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.