Herbivory of invertebrates on submerged macrophytes from Danish freshwaters

1. Invertebrate herbivory on submerged freshwater macrophytes, measured as per cent leaf area lost, was determined for sixteen species and forty-two populations of macrophytes during peak summer biomass in Danish streams and lakes. 2. All seventeen Potamogeton populations and seventeen of the remaining twenty-five non-Potamogeton populations were grazed. Species of Potamogeton were significantly more heavily grazed (mean 4.2%) than non-Potamogeton species (mean 0.8%). Herbivory losses were not significantly different between stream (mean 2.4%) and lake populations (mean 1.9%). Wide ranges in herbivory loss were observed between species from the same locality and within species from different localities. The location of main damage to either old or young leaves was not species specific but varied among localities. Additional data for four macrophyte populations showed that herbivory loss had a strong seasonal variation (e.g. 1.0–26.3% for Potamogeton perfoliatus), with maximum losses during May-June. 3. Although the mean defoliation percentages were low during the period of maximum macrophyte biomass, they were not systematically lower than encountered for terrestrial plants.