GRAZER-EPILITHON INTERACTIONS IN AN AUSTRALIAN UPLAND STREAM

Effects of macroinvertebrate grazers on the distribution of their food resource, epilithon, were examined in a south-eastern Australian stre am. The hypothesis that grazers would significantly alter the developm ent of epilithon was tested experimentally: macroinvertebrates were ex cluded from some experimental substrata and allowed to colonize others . Epilithic chlorophyll a concentration, organic matter content and to tal diatom density were used to monitor the effects of the grazer asse mblage over 35 days. As predicted, epilithon density was higher on bri cks with exclusion barriers than on bricks open to colonization by gra zers. Similarly, diatom densities were significantly higher on bricks from the grazer-exclusion treatment. Patterns in the development of ep ilithon over time point to the importance of prevailing abiotic condit ions in determining the outcome of macroinvertebrate grazing. Differen ces in total epilithon biomass, algal biomass and diatom density betwe en treatments clearly indicate the independent importance of macroinve rtebrate grazing to the microdistribution of epilithon in upland strea ms.