Ecological responses of zoo-benthos to dam removal on the Kennebec River, Maine, USA
Casper, Andrew F.; Thorp, James H.; Davies, Susan R.; Courtemanch, David L.
The removal of dams is frequently promoted as a cost-effective tool for some forms of river rehabilitation, even though the scientific evidence supporting this approach is relatively rare. Our fifteen-month study assessed initial effects of removal of the Edwards Dam at three sites in the Kennebec River, Maine (USA) on zoobenthos, a critical component of a river's food web. Overall changes in both abundance and generic diversity were analysed in relation to feeding strategy. We also evaluated changes in trophic food webs by analysing stable isotope signatures of carbon and nitrogen for major functional feeding groups. The overall density of zoobenthos increased most dramatically (190 %) at the site closest to the former dam, but measures of taxonomic diversity showed variable responses. Neither generic richness nor evenness were significantly altered by removal of the dam. However, the composition of the community changed, with the addition of eight genera including caddisflies (Neureclipsis and Ithytrichia), mayflies (Tricorythodes and Stenonema), oligochaetes (Arcteonais), and various predatory (Ablabesmyia) and non-predatory chironomid midges. Differences in community similarity between a restored site near the former dam and a control, continuously free-flowing site diminished over time. Changes in stable isotope signatures suggested modification of feeding strategies and possible changes in the abundance of aquatic sources of organic matter. Community structure and ecosystem processes are likely to respond to dam removal in either a categorical (effect vs no effect) or a continuous (gradational) manner depending on the type of organism or process examined and the relationship between sizes of the dam and river.