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

Macroinvertebrate community composition, abundance and production were measured in a sand-bottomed, headwater stream on the Coastal Plain physiographic province in the southeastern U.S.A. The stream's watershed had experienced almost no anthropogenic disturbance for over 100 years and thus the stream represented as close to pristine, reference conditions as occurs in this geographic region. Macroinvertebrates were sampled over one year in the three dominant habitats in the stream: sand sediment, submerged wood and macrophytes (Sparganium americanum). Total taxa richness as well as the taxa richness of Ephemeroptera, Plecoptera, Trichoptera and Chironomidae all were greater than in streams flowing through more disturbed watersheds in the area. Annual mean habitat-specific density and biomass were highest in the sediment; density was lowest on the wood and biomass lowest on Sparganium. Habitat-specific production was 39-42 g m -2 y -1 in the sediment and on wood and 16 g m -2 y -1 on Sparganium. The majority of production in all three habitats was by Chironomidae, which comprised 80-92% of total production in each habitat. Taxa in the collector-gatherer and predator functional feeding groups accounted for the majority of production in the sediment and on wood, whereas filter-feeders were predominant on Sparganium. Whole-stream production, calculated by summing habitat-specific values that had been weighted for habitat availability, was 64 g m -2 y -1 , considerably higher than production in more disturbed streams in the region. About 65 % of the total production occurred in the sediment, 26 % on wood, and 9 % on Sparganium. The tanypod chironomid Conchapelopia had the highest production of any taxon in the stream at 19 g m -2 y -1 . The production to biomass ratio for the macroinvertebrate community was 33.3; ratios for six taxa of chironomids exceeded 100. Along with the higher species richness in this stream, production of macroinvertebrates was at least twice as high as that in nearby streams with more disturbed watersheds. These differences may be attributable to the long time since the last anthropogenic disturbance of the stream's watershed, which has led to a mature forest covering nearly all of the watershed and which has resulted in a more stable stream flow and less disturbance of the sediment during high flow than in streams in more disturbed watersheds.