Foodweb Response to Nutrient Enrichment in Rural and Urban Streams

ABSTRACT

Nutrient-diffusing substrates were used to examine nutrient limitation in benthic periphyton and invertebrate grazers, in two rural and three urban stream reaches. Substrates released either nitrate, phosphate, both nutrients, or no nutrients, and were colonized during a five week period. Periphyton biomass did not respond to nutrient addition at any of the sites, while the densities of chironomids and other collector-gatherers showed significant responses to nutrient manipulation. At both rural sites and one urban site, the density of collector-gatherers showed a positive response to phosphate addition. Despite a six-fold range of phosphate (from 5 to 36 μg/1) and relatively invariant nitrate levels (ca. 3 mg/l) across sites, average concentrations of dissolved nutrients in streamwater at baseflow did not predict the pattern of response to nutrient addition. The numerical response of invertebrates to nutrient amendment suggests that the invertebrate fauna of both urban and rural streams are effective in controlling algal biomass and that even at moderately high concentrations, nutrients can limit benthic food webs.