Impacts of Wood Additions on Dissolved and Particulate Nutrient Retention in an Agriculturally Impacted Stream: A Multi-Tracer Injection Study at Whatawhata, New Zealand

Abstract:

Wood is a key component in forested streams, playing an important ecological and physical role in creating step-pool profiles, enhancing habitat heterogeneity, retaining organic matter, and changing water velocity. Wood additions can increase surface water-groundwater exchange, increasing in-stream residence times by slowing water velocities and providing high depositional areas for fine particles (i.e. particulate nutrients C, N, P). Thus, wood additions may create biogeochemical hotspots in streams that allow greater potential for local nutrient cycling and processing. The objectives of this research were to determine if added wood enhances in-stream heterogeneity, results in more complex flow paths, increases natural retention of further organic matter and changes geomorphic characteristics of the stream reach. We conducted a conservative solute and fluorescent fine particle tracer injection study in an agriculturally impacted stream with emplaced wood additions to estimate in-stream retention times in the Whatawhata catchment, North Island of New Zealand. Although similar solute peak concentrations were observed at the different in-stream sampling sites, increased retention was observed near to the wood. Both fine particle deposition and retention time was increased near the emplaced log. Fine particles were also analyzed in situ in sediment and biofilms on cobbles throughout the stream reach following the injection. A direct positive correlation was observed between cobble biofilm biomass and particle accumulation within this retention area. In general, the addition of wood to these agriculturally impacted streams enhanced hydraulic complexity and increased the retention of solute and fine particles.