Longitudinal changes in benthic organic matter and macroinvertebrates in a polluted Colorado plains stream

Abstract

Longitudinal changes in benthic organic matter (BOM), physicochemical variables and macroinvertebrate community structure and functional organization were examined at six sites in the plains section of the Cache la Poudre River, north central Colorado, U.S.A., along a gradient affected by urbanization and agricultural activities. A corer (314 cm{2}) was used to collect BOM and macroinvertebrates and a defined area (11.4 cm{2}) of epilithon from cobble-sized rocks was removed to measure chlorophyll a and periphytic biomass monthly from September, 1993 to August, 1994. BOM dynamics in the stream reach was a consequence of the hydrological flow regime, water chemistry and the growth patterns of macrophytes. The total BOM standing stocks attained maxima at sites 3 and 4 which were primarily impacted by agricultural activities. The concentrations of periphyton chlorophyll a at downstream reaches were two to six times higher that at the upstream reference site. The results from redundancy analysis (RDA) indicated that macroinvertebrate assemblages exhibited the strongest relationship with the distribution and abundance of aquatic vascular plants. The aquatic vascular plants supported a significantly higher diversity of macroinvertebrates. The major roles of aquatic vascular plants were to trap fine particulate organic matter and serve as spatial habitat structure for benthic macroinvertebrates. The spatial variations in the functional organization of macroinvertebrate communities reflected the alterations of physicochemical characteristics and food resource availability in the stream ecosystem. Collector-gatherers formed the dominant group at site 2 which were impacted by the effluents from wastewater treatment facilities. Conversely, the densities of scrapers and shredders were much reduced at the sampling site.