Catchment‐scale controls on groundwater–surface water interactions in the hyporheic zone: implications for salmon embryo survival

The spatial and temporal variability of groundwater–surface water (GW–SW) interactions in the hyporheic zone were investigated in a semi‐pristine upland salmon spawning catchment (Girnock Burn) in the Cairngorm Mountains, northeast Scotland. Stream and hyporheic water quality (200–300 mm depth) were monitored fortnightly at 16 spawning locations distributed throughout the catchment. Hydrochemical tracers were used to assess local GW–SW interactions. Stratified streambed incubators (50–300 mm) provided information on salmon embryo mortality at a sub‐set of ten locations. Hyporheic water quality varied both temporally and spatially according to local GW–SW interactions. It was possible to categorize sites into three broad typologies reflecting local stream–aquifer interactions: (1) groundwater‐dominated; (2) surface water‐dominated; and (3) sites exhibiting transient water table features. Groundwater upwelling occurred in areas where low permeability glacial moraine features caused substantive valley constriction. These locations were also conducive to accumulation of spawning grade gravels and consequently were utilized heavily by spawning salmon. Long residence groundwater was typically characterized by low dissolved oxygen (DO), of sufficiently low quality to be detrimental to salmon embryo survival. At sites dominated by surface water, hyporheic DO remained high throughout and rates of embryo survival were correspondingly high. Survival rates were also high at sites where hydrochemical characteristics indicated a transient water table. This is probably attributable to the hydrological conditions which resulted in increasing DO concentrations towards hatch time when embryo oxygen demand is at its maximum. The degree to which the findings of this study are directly applicable to other catchments is currently unknown. However, similar effects have been observed elsewhere, and based on the information presented here, there are clear implications for fisheries managers who may wish to consider the use of surface incubation facilities to negate the effects of low DO groundwater upwelling where it dominates available spawning habitat. It is suggested that future research should aim to integrate across spatial scales and disciplines to obtain a better understanding of the ways in which hillslope and riparian zone hydrology affect GW–SW interactions, hyporheic zone processes and stream ecology. © Crown copyright 2005. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd.