Implementing the Water Framework Directive in overseas Europe: A multimetric macroinvertebrate index for river bioassessment in Caribbean islands

Neotropical overseas regions of Europe have the same water policy objectives as the continental ones, but were overlooked during recent developments of bioassessment tools that fulfill the Water Framework Directive guidelines. We designed a macroinvertebrate-based multimetric index (IBMA) to assess ecological health in rivers of Martinique and Guadeloupe, two densely populated islands in the Lesser Antilles (Caribbean Sea). Invertebrates were sampled at 114 sites including reference and impacted river reaches following a normalized protocol. Among the 411 biological metrics calculated from our site-specific data, we selected metrics exhibiting the best trade-off between high discrimination efficiency, low specificity, low redundancy, and high stability under reference conditions. We finally retained seven metrics related to taxonomic diversity, species abundance, and preferences for some substratum types. Each metric was weighted by its discrimination efficiency. Using test data sets, we found that the IBMA was sensitive to the full range of disturbances in the area. Also, our index improved the detection of impairments, compared to the former practice by regional administrations. Finally, we suggest that the IBMA might prove relevant to neighboring islands in the biogeographic area.