Physico-chemical criteria to support good ecological status in Europe : report to ECOSTAT by the Task Group on Supporting Physico-Chemical Elements, 2024

This report summarises approaches to setting robust criteria to achieve Water Framework Directive (WFD) objectives for the Annex V physico-chemical supporting elements: transparency, oxygenation, temperature, salinity and acidification. Nutrients have been considered in previous reports. This work is intended to complement existing guidance on statistical approaches to setting thresholds. A general conclusion is that thresholds that were set for older directives (e.g. Freshwater Fish Directive) may not be sufficiently stringent to protect Good Ecological Status and that Member States should check the validity of these. The current approach to data collection and aggregation may also need to be revisited. For example, for oxygen or salinity, concern is warrented for both persistent chronic impacts and episodic acute incidents which are becoming more frequent with climate change events such as heatwaves. We tested the latest version (2023) of the toolkit for setting thresholds on transparency and oxygenation conditions using EU-wide data demonistrating its potential. We identify situations where it may be possible to use historical time series, ecotoxicological data and peer-reviewed literature to set thresholds. However, thresholds are highly context specific, so it is rarely possible to give specific guidelines that apply across the EU. Each chapter ends with a summary table to help guide on the appropriatness of the different approaches to set thresholds.