Abundance of stream invertebrates in winter: Seasonal changes and effects of river ice

The association between anchor ice and stream invertebrate density in the d rift and on the substrate were studied in the Grand River. Ontario, over tw o winters. Under certain climatic and geomorphological conditions, anchor i ce can form a thick blanket on the bottom of rivers and streams. There were almost thirty times more aquatic insects in the drift on mornings after an chor ice developed than when anchor ice did not occur. Both Diptera (mostly chironomids) and Trichoptera were more abundant in the drift when frazil s lush was present. During both winters the total density of insects in the d rift decreased as the seasons progressed, regardless of the presence or abs ence of anchor ice on the day of sampling. Within drift samples taken after anchor ice events this relationship was seen in the orders of Diptera and Trichoptera. There was also a decline in the total number of insects in the surface layer of substrate as the winter progressed, particularly in the f amilies of Chironomidae and Simuliidae. Anchor ice and frazil slush appear to be significant factors in invertebrate dispersal and their role in the w inter ecology of aquatic invertebrates deserves much more study.