Longitudinal changes in macroinvertebrate production in a stream affected by urban and agricultural activities

Benthic samples were collected during a one-year period (September 1993-August 1994) to examine macroinvertebrate production at 6 sites along the plains section of the Cache la Poudre River, Colorado, impacted by urban and agricultural activities. The site least impacted by human activities possessed the lowest secondary production (26 g AFDM m(-2) y(-1)). The site enriched by treated effluents from wastewater treatment facilities and characterized by high inorganic nutrients possessed the highest secondary production (62 g AFDM m(-2) y(-1)). The sites impacted by agricultural activities and characterized by high values of hardness and conductivity and high standing stocks of aquatic vascular plants have intermediate secondary production levels (32-52g AFDM m(-2) y(-1)). Non-Tanypodinae Chironomidae were the most important contributors to the macroinvertebrate production at the 6 sites. Conductivity, hardness, and aquatic vascular plants appeared to play major roles in determining levels of production and functional group composition in the stream reach. Community P/B ratios reflected changes in species composition and environmental conditions at the 6 sites. Smaller and faster growing taxa (e.g., Chironomidae and Simulium spp.) resulted in higher community P/B ratios and production at the sites impacted by urban and agricultural activities.