Dam removal results in extensive geomorphological changes, transforming a lentic ecosystem into a lotic system within the reservoir area. Physical habitat is an important factor in shaping aquatic communities. Following dam removal, existing biotic communities must adjust to reflect physical changes, ultimately forming a new benthic community structure. The ecological theories of habitat template and environmental filtering propose that a new community structure will consist of those organisms that are adapted to the altered environment. Following a dam removal in Pennsylvania, it was hypothesized that the existing macroinvertebrate community of Chironomidae and Oligochaeta, normally associated with lentic ecosystems, would change to a community structure of organisms affiliated with lotic ecosystems, requiring clean substrate and swift water current. Macroinvertebrate samples were taken pre and post dam removal using a lightweight Ponar grab sampler in the deeper reservoir or pools and a 12” Surber sampler was used at wadeable sample locations. Unmanipulated reference sites were sampled approximately 3 miles upstream and 3 miles downstream of the dam site. These sites were selected based on the minor impact received from the dam presence or removal.

Results/Conclusions

Dam removal resulted in the movement of the accumulated sediment within the previous reservoir area. The increased flow resulted in the emergence of two major riffles within the prior dam reservoir, comparable to the riffle/pool sequences found at the upstream and downstream reference sites. Prior to dam removal, dominant organisms found in the reservoir were Chironomids and Oligochaetes. These organisms are well adapted to the oxygen deficient habitat created by the still water in the reservoir, and require soft sediments for burrowing. Following dam removal, the increased current resulted in flushing of fine sediments downstream, eliminating an important habitat requirement of these organisms. Increased current created higher oxygen levels and exposed cobble substrate, providing suitable conditions for organisms associated with lotic ecosystems. Within two months of dam removal, sampling determined that the macroinvertebrate community of the former reservoir area had diversified to include species of netspinning caddisflies and mayflies. These organisms require higher levels of dissolved oxygen and a cobble/gravel substrate with open interstitial spaces. Additional data from sampling performed at a later date confirmed changes from a strictly lentic benthic community to a more diversified community of both lotic and lentic organisms, supporting the theories of habitat template and environmental filtering.