On sources and sinks of bioavailable nitrogen on the Scotian Shelf: Insights from nutrient distributions and nitrate isotope ratios

Abstract

<p>The northwestern North Atlantic, and the extensive Northwest Atlantic Shelf in particular, are among the areas of the world’s ocean most dramatically affected by ongoing climate change. Profound alterations of nitrogen (N) cycling both in the water column and in the sediment are expected in response to rapidly changing ocean and biogeochemical conditions. Despite the importance of bioavailable nitrogen in shaping this marine environment and ultimately sustaining large commercial fisheries, significant uncertainties remain regarding the main sources and sinks of this key macronutrient.</p><p>In this study, we use hydrographic data (T, S, O<sub>2</sub>) and nutrient concentrations collected during the Atlantic Zone Monitoring Program (AZMP) in an extended Optimum Multiparameter Analysis (eOMPA) to quantify the fractional contribution of nearshore versus slope waters on the Scotian Shelf. In combination with nitrate N and O isotope ratios (δ<sup>15</sup>N<sub>NO3</sub> and δ<sup>18</sup>O<sub>NO3</sub>) these results will help to constrain the relationship between physical forcing (on-shelf nutrient transport) and biologically mediated sources and sinks of bioavailable N on the Scotian Shelf. Tracer distributions indicate different hydrographic sources between coastal and offshore slope stations. Nearshore subsurface (> 50 m) waters are characterized by low temperatures and a pronounced deficit in nitrate relative to phosphate (-4 µmol/L; assuming Redfield stoichiometry), highlighting the dominance of cold, fresh water from the Gulf of St. Lawrence along the inner shelf. Off-shelf, higher temperatures along with higher salinity and lower O<sub>2</sub> concentrations indicate the presence of nutrient-rich slope waters, with contributions from both the Labrador Sea and the Gulf Stream. N and O isotope ratios show lower δ<sup>18</sup>O<sub>NO3</sub> (0.0 – 0.4‰) and higher δ<sup>15</sup>N<sub>NO3</sub> (~5.0‰) near the coast relative to stations further offshore (> 2.0‰ and ~4.0‰, respectively). Increased temperatures along with higher values of δ<sup>18</sup>O<sub>NO3</sub> (~2.0‰) and a surplus in nitrate over phosphate (4 µmol/L) reflect the intrusion of warm slope waters into deep basins on the shelf. Tracer distributions further show an imprint of remineralization on bottom water properties along the shelf. These new data and results from the eOMPA will be discussed in the context of the regional circulation and N biogeochemistry.</p>