Responses of turtle assemblage to environmental gradients in the St. Croix River in Minnesota and Wisconsin, U.S.A.

Abstract

We investigated how environmental gradients measured along the St. Croix River in Minnesota and Wisconsin, U.S.A., influenced the turtle assemblage. Among seven species, the five most common species were generalists and had wide distributions throughout the study area. However, patterns in assemblage structure were related to environmental gradients along the river. Sex ratios were male-dominated for the five most common species, and few or no juveniles were captured during the study. The first two canonical axes of a canonical correspondence analysis accounted for 92.7% of the variation in species-environment gradients. Most of the variation in distribution and abundance was attributed to gradients in channel morphology and physical characteristics along the river channel. Abundances of common snapping (Chelydra serpentina), false map (Graptemys pseudogeographica), and painted (Chrysemys picta bellii) turtles were associated with muck substrates and the number of basking sites (i.e., snags, rocks), which increased farther downstream. Abundance of spiny softshell turtles was closely related to increased water velocity and depth, which were related to hydraulic control points in the river. Abundance of common map turtles was associated with the presence of open sandy areas, uniform channel bottom, and gravel substrates. Geomorphic changes along the St. Croix River clearly influence the turtle assemblage and these specific relations should be considered in efforts to preserve and restore components of the assemblage.