Comparative evaluation of the effectiveness of the available ecological indicators of macroinvertebrates health: a field test on a dystrophic crisis event

Biomonitoring with benthic macro-invertebrates is a major tool to evaluate ecological status of aquatic ecosystems. Macro-invertebrates represent also a key Biological Quality Element (BQE) for the evaluation of ecological status of all categories of water bodies in the EU Water Framework Directive (WFD). According to WFD, the evaluation and classification of ecological status is based on the comparison of metric values within the considered aquatic ecosystems with the same metric values in reference 'pristine' ecosystems of the same type. This approach is a major problem for the classification of Mediterranean transitional water ecosystems, since in the Mediterranean Ecoregion almost all relevant transitional waters have been used by human societies since centuries; there are not pristine transitional water in the Mediterranean. Here, we test the adequacy of a reciprocal approach, based on the use of extremely polluted conditions, as reference cases. To this aim, following the strong dystrophic crisis that has occurred to a portion of Lesina lagoon during the summer 2008, we have analysed the spatial and temporal patterns of most of the metrics proposed so far to evaluate the status of benthic macro-invertebrate guilds. They involve, simple and complex metrics, taxonomically based, functionally based and size spectra based, accounting for taxa sensitivity or considering all taxa with the same weight. Globally, more than forty metrics were tested and for the 25th percentile characterized by the highest redundancy and the lowest variability, the patterns of variation between polluted and unpolluted sites and along the temporal scale of the dystrophic crisis are described. The intercalibration of the different metrics and their adequacy to describe the ecological status of transitional waters is discussed.