OVERWINTERING OF SOME NORTH TEMPERATE AND ARCTIC CHIRONOMIDAE: I. THE WINTER ENVIRONMENT

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Abstract
This paper is the first of two concerning overwintering of shallow-pond Chironomidae in regions with cold winters, and deals with winter habitat conditions as a background to a later report on chironomid biology. Some observations on winter temperatures and factors affecting them, especially in terms of whether freezing occurs, are given for shallow ponds near Ottawa, Ont. (45° N.), where mean air temperatures are about −10 °C for the 3 months of midwinter. Ice thickness was inversely proportional to the depth of overlying snow. The high snow cover on a sheltered pond prevented thick ice from forming and the bottom temperature at a depth of 25 cm remained near +2 °C throughout the winter. Up to 40 cm ice occurred on an exposed pond where snow was largely blown away. Maximum ice thickness was not reached until mid-January. The mud surface at a depth of 15 cm froze only on 22 December and at a depth of 25 cm not until 11 January. The minimum temperature near the margin (at a depth of 15 cm) at the mud surface was −2.6 °C, and 5 cm into the mud was −1.7 °C. In 1969 abnormal rainfall in November increased the pond level by over 30 cm just prior to freezing. Of the area subsequently frozen down to the bottom, three-quarters had been dry in the summer. The frozen portion represented 61% of the total pond area. The winter features of shallow pond environments as winter habitats in north temperate regions and in the arctic are reviewed. In north temperate regions, there is great variability in terms of freezing, produced by a complex of interactions between local topography, precipitation, and air temperatures. The minimum in the least favoured areas, maintained for a relatively short period, is only a few degrees Centigrade below freezing, and often considerable areas remain unfrozen especially in sheltered habitats. Part of most summer-permanent habitats remains unfrozen except in unusually severe years. Winter conditions are ameliorated beneath ice and snow insulation by heat stored in the earth below; solar radiation is probably of little importance. Winter stagnation with exhaustion of oxygen may occur in unfrozen areas beneath ice cover, together with increase of concentration of solutes. In the arctic, ponds up to about 1½ to 2 m deep are frozen to the bottom. Minima in the bottom mud may reach −20 °C or −30 °C for several months depending on latitude and on local topography.