Aquatic Invertebrates

Two features, perhaps more than any others, characterize the summer tundra and taiga: countless numbers of puddles, bogs, marshes, ponds, lakes and streams, and enormous populations of biting flies. These two phenomena are, of course, interrelated. Arctic aquatic insects serve as the basis for the food webs for more than 100 species of migratory birds. Predators take prey insects from the air or from the water or both. Alaska longspurs (Calcarius lapponicus) and snow buntings (Plectrophenax nivalis) eat plentifully of adult mosquitoes (Culicidae). Shorebirds, geese, and ducks prey upon caddisfly larvae (Trichoptera) and other aquatic insects.

Aquatic arthropods are of two basic types: those that are entirely aquatic throughout the life cycle, and those that are aquatic in an earlier immature stage or stages, but then become terrestrial as adults (even some of these still require close relations with water). These invertebrates generally follow one or two of three available strategies for survival through the winter. Those that overwinter as adults usually do so in some protected niche (such as under the loose bark of a tree, or under a log, or deep down in a mass of peat moss—the possibilities are endless) and/or under a thick layer of snow. Such hardy individuals are usually equipped with protective physiological adaptations that permit them to survive freezing down to a certain degree. Typical of this strategy is the snow mosquito, Culiseta alaskaensis, which makes its presence felt immediately after the first thaw of early spring in the Alaskan taiga. The females of overwintering arthropods lay their eggs in water (or on ground that will, in all probability, be flooded in spring) before freeze-up (in which case the egg is the overwintering stage—most species of northern mosquitoes fall into this category) or in the first available water of spring. Those aquatic invertebrates that remain aquatic throughout their lives and that occupy habitats that freeze solid in winter typically overwinter in the egg stage or some other cold-resistant stage (some larvae and nymphs may burrow into the substrate of the pond or stream and remain inactive there until warmer temperatures return). Finally, those aquatic invertebrates occupying habitats that are sufficiently deep that they do not freeze to the bottom may also overwinter as eggs or some comparable hardy stage or they may overwinter in an active stage (most probably with activity very greatly reduced), as caddis-fly and stonefly larvae often do.

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