Secondary Production of Epilithic Fauna in a Woodland Stream

Abstract

Epilithic fauna were sampled by placing three sets of 40 cobbles in transects across a riffle in Factory Brook, Massachusetts. One set was sampled monthly from September 1973 to August 1975, allowing 3 months for recolonization. Three Surber samples were collected concurrently during the last 12 months to provide biomass estimates of infauna. Length-wet weight regression equations and dry weight conversion factors were computed for genera; caloric equivalents were obtained from the literature. Surface area of cobbles was determined by a foil-areagraph technique. Secondary production on cobbles was calculated mainly by genus with the modified Hynes method. Mean standing stock in dry weight (DW) was 0.65 gDW/m2 of cobble surface in 1974 and 0.61 gDW/m2 in 1975. Insect production on cobbles in 1974 was 4.80 gDW/m2-year and 4.32 gDW/m2-year in 1975, or between 22.3-24.7 kcal/m2-year. Approximately half of the mean biomass and production on cobbles was attributed to four genera, Epeorus, Ephemerella, Hydropsyche and Acroneuria. Dipterans constituted 56% of the organisms collected but less than 20% of production; mayflies represented 53% of epilithic production. Annual turnover ratios were 7.4 and 7.1 in 1974 and 1975, respectively, and are a first estimate of community-based P/B for stream benthos in New England. Trophic level production efficiency between primary and secondary consumer groups agreed closely with previous studies on benthic communities in streams and may provide a simplified method for estimating secondary production. Total benthic production on the riffle, based on Surber and cobble sampling, was approximately 12.2 gDW/m2-year.