Is there a need for a more explicit accounting of invasive alien species under the Water Framework Directive

Through ratification of the Water Framework Directive (WFD), EU Member States committed themselves to a pressure-based assessment of the ecological status of their water bodies. Invasive alien species (IAS) constitute a major pressure in many aquatic ecosystems, yet are not explicitly accounted for by the majority of WFD assessment methods. Most Member States argue that no explicit assessment of IAS is required, assuming that significant IAS pressures will affect the WFD biological quality elements (BQEs), and be detected by generic WFD status assessments. We tested this assumption for a selection of country-by-surface-water category combinations, covering nearly 40,000 water bodies. For each of the combinations, the pressure by high-impact IAS is higher in water bodies with ecological status varying from bad to moderate than in water bodies in good or high ecological status. Most high-impact IAS show strong associations with low status class categories. Of the 17 most frequently occurring high-impact IAS, only Mustela vison (Schreber, 1777) and Potamopyrgus antipodarum (Gray, 1853) are disproportionately frequent in high status water bodies. The sensitivity of WFD methods varies across BQEs, with macrophyte-based methods showing a consistently high sensitivity to IAS pressures. However, significant pressures are observed in a number of high status water bodies. This points to a need for further optimization of existing methods so that they address the full range of pressures exerted by IAS.