4.3. Surface water quality
The Girnock is an ideal salmon stream in that it is a relatively cool mountain habitat that has the clean, well-oxygenated water that is fundamental to the species at all freshwater life stages. The stream is generally base-rich with a circum-neutral pH. As many salmon life-traits are closely linked to phenology and seasonal temperature changes (e.g. ovulation, migration timing etc.) stream temperature was the first variable measured to complement salmon data. The maintenance of the stream temperature monitoring has, coincidentally, resulted in the longest, continuous sub-daily river temperature record in Scotland (Fig. 8). Early studies suggested that autumn and winter river temperatures decreased by ca. 0.76 °C and 0.57 °C, respectively, between 1970 and 2000, but that spring and summer temperatures had increased by 1.46°C and 1.04°C, respectively, over the same period (Langan et al., 2001; Gurney et al., 2008). Subsequent data collection and updated analysis has revealed a more complex picture with seasonally varying trends and the need to correct for operational and instrument related temporal biases (Jackson et al., In prep.).
Spatial patterns of stream temperatures have also been monitored by a distributed network of temperature sensors which evolved to be consistent with the locations of long-term electrofishing sites (Malcolm et al., 2004b). Mean daily stream temperatures show limited variability across the catchment, though extreme highs and lows can show substantial differences between the upper moorland catchment and the lower forested areas (Malcolm et al., 2008). Where riparian forest cover is present, day time temperatures are lower and night time temperatures remain higher than in more open areas upstream (Garner et al., 2014). Establishment of two automatic weather stations positioned over the river enabled ground-breaking process-based energy budgets to be modelled for forest and moorland stream reaches (Hannah et al., 2004, 2008). These showed that riparian tree cover reduces the rate of heating in the lower catchment by shading which, when combined with cooler water flowing from upstream overnight (advected heat), produces negative downstream gradients in observed water temperatures (Garner et al., 2014). Trees also reduce wind speeds and evaporative heat loss and reduce net losses of long wave at night, thereby reducing daily variability in temperature (Hannah et al., 2008).
Stream temperatures are also known to influence hyporheic temperatures and thus, the environment in which salmon eggs develop. However, this also varies with catchment wetness and local temporal dynamics in groundwater hydraulic gradients (Malcolm et al., 2001; 2008). In general, stream water ingress into the hyporheic zone is dominant in the upper 10cm or so of the stream bed where permeability is highest, though temperature variability is moderated (i.e. decrease in summer and warm in winter) with depth (Birkel et al., 2016). Micro-habitat studies of spatial variations in temperatures in different morphological features (e.g. pools and riffles) provide little evidence of thermal refugia in the Girnock as the stream is generally well mixed and unstratified (Imholt et al., 2013).
Temperature monitoring and modelling at the Girnock proved prescient, given emerging concerns over climate change, highlighting the importance of land use management and riparian woodland. Novel process-based modelling of the effects of riparian woodland (Fabris et al., 2018) combined with new remote sensing techniques (Dugdale et al., 2019) showed that riparian woodland could be an effective climate adaptation strategy, potentially mitigating lethal and sub-lethal effects of high temperatures. Such important insights provided the impetus for a national temperature monitoring network that developed large scale models to prioritise riparian woodland regeneration to locations where river temperatures and climate sensitivity are high (Jackson et al., 2018) and where woodland can have a substantial effect in reducing temperature extremes (Jackson et al., 2021).
Other important water quality parameters also relate to catchment characteristics and land use. The peaty soils of the Girnock generate high Dissolved Organic Carbon (DOC) concentrations during overland flow, especially during higher flows in the summer and autumn, where organic acids also reduce the alkalinity and pH of the stream (Dawson et al. 2011). Spatially distributed hydrological models (see below) have been used to simulate DOC production and transport in different landscape units and under varying degrees of catchment wetness and connectivity (Dick et al., 2015).
With a high population of Red Deer, as well as other riparian animals such as otters, water voles, sheep (in the lower catchment) etc., the potential for significant microbiological pollution of surface waters (indexed by Faecal Indicator Organisms (FIOs)) has been shown (Tetzlaff et al., 2010). Modelling has also used a hydrologically-based spatial framework and has gone some way to explaining the main drivers of FIO delivery in terms of temperatures, antecedent wetness and movements of red deer (Neill et al., 2019). The deer population has been implicated in other water quality issues; for example, detectable nitrate concentrations in parts of the Bruntland Burn corresponded to areas where larger numbers of deer congregate (Blumstock et al., 2015). This was unexpected in streams draining upland areas which are usually oligotrophic and any labile nutrients are usually taken up by aquatic plants.