Effects of Lowhead Dams on Riffle-Dwelling Fishes and Macroinvertebrates in a Midwestern River

—Many studies have assessed the effects of large dams on fishes and macroinvertebrates, but few have examined the effects of lowhead dams. We sampled fishes, macroinvertebrates, habitat, and physicochemistry monthly from November 2000 to October 2001 at eight gravel bar sites centered around two lowhead dams on the Neosho River, Kansas. Sites included a reference site and a treatment site both upstream and downstream from each dam. Multivariate analysis of variance indicated that habitat, but not physicochemistry, varied immediately upstream and downstream from the dams, with resultant effects on macroinvertebrate and fish assemblages. Compared with reference sites, upstream treatment sites were deeper and had lower velocities and downstream treatment sites were shallower and had higher velocities; both upstream and downstream treatment sites had greater substrate compaction than reference sites. Macroinvertebrate richness did not differ among site types, but abundance was lowest at downstream treatment sites and evenness was lowest at upstream treatment sites. Fish species richness did not differ among site types, but abundance was highest at downstream reference sites and evenness was highest at upstream sites. The abundance of some benthic fishes was influenced by the dams, including that of the Neosho madtom Noturus placidus, which was lowest immediately upstream and downstream from dams, and those of the suckermouth minnow Phenacobius mirabilis, orangethroat darter Etheostoma spectabile, and slenderhead darter Percina phoxocephala, which were highest in downstream treatment sites. Although limited to one system during a 1-year period, this study suggests that the effects of lowhead dams on fishes, macroinvertebrates, and habitat are similar to those reported for larger dams, providing important considerations for riverine ecosystem conservation efforts. About 2 million dams exist in the United States, including 75,000 dams over 2 m in height; Kansas ranks second in dam number among all states, with 5,699 dams (Shuman 1995). Dams fragment rivers, reducing connectivity and resulting in negative effects on stream biota upstream and downstream from the impoundment (Doeg and Koehn 1994; Rabeni 1996; Kanehl et al. 1997). Dams block movement of fishes and affect habitat and physicochemical conditions of streams by converting lotic habitats to lentic, changing streamflow, al* Corresponding author: [email protected] 1 Present address: Illinois Natural History Survey, Center for Biodiversity, Champaign, Illinois 61820, USA. 2 Present address: University of Oklahoma, Department of Zoology, Norman, Oklahoma 73019, USA. Received March 10, 2003; accepted November 19, 2003 tering water quality, and modifying channel morphology and bed structure by increasing siltation upstream and erosion downstream (Watters 1996; Helfrich et al. 1999; Porto et al. 1999). These alterations cause changes in assemblage structure of fishes and macroinvertebrates via shifts in composition, abundance, and diversity both upstream and downstream from the impoundment. Although many studies have addressed effects of large dams on fishes (e.g., Martinez et al. 1994; Clarkson and Childs 2000; Wildhaber et al. 2000b), few have examined effects of lowhead dams (,4 m in height) (e.g., Benstead et al. 1999; Helfrich et al. 1999; Beasley and Hightower 2000), and none has done so in a midwestern U.S. stream. Our objectives were to investigate possible effects of two lowhead dams on the fish and macroinvertebrate assemblages, habitat, and physicochemistry of the Neosho River, Kansas. We pre-