292 ALEEM 



2. Its spread is largely determined by the current regime (intensity 

 and direction of flow). 



3. Posidonia builds up a reef through the accumulation of new rhi- 

 zomes added every year, thus producing an elevation of the sub- 

 stratum upon which it grows. Inorganic sediments (sand particles), 

 crustose algae living on the rhizomes, sea urchins and other animal 

 debris further contribute to the growth of the Posidonia-Keei. 



4. The leaves of Posidonia act as a buffer for the water currents flowing 

 over them, thus creating at the back of the Posidonia-Reei calm 

 conditions similar to lagoon formations created by coral reefs. 



5. Organic sediments accumulate in the calm mode thus created, en- 

 riching the substratum and rendering it favorable for the growth 

 of Cymodocea, which tolerates more organic matter than Posidonia. 



6. Caulerpa prolifera occupies the boundary between the Posidonia 

 and Cymodocea and plays an important part in the succession lead- 

 ing to the instalment of Cymodocea. 



7. Other algae like Cystoseira, Sargassum, Hahpteris, Padina and 

 Jania contribute to the succession leading to the instalment of 

 Posidonia. 



8. Transgression of Cymodocea upon Posidonia takes place in gullies 

 and depressions formed in the Posidonia-Reei as a result of erosion 

 by phj^sical or biological factors. Prior to the instalment of Cymo- 

 docea in these gullies, the latter are filled with sand particles. 



9. A state of stability is attained between the Posidonia prairies grow- 

 ing in the exposed mode and forming a reef and those of Cymodocea 

 growing at its back. The balance between the two is maintained 

 primarily by the direction and intensity of currents, which in turn 

 affect the rate of sedimentation of organic and inorganic particles. 

 Unless this equilibrium is disturbed by abnormal climatic changes, 

 by physical factors leading to erosion of the Posidonia-Rcei, or by 

 changes in the topography of the coast, e.g., as a result of subsidence, 

 the two communities in question grow' in close proximity to each 

 other for a very long time. 



Calm modes : 



10. Cytnodocea nodosa flourishes in calm modes such as those found 

 to the east of the Citadel at Abu Qir and reach a "climax" in such 

 modes. Successive stages leading to the establishment of this com- 

 munity are formed by algae, chiefly Caulerpa. Posidonia does not 

 compete with Cymodocea in these calm modes. 



