Indicators and criteria to assess ecological status of the large shallow temperate polymictic lakes Peipsi (Estonia/Russia) and Võrtsjärv (Estonia)

Pressure-based approach, morphoedaphic index and historical data were used in deriving reference conditions and classification criteria for the Estonian large lakes: Peipsi and Võrtsjärv. The comparison of the measured total phosphorus with the reference level based on the morphoedaphic index, shows that the water quality could be estimated as 'moderate' in lake Peipsi and 'good' in Võrtsjärv despite that the measured TP values were higher in the latter. Comparing the changes observed in different quality elements with the normative definitions for quality classes given in the Water Framework Directive yielded quality estimates from 'high' to 'moderate' for both lakes. Applying the \one-out all-out\ principle according to which the ecological status is determined by the biological quality element, which shows the greatest anthropogenic disturbance, both lakes fall unequivocally into the 'moderate' category. Averaging the quality estimates for lakes Peipsi and Võrtsjärv no matter whether on a quality element level or a single parameter level would indicate good status in both lakes that is close to the subjective cognitive expert opinion of the authors.