280 ALEEM 



The only previous detailed account on the ecology of these Mediter- 

 ranean marine phanerogams appears to be that made by Molinier and 

 Picard ( 1952) on the Coasts of France. 



The two distinct phanerogamic communities of Posidonia oceanica 

 Delile and Cymodocea nodosa Ascherson possess, along the Mediter- 

 ranean Coast of Egypt, dissimilar ecological characteristics. The former 

 inhabits exposed localities on open shores and at the head of bays, and is 

 sensitive to temperature and salinity variations; while the latter com- 

 munity thrives mostly in sheltered bays, harbors, and lagoons, where calm 

 water prevails, and thus tolerates a wider range of temperature and sa- 

 linity changes. Posidonia oceanica, moreover, favors a rocky substratum 

 covered with clean sand, with constant aeration and renewal of water 

 by winds and currents. Cymodocea nodosa, on the other hand, grows on 

 muddy sand and tolerates organic pollution in its environment. Under 

 favorable conditions, both communities form prairies on the sea bottom 

 comparable to grass prairies on land. Despite the marked differences in 

 their ecology, there are some localities at which a certain degree of com- 

 petition exists between the two communities and where the ecological 

 factors affecting the growth of one are modified by the presence of the 

 other. Abu Qir, near Alexandria, is one such locality where extensive 

 observations on the two communities in question were made. 



Along the shores of Alexandria and further to the west, a shallow 

 water belt 8 to 10 meters deep and running parallel to the shore, is 

 occupied either by Posidonia or Cymodocea, depending upon the nature 

 of the bottom and the degree of agitation of the water. A second belt, 

 exclusively of Posidonia, lies in deeper water, usually at 20 or more 

 meters below the surface. Surrounding these two beds are vast growths 

 of Cystoseira fimbriata, Halimeda tuna, Caulerpa prolifera, or litho- 

 thamnia, again depending upon the nature of the bottom. Posidonia 

 leaves, rhizomes, and balls formed of macerated tissues of Posidonia 

 mixed with sand particles, are cast along the shore in scattered heaps 

 which also occasionally contain Cystoseira and Sargassum, particularly 

 in winter. The deep-water Posidonia beds extend along the coast of the 

 Libyan Desert west of Alexandria, and are most abundant at Burg El 

 Arab, where the shore is of coarse white sand, completely devoid of fixed 

 algae. In the Delta region, between Rashid and Ras-el-Bar, Posidonia 

 oceanica is scarce, while Cymodocea is more frequent because of the depo- 

 sition of mud particles brought up by the river. From Port Said east to 

 El Arish, Posidonia oceanica is abundant only at the few rocky places 

 between the two ports. 



