Plecopteran communities and annual emergence in five forest streams and two lake outlet streams of central Finland

Abstract

The plecopteran fauna was studied by means of slit traps and hand net during the summers of 1982-1987. A total of 19 plecopteran species comprising c. 4 500 adults and c. 2 900 larvae were sampled in streams. Of these, Capnia atra, Capnopsis schilleri, Nemurella pictetii and Isoperla difformis were recorded for the first time in the streams of the province Pohjois-Häme (Tb). The plecopteran fauna was richest in the small forest streams and relatively poor in the largest outlet. Both, the character of the stream bed and the quantity and quality of food available seemed to be the most important factors affecting the occurrence of the plecopteran species. In the brooks the emergence of stoneflies had two clear maxima: one in spring and another in late autumn. Vernal species were Taeniopteryx nebulosa, Isoperla difformis, Nemoura avicularis, N. flexuosa and Leuctra hippo pus. Dominant species in the autumnal cathes were Leuctrafusca and Leuctra digitata. In the outlet streams the succession of emergence was somewhat different owing to the weak emergence and due to the cold summer with strong floods in 1987. The slit trap method worked well in the catching of stoneflies except during the snow melting seasons and the flood periods.