HYPORHEIC DISTRIBUTION OF LARVAL MIDGE BIOMASS (DIPTERA: CHIRONOMIDAE) IN AN ALABAMA COASTAL PLAIN STREAM: EFFECTS OF SEASON AND HABITAT TYPE.

S.K. Reynolds, Jr. and A.C. Benke. Department of Biological Sciences, University of Alabama, Tuscaloosa, Alabama, USA 35487-0206

Chironomids were sampled to a depth of 50 cm twice monthly over an annual cycle from 4 sites (2 wetland, 2 stream run) in a third order Coastal Plain stream in west-central Alabama. Larval biomass was calculated from length-to-biomass regression equations and these data were analyzed for relationship with depth and season at the family level. Average total biomass values not statistically different between wetland and run habitats, but habitat type had a significant impact on biomass distribution within the hyporheic zone (p < 0.001). When the sites were run separately by habitat type, depth and season were significant predictors of total chironomid biomass (p < 0.05 for all analyses). Eighty percent of biomass was found in the first 10 cm and none below 20 cm in the wetland sites. In contrast, in the run sites 42% of biomass was found in the first 10 cm and 29, 17 and 11% in each 10cm depth interval. The highest biomass values were found in the spring and lowest in the fall or winter. These data suggest that in favorable habitats that the hyporheic zone can harbor a significant proportion of midge biomass relative to the surface sediments.