Grazer-epilithon interactions in an Australian upland stream

Abstract

Effects of macroinvertebrate grazers on the distribution of their food resource, epilithon, were examined in a south-eastern Australian stream. The hypothesis that grazers would significantly alter the development of epilithon was tested experimentally: macroinvertebrates were excluded from some experimental substrata and allowed to colonize others. Epilithic chlorophyll a concentration, organic matter content and total diatom density were used to monitor the effects of the grazer assemblage over 35 days.

As predicted, epilithon density was higher on bricks with exclusion barriers than on bricks open to colonization by grazers. Similarly, diatom densities were significantly higher on bricks from the grazer-exclusion treatment. Patterns in the development of epilithon over time point to the importance of prevailing abiotic conditions in determining the outcome of macroinvertebrate grazing. Differences in total epilithon biomass, algal biomass and diatom density between treatments clearly indicate the independent importance of macroinvertebrate grazing to the microdistribution of epilithon in upland streams.