The effects of weed cutting upon the biota of a large regulated river

Abstract

1. The effects of macrophyte removal from a regulated reach of the River Great Ouse, UK, were recorded. The relationship between macrophyte cover, zooplankton distribution and 0+roach Rutilus rutilus (L.) distribution, diet and growth, were compared before, directly after, and over several weeks following weed-cutting operations. 2. Both zooplankton and fish were significantly associated with the macrophyte zone before weed cutting. It is suggested that this zone provides high food densities and refuge during periods of elevated flows. 3. Removal of all but a 2 m marginal strip of the macrophyte zone led to a rapid decline in the mean densities of planktonic Cladocera, probably as a result of increased washout, fish predation and starvation. This was accompanied by a rapid decline in the growth rate of the roach, which were forced to feed on the less nutritious aufwuchs. 4. It is suggested that river management involving weed cutting adjacent to one bank in alternate years would be sufficient to prevent loss of channel capacity and would provide a refuge for fish and zooplankton.