A GIS Application for the Study of the Structure and the Dynamic of Seagrass Beds at Large Scale: Example of Posidonia Oceanica Beds in the Bay of Saint-Cyr (France, Mediterranean Sea)

Agathe Leriche, Charles-François Boudouresque

Centre d'Océanologie de Marseille, UMR CNRS n°6540, Marseille (FR)

Posidonia oceanica (Linnaeus) Delile is a seagrass endemic to the Mediterranean Sea which constitutes extensive meadows from sea level down to 25-40 m depth (depending on water transparency). From the biological and physical perspective, P. oceanica beds represent a key ecosystem in terms of primary production, export of detritus towards other ecosystems (Pergent et al., 1994), species diversity (Boudouresque et al., 1994), spawning and nursery opportunities for species of fishery interest (Francour, 1997), water movement and sediment flows (Gambi et al., 1989). Due to its ecological role P. oceanica is now a protected species in Spain and France.
A number of studies have shown that P. oceanica beds are very sensitive to disturbance caused by human activity, e.g. coastal development (Meinesz et al., 1991), pollution (Delgado et al., 1999; Pergent et al., 1999), turbidity (Ruiz & Romero, 2001), anchoring (Porcher, 1984; Francour et al., 1999) and trawling (Ardizzone & Pelusi, 1984; Sánchez-Lizaso et al., 1990; Martín et al., 1997; Pasqualini et al., 1999, 2000) and their disappearance has been observed in a number of regions (e.g. Augier & Boudouresque, 1970; Meinesz et al., 1991; Ramos-Esplá et al., 1994). At the fine spatial scale of study, the structure of P. oceanica beds can always be explained by the level of the human impact or by highly unusual natural processes (e.g. freshwater resurgence). At large scale however, there is often a wide diversity of structures in seagrass beds which does not appear to be in direct relation to human activity. The aim of this work (doctoral thesis funded by the Conseil Régional Provence-Alpes-Côte d'Azur and Méditerranée 2000, in collaboration with IFREMER and the University of Corte) is to assess whether structure and spatial and temporal variations of P. oceanica beds are explainable by direct human pressure alone or whether other processes have an influence on them. It requires manipulation and crossing of numerous spatial and temporal data and is thus well suited to GIS analysis.

The database is organized similarly for the four study areas of the thesis project. Here, results are presented for only one site: the Saint-Cyr Bay. Potential human impact on this site takes the form of the port facilities of La Ciotat, two small pleasure boat harbors (Les Lecques and La Madrague), seaside tourism and trawling. Data concerning seagrass beds are the first type of data log in the GIS (ArcView 3.2® software), including:

• Qualitative present data: aerial photographs (taken in July 2000) and sonograms (campaign of side scan sonar in July 2000) of study area are logged in the GIS and geometrically corrected by means of the extension Géoref-Image® version 2.2b of ArcView 3.2® software. Feature extraction process by means of ImageAnalysis® extension with manual corrections is applied. Spatial qualitative data on P. oceanica beds are logged out in the form of a map of seagrass beds present distribution (after scuba diving ground truth).
• Qualitative ancient data: ancient aerial photographs and maps. Aerial photographs (dating from 1944 to 1999) are processed in the same way as the present day photographs. Ancient maps (dating from 1887 to 1999) are digitized, logged in the GIS and the limits of the seagrass bed and the shoreline is manually redrawn after geometric correction. To be usable, the reliability of the ancient maps must be estimated. A reliability index (RI) is calculated for each map and is incorporated in the database.
• Quantitative present data: coverage, shoot density, rhizome baring and plagiotropic rhizome percentage, leaf coverage by epiphytes and grazing pressure were measured by scuba diving on 33 points distributed along 9 transects. These parameters, used for estimating vitality of a P. oceanica bed (Marcos-Diegos et al. 2000), are logged into the database. Geo-statistical GIS analysis underlines spatial structuring of data. These quantitative present data are logged out in the form of an interpolated map for each parameter. A map of normal/abnormal shoot density is established by comparison with reference density according to depth (Pergent et al., 1995).
  
  The comparison between present and ancient qualitative data makes it possible to establish spatial and temporal variations of the P. oceanica distribution in the Saint-Cyr Bay since the beginning of the century. The comparison with quantitative data allows determination of the dynamics and vitality of the seagrass bed of the site and assessment of future possible variations in the distribution of P. oceanica. This type of analysis on the complete database (data for the four sites) will enable the authors to propose a multicriterion grid of seagrass bed vitality, useful for decision makers. The incorporation within the database of a second type of data, concerning environmental conditions (both natural and human induced, e.g. presence of harbors, estimation of anchoring and fishing effort, surface area of coastal development, protected areas, water temperature, meteorological data, etc.) will provide a basis for developing hypotheses about the factors which control the seagrass beds structure and dynamics.

---

References:

• Ardizzone, D. & Pelusi, P. 1984 Yield and damage evaluation of bottom trawling on Posidonia meadows. In: First international workshop on Posidonia oceanica beds (Boudouresque, C.F., Jeudy de Grissac, A. & Olivier, J., eds). Gis Posidonie Publ., Marseille, pp. 63-72.
• Augier, H. & Boudouresque, C.F. 1979 Premières observations sur l'herbier de posidonies et le Détritique côtier de l'île du Levant (Méditerranée, France) à l'aide du sous-marin Griffon de la Marine Nationale. Travaux scientifiques du Parc national de Port-Cros 5, 141-153.
• 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. In: Pour qui la Méditerranée au 21° siècle? Villes des rivages et environnement littoral en Méditerranée. Okéanos, Montpellier, pp. 209-220.
• Delgado, O., Ruiz, J., Perez, M., Romero, J. & Ballesteros, E. 1999 Effects of fish farming on seagrass (Posidonia oceanica) in a Mediterranean bay: seagrass decline after organic loading cessation. Oceanologica Acta 32, 109-117.
• Francour, P. 1997 Fish assemblages of Posidonia oceanica beds at Port-Cros (France, NW Mediterranean): assessment of composition and long-term fluctuations by visual census. P.S.Z.N. Marine Ecology 18, 157-173.
• Francour, P., Ganteaume, A. & Poulain, M. 1999 Effects of boat anchoring in Posidonia oceanica sea-grass beds in the Port-Cros national Park (north-western Mediterranean Sea). Aquatic Conservation: marine and freshwater Ecosystems 9, 391-400.
• Gambi, M.C., Buia, M.C., Casola, E. & Scardi, M. 1989 Estimates of water movement in Posidonia oceanica beds: a first approach. In: Second international workshop on Posidonia beds (Boudouresque, C.F., Meinesz, A., Fresi, E. & Gravez, V., edsit.). GIS Posidonie Publ., Marseilles, pp. 101-112.
• Marcos-Diego, C., Bernard, G., Garcia-Charton, J.A. & Pérez-Ruzafa, A. 2000 Methods for studying impact on Posidonia oceanica meadows. In Introductory guide to methods for selected ecological studies in marine reserves. Goni, R., Harmelin-Vibien, M., Badalamenti, F., Le Diréach, L., Bernard, G. edit., GIS Posidonie publ., Fr., 57-62.
• Martín, M.A., Sánchez-Lizaso, J.L. & Ramos-Esplá, A.A. 1997 Cuantificación del impacto de las artes de arrastre sobre la pradera de Posidonia oceanica (L.) Delile, 1813. Publicaciones especiales Instituto español de Oceanografía 23, 243-253.
• Meinesz, A., Belsher, T., Boudouresque, C.F. & Lefèvre, J.R. 1991 Première éva-luation des pot-en-tialités du satellite SPOT pour la cartographie des peuplements ben-thi-ques superficiels de Méditerranée occidentale. Oceanologica Acta 14, 199-207.
• Pasqualini, V., Clabaut, P., Pergent, G., Benyousse, L. & Pergent-Martini, C. 2000 Contribution of side scan sonar to the management of Mediterranean littoral ecosystems. International Journal of Remote Sensing 21, 367-378.
• Pasqualini, V., Pergent-Martini, C. & Pergent, G. 1999 Environmental impact identification along the Corsican coast (Mediterranean sea) using image processing. Aquatic Botany 65, 311-320.
• Pergent, G., Pergent-Martini, C. & Boudouresque, C.F. 1995 Utilisation de l'herbier à Posidonia oceanica comme indicateur biologique de la qualité du milieu marin littoral en Méditerranée : état des connaissances. Mésogée 54, 3-27.
• Pergent, G., Mendez, S., Pergent-Martini, C. & Pasqualini, V. 1999 Preliminary data on the impact of fish farming facilities on Posidonia oceanica meadows in the Mediterranean. Oceanologica Acta 22, 95-107.
• Pergent, G., Romero, J., Pergent-Martini, C., Mateo, M.A. & Boudouresque, C.F. 1994 Primary prod-uction, stocks and fluxes in the Mediterranean seagrass Posidonia oceanica. Marine Ecology Progress Series 106, 139-146.
• Porcher, M. 1984 Impact des mouillages forains sur les herbiers à Posidonia oceanica. In: First international workshop on Posidonia oceanica beds (Boudouresque, C.F., Meinesz, A., Fresi, E. & Gravez, V., eds), Gis Posidonie Publ., Marseille, pp. 145-148.
• Ramos-Esplá, A.A, Aranda, A., Gras, D. & Guillen, J.E. 1994 Impactos sobre las praderas de Posi-donia oceanica (L.) Delile en el SE español: necesidad de establecer herramientas de ordenamiento y gestión del litoral. In: Pour qui la Méditerranée au 21° siècle? Villes des rivages et environnement littoral en Méditerranée. Okéanos, Montpellier, pp. 64-69.
• Ruiz, J.M. & Romero, O.J. 2001 Effects of in situ experimental shading on the Mediterranean seagrass Posidonia oceanica. Marine Ecology Progress Series 215, 107-120.
• Sánchez-Lizaso, J.L., Guillen-Nieto, J.E. & Ramos-Esplá, A.A. 1990 The regression of Posidonia oceanica meadows in El Campello (SE Spain). Rapports et Procès-Verbaux des Réunions de la Commission internationale pour l'Exploration scientifique de la Méditerranée, 32 (1), 7.