Cleansing our waters: how riffle fish communities in the Spring River of Kansas responded to pollution legislation, mining remediation, and improved water quality.

Metal pollution from mining is a threat to lotic fishes, although legislation that requires improved water quality and provides funding for remediation has been enacted to combat this stressor. The Spring River in southeastern Kansas historically received inputs of cadmium, copper, lead, and zinc that reduced the prevalence of several sensitive and imperiled fishes. Long-term reductions in these toxic metals have occurred since the 1990's because of management activities, but presently it is unknown how riffle fish communities responded to improved water quality. As such, the objective of our research was to quantify changes in riffle fish community structure between 1993-1995 and 2019-2021 using a before-after-control-impact (BACI) study design. Responses were quantified with univariate analyses to examine the interactive effects of time period (i.e., historical versus contemporary), pollution tolerance classification (i.e., intolerant, moderately-intolerant, moderately-tolerant, and tolerant), and site position (i.e., above versus below the most severe historical metal pollution inputs) on relative abundance, occupancy, and species richness of riffle fishes. We used multivariate analyses to examine temporal changes in community structure above and below pollution inputs. Our findings indicated that intolerant fish species relative abundance increased between the historical and contemporary time periods throughout the Spring River. Occupancy of all fishes increased over time in the formerly metal contaminated portion of the Spring River, becoming similar to values in the reference reach. Changes in species richness depended on site position and pollution tolerance, as temporal increases of intolerant and moderately intolerant species richness were most pronounced in the lower Spring River below historical metal pollution inputs. Multivariate analyses revealed a significant interaction between time period and site position, as there was a shift in community structure over time in the lower reach that was less pronounced in the upper reach. Most of the intolerant species that increased in prevalence are of high conservation priority, thus our research suggested that legislation enacted to improve water quality produced a ripple effect that stimulated the recovery of imperiled fish species.