The contribution of different sources of uncertainty in diatom analyses for ecological status assessments, and implications for decision-making

Periphyton communities in streams are naturally variable in space and time and this, along with repeatability and reproducibility, needs to be recognized when using diatoms to inform decisions about the management of freshwater resources. This paper examines five types of uncertainty on four stream waterbodies in northern England, and assesses the implications for decision-making associated with the Water Framework Directive. Effects can be ranked (lowest to highest) as follows: within-analyst variation (repeatability) < site-level variability at the time of sample collection < between-operator variation (reproducibility) < within water body variation ∼ between-season variation. The extent of within water body and between season variation varied between sites, suggesting that generalizations are difficult, although these sources of environmental variation both exceeded most measures of analytical variation. Analytical and within-site variation can be controlled, to some extent by adherence to protocols, but both also include a stochastic component. Within water body and between season variation can be controlled by more intensive sampling, but this comes at a cost. Therefore, it is important that the scale of this be recognized and acknowledged explicitly.