Chapter 10 



Trophic Relationships in Enewetak Atoll 



NELSON MARSHALL* and RAY P. GERBERf 



'Universify of Rhode Island. Kingston. Rhode Island 



02882: current address is P. O. Box 1056, 



St. Michaels. Mari^land 21663 



fSt. Joseph's College. North Windham. Maine 04062 



INTRODUCTION 



Some of the biologists who were attracted to Enewetak 

 Atoll after the marine research laboratory opened have 

 been studying species that are typical of reef environs and 

 plentiful in this setting. Some have been interested in eco- 

 logical features, particularly those of the well-developed 

 windward reefs; and some, who have focused on the reef 

 areas as an ecological subsystem, have been interested in 

 the processes of the atoll as a whole. 



We start by noting three contrasting environments in 

 this large, but typical, atoll. First, there are the coral reefs 

 and knolls, the former almost completely enclosing the 

 atoll, the latter scattered through the lagoon and number- 

 ing over 2000. Then there are the of)en waters of the 

 lagoon. Finally, there is the lagoon benthic environment 

 (other than the coral knolls). In a real sense, and in com- 

 parison with the rest, the reefs and knolls are very produc- 

 tive, even though the oceanic waters surrounding the atoll 

 are low in nutrients and organic food sources. The level of 

 this productivity and the explanation (that the reef com- 

 munity dynamics involve rapid recycling rather than an 

 enrichment from seawater), are discussed in Chapter 9, 

 this volume, also in Odum and Odum (1955), and 

 Johannes et al. (1972). In contrast it would seem that the 

 lagoon waters are not productive; in fact, being extremely 

 clear (often one can see the bottom to depths of 50 m and 

 more), they give the appearance of being rather impover- 

 ished. 



A Trophic Link Between the Reef 

 and the Lagoon 



Rather spectacular populations of fishes are evident in 

 this dual setting of an impoverished lagoon and a produc- 

 tive, recycling coral reef seemingly low in net yield. This 

 raises basic questions as to the food dynamics involved: 



how does the relatively closed reef system, together with 

 the oligotrophic lagoon, support such consumer popula- 

 tions? Part of the answer is because the reef areas are not 

 tight, unyielding environments as early studies suggested, 

 and because there is an impressive flow of detritus, mucus 

 flakes, algal fragments, and aggregated organic matter off 

 the reef. 



When the flow of matter off the reef was first reported, 

 Marshall (1965) suggested "that [particulate] organic 

 matter, including aggregates, transported in suspension 

 from the reef to the mid-atoll areas, may constitute a sub- 

 stantial contribution to the trophic system within the 

 lagoon." Soon thereafter Johannes (1967) made additional 

 observations, noting a considerable flow of what he 

 referred to as organic aggregates as well as detrital frag- 

 ments streaming off the reef. His explanation, that the 

 aggregates were the remnants of coral mucus, flaking off 

 and flowing down-current from the rich reef environment, 

 probably also accounts for some of the material Marshall 

 had seen. Though Johannes provided additional quantita- 

 tive information, the concept that organic particles flowing 

 from the reefs might play an appreciable role in the lagoon 

 trophic system remained a matter of conjecture. 



Marshall had yet another idea as to a potential source 

 of organic particles from off the reef, taking his lead from 

 the publications by Baylor and Sutcliffe (1963) and by 

 Riley (1963) in which they introduced the concept that dis- 

 solved organic matter in seawater might be aggregated 

 into particulate food. The Baylor and Sutcliffe paper had 

 demonstrated, in the laboratory, a mechanism that could 

 explain their formation; namely, that dissolved organic 

 matter in the presence of bubbles would aggregate and 

 thus form particles. They commented on the possible 

 importance of organic particles being produced by wave- 

 induced bubbles at sea. 



Reflecting on the crashing waves at the seaward edge 

 of a typical coral reef, the streaming of bubbles in the path 

 of flow across the reef, and the very high organic produc- 

 tion of typical reef systems, Marshall envisioned a stream- 

 ing of organic aggregates forming on bubbles or other 

 nucleii. In observations on Hogsty Atoll in the Bahamas, 

 he found that amorphous organic particles were indeed 

 abundant in the lagoon but he could not establish a net 



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