Andrews and Rigler 1985

Andrews D, Rigler FH. 1985. The effects of an Arctic winter on benthic invertebrates in the littoral zone of Char Lake, Northwest Territories. Canadian Journal of Zoology 63: 2825-2834.

These authors examined the benthic invertebrates of Char Lake, near Resolute, Nunavut, during both summer and winter. Char Lake freezes to the bottom around its edge, to approximately 2.5m thickness. Surface ice melts first from the edges of the lake, and when the littoral zone is ice-free water temperatures may reach up to 6°C. Deeper areas do not freeze solid in winter, but may never experience total loss of ice cover in summer.

Animals can be roughly categorized as either freezing tolerant, in which ice crystals form inside the body, or as freezing susceptible, in which ice formation is prevented through supercooling. The surrounding liquid water of aquatic habitats may cause tissue freezing by providing nucleating ice crystals at low temperatures, thus aquatic organisms may not be able to supercool, and must therefore either be freezing tolerant or migrate to nonfrozen areas.

The purpose of this study was fourfold. 1. to determine the temperature conditions of the winter benthos in Char Lake; 2. to determine if animals trapped in the zone of freezing can survive; 3. to determine if emigration to deeper, nonfrozen waters occurs; 4. to determine the effects of subzero temperatures on a focus species of harpacticoid copepod.

This paper provides an excellent description of the physical environment of Char Lake, which is located near the Polar Continental Shelf Project in Resolute, and has been intensively studied by many researchers since approximately 1960.

These authors used two distinct sampling methods in summer and winter. Summer sampling of the benthos was by a vacuum system operated by SCUBA divers. Winter sampling was by retrieval of frozen baskets previously embedded in the substrate during summer dives. Plastic chips may still be present in Char Lake from the marker floats used on these benthic baskets, as retrieval was by drilling into the thick ice until the underwater float was reached, followed by expansion of the hole and chiselling out the frozen basket.

Specimens were thawed and examined for survival, though logistical considerations meant that some samples were not as rigorously controlled. Temperature profiles of the lake indicated that the thick layer of ice and snow provided significant insulation, with frozen benthic temperatures never lower than -7.5°C, even with atmospheric temperatures below -20°C. Waters deeper than about 2.5m did not freeze.

The focus harpacticoid species, Attheyella nordenskioldii, persists in Char Lake as two possibly distinct populations. In deeper waters, this species reproduces throughout the year, though most individuals overwinter as adults. In shallower waters, adults may freeze solid for up to 8 months every year, with much more synchronous development than the deeper-water individuals. Both putative populations show 1-year life cycles. The authors explore some hypotheses that could explain how the winter-inactive individuals are able to maintain the same life cycle timing as the winter-active individuals. These hypotheses primarily rest upon energetic considerations, such that the shallow water populations have a summer advantage of temperature and thus metabolic and development rates that can make up for the winter disadvantage of those rates.

Animals trapped in the frozen zone were able to survive, and special overwintering stages were found only in one species of chironomid; all other examined species did not show obvious morphological characteristics associated with overwinter, though it is possible that physiological modifications such as dehydration were occurring. These authors could not separate induced from natural mortality, but were confident that all examined species were capable of high survivorship each winter. Additionally, no significant emigration to deeper, nonfrozen areas was detected, though some levels of such migration probably do occur.

The final part of the discussion of this paper focuses on the different strategies of A. nordenskioldii, and suggests further research on the energetic ecology of this species, this lake, and other seasonally frozen environments is warranted.