Data type :

Posidonia oceanica seagrass meadows

Précision sur le type de donnée :

3 observation scales: overall dynamics (macrostructure), density, buoyage monitoring, rhizomes (microstructure), biometry, lepidochronology (plant)

Measured or observed parameters :

Limit type:

(progressive, clear, erosive or regressive (classification from Meinesz & Laurent, 1978).)

Limitation of position measurement (depth), map of the buoyage (direction, distance, depth), dead clump distance downstream, meadow/buoy photo (3 photos: left, right and above). Description of the meadow structure (plain, hill, uneven), erosion (clumps falling, channels), and nature of the sea bed. Neighbouring species (protected, of national interest, non-native, indicative, etc.).

Density:

(Number of leaf clusters per m². Varies depending on depth and environmental conditions)

Measurements taken in a 20x20cm square, with 3 measurements per buoy (left, right, centre), random but directed sampling. Classification in accordance with Giraud (1977) and Pergent et al. (1995).

Coverage :

(percentage of substrate coverage by posidonia leaves, compared to non covered areas. Varies according to season, depth, and the health of the seagrass meadow).

Vertical coverage measurements, 3m deep with a transparant plastic sheet of 30 x 30 cm, divided into 9 squares as described in the method by Gravez et al. (1995).

Counting the number of squares occupied by the seagrass. Three measurements per buoy, which amounts to more than 30 measurements per marker. In the absence of measurement, a visual estimation of coverage is mage, with 5% tread (but also an observer effect).

Retreat :

(Distance separating the rhizomes from the sediment (measured according the protocol described by Boudouresque et al., 1980))

Retreat is evidence of sediment deficit. Suggested scale for evaluation (<5 cm low, 5-15 cm average, >15 cm high, Charbonnel et al., 2000).

Plagiotropic :

(Proportionof plagiotropic rhizomes.)

Indicator of the seagrass meadow’s good health, tends to increase. The position of the majority of rhizomes (plagiotropic/orthotropic) is noted either side of the marker.

Phenology :

Study of the morphological characteristics of the plant. Marked seasonal variations.

Giraud protocol (1977). Measurements of the number and type of leaves (adult, intermediate, juvenile), length and breadth of leaves, lead area index (LAI) and A coefficient (grazing).

Measurements taken from 15 clusters.

Lepidochronology :

Study of cyclic variations in scale thickness (Pergent, 1987).

Measurement of the number of leaves formed per year. (number of scales present between 2 minima of thickness) and rhizome growth speed per year (length of rhizome stems between 2 minima of thickness). Samples taken from 15 clusters, dissection, weight after drying.

Sampling method : Spatial