coastal waters (Charette et al, 2001;Slomp and Van Cappellen, 2004). The chemical composition of SGD is not only determined by the terrestrial freshwater source(s), but also by the biogeochemical processes occurring along the groundwater flow path, particularly in the bsubterranean estuaryQ where freshwater and seawater mix (Moore, 1999).…”

Tidal variability of nutrients in a coastal coral reef system influenced by groundwater

“…Groundwater input to coral reefs has been shown to be globally important and carry a significant amount of terrestrially derived nutrients to the reef systems (D'Elia et al, 1981;Paytan et al, 2006;Houk et al, 2013). Groundwater discharge is usually enriched in N relative to P with an N : P ratio higher than the Redfield ratio, 16 : 1 (Redfield, 1960), because of more efficient immobilization of P than N in coastal aquifers (Slomp and Van Cappellen, 2004). Such groundwater characterized by a high N : P ratio thus could have significant impacts on coastal reef ecosystems, consid-ering that benthic marine plants are much more depleted in P, with an N : P ratio of about 30 : 1 (Atkinson and Smith, 1983).…”