Agathe Leriche، نويسنده , , Charles-François Boudouresque، نويسنده , , Guillaume Bernard-Granger، نويسنده , , Patrick Bonhomme، نويسنده , , Jacques Denis، نويسنده ,
To assess the dynamics of seagrass beds, it is necessary to be able to compare their present day and former distribution patterns.
In most cases, ancient data are available in the form of maps whose reliability must be estimated before comparison. In the study
area (near Marseilles, France), a series of maps of the seagrass Posidonia oceanica beds spanning more than one century (dating from
1883, 1897, 1958, 1975, 1979, 1991 and 2000, respectively) are available. In addition, a new map was established, using aerial
photography (shallow areas), side scan sonar (deeper areas) and data validation by ground truth. In order to help assess the value of
the ancient maps, a Reliability Index (RI) is proposed. The RI, rated from 0 to 50, weighs three parameters: (i) the initial scale of the
map (source map) and the working scale (target map); (ii) the method of data acquisition (e.g. dredges, grabs, aerial photography,
side scan sonar, scuba diving); and (iii) the method of data georeferencing. Dramatic differences in the distribution of seagrass beds
are apparent between the eight maps. In view of the biological features of P. oceanica (e.g. slow rate of change of the meadow limits
under natural conditions and the persistence of rhizomes after its death), it is possible that some of these differences may not reflect
change over time but may be due to errors in the ancient maps. In contrast, other differences are plausible and validated by field
data, e.g. the regression of the lower limit of the meadow. It was thus possible to compare the calculated RI for each map and the
plausibility of observed differences as a basis for calibrating the RI.
Errors , Posidonia oceanica , Mediterranean Sea , Side scan sonar , Aerial photography , Mapping