Variations à différentes échelles spatiales de l'herbier à Posidonia oceanica (L.) Delile ; effets sur les paramètres physico-chimiques du sédiment

The biometric parameters (shoot density, length and width of leaves, leaf and epiphyte biomasses, leaf surface) of the P. oceanica meadow present a significant spatial variability at the mesoscale level (100m(2)). All the parameters studied, excepting the number of leaves, decrease according to the depth. The horizontal spatial variations (centrifuge and centripetal distributions) reach a maximum in the shallower areas. The analysis of the microstructures (m(2)) of the meadow has not allowed a relationship to be established between the spatial variations of the biometric parameters and the environmental factors in the sediment (pH, water and organic matter content, red-ox potential, nutrient content). The heterogeneity of the meadow in the Revellata Bay quite probably results from the internal shoot and rhyzome growth dynamics. On the other hand, it appears that the presence of the prairie modifies the physical-chemical characteristics of the sediment. In fact, the sandy patches present different physical-chemical characteristics: lower organic matter, different granulometric and much higher interstitial water nutrient levels, as well as a higher pH and oxydo-reduction potential.