DISTURBANCE BY AQUATIC PLANT MANAGEMENT IN STREAMS - EFFECTS ON BENTHIC INVERTEBRATES

The effect of aquatic plant removal on benthic invertebrates and their habitat was studied in two macrophyte-rich streams of the Swiss Plate au. In each stream, habitat conditions (macrophyte biomass, current ve locity, water depth) and invertebrate densities were monitored in a co ntrol reach and in a reach where plants were removed by cutting. Biolo gical samples were taken and physical parameters measured on three dat es before and six dates after plant removal in both reaches. Responses to plant removal were similar in both streams; macrophyte cutting ini tially decreased mean plant biomass (ca. 85%) and total number of inve rtebrates (ca. 65%). Variation between replicates was, however, higher in one of the streams, causing fewer effects on plants and invertebra tes to be statistically significant. Plant cutting affected mainly tax a that used macrophytes as habitat (e.g. Simuliidae, Chironomidae), wh ereas highly mobile taxa (e.g. Ephemeroptera) and taxa living on or wi thin the bed sediments (e.g. Trichoptera, Bivalvia) were less affected . Taxa that decreased after plant removal recovered within 4-6 months, although recovery of macrophytes was quite different in both streams. Invertebrate recovery also seemed to be seasonally dependent, with cu tting having a less severe impact during summer than spring. Our resul ts suggest that macrophytes in streams should be removed only in summe r, preferably leaving some plant beds to act as refugia for phytophilo us invertebrates. (C) 1998 John Wiley & Sons, Ltd.