Olsson 1981

Olsson TI. 1981. Overwintering of benthic macroinvertebrates in ice and frozen sediment in a North Swedish river. Holarctic Ecology 4: 161-166.

This author examined freezing tolerance and freezing resistance in some river-dwelling invertebrates in the Arctic. The study river is one of the few in northern Sweden that has not been dammed for hydroelectric purposes, allowing water levels to fluctuate through a wide range. Ice thickness in winter can exceed 50 cm, and the shallow littoral zone of the river freezes several centimetres into the sediment. Water level is lowest in winter, freezing sediments that are under as much as 4m of flowing water in summer. Spring thaw may occur bottom-to-top in shallow areas, as sunlight penetrates ice and heats underlying sediment, which thaws under a layer of ice; this slow thawing in sediments may be important for winter and spring survival of invertebrates and plants.

Ice and sediment cores taken from the river edge in winter included a range of frozen invertebrates. These animals were returned to the lab and allowed to thaw, to estimate winter survival. Most animals had very high survivorship; one major exception was the isopod Asellus aquaticus, found in a single aggregration of nearly 500 individuals, most of whom were dead upon thawing.

Winter survival was also estimated by freezing some animals in the lab, maintaining them frozen for several months, and thawing. Mechanical damage was inferred to be more severe in the lab than under field conditions as animals without shells or hard cases (e.g. gastropods, trichoptera larvae) such as oligochaetes suffered very high mortalities in the lab, but high survivorship in the field. This author is careful to note that lab freezing conditions included natural sediments and plants, as it has previously been shown that simple freezing of open water (e.g. in a bucket) is lethal to even the most cold-tolerant species, probably due to the mechanical damage incurred by expanding ice crystals that can be avoided by shelter among sediments or plant tissues.

Several cold and freezing putative adaptations were discovered, including the formation of epiphragms in some gastropods, a thin closure of the shell apeture not previously observed in aquatic snails, but common among dessication-resistant land snails. Some trichopteran larvae were found to have blocked their cases, though they were not pupal or prepupal. This blockage may have served to prevent ice formation and associated mechanical damage inside the cases. Some species were found in summer collections but were absent from frozen cores, including gammarid amphipods, suggesting winter migration to unfrozen deeper portions of the river.