Investigations on the organic drift in North Swedish streams

Muller, K.

Organic drift was measured on the basis of catch per 24 hours per 1000 cm2 of net surface, using 1 mm. mesh nets. Plankton nets indicated that the 1 mm. nets under-estimated the actual drift. Even so the drift in a small forest stream, 2.8 sq. meters in cross section, was 6624 individuals or 64.4 grams per day in May. Drift was less later in the summer as water current decreased and as fewer insect larvae were vulnerable. Chironomid, Simuliid, Ephemeropteran and Tri-chopteran larvae were the predominate forms. Hydracarina, Coleoptera and Mollusca were present in small numbers in drift compared to in bottom fauna samples. The drift is maintained by the "colonization cycle". Adult insects fly to headwaters to lay eggs. Over abundance of larvae at headwaters results in dislodgement and spread of population in stream. Drift is a population-regulating factor. A bare area of stream was populated with 924 individuals per 1000 square centimeters in 11 days due to drift. This figure corresponded well to the drift estimated by sampling. Trout feed heavily on drift while grayling are primarily bottom feeders.