Effects of inorganic sedimentation and riparian clearing on benthic community metabolism in an agriculturally-disturbed stream

An in situ chamber technique was used to obtain seasonal estimates of benthic community metabolism at three stations in an agriculturally disturbed stream. Two stations with open canopies were examined. Sand was the dominant substrate at one site, cobble at the other. The third station was shaded by riparian vegetation and had a sand substrate. Seasonal estimates of net community productivity (NCP) and community respiration (CR) at the cobble section were significantly higher than those calculated for the sand sections (p > 0.05). Ratios of gross community productivity (GCP) to 24 h respiration indicated autotrophic conditions in the cobble and extreme heterotrophy in the sand. NCP was significantly higher (p < 0.05) in the open canopy sand than in the riparian shaded sand only when turbidity and discharge were low. Measurements of periphyton ash-free dry mass (AFDM) and chlorophyll a support metabolism estimates. Measurements of loose detrital AFDM were very low and variable compared to others reported in the literature. Therefore, allochthonously derived detritus may not be an important energy source for the benthic community.