Variability of macroinvertebrate community composition in an arctic and subarctic stream

Abstract

The macroinvertebrate community composition was compared in two Alaskan streams (USA) for numeric and species constancy during the ice-free period from 1981 to 1983. Imnavait Creek is a first order arctic stream (60° 39′ N, 149° 21′ W) draining upland tundra in the foothills of the Brooks Range. Caribou-Poker Creek is a 4th order subarctic stream (65° 08′ N, 147° 28′ W) draining the taiga forest north of Fairbanks, Alaska. The aquatic insect larvae and other macroinvertebrates were sampled with drift nets and Hess bottom samplers for four periods, each 1 week long in the ice free season of three years. We found 112 species in the arctic stream and 138 species in the subarctic stream in a chironomid-dominated community. In any sample period the communities contained 51–60 species in the arctic and 49–92 species in the subarctic. Between the four sample periods on average 39% and 50% of the species were present in two sequential samples in the arctic and subarctic stream, respectively. New immigrants, never before found in the system, averaged 37% and 31 % of the community, respectively. These systems are exposed to several intermediate disturbances: prolonged and variable freeze-up, extreme variation in discharge, wide diel and seasonal changes in temperature, and erosion by frazil and anchor ice. The dipterans that compose the most numerous and variable taxa must have variable diapause, ability to grow in cold waters, and good dispersal powers, even migrating across drainages in the arctic. Much of the seasonal dominance pattern appears therefore to be stochastic.