Effects of removal of a small dam on downstream macroinvertebrate and algal assemblages in a Pennsylvania stream

  • Published source details Thomson J.R., Hart D.D., Charles D.F., Nightengale T.L. & Winter D.M. (2005) Effects of removal of a small dam on downstream macroinvertebrate and algal assemblages in a Pennsylvania stream. Journal of the North American Benthological Society, 24, 192-207.


Action: Remove dams or weirs

A before-and-after, site comparison study in 2000–2001 of an urban stream in Pennsylvania, USA (Thomson et al. 2005) found that removing a dam had negative effects on downstream algal abundance and species richness. Downstream of the dam, the abundance of algae growing on rocks declined from 187–200 mg chl a/m2 before the dam was completely removed to 56 mg chl a/m2 6–12 months after full dam removal. Diatom species richness also declined, from 43–46 species/sample to 38 species/sample. Upstream of the dam, there was a decline in algal abundance, but of significantly smaller magnitude than downstream (before: 97–144; after: 67 mg chl a/m2). Diatom species richness increased over time, from 35 species/sample before any intervention, to 44 species/sample after partial dam removal and 40 species/sample after full dam removal. Methods: Between August and November 2000, a 200-year-old, 2-m-high dam was removed from Manatawny Creek. Algae were surveyed in spring–summer before removal (2000), summer–autumn after partial removal (2000) and spring–autumn after full removal (2001). There were two sites 0.2–0.3 km downstream of the dam, and two reference sites 2.0–2.2 km upstream. Algae were brushed from rocks, then chlorophyll a concentrations were measured and diatom species were counted.

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