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The seagrass Posidonia oceanica is extensively monitored in Mediterranean coastal waters and is an ideal candidate for an eco-regional assessment of the coastal ecosystem. The aim of this study is to evaluate the potential of P. oceanica as eco-regional indicator for its assessment at the scale of Mediterranean basin. For this purpose, regional and national P. oceanica monitoring programmes are identified, and their data and metadata are collected and compared in terms of objectives, strategies, sampling designs and sampling methods. The analysis identifies a number of issues concerning data quality, reliability and comparability. In particular, the adoption of different sampling designs and methods may introduce relevant errors when comparing data. The results of this study stress the necessity of carefully planning monitoring programmes. Moreover, it highlights that the adoption of a number of common tools would facilitate all Mediterranean monitoring activities and allows an optimisation of management efforts at an eco-regional scale.

The Biotic Index based on Posidonia oceanica (BiPo) is a classification system for evaluation of the ecological status in Mediterranean coastal waters, developed in accordance with the EU Water Framework requirements. The aim of this study is to verify the applicability and reliability of the BiPo index to different geographical areas of the north-western Mediterranean (France, Spain and Italy), to understand whether such a classification system may be applied more extensively, as so far it has only been applied to coastal waters in Corsica. The ecological status determined for sites is verified against pressures revealed from satellite imagery and from trace metal contamination of plants, to identify the sources of pressure that may be responsible for a low ecological status. The results of this study indicate that: (i) the BiPo index responds reliably to pressures, in different areas of the Mediterranean; (ii) sites with an ecological quality ratio (EQR) close to the good/moderate boundary require particular attention to identify and reduce causes of deterioration; (iii) the support of chemical indicators, in this case metal contamination, is relevant to identify potential sources of pressure.

Human activities generate large volumes of waste that supply marine coastal environments in pathogens, organic matter, nutrients and toxicants. Among the wide range of toxicants are trace elements. Since the latter are toxic for aquatic organisms from threshold levels and as they are therefore likely to cause multiple damage to the population, the community and the ecosystem levels, their environmental occurrence has to be accurately monitored in order to guarantee appropriate environmental management of coastal zones and to preserve marine coastal ecosystems and the goods and services they provide. In the framework of the STARECAPMED project, the present study aimed to monitor, for the first time, the coastal contamination of the entire Mediterranean by As, Ag, Cd, Cu, Hg, Ni and Pb, using Posidonia oceanica as bioindicator species. But sustainable coastal management also requires the development of appropriate contamination classification systems intended, among other purposes, for environmental managers and policy makers. The combined utilization of several complementary monitoring tools, i.e. water quality scale, pollution index (TEPI and TESVI) and spatial analysis (PCA, CA, correlation analysis and GIS mapping) successfully led to the development of an operational classification system of this kind. In particular, the mapping of the trace element contamination according to a new proposed 5-level water quality scale using the quantile method precisely outlined the contamination severity along Mediterranean coasts and facilitated interregional comparisons. The reliability of the use of P. oceanica as bioindicator species was further again demonstrated through several global, regional and local detailed case studies. In conclusion, holistic approaches such as developed in the present study should be privileged to accurately monitor the contamination rate of coastal waters and to transfer relevant information on this composite problem to environmental managers and policy makers.

Biological indicators have the capacity to integrate the temporal changes of contaminants, concentrations or fluxes over various time-scales, and are thus considered as interesting tools for water quality biomonitoring. Since the mid-70ies, French programs have developed water monitoring approaches based on the use of bivalve molluscs; and recently the natural background and the extent of water contamination were bioassessed at the scale of the whole western Mediterranean. But even if bivalve molluscs are viewed as reliable bioindicators, their use is not always made easy as a result of their absence in numerous coastal regions that force their transplantation (cages) during several months before their sampling and analysis. This weakness led several scientists to evaluate the bioindicator abilities of other marine organisms. Seagrasses, whose ability to bioaccumulate contaminants proportionally to environmental contamination levels has been clearly demonstrated, have thus been proposed as an appropriate alternative tool for coastal water quality assessment. Very little studies have however so far considered the combined utilization of these two groups of bioindicator organisms, i.e. caged bivalve molluscs and seagrasses. In the framework of the STARECAPMED project, we therefore compared and discussed the bioaccumulation of trace elements in the Neptune grass Posidonia oceanica and in caged Mediterranean mussels Mytilus galloprovincialis. The sampling was performed at the scale of the western Mediterranean. The two species told two contamination stories which, although sometimes different, showed to be complementary. P. oceanica and M. galloprovincialis bioaccumulated dissolved trace elements from the water column and thus provided information regarding trace element contamination severity integrated over several days to a few months. Seagrasses, strongly rooted in the sediments, reflected the long-term exposure to trace elements since sediments offer a degree of time integration over several years to decades. Caged mussels, as filter feeder artificially maintained in the water column, bioaccumulated trace elements from their particulate phase, and therefore gave valuable information regarding continental-terrigenous inputs to coastal waters. In conclusion, seagrasses and mussels should neither supplant, nor substitute, but rather complement each other in order to provide the full time- and space-integrated coastal contamination story of the Mediterranean.