An intercalibration exercise for benthic macrophyte indices across the Mediterranean Sea coastal lagoons

Within the transitional waters macrophyte Mediterranean Geographical Intercalibration Group (MEDGIG) of the Water Framework Directive intercalibration, three countries (France, Greece, and Italy) compared their methodologies (Exclame, EEI-c, R-MaQI, respectively) for coastal lagoons. All methods classified soft bottom benthic macrophytes (angiosperms, seaweeds) in several sensitivity groups following the concept that “anthropogenic pressure” (stress) drives the ecosystem from a pristine state, where seagrasses are dominant, to a degraded state, where opportunistic species and phytoplankton are dominant. While Greece and Italy assessed the species abundance as coverage (%) in the laboratory, France assessed the species abundance as cover (%) in the field. A database consisting of 105 taxa abundance and pressure data from 55 shallow (depth=1-3m) and vegetated (cover >10%) sites (14 in France, 20 in Greece, 20 in Italy) belonging to meso-, poly- and euhaline (salinity >5‰) coastal lagoons, either confined or not confined, has been created. The 3 methods used a similar scale at biological (species), spatial (site) and temporal (one sampling per year during spring-summer) level, enabling a direct comparison of the 3 indices at biological community level. A common pressure index based on expert judgment was calculated. Multivariate analyses (MDS, Cluster) indicated no biogeographical differences across the Mediterranean Sea. SIMPER analyses confirmed that reference “benchmark” sites communities (pressure index ≤ 6) were characterized by the dominance of angiosperm species (Cymodocea nodosa=49.9%, Ruppia cirrhosa=35.67%, Zostera noltii=10%), while “borderline” communities between good and moderate ecological status were dominated by macroalgae-cyanobacteria in coexistence with angiosperms. Due to ecosystems high natural variability and to relative low number of benchmark sites provided, it was decided to use continuous benchmarking to determine the differences between the countries. Greece appeared more precautious and adjusted its quality class boundaries by lowering both High/Good and Good/Moderate boundaries to 0.7 and 0.4, respectively (France, and Italy: H/G=0.8, G/M=0.6).JRC.H.1-Water Resource