RAPID RESPONSE OF MACROINVERTEBRATES TO DAM REMOVAL IN THE SANDUSKY RIVER, OHIO

The two-meter high St. Johns Dam in Seneca County, Ohio, in the Eastern Corn Belt Plains ecoregion was removed from the fifth-order Sandusky River in November 2003. The macroinvertebrate community was sampled prior to and following removal of the dam to determine the response of that community to free-flowing conditions in the 13-km reach of the former impoundment and to a potential increase in sedimentation downstream. Macroinvertebrates were collected at three sites in the impoundment in November 2003 and at those and four other sites in late summer 2004 using the standard quantitative and qualitative methods employed routinely by the Ohio EPA, and the data were converted to Invertebrate Community Index (ICI) scores.

The proportions of macroinvertebrates sensitive to degraded stream conditions were low at the three sites in the impoundment prior to dam removal but were similar to the proportions at the other four sites afterward. Similarly at those sites, the proportion of macroinvertebrates highly tolerant of degraded conditions declined markedly. On the basis of ICI scores, the macroinvertebrate community within the impoundment before dam removal was rated “Fair” at all three sites and thereby failed to meet the Warmwater Habitat (WWH) criterion for macroinvertebrates established by Ohio EPA as the goal for the Sandusky River. Ohio EPA had rated one of those sites as “Poor” in 2001. In summer 2004, nine months after dam removal, that site was rated “Fair”. Two sites immediately upstream of the former dam were rated “Good” (west side) and “Exceptional” (east side), and a fourth site in the former impoundment near its most upstream extent was rated “Good”. Thus, three of the four sites in the former impoundment met the WWH criterion. A site below the former dam and two sites upstream of the impoundment were rated “Good”. The combined results indicate that the biological quality of the Sandusky River in the formerly impounded reach improved rapidly in response to changes in habitat following removal of the dam.