Longitudinal changes in benthic organic matter and macroinvertebrates in apolluted Colorado plains stream

Longitudinal changes in benthic organic matter (BOM), physicochemical varia bles and macroinvertebrate community structure and functional organization were examined at six sites in the plains section of the Cache la Poudre Riv er, north central Colorado, U.S.A., along a gradient affected by urbanizati on 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 c obble-sized rocks was removed to measure chlorophyll a and periphytic bioma ss monthly from September, 1993 to August, 1994. BOM dynamics in the stream reach was a consequence of the hydrological flow regime, water chemistry a nd the growth patterns of macrophytes. The total BOM standing stocks attain ed maxima at sites 3 and 4 which were primarily impacted by agricultural ac tivities. The concentrations of periphyton chlorophyll a at downstream reac hes were two to six times higher that at the upstream reference site. The r esults from redundancy analysis (RDA) indicated that macroinvertebrate asse mblages exhibited the strongest relationship with the distribution and abun dance of aquatic vascular plants. The aquatic vascular plants supported a s ignificantly higher diversity of macroinvertebrates. The major roles of aqu atic vascular plants were to trap fine particulate organic matter and serve as spatial habitat structure for benthic macroinvertebrates. The spatial v ariations in the functional organization of macroinvertebrate communities r eflected the alterations of physicochemical characteristics and food resour ce availability in the stream ecosystem. Collector-gatherers formed the dom inant group at site 2 which were impacted by the effluents from wastewater treatment facilities. Conversely, the densities of scrapers and shredders w ere much reduced at the sampling site.