184 



MARSHALL AND GERBER 



that for studies at Encwetak this has not been established 

 from direct measurements but is assumed from general 

 oceanographic considerations. Most of what we do know Is 

 inferred from sampling water as it comes in over the reef, 

 already referred to in Chapter 9 where the observations 

 of Odum and Odum (1955) and Johannes et al. (1972) are 

 discussed. These observations could be misleading since 

 they include an unknown net value for the uptake and the 

 release from the outer reef slope. They are, however, com- 

 patible with the generalization that substantial coral forma- 

 tions can develop in relatively impoverished waters (see 

 Lewis, 1977; Kinsey and Davies, 1979, for some of the 

 factors that bear on this). One point that can be made is 

 that, for an atoll system in an isolated setting surrounded 

 by ocean depths, these surrounding oceanic waters are the 

 only source of basic nutrients, except for the nitrogen fixa- 

 tion processes also referred to in Chapter 9 



As mentioned earlier, the usual and plausible explana- 

 tion for reef growth under these conditions is that the reef 

 community, as a biological system, is uniquely adapted to 

 the uptake of low nutrient concentrations and attains high 

 gross productivity through recycling (e.g., Odum and 

 Odum, 1955; Pomeroy and Kuenzler, 1969; and Johannes 

 et al., 1972). It is often suggested that, in upwelling and 

 other enriched areas where nutrient levels are higher, the 

 success of competing ecosystems explains the general 

 absence or p>oor development of reefs. Except that multiple 

 responses and complications were involved, reef deteriora- 

 tion in Kaneohc Bay, Oahu, Hawaii, in the presence of 

 nutrient-rich sewage effluents (also the recovery there after 

 sewage diversion) seems to support this. [For discussions 

 of the Kaneohe Bay story see Banner, 1974, and Smith et 

 al., 1981.] Also, from fertilization experiments, Kinsey and 

 Davies (1979) suggest that nutrient concentrations can 

 suppress coral calcification. There docs seem to be a posi- 

 tive effect from nutrient replenishment, however, since at 

 Enewetak, as in most reef environments (Lewis, 1977), the 

 growth is most luxuriant on the windward side where the 

 greatest cross-reef flow occurs. 



Unfortunately, very little has been done at Enewetak or 

 elsewhere to provide a direct insight into the organic pro- 

 ductivity of the outer reef slope. The nature of biological 

 processes on the slope are probably not substantially dif- 

 ferent in kind from those of the high reef. Rates are 

 undoubtedly reduced as light is attenuated with depth but, 

 as various workers have shown, this reduction does not 

 follow a direct linear relationship since there are some 

 accommodations to reduced light. Some limited observa- 

 tions on calcium carbonate production on the slope (Smith 

 and Harrison, 1977) suggest that, compared to the reef 

 flat, the slope input is minor. Sheppard (1982) provides a 

 comprehensive review of the little that is known about 

 slope environments throughout the world. 



As noted in Chapter 9, the high reef has been inten- 

 sively studied. Generally speaking, gross productivity is 

 extremely high; net productivity is not. Thus it is the lim- 

 ited net productivity of this region, plus that of the less 

 productive outer slope and the input from relatively impov- 



erished incoming oceanic water, that provides the suste- 

 nance described in discussing the trophic link from the reef 

 to the lagoon. 



The productivity input of the coral knolls of the 

 lagoon, probably not great, and basic trophic relationships 

 of the lagoon waters have been explored in the section on 

 trophic links. As to the benthic environment, i.e., the 

 lagoon floor, an impressive feature is the abundance of 

 conspicuous consumer organisms. There are the calli- 

 anasids (ghost shrimps), with mounds so closely spaced 

 that there is often no level bottom between them, and sea 

 urchins (six identified species) in varying densities up to 80 

 m~^ (Colin and Harrison, 1981). Harrison (1983) notes 

 that more carbon is respired by the overall lagoon-bottom 

 community than is produced there; thus we must assume 

 that the bottom fauna must depend to some extent on fall- 

 out from the lagoon detrital and plankton complex. 



This exercise, seeking to grasp the gross trophic rela- 

 tionships of the entire atoll system from fragmented infor- 

 mation, probably serves primarily to offer a sense of what 

 we do not know and need to learn. Even so, Enewetak 

 observations tend to conform to a generalization, discussed 

 by Kinsey (1979) largely from observations elsewhere, 

 which suggest that, considered cumulatively, the reef com- 

 ponents of such a system tend to be autotrophic, while the 

 remaining environments tend to be heterotrophic. Whether 

 this autotrophic/heterotrophic dualism balances out is not 

 known. Referring to reef systems in general. Smith (1983) 

 points out that we do not have a firm answer to this ques- 

 tion. Reflecting on the gross picture at Enewetak, we think 

 a balance does prevail, i.e., overall respiration equals or 

 offsets overall productivity. If this were not the case, one 

 would expect either an accumulation of organic matter on 

 the lagoon floor, in contrast to levels <1% (John T. Harri- 

 son 111, personal communication), or enriched oceanic 

 waters down-current from the atoll. Unfortunately the 

 waters flowing from the atoll have not been analyzed. 

 Perhaps the lack of noticeable pelagic fisheries concen- 

 trated down-current from atolls suggests that enrichment 

 there is not great. 



Implications for Fishery Yields 



Initially two considerations seem to imply a minimum 

 fisheries potential, in spite of the very high gross produc- 

 tivity of the extensive reefs and knolls. On the one hand, 

 many of the environments of the atoll system show little or 

 no net production. Also, as suggested above, the whole 

 atoll system seems to be in balance, with little or no 

 excess productivity. On the other hand, certain consider- 

 ations may offset this: 



1. With systems so highly productive, even a small 

 percentage net release can be appreciable. Furthermore, 

 we have shown that such releases do occur and are being 

 utilized by consumer food chains within the overall system. 



2. Noting that marine fisheries commonly occur in 

 regions that are in a climax or near-climax state, it is sug- 

 gested that capture fishery harvests may involve tapping 



