C.F. BOUDOURESQUE (1), E. CHARBONEL, A. MEINESZ, G. PERGENT, C. PERGENT- MARTINI, G. CADIOU, M.C. BERTRANDY, P. FORET, M. RAGAZZI, V.

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2 Bio]. Mar. Medit. (2000), 7 (2): C.F. BOUDOURESQUE (1), E. CHARBONEL, A. MEINESZ, G. PERGENT, C. PERGENT- MARTINI, G. CADIOU, M.C. BERTRANDY, P. FORET, M. RAGAZZI, V. RICo- RAIMONDINO GIS Posidonie and Centre d'océanologie de Marseille, UMR CNRS n 6540, Campus of Luminy, Marseilles cedex 9, France. A MONITORING NETWORK BASED ON THE SEAGRASS POSIDONIA OCEANICA IN THE NORTHWESTERN MEDITERRANEAN SEA Abstract The Posidonia Monitoring Network (PMN) is based on a biological indicator, the seagrass Posidonia oceanica. It was set-up in 1984 and concerns 33 survey sites located along the Provence and French Riviera coasts, at either the upper or lower limits of the seagrass meadows. Monitoring the lower limit is carried out by positioning permanent cernent markers along this limit and by photographing these markers and studying several P. oceanica vitality parameters every 3 years. Monitoring the upper limit is based on the same rnethods, completed with aerial photographs. Between 1984 and 1999, the percentage of sites at which the P. oceanica meadow was seen to be in expansion increased from 21 to 42%. This is consistent with improvement in the percentage of waste waters which are processed in treatment plants, less than 10% in the early 1980s and close to 100% today. Key-words: Posidonia oceanica, monitoring network, biological indicator. Introduction The Posidonia oceanica (L.) Delile meadow constitutes a key ecosystem in the sublittoral zone (0-40 m depth) of the Mediterranean, from both a biological and physical equilibria perspective. In addition, it harbors a very high species diversity (Boudouresque et al., 1994). P. oceanica beds are very sensitive to disturbances caused by human activity (e.g. coastal development, pollution, turbidity, anchoring, etc.) and their loss has been observed in a number of regions (Meinesz et al., 1991; Boudouresque et al., 1994). A large proportion of the Provence and French Riviera region's economic activity is based on tourism. Each year, this region hosts over 24 million touries, who are mainly attracted by the sea: swimming, pleasure boating, SCUBA diving and snorkeling. The region has therefore made commendable efforts to improve coastal water quality (e.g. setting up of waste water treatment plants, reduction in industrial and domestic pollution levels in the rivers flowing towards the Mediterranean) and preserve natural sites (e.g. establishment of Marine Protected Areas, banning of new harbors and reclamations). In order to evaluate the efficiency of these policies, the administrative authorities have surveillance systems at their disposai, based on the analysis of water chemical composition (nutrients and pollutants). It has appeared necessary, however, to supplement this information with a surveillance network based on a biological indicator, namely P. oceanica. The Posidonia Monitoring Network (PMN; in French: RSP, Réseau de Surveillance Posidonies) was created in 1984, at the request of the regional government of the Provence and French Riviera region (in French: Conseil Régional Provence-Alpes- Côte d'azur), and is a collaboration between the elected representatives, the state maritime services (Bouches-du-Rhône, Var and Alpes-Maritimes regions) and the

3 A monitoring network based on the seagrass Posidonia oceanica scientists belonging to the "GIS Posidonie" association (Boudouresque et al., 1990; Charbonne) et al., 1993). Materials and methods In 1984, 24 surveillance sites were selected along the 650 km of Provence and French Riviera coastline. An additional 9 sites were added in 1994, bringing the number of sites monitored by the PMN to 33. These sites are either located in sensitive areas (where human impact is high and where P. oceanica seagrass beds are likely to regress), in reference areas which are a priori not subjected to important levels anthropogenic impact and where the seagrass beds are likely to be either stable or on the increase, or finally in zones presenting intermediate characteristics (Fig. 1).

4 C.F. BOUDORESQUE, E. CHARBONEL, A. MEINESZ, G. PERGENT, C. PERGENT-MARTINI, G. CADIOR, 330 M.C. BERTRANDY, P. FORET, M. RAGAZZI, V. Rico-RAIMO NDI NO In light of the slow growth rate of P. oceanica, the scientific surveillance of each site is performed every three years. Chronology of the PMN activities has thus been as follows: period (selection of the sites and assessment of initial conditions), period (first-time return to the sites), period (second return), period (third return), (fourth return). The PMN survey sites are located at the two extremes of P. oceanica's bathymetric range: at its upper growth lirait (15 sites) and its lower growth limit (18 sites). Indeed, it is at these limits that the seagrass bed is the most sensitive to changes in environmental conditions. At the lower limits, a dozen or so cement markers are laid down every 5 m. At the upper limit, the laying down of markers is accompanied by the taking of aerial photographs, according to a standardized protocol (altitude, lens, time, angle, resolution, contrast, etc.) and validated by ground truth. For each site, three scales are considered: (i) system scale (aerial photographs, measure of bottom cover), (ii) meadow scale (e.g. photographs of Posidonia around the markers, shoot-density, laying bare of the rhizomes, plagiotopic to orthotropic rhizome ratio) and (iii) the plant itself (biometry, lepidochronology). The surveillance protocol has progressively been improved as experience has been gained and scientific research has evolved (Meinesz, 1977; Lefèvre et al., 1984; Pergent et al., 1995). Results Over the 15 years of surveillance, P. oceanica seagrass bed dynamics have exhibited two opposite trends. At its upper growth limit, a constant decrease in the number of losses can be observed (Tab. 1). Conversely, at its lower limit, losses of P. oceanica beds are on the increase. Globally, the percentage of limits either undergoing a loss or stable has decreased (79 --> 58%), for the benefit of lirait expansions (21-42%). Closer scrutiny of the data, however, reveals that noticeable différences exist among the différent regions (e.g. East vs West of the monitored area), and that the changes in a given site are not always constant: alternating phases of loss and expansion can occur. viscussion ana conclusions In the Provence and French Riviera region, the percentage of urban waste water which undergoes treatment, and the percentage of pollution removed from the riverwaters flowing into the Mediterranean, has risen from less than 10 and 5% in the early 1980s to approximately 95 and 50% today, respectively. The trend in pollutant levels in

5 A monitoring network based on the seagrass Posidonia oceanica 331 littoral waters over this period is often difficult to interpret due to the very high variability of the values recorded. Conversely, based on a biological indicator, the PMN bas clearly demonstrated an overall improvement in the situation. Thus, the PMN is able to provide the elected representatives, local authorities and administrations responsible for the management of coastal regions with a useful, relatively inexpensive and easy to use tool which allows an overall assessment of the quality of the marine environment. The PMN, along with a number of other surveillance networks (based on the measurement of physico-chemical parameters, toxic phytoplankton levels, etc.), are about to be grouped into a single surveillance network for the French Mediterranean coast (in French: RLM, Réseau du Littoral Méditerranéen), whose goal it will be to coordinate these complementary tools and handle the incoming data with greater efficacy. A number of Mediterranean countries are setting up, or are about to set up, surveillance networks similar to that of the PMN. In addition, those parties who signed the Barcelona Convention (UNEP) adopted a plan of action towards the preservation of the marine Mediterranean flora in 1999, which stipulates that the setting up of a P. oceanica surveillance network is a priority. References BOUDOURESQUE C.F., BERTRANDY M.C., BOULADIER E., FORET P., MEINESZ A., PERGENT G., VITIELLO P. (1990) - Le Réseau de Surveillance des herbiers de posidonies mis en place en Région Provence-Alpes-Côte d'azur (France) Rapp. Comm. int. Explor. Mer Médit., 32 (1): 11. BOUDOURESQUE C.F., GRAVEZ V., MEINESZ A., MOLENAAR H., PERGENT G., VITIELLO P. (1994) - L'herbier à Posidonia oceanica en Méditerranée: protection légale et gestion. Pour qui la Méditerranée au 21' siècle? Villes des rivages et environnement littoral en Méditerranée, Montpellier, Fr.: CHARBONNEL E., BOUDOURESQUE C.F., BERTRANDY M.C., FORET P., MEINESZ A., PERGENT G., PERGENT-MARTINI C., RICO-RAIMONDINO V., VITIELLO P. (1993) - Le Réseau de Surveillance Posidonies en Région de Provence-Alpes-Côte d'azur (Méditerranée, France): les premiers résultats. Symp. internation. Protection du Milieu Marin contre la Pollution Urbaine, Marseille, Fr., 3: LEFEVRE J.R., VALERIO C., MEINESZ A. (1984) - Optimisation de la technique aérienne pour la cartographie des herbiers de posidonies. In: Boudouresque C.F., Jeudy de Grissac A., Olivier J., (Eds.), International workshop on Posidonia oceanica beds, GIS Posidonie publ., Fr., 1: MEINESZ A. (1977) - Balisage de la limite inférieure de l'herbier de Posidonia oceanica en rade de Villefranche-sur-mer (Alpes-Maritimes, France). Rapp. Comm. int. Explor. Mer Médit., 24 (6): MEINESZ A., LEFEVRE J.R., ASTIER J.M. (1991) - Impact of coastal development on the infralittoral zone along the southern Mediterranean shore of continental France. Mar. Poll. Bull., 23: PERGENT G., PERGENT-MARTINI C., BOUDOURESQUE C.F. (1995) - Utilisation de l'herbier à Posidonia oceanica comme indicateur biologique de la qualité du milieu littoral en Méditerranée Etat des connaissances. Mésogée, 54: 3-27.