Do we need to identify diatoms to the species level for assessing lake biological quality?

The Water Framework Directive asks to all Member States of the European Union to classify the ecological quality of significant waterbodies on the basis of the biological communities they host. One of the biological communities that must be used for the ecological quality assessment is the periphytic community, mainly composed by diatoms. In Italy, diatom-based lake quality assessment is performed using a specific index, named EPI-L, based on the method of weighted averages. For each species, a trophic score and an indicator weight were calculated using a data set of 80 lakes along a trophic gradient. The calibration data set includes 475 species, but only 109 were used in the calibration being found in at least 3 lakes. In order to reduce the complexity of the lake quality assessment, we calibrated a variant of EPI-L, using diatoms genera instead of species, and we compared the performance of these two variants in terms of correlation with the nutrient level and of different classification of each lake.