A procedure to update quality indices based on species abundances: an example using the EPI-L diatom index

We propose a procedure to update ecological quality indices using species autoecology estimated by gradient analysis when new species are found, or taxonomy is updated. When updating an index, the new values must be comparable to the old ones to avoid recal-ibration and quality class boundary changes. As gradient length influences the values of the species optima, we propose to avoid adding new species to existing lists and we suggest recalibrating the index with a new calibration dataset and rescaling species optima. This final step reduces index updating-induced quality classification changes. An example is shown using the EPI-L diatom index, a quality index for lakes affected by trophic pressure.