Factors Explaining the Distribution and Site Densities of the Neosho Madtom (Noturus placidus) in the Spring River, Missouri

The Neosho madtom, a Federally-listed threatened species endemic to the Arkansas River system, is presently restricted to selected mainstem reaches of the Neosho, Cottonwood, and Spring rivers in Missouri, Kansas, and Oklahoma. These rivers are affected by anthropogenic factors such as municipal waste discharges and agricultural runoff. The Spring River also drains the Tri-State Mining District, where zinc-lead mining occurred in the past. Our objective was to assess effects of water quality degradation, due mainly to mining-related contaminants, on aquatic communities of the Spring River by comparison with those of the Neosho-Cottonwood system. We found higher densities of N. placidus, finer-textured riffle substrate, and lower concentrations of cadmium and lead in benthic macro-invertebrates in the Neosho-Cottonwood system than in the Spring River. In the Spring River, we found no substrate differences between sites with and without N. placidus; however, taxonomic richness of the benthic macro-invertebrate and fish communities were greater, densities of N. placidus and other fishes were higher, and concentrations of zinc and cadmium in benthic macro-invertebrates were lower at sites with N. placidus. Pore waters from three sites in the Spring River system were toxic to Ceriodaphnia dubia; mortality was greater than 80%, and there was no reproduction. Concentrations of zinc and cadmium in pore waters and sediment were high at these sites relative to non-toxic sites, and had SEM/AVS ratios considered potentially toxic. Toxicity tests, concentrations of metals in benthic macro-invertebrates, toxic unit (UT) modeling of pore waters, and an empirical habitat model support a hypothesis of contaminant involvement in the distribution of N. placidus