THE EFFECT OF FETCH ON PERIPHYTON SPATIAL VARIATION

The marked spatial variation in periphyton could reflect differences in exposure, grazing or substratum. To determine if any of these factors was significant, I studied the relationship between the degree of exposure to waves (measured as fetch), grazing intensity (measured as invertebrate biomass) and spatial variation in periphyton biomass in sites of similar substratum along an island in the central mesotrophic basin of Lake Memphremagog (Quebec). In spring, there was a positive relationship between fetch and periphyton biomass both on stones and on artificial substrata. This effect was especially strong for diatoms, for algae between 100-1000 mu-m3, for planktonic forms and for filamentous and long-stalked species. In spring, water renewal encouraged growth of these forms, but the effect disappeared in summer, coincident with silica depletion. Grazers, which were not important in the spatial variation observed in spring, probably contributed to the sudden decline of periphyton in the exposed sites in summer. Observations in nearby lakes indicate that these patterns may be general.