Pesticide Manipulation of a Headwater Stream: Invertebrate Responses and Their Significance for Ecosystem Processes

Abstract

The influence of macroinvertebrates on detrital processing was evaluated by excluding them from one of two small southern Appalachian streams. Exclusion in the treated stream was accomplished by periodic applications of 10 ppm of the insecticide methoxychlor. This caused massive invertebrate drift (>12,000 organisms/m3 of discharge) during the initial treatment and reduced aquatic insect densities and biomass to <10% of the levels within the adjacent untreated reference stream. Community structure in the treated stream shifted from a system dominated by small numbers of large shredding insects (e.g., Peltoperla, Pycnopsyche, Tipula) with comparatively low reproductive rates, to one dominated by large numbers of small collector-gatherers and predators (e.g., Oligochaeta, Chironomidae, Turbellaria) with high reproductive rates. Non-insect invertebrate biomass and density became significantly higher in the treated stream than in the reference stream following initial methoxychlor treatment. We interpreted th...