290 ALEEM 



THE CYCLE OF DEVELOPMENT OF POSIDONIA AND CYMODOCEA 



The initial groups preceding the establishment of either Posidonia 

 or Cymodocea are composed largely of algae and diatoms. The latter 

 help to pave the substratum for the instalment of the seagrasses by accu- 

 mulating sand and organic sediments and building a consistent covering 

 in which the roots and rhizomes of the grasses ramify. The algae enter- 

 ing into this succession differ according to whether the situation is ex- 

 posed or calm. Ordinarily forms like Enteromorpha, Ulva, Padina, 

 Halopteris, Cystoseira, Sargassum, Dasycladus, Jania, and Caulerpa 

 prolifera play an important part. The latter species almost always pre- 

 cedes Cyrnodocea. 



Concerning the biological cycle of development of Posidonia and 

 Cymodocea in the northern Mediterranean, Molinier and Picard (1952, 

 p. 209) distinguish the following three modalities: 



Prairies in exposed superficial modes* In this type, Cymodocea in- 

 stalls itself first on sand moderately enriched with organic sediments. 

 Posidonia invades such a formation initially as small separate rhizomes 

 and later eliminates Cymodocea. Eventually Posidonia builds up a reef 

 which raises the substratum, making it more subject to erosion, and 

 Posidonia thus gradually degenerates. Waves further wash away the sedi- 

 ments and erode the reef. Cymodocea again re-installs itself in the "inter- 

 mats" thus formed. 



Prairies in calm superficial ?nodes. The sand containing much organic 

 sediment in this mode is very favorable for the growth of Cymodocea, 

 which eventually becomes dense. It is invaded by Posidonia and ultimately 

 eliminated by its successor, which then builds up a reef. This reef disturbs 

 the equilibrium between organic and inorganic sediments and leads to 

 the deposition of more mud. Posidonia then disappears and another dense 

 covering of Cymodocea follows. 



Prairies in calm deep modes. Starting from sand containing a certain 

 amount of organic matter, Cymodocea readily establishes itself and is 

 followed by Posidonia, which attains a maximum prosperity in this mode. 

 Under exceptional circumstances such as interference by man, the 

 destruction of Posidonia leads to its replacement by Cymodocea. The 

 authors give only a tentative scheme for this mode. One must not, how- 

 ever, ignore the influence of deep currents in this mode, and it is difficult 

 to suppose that Cymodocea forms the initial stage. 



*The term "mode" is that of the writers; it implies variations in the composition 

 of the •water, and its degree of turbulence in the sense of De Beauchamp, 1914 

 (cf. Feldmann, 1951, p. 321). 



