Ecosystem processes and macroinvertebrate community structure were examined along the longitudinal gradient of Lapwai Creek, a fifth order stream impacted by agricultural nonpoint source pollution. The study ran from May 1988-November 1990, with samples taken seasonally to measure: input, storage, transport, and decomposition of particulate organic matter; chlorophyll a concentration; and macroinvertebrate community structure.; Input of particulate organic matter (POM) from riparian vegetation occurred August-November. Litterfall was lowest for riparian habitat composed of herbaceous vegetation followed by herbaceous/shrub mix, shrub, and deciduous tree habitats. Extensive removal and thinning of climax riparian vegetation and its subsequent replacement by successional species has resulted in annual litterfall rates less than that reported for comparable streams. In response to less litterfall, standing stock of stored POM and concentrations of transported POM were also very low throughout the stream. Rather than responding to increasing stream size, between-site differences in the input, storage, and transport of POM were due to location-specific characteristics of the riparian vegetation. As a result, detrital dynamics of Lapwai Creek did not follow the predictions of the river continuum model. Chlorophyll a concentrations, a measure of primary production, were high at all sites and during all seasons, relative to values reported for comparable Idaho streams. High concentrations of periphyton chlorophyll a can be attributed to increased availability of nutrients via runoff from cropland and increased light penetration with the removal of climax riparian vegetation. Nutrient concentrations for Lapwai Creek exceeded values reported for undisturbed streams and were similar to values reported for other agriculturally-impacted streams.; Macroinvertebrate community composition, as determined using random skewers analysis, did not change along a longitudinal gradient. Cluster analysis revealed that all locations, excluding a site below eutrophicated Winchester Lake, had very similar community composition. Functional feeding groups were dominated by grazers, indicating macroinvertebrate communities had responded to greater availability of algae and decreased availability of POM of terrestrial origin.; Anthropogenic activity, particularly actions related to agricultural practices, have had a profound effect on the ecosystem processes of Lapwai Creek. Rather than shifting from heterotrophic processes in the headwaters to autotrophic processes in the lower reaches, Lapwai Creek is autotrophic from headwaters to mouth. Input of nutrients of agricultural origin and the removal of climax vegetation are responsible for the autotrophic nature of Lapwai Creek and the associated, grazer dominated, macroinvertebrate community structure.
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