From water quality to water quantity: the role of hydrological assessment in freshwater nature conservation

Philip J Boon, Head of Ecosystems and Biodiversity, Scottish Natural Heritage, UK

Abstract
The task of assessing the ‘quality’ of fresh waters, including assessments that are made for the purposes of nature conservation, has evolved steadily over the past 50 years. From an exclusive focus on water chemistry there has been a growing appreciation that habitats and the biological communities they support respond to a far wider array of environmental signals. It is now generally accepted that it is important to retain (or restore) near-natural processes and features in rivers and lakes, such as physical habitat structure, adjacent riparian zones, and hydrological regimes. This paper examines the way that attitudes to freshwater assessment are changing, using illustrations such as the work on river habitats and their flow requirements carried out by the statutory nature conservation agencies in the UK.
Within the European Union, legislation such as the EC Habitats Directive has provided an added incentive for protecting habitats and species, while the EC Water Framework Directive has imposed a statutory requirement for river basin planning where (at least in theory) the needs of freshwater conservation can be addressed. One of the main challenges in setting objectives for rivers of high conservation value is to propose targets for river flow that will protect habitats and their associated biological communities. Techniques for setting ‘ecological flows’ (or ‘environmental flows’) in rivers have moved from a simple assessment of the quantity of water allowed to remain in a river after abstraction to a more sophisticated approach that recognises the importance of flow dynamics such as rates of change and the natural seasonal patterns that characterise rivers. However, much remains to be done – both in understanding the flow requirements of aquatic biota and in establishing an appropriate balance between the ‘ecosystem services’ that rivers provide for human society and the aspiration for biodiversity conservation. In addition, a more flexible approach to ‘adaptive management’ is needed so that flow regulation can take account of adjustments in natural hydrological regimes, especially in response to future climate change.