AbstractPopulations ofA. aquaticuswere sampled quantitatively in 1979–82 at two localities in the river Suså which has a slope of<1 m km−1and large variations in macrophyte biomass, discharge and stream velocity. The latter two differed significantly between years.A. aquaticushad (1)−2 life cycles per year and summer and winter cohort production intervals of 85 and 290 days. Populations ofA. aquaticusvaried between 0 and 28,000 ind. m−2with an exponential increase in the spring and an exponential decrease in the autumn‐winter. TheA. aquaticusrate of decrease varied between 0.62 and 5.61 % d−1and increased with increasing rate of elimination of the macrophyte biomass Differences between the two localities were due to differences in physical heterogeneity. Production varied between 2.8 and 9.1 g DW m−2yr−1and between 12.9 and 55 g DW m−2yr−1at the two localities andP/Bratios were 7.4–9.9 yr−1. Physical limitations are thought to be most important for the populations ofA. aquaticusin the Suså, and the macrophyte biomass played an important role in modifying the physical environment. Differences between streams and lake