Geology as a Structuring Mechanism of Stream Fish Communities

We examined the influence of the Precambrian Shield, a broad-scale geological feature of North America, on stream abiotic conditions and on the associated fish communities relative to an adjacent series of sites located on sedimentary geology (i.e., "off-Shield" streams) in south-central Ontario, Canada. Constrained and unconstrained multivariate analyses were used to quantify relationships in fish species composition, abiotic variables, and fish species' functional traits. The results showed that for low-order lotic systems, streams located on the Precambrian Shield had fish communities that were distinctly different from those of off-Shield streams. Lotic fish communities on the Shield were associated with higher velocity, increased amounts of instream woody debris, higher dissolved oxygen, and shrub-marsh landscapes. In contrast, off-Shield communities were characterized by higher conductivity, alkalinity, water temperature, and turbidity and were frequently surrounded by meadow and cultivated or pastured land. In general, off-Shield sites were more species rich and were associated with species preferring a wide variety of habitats, whereas Shield sites were associated with a subset of the regional species pool that had more specific habitat associations. Overall, our results show that these sharp geological discontinuities result in distinct abiotic environments and fish communities between Shield and off-Shield low-order lotic systems within a small geographic range.