Sources of urea in arctic seas: seasonal fast ice?

Twelve profiles for dissolved nitrogen, including nitrate, ammonia and urea, and for chlorophyll-related pigments, were taken between February 4 and November 23, 1993, as part of a seasonal study of biological oceanography of Resolute Passage, Northwest Territories, Canada. An additional 20 or 23 profiles of salinity, chlorophyll and total dissolved nitrogen obtained between January 16 and December 13, 1993, from up to 10 depths provided background data in the same area. Eight short cores from seasonal fast ice in the same environment were analyzed for dissolved nitrogen species between January 30 and December 4, 1993, omitting the open water period from early to midJuly through the beginning of October. In the water column, nitrate was the predominant dissolved nitrogen source throughout most of the ice season, but it largely disappeared following break-up, as chlorophyll and its derivative pigments increased. By mid-October nitrate reappeared in the subsurface water and was the dominant nitrogen source again by late November. Ammonia and urea concentrations in the water column were relatively low and variable over the winter, declining in spring, but increasing moderately, although erratically, during June at the onset of the seasonal melt, and continuing to fluctuate through summer and fall. In the ice, ammonia and especially urea were the dominant nitrogen sources, culminating in a large, but non-uniformly distributed, standing stock of regenerated nitrogen, with urea the most concentrated, in late May near the time of maximum ice thickness. At freeze-up, regenerated nutrients, and particularly urea, appeared in the new ice, generally at concentrations greater than in the water just under the ice. We argue that a considerable amount of carbon, as part of the urea molecule, is also incorporated into the ice seasonally.