A Burgess

Uncertainty in ecological status assessments of lakes and rivers using diatoms

The EU’s Water Framework Directive requires all surface water bodies to be classified according to their ecological status. As biological communities show both spatial and temporal heterogeneity, expressions of ecological status will, inevitably, have an element of uncertainty associated with them. A consequence of this environmental heterogeneity is that there is a risk that status inferred from one or more samples is different to the true status of that water body. In order to quantify the scale of temporal uncertainty associated with benthic diatoms, replicate samples were collected from sites across the ecological status gradient in lakes and rivers in the UK. Variability (expressed as standard deviation of temporal replicate samples from a single site) could be described using a polynomial function and this was then used to calculate the risk of placing a water body in the wrong ecological status class. This risk varied depending on the distance from the class boundaries and the number of replicates. Based on these data, we recommend that ecological status is determined from a number of samples collected from a site over a period of time.

Assessment of ecological status in UK lakes using benthic diatoms

Abstract: The European Unions Water Framework Directive (WFD) requires that all water bodies in Europe achieve good ecological status (GES) by 2015. We developed an ecological classification tool for UK lakes based on benthic diatoms, a key component of the biological-quality element macrophytes and phytobenthos. A database of 1079 epilithic and epiphytic diatom samples and matching environmental data was assembled from 228 UK lakes. The data set was divided into 3 lake types: low, medium, and high alkalinity. A lake trophic diatom index (LTDI) was developed based on modification of the trophic diatom index (TDI) for rivers, and ecological quality ratios (EQRs) were generated for each lake type. The high/good status boundary was defined as the 25th percentile of EQRs of all reference sites (identified based on independent sedimentary-diatomassemblage data or catchment point-source and landuse data), whereas the good/moderate boundary was set at the point at which nutrient-sensitive and nutrient-tolerant taxa were present in equal relative abundance. The moderate/poor and poor/bad boundaries were defined by equal division of the remaining EQR gradient. Samples from reference sites were used to predict the expected LTDI value for each sample, and these values were compared with the classifications derived from the LTDI. For lakes identified as reference sites, 68% were classified as having high status and 32% as having good. The model predicted 81% of nonreference lakes to have good or worse status. The model was applied to 17 English lakes (10 low- and 7 medium-alkalinity) for which classification based on other WFD tools was available. The classifications based on LTDI gave the same status (within 1 class) as other biological elements for 11 of the 17 lakes (65%). Thus, the LTDI gives a reliable assessment of the condition of the littoral biofilm and is a key component of a WFD-compliant tool kit for classifying UK standing waters.

Comparing aspirations: Intercalibration of ecological status concepts across European lakes for littoral diatoms

Eleven European countries participated in an exercise to harmonise diatom-based methods used for status assessment in lakes. Lakes were divided into low, medium and high alkalinity types for this exercise. However, it was not possible to perform a full intercalibration on low alkalinity lakes due to the short gradient and confounding factors. Values of the Trophie Index were computed for all samples in order that national datasets could all be expressed on a common scale. Not all participants had reference sites against which national methods could be standardised and, therefore, a Generalised Linear Modelling approach was used to control the effect of national differences in datasets. This enabled the high/good and good/moderate status boundaries to be expressed on a common scale and for deviations beyond ±0.25 class widths to be identified. Those countries which had relaxed boundaries were required to adjust these to within ±0.25 class widths whilst the intercalibration rules allowed those countries with more stringent boundaries to retain these. Despite biogeographical and typological differences between countries, there was broad agreement on the characteristics of high, good and moderate status diatom assemblages, and the exercise has ensured consistent application of Water Framework Directive assessments around Europe.