VARIABILITY OF INVERTEBRATE HERBIVORY ON THE SUBMERGED MACROPHYTE POTAMOGETON PERFOLIATUS

1. Invertebrate herbivory was studied in twenty-eight populations of t he submerged macrophyte Potamogeton perfoliatus in Danish streams and lakes in mid-June. All populations but one experienced invertebrate he rbivory and loss ranged from 0 to 11.9% of leaf area among populations . Loss generally increased with leaf age towards the base of the plant s, and young apical leaves were rarely damaged. 2. Herbivory loss was significantly higher in streams (mean 5.0%) than in lakes (mean 2.2%), but varied greatly among populations within the same stream or lake a nd was not correlated to physico-chemical site characteristics, size o r density of plant population, or leaf N and P content. High levels of invertebrate herbivory were therefore not associated with certain typ es of streams or lakes. 3. High herbivore biomass relative to abundanc e of plants was conducive to high loss. In streams, the biomass of the trichopteran Anabolia nervosa accounted for 50% of the variability in loss. No single species appeared to be equally important in lakes, al though loss was correlated to the biomass of the chrysomelid beetle Ma croplea appendiculata. Obligate herbivores, such as lepidopteran larva e, apparently exerted little damage on P. perfoliatus, and leaf mining and channelization from specialist feeders were negligible. It is con cluded that shredders acting as facultative herbivores were the most i mportant invertebrate herbivores on P. perfoliatus in Danish freshwate rs.