Fine scale variability of hyporheic hydrochemistry in salmon spawning gravels with contrasting groundwater-surface water interactions

There is increasing realisation of the importance of groundwater–surface water (GW–SW) interactions in understanding freshwater ecology. A study that assessed the influence of local GW–SW interactions on shallow (<250 mm) hyporheic water quality at two contrasting salmon spawning locations in Scotland, UK is reported. At a groundwater-dominated site, continuous logging sensors revealed that hyporheic dissolved oxygen (DO) concentrations changed rapidly in response to changing hydrological conditions. Low volume (25 ml) spot samples revealed fine-scale spatial variability (<0.05 m) consistent with a vertically shifting boundary layer between source waters. At a surface-water-dominated location, hyporheic water was typically characterised by high DO and electrical conductivity values, characteristic of surface water. Small reductions in DO at this site are hypothesised to be associated with short residence hyporheic discharge. A comparison between in-situ (logging DO sensor data) and ex-situ (small volume sampling) methods revealed good agreement, potentially allowing deployment of the two methods in stratified sampling programmes. This study demonstrates that hyporheic water quality varies over fine spatial and temporal scales and that future studies need to design sampling strategies that consider the scales appropriate to both the ecology and the hyporheic processes of interest.