Articles | Volume 14, issue 13
Biogeosciences, 14, 3321–3336, 2017
Biogeosciences, 14, 3321–3336, 2017

Research article 11 Jul 2017

Research article | 11 Jul 2017

Nitrogen transformations along a shallow subterranean estuary

Mathilde Couturier1,2, Gwendoline Tommi-Morin1,2, Maude Sirois1,3, Alexandra Rao3, Christian Nozais2, and Gwénaëlle Chaillou1,2 Mathilde Couturier et al.
  • 1Canada Research Chair on the Geochemistry of Coastal Hydrogeosystems, BOREAS, group on Nordic System, Département de Biologie, Chimie, Géographie, Université du Québec à Rimouski, Rimouski, G5L3A1, Canada
  • 2Département de Biologie, Chimie, Géographie, Université du Québec à Rimouski, Rimouski, G5L3A1, Canada
  • 3Institut des sciences de la mer de Rimouski, Université du Québec à Rimouski, Rimouski, Quebec, G5L 3A1, Canada

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 (NO3), nitrite (NO2), ammonia (NH4+), 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 was maximal after snowmelt. Inland groundwater supplied high concentrations of NOx 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 NOx and high NH4+ 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 NOx-rich to NOx-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 NOx yr−1 per metre of shoreline and 63 mol DON m−1 yr−1 to the STE, and NH4+ input was negligible. Near the discharge zone, the potential export of N species was estimated around 140, 1.5 and 33 mol yr−1 per metre of shoreline for NH4+, NOx and DON respectively. In contrast to the fresh inland groundwater, the N load of beach groundwater near the discharge zone was dominated by NH4+ 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.

Short summary
At the land–ocean interface, subterranean estuaries (STEs) are a critical transition pathway of nitrogen. Environmental conditions in the groundwater lead to nitrogen transformation, altering the nitrogen species and concentrations exported to the coastal ocean. This study highlights the role of a STE in processing groundwater-derived N in a shallow boreal STE, far from anthropogenic pressures. Biogeochemical transformations provide new N species from terrestrial origin to the coastal ocean.
Final-revised paper