4.2. Groundwater – surface water interactions
Excavation of spawning areas to recover and transplant ova formed part of experimental work in the 1990s to test whether transplanting ova from areas of high to low spawner density might increase juvenile production through reductions in density dependent mortality (Youngson and McLaren, 1998). Indeed, some have advocated for similar approaches more recently, with a focus on moving wild salmon fry rather than ova (Young, 2017).
Surprisingly high ova morality was found at some of the most heavily used, and apparently suitable, spawning sites on the Girnock (Malcolm et al., 2005). It was initially hypothesised by fisheries researchers that fine sediment infiltration was causing anoxic conditions through low interstitial velocity in redds. However, the “fine” sediment fraction in the Girnock was found to be comprised mainly of sand (Moir et al., 2002). Furthermore, interstitial velocities were often high and not clearly correlated with oxygen concentrations (Malcolm et al., 2011). Subsequent process-based investigations, including the first use of continuous optical dissolved oxygen sensors capable of being deployed in the hyporheic zone (the streambed interface between surface water and groundwater), showed that some spawning sites were vulnerable to anoxic conditions where chemically reduced groundwater discharges occurred through the stream bed (Malcolm et al., 2004a; 2006). These areas of groundwater discharge coincided with valley constrictions that forced upwelling groundwater into reaches of the stream where sedimentary conditions were good for spawning but provided sub-optimum habitat in terms of sufficient oxygen levels to sustain ova (Malcolm et al., 2005). Such conditions were shown to be worse in wetter winters, when groundwater fluxes were higher, and less prevalent in drier winters when well-oxygenated stream water dominated the hyporheic zone (Soulsby et al., 2008). So, it seems spawning site selection in the Girnock represents a trade-off between optimal sedimentary conditions and the risk of de-oxygenation in some years.
These local investigations into the hydrology of specific spawning reaches were subsequently placed in a catchment-scale context through characterisation of deeper groundwater flow paths (Fig. 6) as part of hillslope hydrology research in the catchment. This has used a combination of synoptic surveys of environmental tracers in springs and streams (Soulsby et al., 2007; Blumstock et al., 2015; Scheliga et al., 2017); sampling deeper and shallower wells (Blumstock et al. 2016; Scheliga et al., 2018, 2019), geophysical mapping (Soulsby et al., 2016b) and modelling (Ala aho et al., 2017). Much of this work focused in the smaller, more accessible 3.2km2 Bruntland Burn sub-catchment of the Girnock, with broadly similar overall landscape properties (topography, soils, land use etc.) and hydrology where logistical challenges to monitoring are less severe (Birkel et al., 2014).
These studies have shown extensive groundwater storage in glacial deposits in the Girnock which in valley bottom areas can be ~ 40m deep. Generally, groundwater circulation here is slow due to the low permeability of these drifts and deeper groundwater is probably not well-connected to streams. Coarser, shallower drifts on the hillslopes have higher permeability and more dynamic groundwater responses. These appear to be fed by fracture networks in the granite and other rocks exposed on the catchment interfluves which are activated in wetter periods (Scheliga et al., 2017, 2019). Groundwater chemistry is highly variable reflecting geology and residence times; but is mostly strongly alkaline from calcareous rocks in the drifts (Soulsby et al., 2007). Groundwater in granite is less alkaline (Soulsby et al., 1998). The spatial and temporal variability of groundwater inputs has a strong influence on the chemistry of the stream, and the chemistry of the hyporheic zone (and redd environment) depends on local hillslope-aquifer-stream connectivity (Malcolm et al., 2005; Soulsby et al., 2009).