REEF PROCESSES 



171 



amounted to 95% and 85%, respectively, of gut contents 

 of the first two animals; chlorophyll was present in 2% and 

 6%, respectively, of the food mass. Planktivorous fishes 

 were reported to consume, in addition to zooplankion, a 

 substantial amount of detrital algal fragments. Fragments 

 of the nitrogen-fixing blue-green algae Calothrix were abun- 

 dant in the gut samples and were taken to be a primary 

 contributor, both directly and indirectly, to lower C:N 

 ratios in the lagoon. Chlorophyll a and phaeopigment lev- 

 els in the lagoon waters were small, amounting to 0.098 

 mg m"^ and 0.085 mg m~^, compared to total particulate 

 C values of 20.5 mg m"'^. Waters over and behind the 

 reef had lower levels of particulate C and N and higher 

 C:N ratios than incoming oceanic water. Gerber and 

 Marshall concluded that there is substantial input from the 

 reef to the lagoon and that this input probably supports a 

 more abundant zooplankton in the lagoon than would oth- 

 erwise be the case. Gerber and Marshall stated that 

 decomposition of algal fragments by fish digestion may be 

 the first step in its transformation to a form that can be 

 consumed by zooplankton. 



In another study, Johannes and Gerber (1974) exam- 

 ined the import and export of net plankton by a portion 

 of the reef-flat community by placing plankton nets 

 (60-M mesh) immediately upstream and downstream of the 

 coral zones. Differences between these upstream and 

 downstream nets showed a net import of organic C, N, P, 

 benthic algal fragments, fecal pellets, and zooplankton by 

 the coral zones. (However, analyses of small-volume water 

 samples showed a net export of particulate C and particu- 

 late N for the who|p transect.) Benthic algal fragments 

 outweighed all other imported components combined; fecal 

 pellets accounted for the rest of the detritus. Most algal 

 fragments consisted of the red Asparagopsis or the blue- 

 green Calothrix. Johannes and Gerber calculated that sev- 

 eral thousand meroplankters were exported and several 

 thousand holo- and meroplankters were imported daily by 

 each square meter of reef surface. Some removal of algal 

 fragments was due to settling out rather than feeding by 

 animals. Johannes and Gerber concluded that reef commu- 

 nities are efficient traps of net plankton and that this may 

 contribute to downstream changes in community composi- 

 tion and possibly to the limited width of interisland reef 

 systems (through downstream plankton depletion). 



Marshall et al. (1975) made additional observations on 

 particulate and dissolved organic matter in reef waters. 

 They reported that high concentrations of particulate 

 organic carbon occur in the environs of reefs and may be 

 attributed to the reef community itself. Changes in dis- 

 solved organic carbon (DOC) concentrations of waters flow- 

 ing across reef flats are relatively small and inconsistent. 

 The lack of distinct net increases in particulate organic 

 matter (POM) of waters flowing across shallow reefs sug- 

 gests that some of the released particles may be entrapped 

 and consumed by the community. The composition of par- 

 ticulate matter is extremely varied, but there is always 

 very little phytoplankton. As in most waters, particulate 

 organic carbon (POC) levels are an order of magnitude less 



than DOC levels. Marshall et al. stated that there is an 

 impressive increase of POC on rises and reef crests for all 

 reefs studied, and there is a decrease in the ratio of 

 DOC: POC from open ocean waters across reefs and into 

 lagoons (changing from 103 to 33 for material trapp>ed on 

 glass filters). 



The role of detritus in trophic relationships within the 

 lagoon is considered further in Chapter 10 in this volume. 

 The reader is referred to that chapter for a discussion of 

 the magnitude and importance of inputs of detritus from 

 the reef flats to the lagoon and for an integration of the 

 various subsystems of the total atoll ecosystem. 



ENEWETAK RESEARCH 

 IN PERSPECTIVE 



Although much pioneering work was done at Enewetak 

 over the years, interest has now shifted to a number of 

 other localities because of the shutdown of the Enewetak 

 facility, the establishment of facilities elsewhere, and the 

 rapid growth of reef research in the last decade. It is 

 worthwhile to consider how the earlier Enewetak work fits 

 into the broader context of more recent knowledge. 



Advances at the community/ecosystem level have 

 come in considering whole-atoll systems rather than simply 

 reef flats, in examining temporal and spatial variations 

 rather than relying on a restricted set of observations, and 

 in more fully integrating nutrient fluxes into the total meta- 

 bolic picture. Recent work is probably more striking for 

 confirming and amplifying insights derived from Enewetak 

 research than for reversing any major conclusions reached 

 at that level. 



Kinsey (1979, 1983) has considered "standards of per- 

 formance" by reef ecosystems with respect to primary pro- 

 duction and carbon turnover. He summarized metabolic 

 studies and emphasized the considerable uniformity 

 reported for reef flats from different latitudes and with 

 differing biological makeup. The mean gross productivity 

 for 16 studies was calculated tobe7.9gCm~ d (stan- 

 dard deviation [S.D.] is 2.7), with a mean calculated 24-h 

 respiration also of 7.9 g C m~^ d"' (S.D. is 5.0). These 

 are general values which are strikingly similar to the 

 Enewetak-only values discussed earlier in this chapter. 

 Further striking confirmation of the earlier Enewetak work 

 comes from Kinsey's summary of P:R values consistently 

 approximating 1.0 and his summary indicating that plank- 

 ton metabolism is at least an order of magnitude lower 

 than activity of the ecosystem as a whole. Furthermore, 

 the Smith and Kinsey (1976) suggestion, based partly on 

 earlier Enewetak work, of a bimodal model for calcification 

 rates also appears to have been borne out by subsequent 

 work at other localities. Hence, Kinsey (1983) suggested a 

 generalized bimodal picture of (1) reef flats and all exten- 

 sive, present-day metabolically active perimeter zones 

 (whether an outer reef crest or the windward edge of a 

 lagoon ward patch reef) with a gross P of 5 to lOgCm 

 d~ and a calcification rate (G) of 3 to 5 kg CaCOa m~ 

 yr^^, and (2) sand/rubble areas with contrasting rates of 



