182 



MARSHALL AND GERBER 



gain as water crossed the reef (Marshall, 1968). Thus this 

 hypothesis concerning organic particle formation, attractive 

 though it may be, remains unproven. 



Whether one denies or argues that such a bubble- 

 related aggregate formation mechanism is appreciable in 

 the flow over a reef, there is no denying the earlier and 

 continuing observations of detritus, mucus fragments, and 

 other organic particles in the water flow toward the 

 lagoon. In a very simple set of observations on corals held 

 in shipboard tanks, Marshall (1972) showed that stimula- 

 tion of corals by jets of seawater, intended to simulate the 

 effects of breaking waves, did increase the output of 

 organic particles. Qasim and Sankaranarayanan (1970) 

 demonstrated that particulate organic matter greatly 

 increased over the reefs of Kavaratti Atoll in the Laccadive 

 Islands. During Project SYMBIOS, headed by Robert 

 Johannes (Johannes et al., 1972) and with both Johannes 

 and Marshall present, the research team was looking for 

 striking examples of a rich mucus and aggregate flow, but 

 a visible "marine snow" consisting of these small 

 suspended fiarticles was not as evident as that observed 

 earlier by Johannes. Subsequently we have learned, from 

 observations by Gerber and by John T. Harrison 111 (per- 

 sonal communication) that the appearance of "snow" varies 

 from time-to-time and at differing sites behind the reefs. 



Some quantitative information on such inputs, largely 

 from Enewetak data, was summarized by Marshall, Durbin, 

 Gerber, and Telek (1975). Also Johannes and Gerber 

 (1974) report on plankton-net detritus in the reef flow at 

 Enewetak, wherein they indicated a large percentage of 

 algal fragments. 



Such work provided the background for Gerber to take 

 the necessary steps to explore the extent to which the 

 detritus and various amorphous particles in the flow from 

 the reef at Enewetak might be directly utilized by con- 

 sumers in the lagoon. At first he focused on gut contents, 

 analyzing a calanoid copepod, Undinuh uulgahs; a lar- 

 vacean, Oi/cop/eura longicaudata; and seven species of the 

 small pelagic fishes of the Enewetak Lagoon. For the two 

 zooplankton species, he noted that detritus and amorphous 

 p>articles predominated in the guts, while phytoplankton 

 cells were present in only trace quantities. Though the 

 small fish directly ingested reef detritus in the form of 

 suspended algal pigments and fecal pellets, the bulk of the 

 gut contents consisted of copepods and larvaceans. Cou- 

 pling these observations suggests a food chain in which a 

 base of detritus and aggregates is eaten by zooplankton 

 (Gerber and Marshall, 1974a and b), which then can be 

 eaten by the small fishes. 



Later at Enewetak with the help of his wife, Mary, 

 Gerber quantified the particulate food requirements of 

 representative planktivores of the atoll system as he held 

 them in the laboratory in containers filled with water from 

 the lagoon. Ascertaining the particulate organic content of 

 the water before and after feeding and subtracting fecal 

 depKssits, the Gerbers obtained quantitative data on 

 material consumed and assimilated (Gerber and Gerber, 

 1979). 



With this information and with the sampling of zoo- 

 plankton populations carried out by Gerber while doing 

 other work at Enewetak (Gerber, 1981), it was possible to 

 approximate total requirements as noted in Table 1 (con- 

 densed from Gerber and Marshall, 1982). Since there were 

 no assimilation assays for larvaceans to include in the 

 table, it was estimated, on the basis of comparisons of the 

 body content of carbon and nitrogen, that the requirements 

 of this group are about one-sixth that of the small 

 copepxxds (Gerber, unpublished data). Microorganism car- 

 bon requirements are based on summer respiratory rates 

 of concentrated suspended particles from windward reef 

 samples (Johannes ct al., 1972). Winter rates were 

 assumed to be about one-third the summer rates based on 

 relative abundances (Gerber and Marshall, 1982). Nitrogen 

 assimilation rates were estimated to be about one-eighth of 

 the carbon requirements, interpreting a ratio of 8:1 from 

 Vinigradov's (1953) chemical composition data. 



TABLE 1 



Assimilation Requirements of Particulate Organic 



Carbon and Nitrogen by the Major Primary Consumer 



Groups of the Pelagic Lagoon Environment* 



Copepods 3.10 0.40 4.14 0.53 9.77 1.27 

 Pteropods 21 0.03 0.71 0.09 17.40 2.13 

 Larvaceans 28 0.04 0.37 0.05 0.97 0.11 

 Microor- 

 ganisms 1.10 0.14 1.10 0.14 3.30 0.41 

 Rate of as- 

 similation [4.69] [0.61] [6.32] (.81] [31.44] [3.92] 



'Units are in mg m~^ d~' (condensed from Gerber and Marshedl, 

 1982). 



Curious as to whether the reefs might supply an appre- 

 ciable portion of these requirements, we noted that the 

 windward reefs at Enewetak have a net trans- 

 port of seawater into the lagoon amounting to 13.2 X 10* 

 m'^ d^'* during the month of June (Atkinson et al., 

 1981). This figure is multiplied by the quantity of particu- 

 late organic carbon (POC), 34 mg C m~^, flowing off the 

 reef [the average values for Enewetak samples collected on 

 glass fiber and silver filters (Marshall et al., 1975)]. t The 



'Atkinson et al. (1981) give their data in terms of tidal cycles 

 of 12 h 25 min. For quantities per day we simply double their 

 data since none of the values are refined sufficiently to correct for 

 the 50 min difference involved. 



t Other workers (Simmons, 1979, and Westrum and Meyers, 

 1978) have commented on the utilization of such particulate 

 organic carbon by the reef community and, in the 

 Westrum/Meyers paper, quite a point is made of high POC at 

 the reef crest with a rapid decline over the back reef. Our data 

 (Marshall et al., 1975) does not show this marked difference 

 between the reef and the back slope. For this discussion of input 

 to the lagoon we use the back slope veilues. 



