Effects of nutrient loading and insecticide application on the ecology of Elodea-dominated freshwater microcosms. II. Responses of macrophytes, periphyton and macroinvertebrate grazers.

In freshwater model ecosystems nutrient additions resulted in significant short-term increases in periphyton biomass. Effects of applications of both nutrients and the insecticide chlorpyrifos caused a larger and more prolonged increase in periphytic algae. After three to four weeks this bloom of periphytic algae was controlled by non-arthropod grazers, which increased in numbers after insecticide application. The study indicates that the top-down control of periphyton by grazers is an important regulatory mechanism in macrophyte-dominated aquatic ecosystems. Insecticides, by poisoning arthropod grazers, may indirectly stimulate periphytic algae, as long as resources and non-arthropod grazers permit this