Does large woody debris affect the hyporheic ecology of a small New Zealand pasture stream?

Abstract

Rehabilitation of vertical hydrological exchange in small pasture streams is expected to improve hyporheic habitat, invertebrate life and ecosystem functioning, and may be achieved by adding large woody debris (LWD). This study investigated whether hyporheic improvement within 10 to 50 cm below streambed surface is evident around existing wood structures in a small New Zealand pasture stream in the Whatawhata area near Hamilton. Samples were taken around LWD and from areas where no wood was present in the pasture and from a control stream. At the pasture stream site, where the hyporheic zone was impacted by sedimentation and suffered from low dissolved oxygen (DO) levels (on average 8.4, 4.3 and 2.4 mg L−1 at 10, 30 and 50 cm depth, respectively), LWD was not associated with a significant improvement in vertical hydrological exchange, hyporheic habitat quality or invertebrate richness. However, higher densities in hyporheic invertebrates were found where LWD was present (100 vs. 56 individuals per L of sediment), in particular Leptophlebiidae and Oligochaeta. Potential pumping effects of LWD may have been hampered by large-scale deep groundwater upwelling in this reach. We conclude that in pasture streams where sedimentation is a legacy of land clearance on unstable slopes and where low hyporheic DO concentrations prevail, wood addition may not be an effective rehabilitation tool.