Small differences in riparian vegetation significantly reduce land use impacts on stream flow and water quality in small agricultural watersheds

Abstract

Fluvial ecosystem integrity is intimately tied to agricultural intensity in most rural landscapes. Row crop agriculture in particular can elicit a range of deleterious effects on hydrology, surface water quality, and fluvial ecology. Riparian vegetation represents a critical defence of stream systems against environmental perturbations. Vegetation filters surface runoff and encourages infiltration. Furthermore, riparian forests shade streams and are thus able to stabilize in-stream temperature and dissolved oxygen (O). In northwest New Brunswick's potato (Solanum tuberosum L.) growing region, once densely forested riparian corridors now often contain only scattered, irregular patches of residual vegetation, and national water quality standards for streams are regularly exceeded. The objective of this study was to ascertain whether relatively small differences in riparian forest cover are of any significant benefit to stream environments. Specifically, we assessed the capacity of riparian forests to reduce the effects of row crop agriculture on stream flow, water quality, and benthic macroinvertebrate communities of streams draining small subwatersheds near Grand Falls, New Brunswick. The subwatersheds ranged in agricultural intensity from 0.5% to 97% crop cover, which predominantly consisted of potato. Riparian cover and agricultural intensity were highly collinear; hence, riparian forest cover was converted to a binomial categorical predictor (i.e., high and low), with agricultural intensity treated as a continuous covariate. Higher intensity of row-crop agriculture in small subwatersheds corresponded to base flow decline; greater flow pulse frequency; higher dissolved nitrate-nitrogen (NO₃-N), conductivity, pH, and temperature; lower dissolved O; and prevalence of pollution-tolerant macroinvertebrate taxa and grazers. With less than 20% increase in riparian forest cover relative to agricultural intensity, we observed reduced severity of summer low flow periods, significantly lower dissolved phosphate (PO₄) concentrations, lower temperatures, and higher dissolved O. Macroinvertebrate assemblage structure of the riparian cover categories diverged with increasing agriculture, demonstrating the value of even minor amounts of riparian vegetation to benthic community condition. Our results conclusively show that relatively small increases in riparian forest cover will lead to statistically detectable and ecologically meaningful improvements to stream health.