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.