WHAT BIOTIC INDICES TELL US ABOUT ECOSYSTEM CHANGE: LESSONS FROM THE SEAGRASS POSIDONIA OCEANICA INDICES APPLICATION ON HISTORICAL DATA

Anthropogenic pressure on marine ecosystems is affecting water quality and seafloor integrity. Mediterranean seagrass meadows of endemic Posidonia oceanica (Linnaeus) Delile are considered a priority habitat under the European Directive 92/43/ CEE, given their ecological and economic importance and being an environmental quality indicator. Availability of historical data on three P. oceanica meadows along the Ligurian coast (NW Mediterranean Sea) allowed assessing change in the meadow status over time. A number of indicators and ecological indices at different levels of ecological complexity, including the multimetric PREI (Posidonia Rapid Easy Index) adopted by the environmental agencies, were employed and compared. This paper aims at a) defining the health status of the P. oceanica meadows and measuring their changes through time collating available historical information; b) evaluating the discriminating power of the different indices and assessing their consistency with each other. The different indices adopted revealed little consistency thus suggesting that no single index can define the health status of P. oceanica meadows; as a consequence the use of an indices set is highly recommended to monitor meadow evolution over time.