SUBTIDAL ENVIRONMENTS AND ECOLOGY 



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Fig. 20 Reentrants of the reef face, southwest islands, Enewetak Atoll. Upper left: Sand channel in between elevated reef fingers, 

 depth on sand approximately 12 m. Upper right: upper portion of a reentrant on the outer slope at 30 m depth. Lower left and 

 right: Sediment transport via reentrant down the outer reef face, southwestern islands, depth 40 m. 



composition (Gerber, 1981), but these are only a bare 

 beginning. The open lagoon is generally rough and less 

 than ideal for working in a small boat. Navigation and posi- 

 tioning are difficult because the islands of the atoll rim are 

 so low that in the center of the lagoon little land is visible. 



Water column productivity within the lagoon has not 

 been well documented. The author has seen, on several 

 occasions, large blooms of phytoplankton in the northern 

 and western lagoon. These were sharply differentiated 

 areas of "brown water" many kilometers in length and, on 

 two occasions, as surface slicks many centimeters thick. 

 The surface slicks occurred under extremely calm condi- 

 tions and were nearly linear masses of tan phytoplankton, 

 tens of meters broad and over 1 km in length. Thickness 

 was not determined but was believed to be at least 30 cm. 

 The blooms may be associated with water of lengthy 

 residence time in the lagoon since they have been 

 observed only from areas where this is typically the case. 



Dense swarms of zooplankton were often observed in 

 the lagoon by scuba divers, often within a discrete portion 

 of the water column. During the summer, particularly huge 



numbers of salps and ctenophores were observed many 

 times on the reef. Gerber and Marshall (1982) documented 

 a "bloom" of pteropods and a subsequent population 

 decrease in the central lagoon during a 4-week period. 



The unidirectional flow of water from windward reefs 

 across the reef flat to the lagoon is significant not only in 

 the physical flushing of the lagoon but as a mechanism by 

 which increased nitrogen, produced by nitrogen fixation on 

 the shallow reef flat (Webb et al., 1975), reaches the 

 lagoon. 



Webb et al. (1975) felt there were three important 

 routes by which Calothrix Crustacea fixed N2 enters the 

 remaining reef ecosystem. First, fish grazing and the low 

 assimilation efficiency (Chartoch, 1972) of herbivorous 

 fishes makes the fixed nitrogen available. Second, fragmen- 

 tation of Calothrix and lagoon transport makes it available 

 to herbivores and detritivores in the lagoon. Third, 40 to 

 60% of the nitrogen fixed is released in solution and is 

 available for other organisms. 



Gerber and Marshall (1974) have shown that detritus 

 flowing off the shallow reefs forms a major component of 



