EFFECTS OF NUTRIENT LOADING AND INSECTICIDE APPLICATION ON THE ECOLOGY OF ELODEA-DOMINATED FRESH-WATER MICROCOSMS .1. RESPONSES OF PLANKTONAND ZOOPLANKTIVOROUS INSECTS

Responses of phytoplankton, zooplankton and zooplanktivorous insects t o nutrient additions, and to the combination of nutrient loading and i nsecticide application, were studied in indoor microcosms intended to model drainage ditches. The nutrient additions alone did not result in a pronounced increase in phytoplankton chlorophyll-a. Grazing by zoop lankton and uptake of nutrients by macrophytes prevented phytoplankton from blooming. Effects of nutrient additions were predominantly obser ved in an increase in zooplankton abundance (Cladocera and Copepoda) f ollowed by an increase in numbers of the zooplanktivorous insect Plea minutissima. The application of nutrients in combination with the inse cticide chlorpyrifos, however, caused a more pronounced increase in th e abundance of phytoplankton and Rotatoria, most probably due to the d ecrease in arthropod grazers and Chaoborus larvae.