THE IMPACT OF GRAZING WATERFOWL ON SUBMERGED MACROPHYTES - IN-SITU EXPERIMENTS IN A SHALLOW EUTROPHIC LAKE

The impact of grazing waterfowl on submerged macrophytes was studied i n shallow (mean depth 0.8 m), eutrophic Lake Stigsholm, Denmark (area 21 ha). Two types of experiment were conducted, small-scale exclosure experiments(1 m(2) plots) and large-scale enclosure experiments (100 m (2) plots). In both experiments, shoots of Potamogeton crispus L. were planted in densities ranging from 1 to 8 m(-2). Potamogeton pectinatu s L, and Callitriche hermaphroditica L. were also present. The herbivo rous waterfowl community foraging in the lake comprised mainly coot (F ulica atra L.), densities ranging from 0 to 9 individuals ha(-1), and a few mute swans (Cygnus olor Gmel.), 0.2 individuals ha(-1). During t he growing season, macrophyte biomass, shoot length, shoot number per square metre and shoot height became significantly greater in exclosur es/enclosures protected against grazing by waterfowl than in unprotect ed exclosures/enclosures. The largest differences recorded were in tot al shoot length and number of shoots per square metre, the maximum val ues found in the protected exclosures being 18.0 m and 156 m(-2), resp ectively, compared with 5.5 m and 53 m(-2) in the unprotected exclosur es. In addition, the percentage of stubble (number of shoots lacking a n apex relative to the total number of shoots) was greatest in the unp rotected exclosures, with a maximum summer percentage of 25% compared with only 12% in the protected exclosures. The study provides further evidence that waterfowl may suppress macrophyte biomass in lakes with a low abundance of submerged macrophytes. We predict that by grazing o n submerged macrophytes, waterfowl may have a negative impact on lake water quality, particularly in lakes that are in the recovery phase an d in which macrophytes are about to re-colonise following a reduction in external nutrient loading.