Integrating plankton indicators to assess the state of pelagic habitats in the Northeast Atlantic

Pelagic habitats in the Northeast Atlantic (NEA) have undergone substantial ecological change over the past six decades due to pressures such as climate change, overfishing, and nutrient pollution. To assess Good Environmental Status (GES; under the EU Marine Strategy Framework Directive) we present an integrated assessment of NEA pelagic habitat status using two plankton biodiversity indicators, representing community composition and plankton biomass/abundance, alongside an informative assessment of plankton diversity. We applied a stepwise approach, first combining component-level results across assessment units and fixed-point stations for four pelagic habitat types, then integrating status across indicators and habitat types to derive regional environmental status. Because operational thresholds are lacking, ‘Uncertain’ was the default status, while ‘Not Good’ required consistent, spatially representative biological change plausibly linked to anthropogenic pressures.

Across the assessed indicators, we found regional and habitat-specific changes in plankton lifeform abundance, and general declines in phytoplankton biomass and zooplankton abundance. Six habitat-region combinations were assessed as ‘Not Good’, three as ‘Uncertain’, and one was ‘Unassessed’ due to lack of data. No pelagic habitats or regions were found to be in GES. It was only possible to designate ‘Not Good’ or ‘Uncertain’ status due to lack of suitable baseline data and uncertainty around what constitutes ‘Good’ status in the context of NEA pelagic habitats. Sea surface temperature and nutrients were the most important pressures associated with change. These results highlight the need to reduce nutrient pollution and meet international climate targets to conserve pelagic habitats and their ecosystem services.