1. Rivers and their catchments are complex, dynamic and non-equilibrium systems. Although the general functioning of these ecosystems is familiar, the characteristics of particular sites are often poorly known and accurate prediction of future behaviour to inform management decisions is extremely difficult.
2. Simple relationships between river nutrient concentrations and the health of river ecosystems do not exist. Natural variability and practical and technical constraints reduce our ability to set nutrient targets to protect river ecosystems. Particularly challenging is the use of simple dose–response relationships as regulatory threshold targets, whereby sections of river are classified as either passing or failing to meet good ecological status.
3. Ecologically meaningful frameworks are needed that take account of the epistemic uncertainty associated with our predictions. Such frameworks should have clearly defined goals; be holistic; allow for natural variability; help to define ecologically acceptable environmental regimes; recognise that significant uncertainties mean that we will often be using indices rather than absolute measures; and use measures that explicitly include uncertainty estimates. Such approaches allow those outside the decision-making process to understand the level of environmental precaution included in the management of complex systems.