POSIDONIA AND CYMODOCEA 291 



It appears that in almost all cases the instalment of Posidonia is 

 preceded by that of Cymodocea in the three modalities just listed. My 

 observations on the Egyptian coast, especially at Abu Qir, lend little 

 support to this view. For example, Cymodocea prairies on the east side 

 of the Citadel in Abu Qir Bay, a superficial calm mode with much 

 organic sediment, have been in place since my first visit in 1943, having 

 been neither eliminated nor encroached upon by Posidonia, which grows 

 more toward the open sea. This is true also with respect to strongly 

 exposed modes where Posidonia rarely, if ever, has been replaced by 

 Cymodocea, unless one assumes the interference of serious topographic 

 changes that alter the modality itself, which would be very exceptional. 

 It is true that in certain moderately exposed modes such as those found 

 at the back of rocks or barriers facing a strong current or as a result of 

 special configurations of the shore line itself, competition between 

 Cymodocea and Posidonia takes place and the latter might eventually 

 eliminate the former. Molinier and Picard give a good example of this 

 (1952, p. 167) and I have noted such cases frequently at Alexandria. 



I have already stressed that the vast prairies of Cymodocea on the 

 western side of the Ridge at Abu Qir owe their existence to the special 

 conditions created by the Posidonia-Reei. In other words, Cymodocea 

 comes as a direct result of the growth of this reef. This particular 

 Cymodocea has also been in situ for a very long time. Under no circum- 

 stances has Posidonia been found in this community. Aside from the 

 peripheral parts near the shore, where the two grasses meet, each of 

 them has attained a state of stability. On the other hand, the trans- 

 gression of Cymodocea upon the neighboring Posidonia takes place only 

 in the gullies and depressions on the Posidonia-Reei, as previously 

 described, and it cannot grow on the same level as Posidonia. All these 

 invasions occur after the reef has been established. Molinier and Picard 

 seem to be aware of these facts and have also observed "formation 

 lagunaire" (p. 230) at the back of Posidonia reefs. 



SUMMARY AND CONCLUSIONS 



A detailed study of the submerged algal and phanerogamic com- 

 munities inhabiting the sea area at Abu Qir, near Alexandria, has re- 

 sulted in the conclusions which may be summarized as follows : 



Exposed modes : 



1. Posidonia oceanica forms extensive "prairies" on the exposed sea area 

 lying west of the Ridge at Abu Qir. 



