166 



MARSH 



zooxanthellae contains no P; hence, tha coral-algal associa- 

 tion should require new P only for growth and replacement 

 of zooxanthellae. Corals may thus recycle phosphorus (and 

 nitrogen) so effectively that their requirements are met by 

 the solid food they ingest, with no uptake from ambient 

 water being necessary; this is possible only for a slow- 

 growing community. 



With respect to transfers between trophic levels, it was 

 not clear to Pomeroy and co-workers which consumers 

 benefit from the coral production, although mucus may be 

 one of the major coral products (a recurrent theme which 

 will be discussed further in the next section). Phosphorus 

 in the algal filaments cropped by fish is probably recycled 

 largely to P04"^ by excretion. At least 75% of such P may 

 be recycled in 1 day in this kind of system. The authors 

 found significant recycling of P in all reef communities 

 examined. They concluded that the principal reef commu- 

 nities at Enew?tak are not limited by P and have evolved 

 either internal (biochemical) or external (food chain) recy- 

 cling loops to satisfy apparent P demands. These conclu- 

 sions were thus consistent with the ways that ecologists 

 have thought of reefs since publication of the Odutn and 

 Odum paper. 



Arsenate uptake and reduction by the coral Pocilhpora 

 verrucosa are somewhat related to ohosphorus cycling and 

 were examined by Pilson (1974). In incubation expcri 

 ments, P. verrucosa was found to remove arsenate from 

 solution and convert some of it to arsenite. which reap 

 peared in the ambient seawater. This suggests that organ 

 isms other than bacteria may be responsible for maintain- 

 ing som.c of the arsenic in seawater in a reduced form. 

 Reduction of arsenate may be a mechanism to allow the 

 loss from the liviiig coral cells of arsenate incidentally 

 transported in along with phosphate. 



D'Elia (1977) further examined the uptake and release 

 of dissolved phosphorus by corals in incubation exp>eri- 

 ments. He found that the net uptake of reactive phos- 

 phorus from seavater by coral? containing zooxanthellae 

 v/as not sufficient to offset simultaneous losses of organic 

 phosphorus; hence, there was a net loss of total P. A coral 

 without zooxanthellae was unable to remove net amounts 

 of reactive P from solution, evon at levels greater than the 

 normal ambient levels in reef waters. Reactive P uptake 

 was found to be light sensitive, was highly temperature 

 dependent, showed characteristics of Michaclis-Menton 

 enzyme kinetics, and could be inhibited by arsenate An 

 active-transport mechanism, thus appeared to be involved 

 in P uptake. The kinetics of net reactive P uptake were 

 described by a Michaelis-Menton equation modified to 

 include a correction for an efflux of reactive P going on at 

 the same time. The m.ean half-saturation constant was 377 

 nM and the mean maximum rate of uptake was 29.3 ng- 

 atoms P mg chl o ' h V DElia concluded that corals con- 

 taining symbiotic algae are ti.us unable to obtain all their P 

 requirements by means of reactive P uptake at typical 

 environmental concentrations but that their ability to obtain 

 part of the P requirements in this fa<:hion may help them 

 to flourish in water low in available P. This is probably 



further enhanced by the presence of mechanisms for effi- 

 cient recycling of P within the symbiotic association. 



Finally, Webb et al. (1977), in the paper previously dis- 

 cussed regarding nitrogen flux, briefly considered phos- 

 phorus along with nitrogen in the nutrient-flux measure- 

 ments of Holothuria atra populations They reported that 

 the release of P followed the general rules in the literature 

 for size-metabolism relationships. 



TROPHIC TRANSFERS 



Energy and nutrient transfer between trophic levels is 

 perhaps the area of energy and materials flux which has 

 been the least studied with respect to the amount of 

 research that is required for a comprehensive understand- 

 ing This is an area of great interest for ecologists and for 

 those concerned with the increasingly important area of 

 reef management A major effort would be required for 

 such a comprehensive understanding. As with so many of 

 the other topics discussed in this chapter, the first general 

 effort in this area was that of Odum and Odum (1955). In 

 addition to a description of trophic pathways and quantita- 

 tive trophic pyramids discussed earlier, they made some 

 preliminary attempts to quantify trophic transfers. While 

 their work pointed the way for future research, it was 

 hardly definitive; other researchers, so far, have been slow 

 to take up the challenge. 



Efforts since the Odums' study have dire'"ted attention 

 to three main a'eas of trophic relationships. The greatest 

 interest, cutting across these areas, appears to have been 

 on transfers from the windward reef flats into the lagoon, 

 with much less attention directed to transfers between 

 trophic levels within the reef flat subsystem Aside from 

 the relatively small-scale effort by Webb et al. (1977) to 

 understand nutrition of the large populations of sea 

 cucumbers on the reef flats (discussed previously), the 

 greatest interest at Enewetak, as elsewhere, has been with 

 the feeding relationships of the diverse fish community. 

 Most of the interest has been either on the recurrent 

 theme of "pseudoplankton'" (relatively large algal fragments 

 broken off from seaward reef zones and carried lagoon- 

 ward) or on the role of coral mucus in trophic relation- 

 ships, particularly those of the lagoon community. The 

 mucus interest, at least, fits in with the general interest of 

 many marine ecologists in the role of detritus and particu- 

 larly of organic aggregates that first came to prominence in 

 the late 1960s and carried over into the 1970s. It would 

 be desirable to have a careful evaluation of the general 

 framew'>rk of trophic relationships which could then be 

 used to point out directions for future trophic research. A 

 new overview paper such as that of Odum and Odum is 

 needed because of recent understandings of marine ecosys- 

 tems in general and reef ecosystems in particular. 



Trophic Rplationships of Fishes 



A major early paper on trophic relationships of reef 

 fishes was that of Hiatt and Strasburg (1960). who exam- 

 ined the feeding habits of 233 species. They distinguished 



