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

Abstract

The association between anchor ice and stream invertebrate density in the drift and on the substrate were studied in the Grand River, Ontario, over two winters. Under certain climatic and geomorphological conditions, anchor ice 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 anchor ice developed than when anchor ice did not occur. Both Diptera (mostly chironomids) and Trichoptera were more abundant in the drift when frazil slush was present. During both winters the total density of insects in the drift decreased as the seasons progressed, regardless of the presence or absence 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 families of Chironomidae and Simuliidae. Anchor ice and frazil slush appear to be significant factors in invertebrate dispersal and their role in the winter ecology of aquatic invertebrates deserves much more study.