Defining thresholds for good environmental status using structured expert elicitation

Abstract Thresholds for good status are required under the EU Marine Strategy Framework Directive (MSFD) and the UK Marine Strategy (UKMS) to assess if good environmental status is being achieved and how human activity affecting marine environments is to be managed. Thresholds sit within a broader ecosystem-based management framework, where Regional Seas Conventions and sectoral bodies support implementation of environmental policy. ‘Good status’ thresholds must be clear to inform regulatory and management decisions with direct implications for coherence across EU, UK, and regional marine governance. The IDEA protocol is a structured elicitation approach, obtaining expert judgment in a formalized, transparent, and replicable way to reduce subjectivity and biases in expert-estimated thresholds. We used this protocol to estimate quality (i.e. biomass) and extent (i.e. area) thresholds for Seafloor Integrity (MSFD D6) using three example systems (horse mussel bed, soft-sediment bivalve community, and common cockle bed). It emerged there were two different interpretations of the point at which a benthic system was considered ‘adversely affected’ amongst experts. Thresholds interpreted as whether the system was currently in a good state were broadly consistent across systems (quality thresholds: 62%–69% of unimpacted biomass, extent thresholds: 76%–83% of total area). In contrast, thresholds interpreted as whether the system could recover to a near-unimpacted reference state in the future, were highest for the most sensitive (horse mussel) system (quality: 59%, extent: 80%) and lower for others (quality: 24%–45%, extent: 59%–65%). It is unclear which of these threshold values are most applicable to assess benthic ecosystem condition under marine legislation (e.g. MSFD D6), as there was no alignment amongst experts on an interpretation of ‘adversely affected’. Therefore, we present two divergent thresholds for policymakers to review in terms of their applicability for assessing Seafloor Integrity and suggest that defining what ‘good status’ refers to in practical terms is an urgent need. The IDEA protocol presents a transparent, indicator-independent approach to estimate thresholds for good status, however, consensus on the practical interpretation of ‘good’ is required to produce operational thresholds that can be applied consistently across ecosystem indicators, regions, and legislative frameworks, to support robust marine environmental assessments.