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USING THE STRESSOR IDENTIFICATION GUIDANCE TO IDENTIFY THE CAUSE OF DEGRADATION OF THE BENTHIC INVERTEBRATE COMMUNITY IN THE PRESUMPSCOT RIVER, MAINE
Citation:
Cormier, S M., S. Davies, P. Mitnik, AND J. Gerritsen. USING THE STRESSOR IDENTIFICATION GUIDANCE TO IDENTIFY THE CAUSE OF DEGRADATION OF THE BENTHIC INVERTEBRATE COMMUNITY IN THE PRESUMPSCOT RIVER, MAINE. Presented at North American Benthological Society, La Crosse, WI, June 3-8, 2001.
Impact/Purpose:
The purpose of this research project is to provide methods, tools and guidance to Regions, States and Tribes to support the TMDL program. This research will investigate new measurement methods and models to link stressors to biological responses and will use existing data and knowledge to develop strategies to determine the causes of biological impairment in rivers and streams. Research will be performed across multiple spatial scales, site, subwatershed, watershed, basin, ecoregion and regional/state.
Description:
From 1984 to 1996, an impounded segment of the Presumpscot River, downstream of a major point source, failed to attain Maine's Class C aquatic life standards based on benthic invertebrate assemblages. Eight candidate causes were hypothesized: 1) toxic chemicals; 2) floc resulting in low dissolved oxygen (DO); 3) settleable floc causing smothering; 4) excess nutrients; 5) impoundment increasing sedimentation; 6) impoundment decreasing flow velocity; 7) impoundment causing low DO; and 8) impoundment causing habitat loss. All causes with low DO were eliminated because concentrations were adequate for maintenance of aquatic life above and below the discharge. Total phosphorus was elevated below the discharge but chlorophyll a concentrations were negligibly increased. A strength of evidence analysis identified the cause from among the remaining four. Toxic chemicals were unlikely due to low concentrations. The two impoundment hypotheses were unlikely because other hydraulically similar impoundments attained aquatic life standards; and an upstream site within the same impoundment met aquatic life use criteria. Evidence favored the floc hypothesis. The exposure pathway was complete and plausible. Other rivers with similar elevated floc had impaired biological assemblages. In another river, the Androscoggin, reductions in the discharge reduced the settled floc and the biological assemblage recovered. Floc was identified as the cause.