Nutrient dynamics in the western Canadian Arctic. II. Estimates of new and regenerated pro duction over the Mackenzie Shelf and Cape Bathurst Polynya

Abstract

Uptake of 15N-labelled nitrate, ammonium, and urea was measured over a quasiannual cycle in the Cape Bathurst Polynya in the Amundsen Gulf and on the Mackenzie Shelf, during the Canadian Arctic Shelf Exchange Study (CASES) in 2003 and 2004. Before the phyto - plankton bloom and in autumn, nitrogen uptake was slow, representing less than 5% of annual consumption. Uptake rates increased exponentially after ice retreat and within 3 wk reached amaximum of 38.6 mmol N m-2 d-1. During spring, NO3- uptake supported new production of 166 mmol N m-2 and f-ratios rose from 0.1-0.2 to 0.6-0.9. Filter fractionation showed that GF/F filters retained 93.1 ± 1.3% of the 15N incorporated into particulate matter, suggesting that phytoplankton were responsible for the majority of the N uptake. Although free-living bacteria took up relatively more 15N in autumn and in the lower part of the euphotic zone than phytoplankton, their assimilation of inorganic N had little effect on water column integrated f-ratios or new production. Urea supplied almost half the N assimilated by phytoplankton annually and about 80% of the regenerated production during the spring bloom. Total new production, estimated from water column integrated 15N-nitrogen uptake rates and linear models that interpolated rates over unsampled periods, was 342-415 mmol N m-2 yr-1. Total annual N production for the region was 1.24-1.48 mol N m-2 yr-1. © Simpson, Tremblay, Brugel, Price, and Fisheries and Oceans Canada 2013.