Invertebrate drift in a glacial river and its non-glacial tributary

1. Invertebrate drift was studied in a glacially fed river and a non-glacial tributary in western Norway. Samples were taken during two consecutive 24-h periods in May, July and October 1997. The 3 months are characterized by snowmelt, ice melt and rainfall runoff, respectively. The main glacial river has colder, more turbid water, especially during the period of maximum ice melt during summer. 2. Chironomidae, especially the genus Diamesa, dominated the drift in the main river in May and October, constituting 97 and 99% of total numbers, respectively. Simuliidae, Plecoptera, Ephemeroptera and Trichoptera. were the other main components. 3. A comparison of drift and benthos data revealed that the tributary was of little significance for colonization of the main glacial river. Only some additional species in very low numbers were recorded downstream of the confluence. 4. During July significant differences in diel drift pattern of Chironomidae and Simuliidae existed between the glacial and non-glacial reaches. There was a mid-day peak independent of discharge in the glacial river, but this peak was not noted in the tributary. Species of the genus Diamesa appear to be adapted for daytime drift, possibly evolved through the absence of predators and competitors that are typical of rhithral systems where nocturnal drift is more usual.