Chronosequence of morphological change in a stream fish following impoundment

Water impoundment is among the most prevalent and potent forms of anthropogenic environmental modification worldwide. Flow modification can drive morphological changes in riverine species that persist after dam closure, but little is known about the tempo and trajectory of phenotypic responses to impoundment. Through geometric morphometric analysis of natural history collections from behind Claiborne Lock and Dam (i.e. in Claiborne Lake reservoir) and nearby Chilatchee Creek (Alabama River Basin, Alabama, U.S.A.), we reconstructed multidecadal chronosequences of morphological change in a stream fish (Cyprinella venusta Girard) to characterise phenotypic responses to impoundment and unrestricted flow. We then conducted cross‐correlation analysis of hydrological and morphological time series to determine whether morphological responses to impoundment are consistent with expected outcomes of evolution, phenotypic plasticity, or some combination thereof. We found evidence of marked phenotypic change shortly after impoundment. Within a year of dam closure, individuals from Claiborne Lake exhibited a reduction in head size, an increase in body depth, and a decline in the length of the dorsal fin base, with further responses becoming apparent c. 6 years thereafter. The morphology of individuals from the nearby stream consistently differed from individuals in the reservoir. We found that fish from Chilatchee Creek exhibited gradual directional shifts and morphological fluctuations over time, which differed from patterns of change observed in C. venusta from Claiborne Lake. There also were indications of multidecadal lags at both sites, probably reflecting common responses to periodic climatic variation that were mitigated by site‐specific differences in physiography and ecology. Our findings are consistent with the hypothesis that impoundment can elicit punctuated responses attributable to within‐generational phenotypic change and rapid evolution over as few as two generations. Our findings also indicate that hydrological variation in unrestricted streams can elicit pronounced shifts in body shape, suggesting that space‐for‐time substitutions may not effectively capture the nature of responses to flow modification. Additional investigations based on more extensive and complete chronosequences would further illustrate the value of natural history collections and offer more insight into the mechanistic basis of biotic responses to environmental change.