Size spectra show common patterns of variation among ecosystem types, functional guilds and taxonomic groups, as well as predictable responses to pressures. Here, we extend the size spectra approach to macroinvertebrate ecological status assessment in transitional waters, by developing, testing and validating a multi-metric index of size spectra sensitivity (ISS), which integrates size structure metrics with metrics describing the sensitivity of size classes to anthropogenic disturbance and species richness measures. The ability of different theoretical models of size spectra sensitivity to discriminate between undisturbed and disturbed ecosystems and levels of environmental stress was evaluated. We have used data on benthic macroinvertebrates within 12 Mediterranean and Black Sea transitional water bodies (i.e. coastal lagoons) from Italy, Albania, Greece, Bulgaria and Romania and compared the models’ efficiency through their pressure-impact response along organic load and enrichment gradients. Data from a thirteenth Mediterranean lagoon was used for validation purposes. Asymmetric models of size class sensitivity, assuming higher sensitivity of larger body size classes, were found to be more effective than symmetric models, with higher sensitivity of less common size classes, in distinguishing undisturbed from disturbed lagoons and lagoon stations. The ISS based on the asymmetric sensitivity model that was most efficient in identifying the anthropogenic impacts also showed significant dose–response relationships along environmental parameters describing the organic load and enrichment gradients. A scheme for the classification of Ecological Quality Status based on the macroinvertebrate ISS is proposed and validated. The validation procedure found that ISS is an effective and sensitive monitoring tool, easy to apply and to inter-calibrate among laboratories. Moreover, the tests performed here over a very large ecoregional area have proved the spatial robustness of the new index.
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