Water Framework Directive: ecological classification of Danish lakes

Summary The European Water Framework Directive (WFD) requires that all European waterbodies are assigned to one of five ecological classes, based primarily on biological indicators, and that minimum good ecological quality is obtained by 2015. However, the directive provides only general guidance regarding indicator definitions and determination of boundaries between classes. We used chemical and biological data from 709 Danish lakes to investigate whether and how lake types respond differently to eutrophication. In the absence of well‐defined reference conditions, lakes were grouped according to alkalinity and water depth, and the responses to eutrophication were ordered along a total phosphorus (TP) gradient to test the applicability of pre‐defined boundaries. As a preliminary classification we suggest a TP‐based classification into high, good, moderate, bad and poor ecological quality using 0–25, 25–50, 50–100, 100–200 and > 200 µg P L −1 boundaries for shallow lakes, and 0–12·5, 12·5–25, 25–50, 50–100 and > 100 µg P L −1 boundaries for deep lakes. Within each TP category, median values are used to define preliminary boundaries for the biological indicators. Most indicators responded strongly to increasing TP, but there were only minor differences between low and high alkalinity lakes and modest variations between deep and shallow lakes. The variability of indicators within a given TP range was, however, high, and for most indicators there was a considerable overlap between adjacent TP categories. Cyanophyte biomass, submerged macrophyte coverage, fish numbers and chlorophyll a were among the ‘best’ indicators, but their ability to separate different TP classes varied with TP. When using multiple indicators the risk that one or more indicators will indicate different ecological classes is high because of a high variability of all indicators within a specific TP class, and the ‘one out – all out’ principle in relation to indicators does not seem feasible. Alternatively a certain compliance level or a ‘mean value’ of the indicators can be used to define ecological classes. A precise ecological quality ratio (EQR) using values between 0 and 1 can be calculated based on the extent to which the total number of indicators meets the boundary conditions, as demonstrated from three Danish lakes. Synthesis and applications . The analysis of Danish lakes has identified a number of useful indicators for lake quality and has suggested a method for calculating an ecological quality ratio. However, it also demonstrates that the implementation of the Water Framework Directive faces several challenges: gradual rather than stepwise changes for all indicators, large variability of indicators within lake classes, and problems using the one out – all out principle for lake classification.