Testing MEDOCC and BOPA indices in shallow soft-bottom communities in the Spanish Mediterranean coastal waters

The implementation of the European Water Framework Directive has enhanced the development of several indices based on benthic invertebrate communities. Some of them, such as BOPA, simplify the calculation employing the taxonomic sufficiency principle to assess the ecological status of ecosystems since they only consider opportunistic polychaetes and amphipods; others, such as MEDOCC, include all the species found in the community, and assign them to four ecological groups: sensitive, indifferent, tolerant, and opportunistic. Anyway, there is the need for testing, validating and improving these indices in different regions and communities. In this study we test the application of MEDOCC and BOPA along the Spanish Mediterranean coastal waters (Catalonia, Balearic Islands, Valencia, Murcia, and Andalusia regions) in order to investigate whether benthic indices with different taxonomic resolution provide similar ecological status assessment. Shallow fine soft-bottom communities were sampled from 2002 to 2010 in a total of 241 locations. The comparison between both indices demonstrated that the concordance for the studied communities reached up to 79% in some areas, although it was less than 30% in natural and anthropogenically modified ecosystems dominated by stress-tolerant species. Even though BOPA index provides good results, in general it evaluates in higher ecological status than MEDOCC does and it does not always allow discriminating more disturbed situations. The assignation of a same species to different ecological groups (some opportunistic polychaetes of one method are classified as indifferent by the other) and the fact that all the amphipods (except genus Jassa) are considered as sensitive species in BOPA index but not in MEDOCC, could contribute to the observed discrepancies. Some suggestions to improve the agreement between both methods are pointed and discussed.