A new perspective on coastal hypoxia: The role of saline groundwater

Abstract

Processes leading to the development of coastal hypoxia are usually attributed to anthropogenic nutrient loading. We present new evidence documenting a natural process capable of producing large-scale water masses undersaturated with respect to dissolved oxygen, independent of human-caused nutrient loading and subsequent nutrient-enhanced productivity. Along the heavily developed coastline of Myrtle Beach, South Carolina, the offshore discharge of saline, anoxic groundwater generates undersaturated bottom water masses which can be advected into nearshore waters. Highly elevated radium isotope activities measured in lowoxygen bottom waters cannot be supported by any known terrestrial source. Rather, discharge of cold, anoxic, high-radium water from offshore carbonate aquifers is the likely source. Density stratification inhibits mixing of these discharged waters during inshore transport. During a lowoxygen event in August 2012, this restricted mixing is likely the mechanism for the observed oxygen deficit in bottom waters. This discovery suggests benthic layers undersaturated with respect to dissolved oxygen may be considerably more prevalent than currently recognized, particularly around the carbonate-platform of the US east coast.

Small-scaled lateral variations of an organic-rich formation in a ramp-type depositional environment (the Late Jurassic of the Boulonnais, France): impact of the clastic supply

“…The larger the transgression, the larger the volume of reworked and remobilized sediments, which will impact the possible release of iron, coating fine particles; -the flux of river-borne (Severmann et al, 2010) or airborne (e.g., Baddock et al, 2013;Buck et al, 2013) ironcoated particles; -the flux of OM possibly associated with reactive iron (e.g., Barber et al, 2014;Bressac and Huieu, 2013;Krachler et al, 2012;Shigemitsu et al, 2013). Notably, Krachler et al (2012) indicated that peatland-derived iron-bearing lignin particles may have a sufficiently long half-life in ocean water to sustain iron concentration in extended regions of the ocean; -the offshore discharge of anoxic ground waters may generate O 2 -undersaturated bottom water masses that can be advected into nearshore waters (Peterson et al, 2016). These anoxic water masses may thus carry dissolved iron.…”

Evaluation of Water Residence Time, Submarine Groundwater Discharge, and Maximum New Production Supported by Groundwater Borne Nutrients in a Coastal Upwelling Shelf System

“…The SGD occurs in coastal aquifers the scales varying from intertidal, embayment, to continental (Bratton, 2010). The SGD study has been integrated with that of coastal ecosystem dynamics because SGD-borne nutrients are considered to lead to the coastal eutrophication and hypoxia, and maybe be the causative factor of red tide outbreaks (Gonneea & Charette, 2014;Luo & Jiao, 2016;Peterson et al, 2016;Talbot et al, 2003). SGD can occur through the continental shelf due to outcrops or springs of offshore aquifers (Lamontagne et al, 2008(Lamontagne et al, , 2015Swarzenski et al, 2001), advective flow driven by storms, buoyancy, and large tidal fluctuations (Kim et al, 2005;Moore & Wilson, 2005), or the pore water exchanges by other physical driven forces such as flow and topography-induced pressure gradient, fluid shear, density-driven convection, and compaction, etc.…”

Investigation of submarine groundwater discharge to tidal rivers: Evidence for regional and local scale seepage

“…While other physical factors certainly contribute toward the observed hypoxia, such as biological productivity, low current velocity, sluggish mixing, and flushing with Forge River, it is apparent that the N load from groundwater discharge has an important influence on the geochemistry of the adjacent coastal surface waters, which in turn has a profound negative effect on the local biota (Gobler, DePasquale, Griffith, & Baumann, ). Conversely, the groundwater discharge during this time period may have been inherently anoxic, as demonstrated, for example, by large volumetric fluxes of saline SGD off the coast of South Carolina (Peterson et al, ). Swanson et al .…”