Instream and riparian implications of weed cutting in a chalk river
Introduction
Southern and eastern England hosts the largest chalk river resource in Europe (UK BAP Steering Group for Chalk Rivers, 2004). Of the 161 chalk rivers and streams identified, ten are designated for their wildlife interest as river Sites of Special Scientific Interest (SSSI). Four of these are of European interest and designated as candidate Special Areas of Conservation (cSAC) under the Habitats Directive. They represent unique freshwater habitats that are listed in the UK Biodiversity Action Plan as priority habitats for protection.
A pristine chalk river is likely to consist of multiple channels that are largely shaded by trees (alder and willow); supporting patchy macrophyte cover. River water would be characterised by low nutrient and suspended solids concentrations (Mainstone, 1999). However, it is very difficult to accurately define reference conditions for chalk rivers (Acreman and Ferguson, 2010) although they are needed in the implementation of the European Water Framework Directive (European Commission, 2000).
For more than 2000 years man has modified these chalk river systems by clearing riparian woodlands, modifying channels (widening, deepening and straightening), elevating nutrient and suspended solids concentrations (from agricultural and urban sources) and reducing flows through abstraction (WWF-UK, 2009). In some chalk rivers low flows, high nutrient concentrations and deep accumulations of fine sediment have resulted in few macrophytes. Likely reasons for this include limited photosynthesis caused by prolific epiphytic algal growth and/or turbid water (e.g. Phillips et al., 1978), limited diffusion of nutrients to leaves owing to low velocity flow (e.g. Madsen et al., 2001) and/or epiphytic algal growth and unsecure rooting in surficial nutrient rich fine sediment (e.g. Spink et al., 1993).
In other rivers, the reduction in natural shade has increased the productivity and coverage of aquatic plants (cf. Dawson and Kern-Hansen, 1979). Consequently extensive and frequent weed cutting has been undertaken for many years in numerous rivers to maintain their key functions, which include flood water conveyance, riparian water level control and viable fisheries (e.g. Baattrup-Pedersen and Riis, 2004, Nikora et al., 2008).
The instream hydraulic and ecological significance of weed is widely accepted and whether or not to cut is a heavily debated subject. It has been suggested that the most satisfactory approach would be to address the causes of excessive growth which include high levels of nutrients and unnaturally low shading of rivers (e.g. Dawson, 1978, Swales, 1982). Without management Ranunculus biomass would also naturally be lower (Dawson, 1976) and it would naturally wash out earlier (e.g. Ham et al., 1982). Franklin (2007) also suggests that Ranunculus growth would be self regulating owing to the feedbacks between plant growth and velocity. Given that such self-regulation operates over a longer timescale, weed growth may not be adequately controlled in this way and there is likely to be a shorter term need for weed cutting in certain places.
In response the UK Environment Agency only cut weed where it is essential. For example, on the River Avon in Hampshire weed is only cut where there is a real flood risk to multiple properties or damage to infrastructure. To minimise impacts of weed cutting best practice guidance is summarised in several publications (e.g. Mainstone, 1999, Wheeldon, 2003) and provided by the Environment Agency, Natural England and various angling/wildlife associations.
However, scientific accounts of coincident multiple instream and wider riparian impacts of cutting are limited. Existing accounts include impacts on invertebrate populations (Dawson et al., 1991), transport of fine particulate organic matter (Warren et al., 2009), fish habitat (Swales, 1982) and plant communities (Baattrup-Pedersen and Riis, 2004). None of these accounts consider the ecological impacts on riparian wetlands, which are common in chalk river systems.
In this study a multidisciplinary approach is adopted to quantify key physico-chemical impacts that result from weed cutting and to discuss their potential ecological implications. Specific objectives are to quantify, for the first time, the coincident impacts on river hydraulics (including conveyance capacity), the adjacent wetland, and river water quality.
Section snippets
Study area
The Lambourn catchment is located within the Berkshire Downs, southern England. The ephemeral head of the river is located in Lynch Wood (51.512°N, 1.529°W) at an elevation of approximately 130 m above sea level, with the perennial head situated 6–7 km downstream at Maidencourt Farm (51.481°N, 1.464°E). At Shaw, where the catchment area is 234 km2 it has a mean discharge of 1.73 m3/s, a median annual flood of 3.6 m3/s and a base flow index of 0.96 (Marsh and Hannaford, 2008) which clearly
Macrophyte coverage in July 2008
The objective of the weed cuts was to remove approximately 40% of the instream macrophyte coverage comprised mainly of Ranunculus. The observed reduction in coverage after the weed cut was evident in the MTR surveys. At three of the four surveyed reaches the recorded coverage reduced from >75% to 25–50% whereas at the other reach it reduced from 50–75% to 10–25%. The coverage of floating plant species (Azolla filiculoides, Lemna minor and Lemna minuta) showed less obvious changes in cover. They
Conveyance of flood flows versus maintenance of low flow water levels
While observations of the hydraulic impact of vegetation growth have been reported previously (Wharton et al., 2006) quantification of hydraulic impacts of weed cutting, primarily undertaken to mitigate flood risk, is rare.
The consequent increase in the conveyance capacity of the River Lambourn (89 to 141%) monitored here in response to reduced flow resistance from vegetation demonstrates the effectiveness, at least in the short term, of weed management over the spring and summer period.
Conclusion
This paper has quantified, for the first time, a wide range of coincident physical and chemical impacts of weed cutting and both instream and riparian environments were shown to be affected. Thus, when deciding on whether, when and how much weed to cut a wide range of potential implications should be evaluated. Measurements clearly demonstrated how weed cutting enhanced flood flow conveyance, reduced water levels (river and wetland) and increased velocities, and mobilised fine sediment with its
Acknowledgements
This study represents CEH core science that is funded by the Natural Environment Research Council. The authors would like to thank Ms Luff for granting access to Water Quality station 3 and Mr. B. Wheeler and his colleagues for conducting the weed cuts. Comments on drafts of this paper from CEH and BGS colleagues were much appreciated.
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