Responses of Invertebrate Drift to Streamside Brush Removal

Abstract

Invertebrate drift, a principle food of trout, was sampled in the Little Plover River once a month from June 1976 through August 1977 to determine if removal of woody streamside vegetation increased drift number and biomass, and altered taxonomic composition. Simultaneous drift net samples of invertebrates and detritus were collected in a Meadow Zone, two Brushy Zones, and a Treatment Zone where the streamside brush was removed from both stream banks in spring 1973. Results were compared between zones and to pretreatment drift and benthos studies conducted in 1972-73. Brush removal was initiated to improve trout habitat and the sport fishery. Numbers and biomass of drifting aquatic invertebrates in the Treatment Zone were not significantly different from drift in the Upper and Lower Brushy Zones in 1976-77, four years after brush removal, or from drift in the Treatment Zone in 1972, before removal. Benefits of brush removal to invertebrate growth and reproduction from increased area of productive substrate (aquatic vegetation) in the Treatment Zone may have been nullified by unusually high water temperatures, low stream flow, and thick ice. Numbers and biomass of aquatic drift in the Meadow Zone were significantly higher than in the other zones in 1976-77 as well as in 1972. Greater abundance of aquatic drift in the Meadow Zone was probably related to greater amounts of productive substrates (aquatic vegetation and gravel) and a richer detritus food base than in the other zones. There was no significant difference in total drift between zones. Drifting detritus in the Treatment Zone was significantly greater than in the Lower Brushy Zone and similar to detritus. in the Meadow and Upper Brushy Zones. Mean monthly drift per 100m^3 of discharge in the Meadow Zone, Treatment Zone, Upper Brushy Zone, and Lower Brushy Zone were: total numbers, 814, 677, 327, and 798 organisms; aquatic numbers, 660, 192, 153, 146 organisms; aquatic biomass, including molluscs, 9.45, 2.12, 1.30, and 3.91 grams, and without molluscs, 2.13, 0.43, 0.31, and 0.30 grams; detritus weight, 15.6, 8.3, 11.0, and 5.5 g dry wt/100m^3. Aquatic drift comprised 44.3% of the number and 96.0% of the biomass of the total invertebrate drift in all zones combined. Terrestrial drift was much more abundant in the Treatment and Lower Brushy Zones than in the Meadow and Upper Brushy Zones. Only in the Lower Brushy Zone were aquatic numbers significantly higher in 1976-77 than in the pretreatment drift study in 1972, however, terrestrial and total drift were significantly greater in all zones in 1976-77. There was no significant difference in drifting detritus between studies. The monthly mean number of drifting taxa in the Treatment Zone (29} was significantly greater than in the Upper Brushy Zone (19}, similar to that in the Lower Brushy Zone (30}, and less than that in the Meadow Zone (37). All zones except the Upper Brushy Zone had significantly higher numbers of taxa in 1976-77 than in 1972. Gammarus spp. decreased in importance from 1972 to 1976-77, whereas Lymnaea spp. and Chironomidae increased, probably due to higher water temperatures.