Exploring the robustness of macrophyte-based classification methods to assess the ecological status of coastal and transitional ecosystems under the Water Framework Directive

Identifying and quantifying the factors that contribute to the potential misclassification of the ecological status of water bodies is a major challenge of the Water Framework Directive (WFD). The present study compiles extensive biomonitoring data from a range of macrophyte-based classification methods developed by several European countries. The data reflect spatial and temporal variation as well as inter-observer variation. Uncertainty analysis identified that factors related to the spatial scale of sampling generally contributed most to the uncertainty in classifying water bodies to their ecological status, reflecting the high horizontal and depth-related heterogeneity displayed by macrophyte communities. In contrast, the uncertainty associated with temporal variation was low. In addition, inter-observer variation, where assessed, did not contribute much to overall uncertainty, indicating that these methods are easily transferable and insensitive to observer error. The study, therefore, suggests that macrophyte-based sampling schemes should prioritize large spatial replication over temporal replication to maximize the effectiveness and reliability of water body classification within the WFD. We encourage conducting similar uncertainty analyses for new/additional ecological indicators to optimize sampling schemes and improve the reliability of classification of ecological status.