Grazing on epiphytes1

A correlation was found between the seasonal epiphyte cycle in Lake Memphremagog (Québec‐Vermont), with a maximum in mid‐June, and the abundance of grazers (mainly oligochaetes and chironomids). Evidence for a cause and effect relationship was provided by exclosure‐enclosure experiments. When grazers were excluded epiphyte biomass did not decline as it did in the lake. Addition of grazers to a dense epiphyte cover resulted in a decline qualitatively and quantitatively similar to that in the lake. Grazing pressure, calculated from general empirical equations for aquatic deposit feeders, was of the required order of magnitude to explain the decline in epiphyte biomass.Because similar seasonal epiphyte cycles have been described elsewhere these findings should have a wider relevance.