the influence of pollution, reduced sediments may appear in the lagoon, 

 containing sulfides and hydrogen sulfide (Sorokin, 1971b). The bottom 

 of the lagoon in atolls is covered with coral sand. If the movement of 

 the water is low, it is inhabited by macrophytes ( Zostera , Thalassia , 

 Halophila , Halodul e , Syringodium , Enhal us ) and psammophilic green 

 calcareous algae ( Halimeda , Amphirhoa , Avrainvillea ) . The sand on the 

 bottom of the lagoon also contains a rich microflora and phytobenthos. 

 They form the diet of the gastropod fauna living here ( Strombus , 

 Terebra , Cymatium , Cerithiidae, Leucozonia , Oceanebra , Astraea , Conus , 

 Mitra , 01 iva . Morula , Hyal ina , Tegula , Turridae) . Among the 

 psammophilic fauna, the Ascidia , Echinodermata (the sea stars Linckia , 

 Culci ta , the urchins Diadema and particularly the Stichopus , Holothuria 

 and Ophiodesma ). The Holothuria sometimes form massive accumulations, 

 in which their biomass per m of bottom surface reaches 1 kg or more. 

 These accumulations are formed, e.g., by Ophiodesma spectabili^ in the 

 lagoon of Oahu Island (over 10 indiv./m^, biomass about 3 kg/m^) or 

 Holothuria difficil is in the lagoon of Eniwetak Atoll (over 100 

 indiv./m^, biomass about 10 kg/m^) (Bakus, 1973, and our own 

 observations). The sand of the lagoon is inhabited by a mass of 

 polychaetes ( Heteroptera , Sabellidae), as well as varied myobenthos 

 (foramini fera , ostracods, micromol 1 usks) and microbenthos 

 (microphytobenthos and bacteria). An abundant population of the sandy 

 soils of the lagoon gives them high trophic value. The fauna and 

 detritus which can be extracted from the sand form the diet of many 

 benthophagous fish. 



Among the corals in the lagoon there Are almost always forms which 

 prefer higher levels of illumination ( Porites , Ocul ina , Favia , 

 Siderastrea , fentipora , Pavona , branched forms of Acropora and 

 Millepora ) . Conditions for growth of corals in lagoons are frequently 

 unfavorable due to significant water turbidity and a low rate of water 

 exchange. If this is the case, a mass of dead coral and fragmentary 

 material accumulates on the floor of the lagoon. The living coral grows 

 in the lagoon in certain areas, forming "patch reefs," which rise over 

 the bottom of the lagoon in the forms of hills or ridges covered with 

 living coral and corallines. 



The R (rear) zone is situated at the internal edge of the reef, 

 depth 1-3 m. This is the re^r slope of the reef plateau. It is usually 

 covered with rubble carried down from the reef flat, and is crossed by a 

 network of channels, the bottom of which is covered with sand and 

 clastic materials. Zone R is characterized by significant turbulence 

 from surf waves, which roll over the edges of the reef, as well as tidal 

 currents, and also by a high level of illumination. This creates an 

 optimal condition for the growth of corals, calcareous algae and 

 macrophytes, which inhabit the remainders of the plateau and form 

 independent, broad colonies. The dominant species of corals here are 

 usually the branching forms (Porites compressa , Acropora , Seriatopora , 

 Millepora ) , as well as the fan-shaped and massive forms ( Diploria , 

 ^bntrastrea , Favia , Favites , Hydropora , Dichocoenia , Acropora cuneata , 

 itorrUpora^, Porites lobata , P_. lutea , _P. asteroidesT . On the bottom of 

 the channels between the patch reefs and beneath the lip of the plateau, 

 we frequently see the solitary corals, Fungia . Soft corals 

 (Alcyonaria) , macroalgae ( Chlorodesmium , Thalassia, Padina, Sargassum, 



166 



