Taxonomy- or trait-based ecological assessment for tropical rivers? Case study on benthic diatoms in Mayotte island (France, Indian Ocean).

Diatom-based ecological quality assessment methods have been implemented and used regularly in the Water Framework Directive. These indices use the species' abundance profiles along a specific environmental gradient, which they aim to assess. However, this approach has several problematic issues including the unstable and fast-changing diatom taxonomy. The use of traits can be a solution if their responses to the environmental pressure are well-defined. Here, we developed taxonomy-based and trait-based diatom assemblage indices to assess the ecological status of riverine sites on a tropical island. The two indices are based on two sub-indices that measure the diatom assemblage response to a nutrient and organic matter/turbidity gradient. Both taxonomy- and trait-based indices correlated significantly with the selected environmental gradients of the test database, which was not used during index development. We showed that traits could be used for quality assessment of the Mayotte rivers and require much less effort than taxonomy-based indices. There were differences between the two types of indices, which are discussed in this paper. As a perspective for further studies, tests of trait-based indices among different eco-regions would be challenging.