Springtime sensible heat, nutrients and phytoplankton in the northwater Polynya, Canadian Arctic

Sampling was conducted in the Northwater Polynya (between 70°20' and 77°20'N), on 17 and 19 May 1991. At each of the 14 sampling stations, CTD profiles were recorded from surface to bottom and nutrients and phytoplankton were determined at four depths down to 30 m. The presence, between 220 and 400 m, of water temperatures > 0°C is an indication that, in winter, the West Greenland Current enters the Northwater along the Greenland coast. The worm water is progressively mixed as it moves northward and eastward. It was thus hypothesized that sensible heat is as an important factor in keeping the Northwater open. Measured chemical and biological variables were quite homogeneous on the vertical down to 30 m and they showed longitudinal gradients. From east to west, the average concentrations of nutrients increased (phosphate from 0.5 to 1.4, nitrate from 3.7 to 10.8, and silicate from 6.8 to 34.2 mmol/m3), whereas the areal concentrations of phytoplankton decreased (from 47 to 9 x 10 to the ninth power cells/m2 and from 506 to 50 mg Chla/m2). Nutrient ratios indicated possible silicon deficiency in the easternmost part of the polynya. Diatoms dominated cell numbers (greater or equal to 87% at all stations). Concentrations of the three nutrients were inversely correlated with both Chla and cell numbers. The Y-intercepts of regressions of Chla on nutrients provided an estimate of potential maximum biomass in the upper 30 m, which was ca. 600 mg Chla/m2, or lower if there was silicon limitation. The overall picture was that of a diatom bloom, moving westward and progressively exhausting the nutrients. Initiation of the bloom appeared to have been linked to the absence of sea ice. A source of heat for this would have been the above sensible-heat process. (Résumé d'auteur