Submarine Groundwater Discharge Exceeds River Inputs as a Source of Nutrients to the Great Barrier Reef

Abstract

Rivers are often assumed to be the main source of nutrients triggering eutrophication in the Great Barrier Reef (GBR). However, existing nutrient budgets suggest a major missing source of nitrogen and phosphorus sustaining primary production. Here, we used radium isotopes to resolve submarine groundwater discharge (SGD)-derived, shelf-scale nutrient inputs to the GBR. The total SGD was ∼10−15 times greater than average river inputs, with nearshore groundwater discharge accounting for ∼30% of this. Total SGD accounted for >30% of all known dissolved inorganic N and >60% of inorganic P inputs and exceeded regional river inputs. However, SGD was only a small proportion of the nutrients necessary to sustain primary productivity, suggesting that internal recycling processes still dominate the nutrient budget. With millions of dollars spent managing surface water nutrient inputs to reef systems globally, we argue for a shift in the focus of management to safeguard reefs from the impacts of excess nutrients.

Rebuttal to Correspondence on “Submarine Groundwater Discharge Exceeds River Inputs as a Source of Nutrients to the Great Barrier Reef”

“…We do not dispute this, as the total SGD (recirculated seawater plus freshwater discharge) is not governed solely by the amount of freshwater discharge. Numerous passages in our paper 1 clearly suggest that recirculated seawater is by far the most dominant form of SGD in the GBR. Our paper goes on to highlight the importance of resolving the input of fresh SGD going forward.…”

“…We thank Lewis et al for their comment on our paper Submarine Groundwater Discharge Exceeds River Inputs as a Source of Nutrients to the Great Barrier Reef and are thankful for the opportunity to clarify their interpretation. The three main points of their correspondence are addressed below.…”

“…■ NEARSHORE VERSUS CATCHMENT-DERIVED SGD Lewis et al are critical that Tait et al 1 did not "differentiate between catchment-derived nearshore groundwater and recirculated nearshore marine waters". Indeed, fresh groundwater inputs, or as Lewis et al term them, the "catchmentderived" component of submarine groundwater discharge (SGD), were not calculated.…”

Iodide, iodate & dissolved organic iodine in the temperate coastal ocean

“…In freshwater, the primary iodine species are iodide or DOI (Schwehr and Santschi, 2003;Tang et al, 2013;Zhang et al, 2013). Freshwater chemical species entering the marine system because of submarine groundwater discharge (Tait et al, 2023) or from estuaries may first undergo chemical changes and removal and resuspension cycles due to ion exchange and precipitation, though whether dI T behavior is conservative is under debate (Smith and Butler, 1979;Ullman and Aller, 1980;Takayanagi and Cossa, 1985;Francois, 1987;Luther and Cole, 1988;Abdel-Moati, 1999;Cook et al, 2000;Jones and Tebo, 2021;Lin, 2023). In the case of iodine, estuarine processes may even entirely remove any freshwater iodine such that the only effect of freshwater is expected to be dilution of the seawater concentration, i.e., it can be described by Equation 1 (Takayanagi and Cossa, 1985).…”

Correspondence on “Submarine Groundwater Discharge Exceeds River Inputs as a Source of Nutrients to the Great Barrier Reef”

“…Tait et al’s (hereafter Tait) recent 2023 ES&T publication uses observations of sea surface concentrations of radium isotopes to infer that submarine groundwater discharge (SGD) exceeds river inputs as a source of nutrients to the Great Barrier Reef (GBR). Specifically, they claim that total SGD is ∼10–15 times greater than average river inputs and that total SGD accounts for >30% of all known nutrient inputs.…”