Hyporheic zone processes in a canalised agricultural stream: implications for salmonid embryo survival

Agricultural practices have the potential to influence hyporheic exchange through increased fine sediment loads and through reduced hydraulic conductivity, bed roughness and morphological diversity. These impacts can reduce connectivity between stream and hyporheic waters to the detriment of salmonid embryos. Salmonids bury their eggs in streambed gravels, typically to depths of up to 300 mm and for incubation periods of up to 6 months. Embryo survival is dependant on a complex range of factors which critically includes the delivery of oxygen from surface waters. This paper investigates hyporheic zone exchange processes, oxygen concentration and embryo survival in a canalised agricultural stream at a range of nested spatiotemporal scales. Results are contrasted with those from the Girnock Burn, a relatively undisturbed catchment. Conservative tracer experiments were used to assess reach average hyporheic exchange using the USGS OTIS model. Artificial redds were used to assess reach scale variability in hyporheic processes. Incubation stacks were used to assess embryo mortality and hyporheic water quality with depth at the scale of individual redds. Optodes (optical dissolved oxygen sensors) were used at a sub-set of sites to assess fine scale temporal variability in hyporheic oxygen and temperature. Stream and hyporheic hydrochemistry were used to infer source water contributions and provenance. Data showed that reach average hyporheic exchange in the Newmills Burn was ca. five times less than in the Girnock Burn. Dissolved oxygen (DO) concentrations related strongly to spatiotemporal patterns of groundwater (GW) discharge. Embryo survival was significantly correlated with DO. It is suggested that low DO in the Newmills Burn relates to groundwater upwelling and limited hyporheic exchange caused by a combination of low morphological diversity, hydraulic conductivity and bed roughness. It is suggested that future studies of embryo survival should look beyond the single issue of fine sediment effects to include a broader understanding of hyporheic zone processes.