Comparison of threatened Neosho madtom (Noturus placidus) densities between riverscapes differing in anthropogenic stressors, with a particular focus on recovery from mining-derived metal pollution

Water pollution imperils the Neosho madtom (Noturus placidus), which is threatened federally and in Kansas. Within Kansas, madtom densities were historically lower in the Spring River compared to the Cottonwood and Neosho Rivers, especially within the Spring River below tributary inputs that delivered cadmium, copper, lead, and zinc pollution from the Tri-State Mining District of Missouri, Kansas, and Oklahoma. Neosho madtom are less numerous in waters containing elevated metal concentrations because of direct toxicity and lower benthic macroinvertebrate (i.e., food) availability. Long-term reductions in metal concentrations in the Spring River have occurred, but no study has examined whether madtom and macroinvertebrate densities have responded to improved water quality. We addressed this question by comparing madtom and macroinvertebrate abundances between the Neosho-Cottonwood system and the Spring River above and below metal pollution inputs. However, madtoms are influenced by environmental factors and anthropogenic stressors beyond mining-derived metal pollution, so we also examined if food availability, local habitat variables, and watershed characteristics were related to madtom densities. We found that madtom and macroinvertebrate population densities in the Spring River below metal pollution were similar to those in the Spring River above metal pollution and the Neosho-Cottonwood River system. Furthermore, macroinvertebrate availability and watershed characteristics were not associated with madtom abundance. However, two local habitat variables (turbidity and depth) were associated with madtom densities, such that an increase in turbidity or decrease in depth resulted in higher madtom densities. Our results highlight the benefits that water quality improvements can have on imperiled stream organisms.