POSIDONIA AND CYMODOCEA 281 



THE POSIDONIA-CYMODOCEA COMPLEX AT ABU QIR 



Abu Qir suburb, about 25 kilometers east of Alexandria, lies on a 

 small peninsula, with the Citadel forming the head of an arrow which 

 separates the calm, sandy bay of Abu Qir on the east from the open rocky 

 shore to the west. Prairies of Cymodocea nodosa grow in the shallow 

 water of the bay, where organic decomposition, especially in summer, 

 produces H2S in the substratum. Posidonia oceanica flourishes on the 

 rocky open coast. As the littoral rocks here are almost flat, the littoral 

 algal belts are broad and distinct, despite the low amplitude of tides 

 (30-50 cm). 



The area studied (fig. 1) covers roughly 10,000 sq. m. and is for 

 the most part submerged. The rocky ridge, extending for about 60 meters 

 into the sea and separating it into two distinct subregions, is broken in 

 the middle by a channel through which a strong current flows from the 

 east. A region of submerged rocks and islets extends parallel with the 

 coast, a short distance out from the end of the ridge. The subregion west 

 of the ridge is the deeper and more exposed to currents and waves; that 

 to the east is shallower and more protected, and is referred to as the 

 "lagoon." Little seasonal variation has been found in the chief algal and 

 phanerogamic communities of these regions. As the greater part of the 

 work was done during the summer, the profiles presented here are those 

 for the months of June and July. 



The ridge (figs. 1-2) is splashed by waves during rough weather, 

 forming on the higher parts of the rocks a "spray zone" occupied mainly 

 by blue green algae. In the small rock-pools and small ditches on the 

 ridge, where the water remains unchanged for longer periods and tem- 

 perature and salinity variations are prominent, a special algal flora 

 grows, composed mainly of blue greens, Enteromorpha cornpressa and 

 Polysiphonia pJileborhiza. On the rather abruptly sloping side of the 

 ridge to the west, the algal communities are arranged in well-defined 

 descending belts, described below. Some of these belts, it will be noted, 

 disappear with the advent of hot weather. 



The Littoral Region : 



1. Blue green-belt. This is formed chiefly by Rivularia polyotis and 

 Brachytrkhia balanij intermingled with Lyngbya. Near the shore, where 

 the vegetation is subject to continuous splashing by waves, grows Cla- 

 dophora sp., higher on the ridge than the blue greens but receiving a 

 greater amount of spray. This place is usually occupied by Nemalion 

 hebninthoides during the spring, but it disappears early in June. 



