Longitudinal zonation and food of larval chironomids (Insecta: Diptera) along the course of a river in temperate Canada

The chironomid larvae of the Rouge River, Ontario, Canada, showed a longitudinal zonation which may have been influenced by water temperature and substrate composition. Species of Orthocladiinae predominated in the cooler headwaters while species of Chironomini dominated the comparatively warmer water near the estuary. In the middle section the chironomid assemblage was of a more mixed nature. Differences occurred in both food availability and feeding between sites along the river. Within genera, there were differences in the proportions of major food items between the larval guts and the available food material sampled from the same area. In most cases (e.g. Cryptochironomus, Polypedilum and Orthocladius ) larvae selected detritus over diatoms. Some genera (e.g. Microtendipes ) changed the proportions of food items ingested seasonally, while others (e.g. Psectrocladius ) exhibited a more stable diet throughout the year. Second instar larvae appeared to seasonally adjust their diet most often (e.g. Cricotopus. Tanytarsus and Eukiefferiella ), however these changes were not just in the types of food eaten, but also in the proportions of food types consumed. Larvae of all genera continued feeding throughout the cold water conditions (<5°C) of winter. Incidence of predation was very low, for example <10% in the Tanypodinae. In the laboratory, species of Chironomini ingested and assimulated, to varying degress, isolated populations of diatoms, detritus and bacteria.