286 ALEEM 



linearis. The more muddy places are covered with a mat of the blue 

 green algae Microcoleus chthonoplastes and Lyngbya. Occasionally there 

 are small patches of Dasycladus clavaeformis, and Ulva lactuca with its 

 characteristic epiphytic brown alga Myrionema strangulans. Cystoseira, 

 Halimeda tuna, Valonia macrophysa, and Udotea petiolata occur less 

 frequently, the last three usually in concealed places. Caulerpa prolifera 

 grows abundantly between the Cymodocea patches. Derbesia tenuissima 

 and Ceramium gracillimum var. byssoideum are found as epiphytes on 

 the older fronds of Padina. The wet sand of the shore sometimes displays 

 a brown color due mainly to diatoms and to the peridinian Exuviaella 

 marina. 



The flora and fauna inhabiting the crevices and grottos on the lagoon- 

 side of the ridge differ markedly from those in the grottos to the west. 

 Of interest is the gradual replacement, in these grottos, of the calm water 

 algae by more open sea flora, as one proceeds to the open sea. 



On the whole, the lagoon vegetation is dense and easily detached from 

 the substratum, which is composed mainly of sandy mud. Fig. 5 shows 

 the distribution of the common algae and sea grasses in the lagoon. 



STRUCTURE OF THE POSIDONIA AND CYMODOCEA COMMUNITIES 



The PosrooNiA Community 



A dense and diverse flora of algae finds shelter as epiphytes on leaves 

 and rhizomes of Posidonia. Light is an important factor governing the 

 distribution of these algae. Species growing on the rhizomes or between 

 them are usually shade-algae that thrive under the reduced hght con- 

 ditions created by the overlying thick canopy of leaves. The algae grow- 

 ing on the upper parts of the leaves are those most adapted to the pre- 

 vailing light in their environment. The algae of this community may 

 thus be classified into (a) epiphytes on rhizomes in shallow water and 

 in deep water; (b) epiphytes on leaves in shallow water and in deep 

 water. 



Whether dead or alive, the rhizomes of Posidonia provide a good 

 substrate for a number of the larger algae. In relatively shallow water, 

 particularly when the leaves are dense as at Abu Qir, the following algae 

 are invariably found as epiphytes on the rhizomes. 



