TROPHIC RELATIONSHIPS 



183 



product of these values divided by the volume of 

 Enewetak lagoon, 420.5 X 10^ m^ (Atkinson et al., 

 1981), yields an effective reef input of particulate carbon 

 of 1.07 mg C m^'^ d ' in summer. The effective reef 

 input of particulate organic nitrogen (PON) is estimated to 

 be one-sixth of the carbon rate, based on the C:N ratio of 

 6.6:1 for the particulate organic matter exported from the 

 windward reef tract at Enewetak (Webb et al., 1975). 



Because of the lack of entering data, values for the 

 flux of POC and PON across the windward reef in winter 

 are even more sp>eculative. Though winter and summer are 

 not differentiated as the data are presented by Atkinson et 

 al. (1981), our interpretation of the lumped values and 

 their range is that the currents across the reef, which as 

 they point out are driven largely by surf, would be three 

 times as great in winter when the trade winds prevail. 

 Since the increased surf and current must cause a greater 

 release of mucus from corals and of other detritus particles 

 of reef origin, we have suggested that the reef input of C 

 and N in winter is about three times greater than in sum- 

 mer, or >3 mg C m~'^. The fxjtcntial reef input is appreci- 

 ably increased if one speculates, quite reasonably, that at 

 least some of the dissolved organic matter flowing off the 

 reef (about three times that of the POC according to 

 Marshall et al., 1975) is aggregated into particulate form 

 available to consumers. 



In winter such estimated inputs from the windward reef 

 alone would seem to meet the estimated C and N require- 

 ment of the lagoon primary consumers. In summer, when 

 reef inputs apparently are not as great yet consumer 

 demands may increase, this input seems to fall far short of 

 demand. Other sources that may be involved to meet 

 estimated consumer requirements would be: 



1. POC entering from other reef areas (i.e., from 

 other than the windward sectors) 



2. POC entering via the Deep Channel, the channel 

 toward the Southwest, and through other passes 



3. POC from coral knolls— there are 2300 of these 

 with a total area of 9.8 X 10^ m^ (Emery et al., 1954) 



4. Photosynthetic inputs 



a. From plankton 



b. From benthic macroflora 



c. From benthic microflora 



Since Atkinson et al. (1981) repjort no net inward flow 

 from across leeward reefs and through the passes, contri- 

 butions via (1) and (2) probably are not very large. Also, 

 since the depths of the crests of the coral knolls average 

 36 m below the surface and the flow is not great (2 to 4 

 cm s"' according to Atkinson et al., 1981), it seems 

 unlikely that there is a major input from that source. 

 Among the photosynthetic inputs, the role of benthic 

 macroflora must be minor since, as Gilmartin (1960) points 

 out, such vegetation is not abundant. Similarly one can 

 expect very little from benthic microflora in view of the 

 lagoon depths of about 50 m, though it is possible that 

 algae symbiotic in foraminifera, particularly in the shallows, 

 contribute significantly. At Takapota Atoll, Sournia (1976) 



attributed high benthic productivity to such symbionts, and 

 Lee (1978) suggests that this can occur in low light; how- 

 ever, there are no observations directly relating such an 

 input at Enewetak. 



This leaves photosynthesis by plankton as the likely 

 major additional POC source. Unfortunately, there is only 

 one set of determinations (Doty and Capuro, 1961), indi- 

 cating a production of 5.76 mg C m~^ d~' on a winter 

 day. This scant information suggests that phytoplankton 

 production may equal the combined inputs from the wind- 

 ward reefs and the other sources listed above. Further- 

 more, since Gerber and Marshall (1982) found that phyto- 

 plankton were more than twice as abundant in summer 

 than in winter, it seems likely that such production is a 

 major factor in meeting summer consumer requirements. 

 (No consideration is given to dissolved organic matter gen- 

 erated from productivity within the lagoon since this would 

 not constitute additional input but, rather, part of the pro- 

 duction, release, and reformation processes taking place 

 within that part of the system.) 



To summarize, it appears that reef inputs constitute an 

 important part of the lagoon trophic regime, especially in 

 winter. Photosynthesis by the lagoon phytoplankton may 

 be at least equally important, with summer inputs probably 

 being the greatest. The average of consumer requirements 

 in summer is not as great as Table 1 suggests since peaks 

 in the abundance of pteropods and larvaceans (Gerber and 

 Marshall, 1982) and concentrations of jellyfish described by 

 John T. Harrison Hi (personal communication) are probably 

 of short duration. 



Comments on Organic Fluxes of the 

 Atoll System as a Whole 



The foregoing historical and narrative account of 

 organic inputs and utilization in the lagoon deals with only 

 one facet of the trophic relationships in the entire atoll 

 ecosystem. Considered in a more comprehensive way, 

 whether at Enewetak or elsewhere, the chief compart- 

 ments of a reef and the adjacent shallow water ecosystem 

 are the inputs from the 



1. Surrounding oceanic waters 



2. Outer reef slope 



3. High reef 



4. Knolls and patches in the lagoon 



5. Overlying lagoon waters 



6. Benthic environment of the lagoon * 



These compartments, except for the oceanic waters, are 

 lumped by some authors under the inclusive heading: 

 "coral reef ecosystems." 



It was mentioned previously that the inputs from sur- 

 rounding oceanic waters are low. It is admitted, however. 



'For some reef settings there are also extensive mangroves 

 inshore of the lagoon or other coastal shallows, and these 

 represent an additional input to the system; however, mangrove 

 areas are not developed at Enewetak and generally are not exten- 

 sive on atolls. 



