Factors influencing algal consumption and feeding rate in Heterotrissocladius changi Saether and Polypedilum nebeculosum (Meigen) (Chironomidae: Diptera)

Summary Factors influencing the consumption of algae and the rate of feeding by the larvae of Heterotrissocladius changi Saether and Polypedilum nebeculosum (Meigen) (Chironomidae: Diptera) were determined from collections made between July 1975 and May 1977 in Yellowknife Bay, situated in the Canadian subarctic. Although both species fed heavily on algae during the summer, few cells were ingested in the winter, coincident with an overall reduction in feeding intensity. H. changi fed primarily on benthic species (Achnanthes minutissima, Achnanthes pinnata, Fragilaria pinnata, Fragilaria vaucheriae, Navicula spp., Scenedesmus spp.) Whereas P. nebeculosum was restricted to planktonic forms (Dinobryon spp., Scenedesmus spp.). The consumption of algae by H. changi depended largely on the density of the microflora in the environment. This factor had little influence on P. nebeculosum and was replaced by size selection, which prevented the ingestion of many planktonic algal species. The importance of algae to both chironomids also depended on the susceptability of certain algal forms to digestion. Temperature generally regulated the intensity of feeding of both H. changi and P. nebeculosum. However, changing day-length initiated heavy feeding during April and May, despite low water temperatures. The depth of water and the organic content of the substrate had no detectable effect on the amount of material in the guts.