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The impact of grazing waterfowl on submerged macrophytes: In situ experiments in a shallow eutrophic lake

1996; Elsevier BV; Volume: 53; Issue: 1-2 Linguagem: Inglês

10.1016/0304-3770(95)01013-0

1879-1522

Martin Søndergaard, Lise Bruun, Torben L. Lauridsen, Erik Jeppesen, Tom Vindbæk Madsen,

Freshwater macroinvertebrate diversity and ecology

The impact of grazing waterfowl on submerged macrophytes was studied in shallow (mean depth 0.8 m), eutrophic Lake Stigsholm, Denmark (area 21 ha). Two types of experiment were conducted, small-scale exclosure experiments (1 m2 plots) and large-scale enclosure experiments (100 m2 plots). In both experiments, shoots of Potamogeton crispus L. were planted in densities ranging from 1 to 8 m−2. Potamogeton pectinatus L. and Callitriche hermaphroditica L. were also present. The herbivorous waterfowl community foraging in the lake comprised mainly coot (Fulica 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 the growing season, macrophyte biomass, shoot length, shoot number per square metre and shoot height became significantly greater in exclosures/enclosures protected against grazing by waterfowl than in unprotected exclosures/enclosures. The largest differences recorded were in total shoot length and number of shoots per square metre, the maximum values found in the protected exclosures being 18.0 m and 156 m−2, respectively, compared with 5.5 m and 53 m−2 in the unprotected exclosures. In addition, the percentage of stubble (number of shoots lacking an apex relative to the total number of shoots) was greatest in the unprotected 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 on submerged macrophytes, waterfowl may have a negative impact on lake water quality, particularly in lakes that are in the recovery phase and in which macrophytes are about to re-colonise following a reduction in external nutrient loading.