Nitrogen transformations along a shallow subterranean estuary

Abstract

<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 Îles-de-la-Madeleine (Québec, Canada). This study reveals the vertical and horizontal distribution of nitrate (NO<sub>3</sub><sup>−</sup>), nitrite (NO<sub>2</sub><sup>−</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>−</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>−</sup> and high NH<sub>4</sub><sup>+</sup> concentrations in the discharge zone. The long residence time of groundwater within the beach (~ 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 mol m<sup>−1</sup> y<sup>−1</sup> of NO<sub>x</sub> and 63.57 mol m<sup>−1</sup> y<sup>−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 % of the DIN flux to coastal waters, at 42.80 mol m<sup>−1</sup> y<sup>−1</sup>. Since N fluxes to the coastal bay (88 mol m<sup>−1</sup> y<sup>−1</sup>) are slightly lower than N fluxes from fresh inland groundwater (102 mol m<sup>−1</sup> y<sup>−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>

Seasonality of Organic Matter Degradation Regulates Nutrient and Metal Net Fluxes in a High Energy Sandy Beach

“…Whereas the first is governed by pelagic algae biomass and other NO 3 − sinks, the latter mainly depends on microbial respiration rates. The temporal variability in DIN fluxes has been shown in prior studies (Couturier et al, ; Gonneea & Charette, ; Liu et al, ) where fluxes were in the same range as at our study site, ranging from 52 mmol DIN/day per meter shoreline (Gonneea & Charette, ) to 388 mmol DIN/day per meter shoreline (Couturier et al, ). However, a temporary net removal of DIN has not been observed in these environments.…”

“…However, a temporary net removal of DIN has not been observed in these environments. One main difference between Spiekeroog Island and Waquoit Bay (Cape Cod, USA; Gonneea & Charette, ), Martinique Beach (Gulf of St. Lawrence, Canada; Couturier et al, ), and Tolo Harbour (Hongkong; Liu et al, ) is the high tidal range of 2.7 m and the wave exposition of the Spiekeroog study site. Both potentially intensified the intertidal benthic‐pelagic coupling and made the system sensitive to water column composition changes governing electron donor, electron acceptor supply and thus microbial rates.…”

“…Nitrate reduction in subterranean estuaries can be driven by inputs of NO 3 − from fresh groundwater and inputs of marine organic matter (Couturier et al, ; Kim et al, ) as well as by seawater supplied NO 3 − . In combination with coupled nitrification‐denitrification, the reduction of NO 3 − to nitrogen gas through denitrification can lead to substantial N‐loss as N 2 and N 2 O, which has been observed for permeable intertidal and subtidal sediments (Marchant et al, ; Marchant et al, ).…”

Seasonal and Spatial Variation in the Location and Reactivity of a Nitrate‐Contaminated Groundwater Discharge Zone in a Lakebed

“…The relevance of water movement to N cycling processes within the sediments was not readily apparent based solely on process rate potentials and functional gene quantification. The Ashumet Pond results contrast with studies of subestuarine NO 3 − discharge in which NO 3 − removal rates were greater in deeper sediments, with decreasing activity and NO 3 − concentrations near the surface (Couturier et al, ; Kroeger & Charette, ; Slater & Capone, ). Differences in sediment process distribution between freshwater and subestuarine systems result in part from tidal influences and steep salinity gradients along groundwater flow paths in subestuarine systems, neither of which are factors in inland lakes.…”

The Subterranean Estuary: Technical Term, Simple Analogy, or Source of Confusion?

“…Thus, although the definition is physical, the implication is biogeochemical, “to emphasize the importance of mixing and chemical reaction in these coastal aquifers” (Moore, ). The reactions resulting from mixing of waters of different origin have important implications for processes of particular interest to the marine community, including nutrient sources and attenuation (Couturier et al, ), temporal and spatial variability of biochemical conditions (Santos et al, ), and the impact of coastal groundwater discharge on global chemical fluxes (Moore, ).…”