Using chlorophyll a and cyanobacteria in the ecological classification of lakes

Phytoplankton is one of the four key biological quality elements to be used in the ecological classification of lakes in Europe according to the Water Framework Directive (WFD). Chlorophyll a (Chla) has so far been used as the main – and sometimes only – metric to define class boundaries. Chla is often a key metric for lake managers and is used to determine whether and how much action should be taken to reduce the external nutrient loading. In this paper we present the analyses of empirical relationships between nutrient (total phosphorus, TP, total nitrogen, TN) concentrations versus Chla and the proportion of cyanobacteria of total phytoplankton biomass based on data from 440 Danish lakes (1800 lake years). These data represent one eco-region sampled using standardised methodology, thereby minimising the heterogeneity often seen in large datasets. Sampling frequency is important for the precision by which Chla can be determined and the precision is always low with less than 15 summer measurements. As expected Chla was related significantly to TP, but the variability was high, with R2 reaching only 0.47, 0.59 and 0.61 in shallow, stratified and siliceous lakes, respectively, based on summer averages. The correlation was strongest in late summer (R2 up to 0.80) and weak in winter. Chla is also related to TN, but the correlation coefficients were low throughout the year, and in a multiple regression with TP included, TN only added little to the total variability. Similarly, the proportion of cyanobacteria increased significantly with TP, but the correlation was weak. Seasonal and yearly data from five lakes with relatively stable TP show considerable variations in Chla and cyanobacteria abundance during a 20-year monitoring period. It is concluded that despite clear nutrient phytoplankton relationships it will be difficult to define the proposed WFD ecological classes – particularly regarding cyanobacteria. To ensure a high degree of certainty for meeting a specific water quality threshold, lake managers must reduce the external phosphorus loading more strongly than expected from existing simple empirical external loading-inlake TP–Chla relationships.