Orbetello lagoon (Tuscany, Italy) is a meso-eutrophic, shallow-water ecosystem which has undergone cyclic shifts in macrophyte dominance since 1970. Field data on the total standing crops of Chlorophyceae and Rhodophyceae (from 1983 to 2011) and Angiospermae (seagrass; from 1998 to 2007), produced each year by the ecosystem, was acquired. A general lagoon environment quality score (decay level DL, categories 0e4) was attributed (a posteriori) for each annual dataset examined. Univariate and multivariate statistical analyses were used to relate total macrophyte standing crops of 29 years of monitoring to the general quality score. Two indices were proposed (Abundance of Macroalgae Index, AMI, and Abundance of Seagrass Index, ASI, the latter in two versions, ASI-1 and ASI-2) and tested against DL. The results showed that biomass data did not accurately describe general environmental quality of the lagoon ecosystem, whereas the indices gave a better fit. AMI showed the best performance, demonstrating that macroalga data was much more informative than seagrass data. Values of AMI over 4.0 were significantly associated with critical general quality scores (DL > 2).
