Beyond Anadromy: Ecological Influences of Dams and Their Removal on Resident Fish and Benthic Macroinvertebrates

While the negative influence of dams as barriers to anadromous fishes is widely documented and accepted, impacts to other aspects of lotic ecosystems have received less attention. Staff from the Maryland Department of Natural Resources explored these other impacts in two studies.  The first study focused on the impacts of sediment movement following the removal of Simkins Dam on the Patapsco River. When Simkins Dam was removed, fine sediment stored behind it was allowed to erode freely into the river channel. These sediments buried coarse substrate habitats downstream while uncovering coarse substrate habitats upstream, causing concomitant changes in the distribution and abundance of burrowing invertebrates, EPT taxa, and benthic fishes. The second study examined trophic dynamics upstream from dams that exclude the catadromous American Eel from certain Maryland streams. The American Eel is a top predator in many Atlantic coastal streams. Its absence upstream of large dams resulted in top-down trophic effects on stream fish and benthic macroinvertebrate densities.  Benthic fishes were significantly higher and benthic macroinvertebrates significantly lower in density upstream from dams where eels were absent compared to where they were present.