SPRINGTIME SENSIBLE HEAT, NUTRIENTS AND PHYTOPLANKTON IN THE NORTHWATER-POLYNYA, CANADIAN ARCTIC

Sampling was conducted in the Northwater Polynya (between 70 degrees 2 0' and 77 degrees 20'N), on 17 and 19 May 1991. At each of the 14 samp ling stations, CTD profiles were recorded from surface to bottom and n utrients and phytoplankton were determined at four depths down to 30 m . The presence, between 220 and 400 m, of water temperatures >0 degree s C is an indication that, in winter, the West Greenland Current enter s the Northwater along the Greenland coast. The warm water is progress ively mixed as it moves northward and eastward. It was thus hypothesiz ed that sensible heat is as an important factor in keeping the Northwa ter open. Measured chemical and biological variables were quite homoge neous on the vertical down to 30 m and they showed longitudinal gradie nts. From east to west, the average concentrations of nutrients increa sed (phosphate from 0.5 to 1.4, nitrate from 3.7 to 10.8, and silicate from 6.8 to 34.2 mmol m(-3)), whereas the areal concentrations of phy toplankton decreased (from 47 to 9 x 10(9) cells m(-2) and from 506 to 50 mg Chla m(-2)). Nutrient ratios indicated possible silicon deficie ncy in the easternmost part of the polynya. Diatoms dominated cell num bers (greater than 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 provide d an estimate of potential maximum biomass in the upper 30 m, which wa s ca. 600 mg Chla m(-2), or lower if there was silicon limitation. The overall picture was that of a diatom bloom, moving westward and progr essively 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. Copyright (C) 1996 E lsevier Science Ltd.