Effects of Disturbance on Benthic Functional Structure and Production in Mountain Streams

Abstract

To assess the role of macrofauna consumers in organic matter dynamics of headwater streams, we applied seasonal insecticide treatments to a southern Appalachian Mountain stream and compared benthic community structure between this and two other streams. Production was estimated in the two major habitats: mixed cobble-gravel-sand substrate and bedrock outcrop. Using the proportional availability of the habitats in each stream, production over the entire stream was measured. Annual habitat-weighted production in the untreated streams during the two study years ranged from 8.9 to 14.0 g AFDM m^-2 yr^-1, 81 to 86% of which was attributed to insects. Habitat-weighted production was distributed rather evenly among the collector-gatherer (32-41%), shredder (23-31%), and predator (28-33%) functional groups. Collector-filterer and scraper groups accounted for less than 9% of total habitat-weighted production in all three streams. Insecticide applications resulted in dramatic changes in the macrofaunal community. Annual habitat-weighted production (4.4 g m^-2 yr^-1) in the treated stream decreased by 62% from Yr 1 to Yr 2, with insects contributing only 45% to total production. Oligochaetes, several Diptera, dragonflies, and copepods composed most of the production during treatment. Although habitat-weighted production of all functional groups was lower in the treatment year than in the pre-treatment year, collector-gatherer production decreased the least (21% reduction) and dominated overall production during treatment. Production of the other functional groups decreased by 71-94% between the two years. The changes in macrofaunal community structure and production observed in the insecticide treated stream were much greater than between-year changes in the reference stream, despite the occurrence of a record drought in Yr 2. The effect of the two different disturbances on overall production differed not only in magnitude but also in direction, with production in the reference stream increasing (12% over Yr 1) during the drought year. Resource ingestion was estimated using values of functional group production. Estimates of ingestion suggested that the major functional groups were ingesting a fairly large portion of available resources in the untreated streams. Collector-gatherers consumed 34-64% of total annual habitat-weighted fine particulate organic matter. Shredders consumed an estimated 28-47% of average annual standing crop of leaves, and predators ingested 67-79% of total macrofaunal production. Insecticide treatments resulted in much lower levels of resource ingestion: 26%, 4%, and 50% for collector-gatherers, shredders, and predators, respectively. Our results show the importance of using estimates of production when assessing disturbance effects on macrofaunal communities, since abundances greatly underestimated the impact of insecticide treatments. Biomass estimates, though providing better measures of disturbance than abundances, tended to overestimate overall macrofaunal losses because of the disproportionate influence of large, slow growing taxa such as crayfish.