Abstract. The transformations of chemical constituents in subterranean estuaries (STEs) control the delivery of nutrient loads from coastal aquifers to the ocean. It is important to determine the processes and sources that alter nutrient concentrations at a local scale in order to estimate accurate regional and global nutrient fluxes via submarine groundwater discharge (SGD), particularly in boreal environments, where data are still very scarce. Here, the biogeochemical transformations of nitrogen (N) species were examined within the STE of a boreal microtidal sandy beach located in the Magdalen Islands (Quebec, Canada). This study revealed the vertical and horizontal distribution of nitrate (NO, dissolved organic nitrogen (DON) and total dissolved nitrogen (TDN) measured in beach groundwater during four spring seasons (June 2011(June , 2012(June , 2013(June and 2015 when aquifer recharge was maximal after snowmelt. Inland groundwater supplied high concentrations of NO x and DON to the STE, whereas inputs from seawater infiltration were very limited. Non-conservative behaviour was observed along the groundwater flow path, leading to low NO x and high NH + 4 concentrations in the discharge zone. The long transit time of groundwater within the beach (∼ 166 days), coupled with oxygen-depleted conditions and high carbon concentrations, created a favourable environment for N transformations such as heterotrophic and autotrophic denitrification and ammonium production. Biogeochemical pathways led to a shift in nitrogen species along the flow path from NO x -rich to NO x -poor groundwater. An estimate of SGD fluxes of N was determined to account for biogeochemical transformations within the STE based on a N-species inventory and Darcy's flow. Fresh inland groundwater delivered 37 mol NO x yr −1 per metre of shoreline and 63 mol DON m −1 yr −1 to the STE, and NH and DON. Our study shows the importance of tidal sands in the biogeochemical transformation of the terrestrial N pool. This local export of bioavailable N probably supports benthic production and higher trophic levels leading to its rapid transformation in surface sediments and coastal waters.
<p><strong>Abstract.</strong> The transformations of chemical constituents in subterranean estuaries (STE) control the delivery of nutrient loads from aquifers to the coastal ocean. It is important to determine the processes and sources that affect nutrient concentrations at a local scale in order to accurately estimate global nutrient fluxes via submarine groundwater discharge (SGD), particularly in boreal environments, where data are still very scarce. Here, the biogeochemical transformations of nitrogen (N) species were examined within the STE of a microtidal boreal sandy beach located in the &#206;les-de-la-Madeleine (Qu&#233;bec, Canada). This study reveals the vertical and horizontal distribution of nitrate (NO<sub>3</sub><sup>&#8722;</sup>), nitrite (NO<sub>2</sub><sup>&#8722;</sup>), ammonia (NH<sub>4</sub><sup>+</sup>), dissolved organic nitrogen (DON) and total dissolved nitrogen (TDN) measured in beach groundwater during four spring seasons (June 2011, 2012, 2013 and 2015) when aquifer recharge is maximal after snow melt. Inland groundwater supplied high concentrations of NO<sub>x</sub><sup>&#8722;</sup> and DON to the STE, whereas inputs from seawater were very limited. Non-conservative behaviour was observed along the groundwater flow path, leading to low NO<sub>x</sub><sup>&#8722;</sup> and high NH<sub>4</sub><sup>+</sup> concentrations in the discharge zone. The long residence time of groundwater within the beach (~&#8201;82 days), coupled with oxygen-depleted conditions and high carbon concentrations created a favourable environment for N transformations such as heterotrophic denitrification and ammonium production. An estimate of SGD fluxes of N was determined to account for biogeochemical transformations within the STE. Fresh inland groundwater delivers 37.54&#8201;mol&#8201;m<sup>&#8722;1</sup>&#8201;y<sup>&#8722;1</sup> of NO<sub>x</sub> and 63.57&#8201;mol&#8201;m<sup>&#8722;1</sup>&#8201;y<sup>&#8722;1</sup> of DON to the STE, and NH<sub>4</sub><sup>+</sup> input was negligible. But the N load to coastal waters is dominated by NH<sub>4</sub><sup>+</sup> and DON, due to N transformations along the flow path. NH<sub>4</sub><sup>+</sup> represents 99&#8201;% of the DIN flux to coastal waters, at 42.80&#8201;mol&#8201;m<sup>&#8722;1</sup>&#8201;y<sup>&#8722;1</sup>. Since N fluxes to the coastal bay (88&#8201;mol&#8201;m<sup>&#8722;1</sup>&#8201;y<sup>&#8722;1</sup>) are slightly lower than N fluxes from fresh inland groundwater (102&#8201;mol&#8201;m<sup>&#8722;1</sup>&#8201;y<sup>&#8722;1</sup>), the STE appears to be a sink of terrestrially-derived N. The net transformations of N in the STE led to N removal along the groundwater flow path along the groundwater flow path.</p>
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