Bio]. Mar. Medit. (2000), 7 (2): 328-331
C.F. BOUDOURESQUE (1) , E. CHARBONEL, A. MEINESZ, G. PERGENT, C. PERGENTMARTINI, G. CADIOU, M.C. BERTRANDY, P. FORET, M. RAGAZZI, V. RICoRAIMONDINO
GIS
Posidonie and Centre d'Océanologie de Marseille, UMR CNRS n°6540, Campus of Luminy, 13288
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-AlpesCôte d'Azur), and is a collaboration between the elected representatives, the state
maritime services (Bouches-du-Rhône, Var and Alpes-Maritimes regions) and the
A monitoring network based on the seagrass Posidonia oceanica
329 -
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).
330
C.F.
BOUDORESQUE, E. CHARBONEL, A. MEINESZ, G. PERGENT, C. PERGENT-MARTINI, G. CADIOR,
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: 1984-87 period (selection of the sites and assessment of initial conditions),
1988-90 period (first-time return to the sites), 1991-93 period (second return), 1994-96
period (third return), 1997-99 (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
i mproved 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
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.
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