Invertebrate grazer‐periphyton interactions in a eutrophic marsh pond

SUMMARY. 1. The effects of two different invertebrate grazer guilds on the periphytic algal community were studied in a marsh pond in Manitoba, Canada over one entire open‐water season. 2. Microcrustaceans (cladocerans, copepods, ostracods), which were numerically dominant in the unperturbed grazer community, effectively reduced the periphytic algal biomass to a persistent, depauperate residual assemblage throughout the season. 3. The perturbed grazer community, enclosed and treated with ro‐tenone, was composed primarily of chironomids and oligochaetes. This community permitted a more diverse algal community to develop except when grazer numbers peaked. 4. When grazer numbers in the perturbed community were at a maximum, the algal assemblage appeared to be ‘overgrazed’. Algal biomass was reduced below the minimum characteristic of the unperturbed community. 5. There was an inverse relationship between algal biomass and grazer numbers in both perturbed and unperturbed communities, but variation in degree of response was much greater in the perturbed system than in the unperturbed one.