Dissolved Nitrogen Cycling in the Eastern Canadian Arctic Archipelago and Baffin Bay From Stable Isotopic Data

Abstract

Climate change is expected to alter the input of nitrogen (N) sources in the Eastern Canadian Arctic Archipelago and Baffin Bay due to increased discharge from glacial meltwater and permafrost thaw. Since dissolved inorganic N is generally depleted in surface waters, dissolved organic N (DON) could represent a significant N source fueling phytoplankton activity in Arctic ecosystems. Yet, few DON data for this region exist. We measured concentrations and stable isotope ratios of DON (δ¹⁵N) and nitrate (NO₃⁻; δ¹⁵N and δ¹⁸O) to investigate the sources and cycling of dissolved nitrogen in regional rivers and marine samples collected in the Eastern Canadian Arctic Archipelago and Baffin Bay during the summer of 2019. The isotopic signatures of NO₃⁻ in rivers could be reproduced in a steady state isotopic model by invoking mixing between atmospheric NO₃⁻ and nitrified ammonium as well as NO₃⁻ assimilation by phytoplankton. DON concentrations were low in most rivers (≤4.9 μmol N L⁻¹), whereas the concentrations (0.54–12 μmol N L⁻¹) and δ¹⁵N of DON (−0.71–9.6‰) at the sea surface were variable among stations, suggesting dynamic cycling and/or distinctive sources. In two regions with high chlorophyll-a, DON concentrations were inversely correlated with chlorophyll-a and the δ¹⁵N of DON, suggesting net DON consumption in localized phytoplankton blooms. We derived an isotope effect of 6.9‰ for DON consumption. Our data helps establish a baseline to assess future changes in the nutrient regime for this climate-sensitive region.