Macroinvertebrate community structure and production in a low-gradient stream in an undisturbed watershed

Macroinvertebrate community composition, abundance and production were meas ured in a sand-bottomed, headwater stream on the Coastal Plain physiographi c province in the southeastern U.S.A. The stream's watershed had experience d almost no anthropogenic disturbance for over 100 years and thus the strea m represented as close to pristine, reference conditions as occurs in this geographic region. Macroinvertebrates were sampled over one year in the thr ee dominant habitats in the stream: sand sediment, submerged wood and macro phytes (Sparganium americanum), Total taxa richness as well as the taxa ric hness of Ephemeroptera, Plecoptera, Trichoptera and Chironomidae all were g reater than in streams flowing through more disturbed watersheds in the are a. Annual mean habitat-specific density and biomass were highest in the sed iment; density was lowest on the wood and biomass lowest on Sparganium. Hab itat-specific production was 39-42 g m(-2) y(-1) in the sediment and on woo d and 16 g m(-2) y(-1) on Sparganium. The majority of production in all thr ee habitats was by Chironomidae, which comprised 80-92% of total production in each habitat. Taxa in the collector-gatherer and predator functional fe eding groups accounted for the majority of production in the sediment and o n wood, whereas filter-feeders were predominant on Sparganium. Whole-stream production, calculated by summing habitat-specific values that had been we ighted for habitat availability, was 64 m(-2) y(-1), considerably higher th an production in more disturbed streams in the region. About 65% of the tot al production occurred in the sediment, 26% on wood, and 9% on Sparganium. The tanypod chironomid Conchapelopia had the highest production of any taxo n in the stream at 19 g m(-2) y(-1). The production to biomass ratio for th e macroinvertebrate community was 33.3; ratios for six taxa of chironomids exceeded 100. Along with the higher species richness in this stream, produc tion of macroinvertebrates was at least twice as high as that in nearby str eams with more disturbed watersheds. These differences may be attributable to the long time since the last anthropogenic disturbance of the stream's w atershed, which has led to a mature forest covering nearly all of the water shed and which has resulted in a more stable stream flow and less disturban ce of the sediment during high flow than in streams in more disturbed water sheds.