4.4. Primary production, invertebrates and drift
Continuous stream monitoring for dissolved oxygen (DO) has been carried
out in conjunction with modelled estimates of net primary production
from in-stream plants. Whilst the dominance of near surface runoff
sources, rough channel topography and cool temperatures maintain
relatively high dissolved oxygen concentrations in the Girnock; seasonal
and diurnal variations reflect the balance of primary production and
respiration from heterotrophic consumption in the stream (Dick et al.,
2016). Algae and mosses on the stream bed drive the primary production
through growth early in the year and later in summer, with primary
production being highest in the upper, non-forested parts of the
catchment where radiation inputs are highest (Birkel et al., 2012).
Similarly, respiration is higher in the upper catchment, presumably
reflecting the higher substrate for heterotrophs and warmer temperatures
which make the stream heterotrophic overall.
The Girnock also has a diverse and abundant benthic invertebrate
community, dominated by stoneflies, mayflies and caddis flies (Gibbins
et al., 2016). This largely reflects the good water quality with high
alkalinity, high DO, low nutrient levels and gravel-rich substrate.
These animals graze on the primary producers, detritus and particulate
organic carbon. Despite, strong groundwater influence in the hyporheic
zone in some parts of the catchment, there are few anaerobic-tolerant
invertebrates (Gibbins et al., 2016) implying that overall, strong
well-oxygenated hyporheic flows dominant throughout most of the
catchment for most of the time.
Invertebrates provide the main food for juvenile salmonids.
Investigations into invertebrate drift in the Girnock have shown that it
peaks early in the spring and drives the highest rapid seasonal growth
rates in juvenile salmon. This complex interaction between food
availability and temperature explains the strong disparity between
juvenile growth rates predicted from laboratory-based temperature models
with ad-lib rations (Elliot and Hurley, 1997) and those growth rates
observed in the stream (Bacon et al., 2005). Later in the summer,
reduced food availability and high metabolic costs associated with
higher temperatures constrain growth potential (Jones et al., 2002).
Furthermore, within the river channel, the highest availability of drift
was correlated with areas of higher flow velocity linking hydraulic
habitats with potential for growth, with most bio-energetic models
assuming a trade-off between prey delivery and probability of capture
which reduces at higher velocities. Nevertheless, optimal habitat
utilisation of such physical controls on food availability is also
affected by fish behaviour and the density-dependent competition between
juveniles.