Changes in benthic animal production of a weir basin after eight years of succession

Benthic animal production was studied in a weir basin area of western Norway in 1984, eleven years after construction of the basin and eight years after the first benthic study. Compared to estimates from 1976, a marked change in production and species composition had occurred. Benthic animal production at two stations inside the basin was 199 and 169 KJ m−2 yr−1, an increase of 440 and 270 per cent respectively compared to 1976. In a riffle upstream of the basin, the production estimate was 50 KJ m−2 yr−1, a decrease of 40 per cent since 1976. Chironomids formed the main contribution to the increased production in the weir basin, mainly through increased abundance of larger species like Stictochironomus pictulus and Micropsectra spp. Also other species more adapted to lentic water, like the ephemeropterans Siphlonurus aestivalis and Ameletus inopinatus, showed increased density. The fauna succession and increased production in the weir basin was mainly due to improved habitats and food conditions, as the amount of organic sediments had increased since the first production study.