Development of novel diatom metrics to assess ecological status of phytobenthos in Greek lakes

Abstract

Mediterranean lakes present particular challenges when developing ecological assessment techniques, due partly to long histories of human activity in their catchments but also to strong interactions with climate, intensifying in recent decades due to global warming. A survey of diatoms in lakes in Greece revealed both a strong nutrient gradient and a salinity gradient. It is necessary to disentangle these when assessing ecological status in order that appropriate measures for restoring good ecological status are applied. Two approaches were used to develop a Water Framework Directive-compatible metric for assessing ecological status using diatoms. The strongest relationship was obtained using weighted averaging, but an alternative metric, generated by rescaling a well-established and widely used diatom metric performed only slightly less well. The limited number of lakes in Greece resulted in a small dataset, creating a risk of overfitting models and the rescaled metric has the advantage of drawing on a broader autecological knowledge base. Supplementary metrics to evaluate salinity are also proposed in order to ensure that lakes subject to this pressure are clearly differentiated from those where nutrient impacts predominate. Approaches to defining reference conditions and status class boundaries draw on established criteria developed for Water Framework Assessments in Europe to ensure that status assessments in Greece reflect similar levels of ambition to those established elsewhere in the European Union.