284 ALEEM 



facing the onshore currents and acting as a buffer against them. This form 

 is evidently determined by the currents and waves, as the maximum de- 

 velopment occurs where they are strongest. Toward the shore the develop- 

 ment of Posidonia is at a minimum and competition with Cymodocea 

 is great, A strong current flowing from the east through the channel 

 intersecting the ridge has apparently caused the Posidonia community 

 to extend west along its direction of flow, thus producing a bulge on the 

 western side of the triangle. 



Apart from modifying the intensity of the current, the Posidonia 

 leaves also act as a trap for floating algae and organic debris, which are 

 deposited on the leeward side of the reef. There, the calm water and 

 accumulated organic debris make conditions favorable for the establish- 

 ment of Cymodocea nodosa, in a rough triangle next to that formed by 

 Posidonia but with its maximum development toward the shore, where 

 relative calm prevails. The substratum is of muddy sand, and the slope 

 toward the open sea is greater than that of the Posidonia-Reei. In its 

 maximum development toward the shore, Cymodocea grows at a depth 

 of approximately 3 meters ; at its distal end, the depth is 8 meters. The 

 leaves of Cymodocea nodosa attain a length of 150 cm or more; but its 

 rhizomes are thin and unable to build up a reef similar to that built 

 by Posidonia. The boundary between these two great sea-grass communi- 

 ties is occupied by Caulerpa prolifera, as previously noted. 



Cymodocea encroaches upon Posidonia from different directions. It 

 establishes itself in depressions and gullies resulting from the destruction 

 or erosion of certain parts of the reef from the action of stones, animals, 

 marine fungi, or other agents. These depressions become filled with 

 sand and are occupied not only by Cymodocea, but also by Caulerpa 

 and other algae such as Halopteris, Sargassum, Cystoseira, and Padina, 

 whose ramified stolons help to bind the sand in place. 



Cymodocea has also established itself in a narrow belt running from 

 north to south, parallel to the Caulerpa-community (fig. 1) and occupy- 

 ing the distal edge of the Posidonia-Reei. The intervening space between 

 Cymodocea and Caulerpa is inhabited by degenerating Posidonia, which 

 apparently has been choked by the long dense leaves of Cymodocea. A 

 close examination shows that long cracks form as a result of the erosion 

 of the sandy substratum by water movements, causing the collapse of 

 the edge of the reef. These cracks eventually are colonized by Cymodocea. 

 From the observations made at Abu Qir, it appears that the 

 Cymo^oc^fl-community in the area back of the Posidonia-Reei owes its 

 existence largely to the latter's role in modifying the currents. Another 



