Spatio-temporal effects of river regulation on habitat quality for Atlantic salmon fry
Introduction
Regulation of rivers is evident at a global scale. Around 48% of the world’s river volume is moderately to severely regulated, fragmented, or both, this figure could rise to 93% by 2030 (Grill et al., 2015). A major cause of this change is the proposed construction of large dams in coming decades (Grill et al., 2015; Zarfl et al., 2014). The effects of regulation on flow regime can differ substantially depending on the activity or water user e.g., water supply, hydropower, flood defence, or a combination of these (Acreman and Dunbar, 2004). It is intuitive that the timing of operation (i.e., seasonality, diel patterns), as well as the type of use (e.g., hydropower generation – a major driver for new schemes, compensation release, inter- or intra-basin water transfers) and location of regulation infrastructure (e.g., headwaters, tributaries, close to river mouth) all have a potential bearing on the effects it can have on the functioning of ecosystems.
For many fish species some degree of migration or dispersal is an integral part of their life-history, and for salmonid species the spatial and temporal scales of these migrations vary substantially between different life-stages (McCormick et al., 1998). Consequently, dams and weirs have a bi-directional (cumulative) effect on longitudinal connectivity (e.g., Buddendorf et al., 2017, McKay et al., 2013, Norrgård et al., 2013, Schick and Lindley, 2007). However, in-channel structures like dams and weirs also affect the downstream hydraulic and hydrological conditions by altering the natural flow regime with potential consequences for habitat quality (Gibbins et al., 2001, Poff et al., 1997, Richter et al., 1997). The effects of flow regulation can occur at local scales (habitat/reach scale) immediately downstream of a dam or weir, but they can also be propagated downstream where their impacts on hydraulic habitat can vary spatially depending on channel morphology (Petts and Thoms, 1987, Postel and Richter, 2003). Numerous studies have investigated the effects of flow regulation on lotic ecosystems (e.g., Bain et al., 1988, Cowx et al., 2012, Freeman et al., 2001, Pringle et al., 2000, Tockner et al., 2011). It has long been hypothesised that changes in flow variability are instrumental in reducing the diversity and stability of such ecosystems (Dudgeon et al., 2006, Poff et al., 1997). For salmonids in particular, the potential importance of river flow and the effects of regulation on different life-stages have been studied extensively (Armstrong and Nislow, 2012, Bendall et al., 2012, Dunbar et al., 2012, Enders et al., 2009, Gibbins and Acornley, 2000, Gibbins et al., 2001, Malcolm et al., 2012, Milner et al., 2012, Nislow and Armstrong, 2012, Puffer et al., 2015, Scruton et al., 2008, Warren et al., 2015).
In recognition of the importance of flow variability (Acreman and Dunbar, 2004, Arthington et al., 2006, Carlisle et al., 2010, Poff and Schmidt, 2016, Razurel et al., 2016, Renöfält et al., 2010), environmental regulation increasingly incorporates a certain degree of natural variation to protect the integrity and functioning of lotic ecosystems. For example, in the UK this is done using the building block approach as described in UKTAG (2013). Nevertheless, there remains considerable debate as to the most appropriate approaches for classifying hydrological alteration and how such assessments should be included in impact assessment and river management (Macnaughton et al., 2017, Penas et al., 2016).
The lack of pre-regulation data is often a major impediment to assessing and understanding the effects of flow regulation on in-stream ecology (Macnaughton et al., 2017). Consequently, it is necessary to develop assessment techniques that allow the spatio-temporal effects of river regulation to be assessed in the absence of baseline data. Such techniques should ideally be broadly applicable and preferably transferable between river systems.
This study aimed to investigate the spatio-temporal effects of river flow regulation on hydraulic habitat quality for juvenile Atlantic salmon (S. salar L.) in their first year after hatch (hereafter referred to as fry for simplicity) in the River Lyon, a major tributary to the River Tay, Scotland’s largest catchment (4587 km2). The River Lyon was selected because it is an important salmon river, and it has a relatively simple linear river network, while still being subject to different types of river regulation. Previous studies have shown substantial changes in invertebrate communities (Jackson et al., 2007), and the hydrology of the Lyon catchment (Geris et al., 2015, Soulsby et al., 2015). In addition Buddendorf et al. (2017) assessed the effects of impoundments on longitudinal connectivity. There is some limited evidence that suggests that regulation in the Lyon has brought about change (i.e., a decline) in the population of Atlantic salmon (Summers, 2000). However, there have been no previous studies of the effects of regulation on habitat quality for salmon fry. A pre-existing rainfall-runoff model to estimate natural flows (Geris et al., 2015), a discharge time series under regulated conditions, detailed bathymetric surveys, and transferable hydraulic habitat quality models for Atlantic salmon fry (Millidine et al., 2016) were used to assess the likely impacts of river regulation on habitat quality. Three sites were chosen to illustrate the effects of different common types of regulation: re-naturalised, compensation release, and hydropower generating flow conditions. The compensation release and re-naturalised sites are located increasingly downstream of a reservoir, whereas the hydropower generating site is located nearer to the headwaters of the catchment, downstream of a high-head hydropower dam (Fig. 1).
The study aimed to answer the following research questions: 1) compared to natural flow conditions, do the three common types of flow regulation have a negative impact on salmon fry habitat quality? 2) is there a change in habitat quality among seasons? 3) is temporal variability in habitat quality under regulated flow conditions lower and this is associated with a smaller variability in discharge?
Section snippets
Study site
The River Lyon is situated in the Scottish Highlands, UK (Fig. 1). The catchment area is 390 km2, and is characterised by a steep hillslope geometry with high gradient tributary streams containing waterfalls that prevent access for Atlantic salmon (Buddendorf et al., 2017). Average precipitation is approximately 2300 mm per year. Land use in the catchment is primarily agricultural with pastures grazed by sheep throughout the year. There is some commercial forestry and there are several small
Regulated versus natural flow regimes
There were marked differences between the regulated and natural flow regimes over the period 2008–2011 (Fig. 4). Under regulated conditions, the range of flow variability tends to be lower than for natural conditions at all sites. However, the magnitude of differences between regulated and natural conditions varied distinctly between sites. At the RN-site regulated flows were similar to natural conditions, whereas the C- and HP-sites show a time series for regulated conditions that is markedly
Research question 1: compared to natural flow conditions, do the three common types of flow regulation have a negative impact on salmon fry habitat quality?
Based on the combination of hydraulic models and hydraulic habitat models used in this study there is reasonable evidence that suggests the HP flow regime has a detrimental effect on habitat quality for fry compared to a natural flow regime during the spring and autumn. During the summer, it appears that regulated flow may provide better habitat quality, however, within the time series there have been prolonged periods in summer where hydropower was not generally operating and thus a
Acknowledgements
Thanks to the Scottish Government Hydro Nation Scholarship program for funding WBB. Also, many thanks to Gianluca Lazzaro and Aaron Neill for their help doing fieldwork. Thanks to Scottish and Southern Energy and the Scottish Environmental Protection Agency for providing regulatory compliance data and river level data used in constructing the regulated discharge time series. The authors thank the two anonymous referees for their feedback on the manuscript.
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