Freshwater lotic communities upstream and downstream of a low head dam before and after removal

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Leslie A. Riley, Garrett S. Duktig, Everett D. Meredith, Tyler T. Tanto, Kenneth J. Oswald and Robert G. Verb, School of Science, Technology, and Math, Ohio Northern University, Ada, OH, Zachary A. Bragg, Entomology, University of Kentucky, Lexington, KY

Background/Question/Methods
Low head dams can negatively impact freshwater lotic systems by interrupting stream community connectivity, trapping sediments and altering hydrologic patterns. Removal of low head dams can restore the natural flow regime in rivers, but the effects on stream communities have been mixed, limiting our understanding of ecosystem recovery post-removal. The purpose of this study is to document stream communities upstream and downstream of a low head dam prior to, and after, removal. The Allentown Dam, removed in August 2020, was located in the Ottawa River (Maumee River Watershed) in northwest Ohio. For two years prior to removal (September 2018, September 2019), upstream and downstream communities were monitored along with communities at two reference sites. Periphyton (N=24), macroinvertebrates (N=24), fish (N=4) and selected physical and chemical parameters (N=24) were collected over this two-year period. In September 2020, the same sites were resampled after dam removal and will be resampled again in September 2021. Using ANOVAs, we tested for differences in selected community indices (Shannon diversity, taxa richness, total biomass) and physical and chemical parameters. We also examined whether community structure differed significantly across sites using a Multi Response Permutation Procedure (MRPP).
Results/Conclusions
Preliminary results prior to dam removal indicate that community indices and physical and chemical parameters were similar upstream and downstream of the dam. In addition, the benthic riffle community (periphyton, macroinvertebrates) directly upstream of the dam was not significantly different from the downstream community, but one reference site was significantly different from both dam sites and the second reference site (MRPP: T = -4.33, Δ = 0.38, A = 0.25, p < 0.001). On the other hand, fish biomass and fish diversity were higher at the downstream dam site compared to upstream of the dam and predatory fish comprised a higher proportion of the community at the downstream site. Preliminary analyses suggest that this low head dam did not significantly impact the benthic riffle community (periphyton, macroinvertebrates), but did serve as a barrier for some larger, predatory fish moving upstream. Ongoing sample processing and analyses will allow for the inclusion of comparisons between the 2020 stream communities after dam removal to the previous two years.