Enhanced hyporheic exchange flow around woody debris does not increase nitrate reduction in a sandy streambed

Anthropogenic nitrogen pollution is a critical problem in freshwaters. Although riverbeds are known to attenuate nitrate, it is not known if large woody debris (LWD) can increase this ecosystem service through enhanced hyporheic exchange and streambed residence time. Over a year, we monitored the surface water and pore water chemistry at 200 points along a ~50m reach of a lowland sandy stream with three natural LWD structures. We directly injected 15N-nitrate at 108 locations within the top 1.5m of the streambed to quantify in situ denitrification, anammox and dissimilatory nitrate reduction to ammonia, which, on average, contributed 85%, 10% and 5% of total nitrate reduction, respectively. Total nitrate reducing activity ranged from 0-16µM h-1 and was highest in the top 30cm of the stream bed. Depth, ambient nitrate and water residence time explained 44% of the observed variation in nitrate reduction; fastest rates were associated with slow flow and shallow depths. In autumn, when the river was in spate, nitrate reduction (in situ and laboratory measures) was enhanced around the LWD compared with non-woody areas, but this was not seen in the spring and summer. Overall, there was no significant effect of LWD on nitrate reduction rates in surrounding streambed sediments, but higher pore water nitrate concentrations and shorter residence times, close to LWD, indicated enhanced delivery of surface water into the streambed under high flow. When hyporheic exchange is too strong, overall nitrate reduction is inhibited due to short flow-paths and associated high oxygen concentrations

Simple large wood structures promote hydromorphological heterogeneity and benthic macroinvertebrate diversity in low-gradient rivers

This work has been carried out within the SMART Joint Doctorate Programme ‘Science for the MAnagement of Rivers and their Tidal systems’ funded by the Erasmus Mundus programme of the European Union

Environmental Flows: Habitat Modeling

Aquatic habitat modelling is commonly used in riverine wetlands for environmen.tal flow assessments as a means of defining the empirical relationship between environmental variables, and usable habitat for selected target species, life stages or aquatic communities. Aquatic habitat modelling is used to determine environmental flow strategies by estimating the effects of historic, current or future flow scenarios on habitat availability. The origins of aquatic habitat modelling for determining environmental flows can be traced to the development of the Instream Flow Incremental Methodology (IFIM) and the set of computer programs required to implement an IFIM study, i.e. the Physical Habitat Simulation (PHABSIM) system. More recently, other models have used alternative approaches, such as CASiMiR (Computer Aided Simulation Model for Instream Flow and Riparia) that utilises a mix of expert opinion and fuzzy logic based rules to describe the habitat use of target species or the application of multidimensional, i.e., two-dimensional and to a lesser extent three-dimensional hydraulic-habitat models. These enable an enhanced representation of the hydraulic environment and allow the calculation and modelling of turbulent flow properties. However, a knowledge of how biota respond to and are influenced by these properties remains an ongoing challenge to incorporate them into environmental flow setting