Defining ecologically derived transparency boundaries for assisting Greek natural lakes ecohydrological management

Water transparency is a simple qualitative feature widely used as a proxy for lake trophic state and eutrophication pressure. The Water Framework Directive (WFD) requires boundaries for physical and chemical supporting elements to ensure good ecological status, yet many Member States lack type-specific transparency boundaries linked to biological responses. Secchi depth represents an ideal indicator for both expert and non-expert monitoring, integrating ecohydrological processes into an accessible, cost-effective measure. This study aims to designate water transparency boundaries supporting good ecological status for three Greek natural lake types: very shallow, shallow and deep lakes, for monitoring and management. Using national monitoring data (2013–2023) from 19 natural lakes, we applied binary logistic regression to establish transparency boundaries calibrated against two biological quality elements (BQEs): phytoplankton and macrophytes. Model performance showed moderate to good predictive capacity with a conservative error balance favoring environmental protection. The analysis yielded type-specific boundaries of 0.6 m for very shallow lakes (phytoplankton), 1.4–1.5 m for shallow lakes (macrophytes and phytoplankton, respectively) and 3.1 m for deep lakes (phytoplankton). These empirically derived, BQE-validated boundaries provide operational targets for WFD compliance, restoration planning and citizen science monitoring programs, supporting adaptive ecohydrological management of Mediterranean lake ecosystems while enabling accessible long-term trend assessment.