REEF PROCESSES 



167 



tration (57 ± 47 nM) below which uptake did not occur. 

 A limited number of organic nitrogen determinations indi- 

 cated that a slight efflux of such comf)ounds occurred. A 

 model of nitrogen flux in Pocillopora speaes was produced 

 in which, at typical ambient levels, ammonium uptake 

 appeared to be about twice as great as nitrate uptake. 

 D'Elia and Webb concluded that, while dissolved nitrogen 

 taken by corals might be nutritionally important, it was not 

 likely that the organisms could sustain high growth rates 

 on this nitrogen source alone; hence, some ingestion of 

 particulate nitrogen was probably necessary. 



In a later paper, Muscatine and D'Elia (1978) studied 

 ammonium flux in intact coral colonies held in incubation 

 chambers. Of several species tested, only those containing 

 zooxanthellae were found to take up and retain NH4 . 

 Uptake and retention were enhanced by light, and the 

 authors concluded that uptake during a normal daylight 

 period is sufficient to sustain NH4^ retention during the 

 night. The pattern of uptake kinetics for several species 

 indicated that a two-process mechanism might be involved. 

 When a diffusion correction was made, uptake kinetics 

 could be characterized by the Michaelis-Menton equation. 

 Corals without symbiotic algae were found to release NH4 

 during incubation experiments. 



Webb et al. (1977) assessed the biomass and nutrient 

 flux in populations of the sea cucumber Holothuria atra 

 which are abundant on the windward reef flats of 

 Encwetak. They estimated average densities of three 

 animals m~^, with a median weight of 60 g, in the 

 zone of small heads on a coralgal transect. Size distribution 

 of individuals was found to be negatively correlated with 

 water current velocity. 



Ammonium release by the population was equivalent 

 to 9% of total release by the reef community and was 

 weight-specific in H. atra, H. difficilis. and Actinopi;ga mau- 

 ritiana (I.e., the size-specific release rate was constant for 

 different-sized organisms). Urea was also released by 

 animals in incubation chambers. For H. difficilis, the total 

 release consisted of 57% ammonium, 17% urea, and 26% 

 unidentified nitrogen compounds in a 2-g animal. For all 

 three species, the ratio of N:P release was 42:1 for 60-g 

 animals and 25:1 for 1-g animals. 



Webb et al. (1977) also analyzed reef-flat sediments 

 and the gut contents and fecal pellets of H. atra for total 

 and organic carbon. Of the total carbon in the sediments, 

 3% was estimated to be organic; 10% of the carbon in 

 fecal pellets was estimated to be organic. The authors 

 stated that this finding could be accounted for by the dis- 

 solution of CaCOs in the animals' guts, by selective feed- 

 ing, or by both of these processes. The sediments were 

 estimated to consist of 0.4% organic C and 12% inorganic 

 C by dry weight; fecal pellets were 1.3% organic C and 

 11.6% inorganic C. The dissolution of CaCOs by 

 holothurian populations was estimated to be approximately 

 2.5 g m d~\ equivalent to about 25% of the net calcifi- 

 cation rate for the reef flat as a whole. Webb et al. con- 

 cluded that H. atra is a selective feeder and that it ingests 

 euid egests materials considerably richer in organic carbon 



than the general sediment; assimilation efficiency was 

 estimated to be approximately 40%. 



Phosphorus Flux in Individual Populations 



Studies of phosphorus flux in individual populations 

 started before those of nitrogen flux and have been more 

 numerous and concerned with a larger variety of organ- 

 isms. The first was reported by Pomeroy and Kuenzler 

 (1967), who examined phosphorus content and elimination 

 rates for dominant reef animals of several taxa; these two 

 measurements were then integrated to estimate the flux of 

 P through the organism.s, expressed as turnover times. 

 Herbivorous fishes appeared to receive just enough P for 

 growth and reproduction, as reflected in somewhat lower 

 excretion rates and longer turnover times than would be 

 predicted for organisms of their size by standard relation- 

 ships. In the important herbivorous fish Acanthurus (turn- 

 over time, 410 days), the ingestion of small quantities of 

 animal food, even if taken only incidentally to grazing on 

 algal filaments, might be an important phosphorus source. 

 Carnivorous animals and deposit feeders were calculated to 

 receive excess P in their diets, with turnover times (12 to 

 4000 days for a large size range) not significantly different 

 than they would be in ecosystems with a more abundant 

 supply of that element. Of five coral species, four showed 

 very long turnover times (10^ to 10^ days) and little P loss 

 to the environment. The giant clam Tridacna crocea 

 showed a typical turnover time (about 900 days) for a mol- 

 lusc of its size, and most of its P loss was that incor- 

 porated into living zooxanthellae which were subsequently 

 lost. In general, the phosphorus economy of animals con- 

 taining algal symbionts seemed to be quite varied. Zoo- 

 plankton had turnover times approximating 1 day. 



Additional studies of the exchange of phosphorus 

 between organisms and reef waters were conducted by 

 Pomeroy et al. (1974). Although algal mats dominated by 

 the blue-green Schizothrix showed an active uptake of 

 radioactively labeled P04"^ in the light, they also showed a 

 continuous loss. Pomeroy et al. thought that the loss was 

 at high enough rates that, if a bell jar were placed over the 

 pavement community for a short time in the dark, changes 

 in dissolved phosphorus could be detected chemically in 

 the enclosed water mass. They also considered it possible 

 that as much as 50% of the photosynthetic products of the 

 pavement community might be released as soluble organics 

 containing no P. Algal mats dominated by the articulated 

 coralline red Jania showed no net uptake or loss of phos- 

 phorus when incubated. Algal-encrusted pebbles showed an 

 insignificant suggestion of uptake and no evidence of loss. 

 The corals Acropora and Heliopora showed no net uptake 

 or loss after equilibration, while Millepora showed continu- 

 ous uptake. Compartmental analysis of the former two spe- 

 cies suggested that there was a pool of phosphorus in the 

 coral tissue in equilibrium with P04^'^ in the ambient water 

 and isolated from the P demand of the zooxanthellae. 

 Pomeroy et al. cited Muscatine's earlier (1967) work as 

 evidence that much of the photosynthate released from the 



