Bennion, H; Burgess, A; Kelly, M; Juggins, S; Reddihough, G; Yallop, M; (2011) Assessment of ecological status in UK lakes using diatoms - Draft. (Environment Agency Science Report SC070034 ). Environment Agency: Bristol.

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Abstract

1. The European Union’s Water Framework Directive requires all water bodies to achieve ‘good ecological status’ by 2015 and this chapter describes an ecological classification tool for lakes based on diatoms, a key component of the biological quality element ‘macrophytes and phytobenthos’. 2. A database of benthic diatom samples collected since 2003, specifically for development of the tool, was assembled. In total 1079 samples from 228 lakes were included in the database with matching environmental data. The dataset was divided into three lake types - low, medium and high alkalinity. 3. ‘Reference sites’, relatively unimpacted by human activity, were selected from this database based on a range of criteria including evidence of degree of change based on palaeolimnological data, and were used to develop a type-specific reference typology. 4. A lake trophic diatom index (LTDI) was developed and ecological quality ratios (EQRs) were generated for each lake type that are ≥1, where the diatom assemblage showed no impact, to (theoretically) 0, when the diatom assemblage was indicative of major anthropogenic activities. 5. The boundary between ‘high’ and ‘good’ status was defined as the 25th percentile of EQRs of all reference sites. The boundary between ‘good’ and ‘moderate’ status 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 then defined by equal division of the remaining EQR gradient. 6. The reference data were employed to predict the ‘expected’ class at each site and these were compared with the classifications derived from the tool. For lakes identified as reference sites, 68% were classified as high with the remainder classified as good. For non-reference lakes the model predicted the majority of sites to be good or worse. The model appears to perform well. 7. The performance of the model was further tested by application to a set of 17 Cumbrian lakes for which classification based on a range of other tools was available. There was a tendency for the diatom tool classifications to be more relaxed than those using other tools with 76% of lakes being placed in a higher status class than that indicated by the entire suite of biological metrics. The findings suggest that the reference community in low alkalinity lakes might be 'buffered' against change as the dominant taxa appear to be relatively tolerant to mild enrichment. 8. In an attempt to improve the predictions for low alkalinity lakes the response of surface sediment diatom assemblages along the nutrient gradient was explored, as these potentially contain a more diverse benthic diatom assemblage as well as planktonic taxa. However, a set of weighted averaging transfer functions offered no improvement over the diatom tool based on epilithic and epiphytic assemblages. 9. In conclusion, for low alkalinity lakes in particular, DALES is more relaxed than other UK tools. However, DALES gives an accurate assessment of the condition of the littoral biofilm and thus provides a WFD compliant tool to form part of the toolkit for classifying UK standing waters.

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