ESMI: a macrophyte index for assessing the ecological status of lakes.

The paper introduces the Ecological State Macrophyte Index (ESMI), a method compliant with the Water Framework Directive (2000/60/EC) for assessing the ecological status of lakes based on macrophytes. A description of the elaboration of macrophyte metrics, relevant reference conditions and a classification system for two types of high-alkalinity lowland lakes (stratified and polymictic), and a customised field survey procedure based on belt transects are presented. The ESMI evaluates two aspects of macrophyte community: taxonomic composition (index of evenness J) and abundance (colonisation index Z), which are combined into one multimetric. ESMI values range from 0 to 1, where 1 denotes pristine conditions and 0, highly degraded habitats. The high/good class boundary (H/G) was set at the first quartile of ESMI values of reference lakes. For the other classes, boundaries were set by dividing the range of ESMI values between the H/G boundary and the minimum value recorded in the dataset in a logarithmic scale into four. The ESMI correlated best with water transparency (Pearson's R = 0.62 in stratified lakes and 0.79 in polymictic ones), whereas the correlations with phosphorus and nitrogen concentrations were somewhat weaker (R = -0.48 to -0.57). Based on the results of international intercalibration, the original class boundaries were modified (merged for stratified and polymictic lakes, the good/moderate boundary tightened to approximately 20%), to make ESMI-based assessment results comparable with the outcomes of other European methods.