EFFECTS OF NUTRIENT LOADING AND INSECTICIDE APPLICATION ON THE ECOLOGY OF ELODEA-DOMINATED FRESH-WATER MICROCOSMS .3. RESPONSES OF MACROINVERTEBRATE DETRITIVORES, BREAKDOWN OF PLANT LITTER, AND FINAL CONCLUSIONS

The effects of nutrient loading, and a combination of nutrient loading and insecticide application, on aspects of decomposition, densities o f detritivorous macro-invertebrates and dissolved oxygen concentration s were studied in indoor microcosms intended to model drainage ditches . Nutrient additions hardly affected the breakdown of particulate orga nic matter (POM) in litter bags and only minor increases in the abunda nce of detritivorous macroinvertebrates were observed. Dissolved oxyge n concentrations, however, increased for a prolonged period after the addition of a second high dose of nutrients. Application of the insect icide chlorpyrifos in nutrient enriched cosms caused an elimination of Gammarus pulex, and a collapse of the populations of Asellus aquaticu s and Proasellus meridianus (primary effect). This Loss of shredder po pulations re suited in a decreased decomposition of POM in litter bags . Decomposition rates of POM rose again later in insecticide treated c osms, and were even the highest at the end of the experiment, probably due to the recovery of Proasellus meridianus and an increase in the n umbers of the oligochaete Lumbriculus variegatus. Another, probably se condary, effect of chlorpyrifos application observed in the microcosms was a significant decrease in the oligochaete Dero digitata, which is explained by increased predation. Finally, a synthesis is given of th e overall impact of nutrient loading and insecticide application on th e structure and function of the microcosms by incorporating the conclu sions of the present and two preceding papers.