Résumé
Transitional waters as ecotones between terrestrial, freshwater and marine ecosystems, characterized by high spatial heterogeneity and temporal variability are examined. The EU Water Framework Directive (WFD) poses to the scientific community the challenge of ecosystem types. A niche theory approach is proposed to identify the limiting forcing factors organizing biological quality elements such as the limiting niche dimensions. The analysis of a macro-invertebrate dataset from published papers on 36 Italian lagoons suggests a two-level typological classification of Mediterranean lagoons. It is found that basic ecological theories such as niche and island biogeography theories, have fundamental implications for the process of developing a typological classification for all aquatic ecosystems, as required by the WFD.