Stream periphyton response to grazing and changes in phosphorus concentration |
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Authors: | Sharon P. Walton Eugene B. Welch Richard R. Horner |
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Affiliation: | (1) Department of Civil Engineering, Environmental Engineering and Science Program, University of Washington, Seattle, Washington, 98195;(2) Present address: King County Department of Public Works, Surface Water Management Division, Seatle, WA 98104, USA |
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Abstract: | Grazing by the large caddisfly larva, Dicosmoecus gilvipes (Trichoptera; Limnephilidae), drastically reduced periphyton biomass in laboratory channels at a current velocity of 20 cm s–1. Reduction in biomass as chl a and AFDW ranged from 88 to 93% and 82 to 85%, respectively. On average, grazing rate increased with in-channel SRP (soluble reactive phosphorus) content from 6 to 10 µg 1–1. Grazing rates averaged 25.9–29.3 µg chl a m–2 d–1 and 10.8–12.2 µg chl a mg–1 d–1 based on area and grazer biomass, respectively, with most variability among treatments being due to the grazing effect. Grazing tended to shift the algal community increasingly to filamentous blue-green algae regardless of enrichment. After three weeks, Phormidium comprised over 61% of the community in grazed treatments but only 35% in ungrazed treatments. The stalked diatom Gomphonema comprised only 4% of the grazed community, but 11% in the three ungrazed channels with similar values for Scenedesmus. A model that includes grazing was calibrated to the data and produced a reasonable expectation of periphyton biomass over a range in SRP concentrations. While the model with constant grazer abundance predicts a gradually increasing grazed biomass as SRP increases, grazer production in natural streams may actually increase to accommodate the increased food production. |
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