Répartition et conditions d'installation de Posidonia oceanica Delile et Cymodocea nodosa Asherson dans le golfe de Tunis.

Abstract

The existence of a cyclic pattern of sedimentology is demonstrated for the accidental west coast of the Gulf of Tunis. During the winter and spring, neritic silting occurs via the sediment carried by the outfall of the Oued Medjerdah and Miliane; the cessation of rain runoff (May, June) changes in the direction of prevailing winds influences resuspended sediments to be deposited in shallower depths occurring throughout summer to Sept Oct; exceptions from this pattern are specific areas in 0-20 m depth with sand-mud substrates. In contrast on the eastern coast of the gulf little sedimentation occurs, and the substrate is one or a combination of sand, rocks, or dietritus. The occurrence of C. nodosa and P. oceanica depends principally upon the stability of the substration, the extent of the grass beds of Posidonia especially is effected by turbidity/sedimentation, over wide ranges of salinity and temp levels. Cymodocea thrives more on unstable substrates where sand or silty deposts are 'cycling'; whereas Posidonia thrives in areas of stable substrate, or certain alluvial areas where wave action prevent siltation. The amount of organic matter for an area is a result of these factors. In action of temp and salinity upon development of the 2 spp of marine grasses was tested to indicate some correlation with the nature of the substrate.