EFFECTS OF NUTRIENT LOADING AND INSECTICIDE APPLICATION ON THE ECOLOGY OF ELODEA-DOMINATED FRESH-WATER MICROCOSMS .2. RESPONSES OF MACROPHYTES, PERIPHYTON AND MACROINVERTEBRATE GRAZERS

Responses of macrophytes, periphyton and macroinvertebrate grazers to nutrient additions, and to the combination of nutrient loading and ins ecticide application, were studied in indoor microcosms intended to mo del drainage ditches. Nutrient additions resulted in significant incre ases in biomass of macrophytes and in stocks of nitrogen and phosphoru s stored by them. Only short-term increases in periphyton biomass were observed. Effects of the application of both nutrients and the insect icide chlorpyrifos caused a larger and more prolonged increase in peri phytic algae on Elodea due to the loss of arthropod grazers. After 3 t o 4 weeks this bloom of periphytic algae was controlled by the non-art hropod grazers Stylaria lacustris and Lymnaea stagnalis, both increasi ng in numbers after insecticide application. Our study indicates that, even in nutrient enriched systems, the top-down control of periphyton by grazers is an important regulatory mechanism in macrophyte-dominat ed aquatic ecosystems. Insecticides, by poisoning arthropod grazers, m ay indirectly stimulate periphytic algae, as long as resources and non -arthropod grazers permit this.