INTERTIDAL ECOLOGY 



143 



(Webb and Wiebe, 1975), Wiebe, Johannes, and Webb 

 (1975) concluded that Calothrix is the most abundant and 

 important nitrogen-fixing organism. It is extremely tolerant 

 of the high range of temperatures and salinities characteriz- 

 ing its environment. Under experimental conditions, rates 

 of N fixation doubled between 27°C and 36°C; the limit- 

 ing temperatures were 24°C and 39°C. Experimental salin- 

 ities between 3% and 45% did not affect fixation rates 

 (Wiebe, Johannes, and Webb, 1975). 



Predominantly algal reef flats are thus extremely impor- 

 tant as a source of fixed nitrogen for adjacent communities 

 and hence a critical source of their high productivity. 

 Wiebe, Johannes, and Webb (1975) concluded that fixed 

 nitrogen enters the rest of the reef ecosystem via three 

 routes: (1) herbivorous parrot fishes and surgeonfishes 

 that graze on Calothrix Crustacea have low assimilation effi- 

 ciency, and much of the organic matter in their food is 

 liberated as feces; (2) Calothrix growing in the surf zone is 

 broken off the substrate and moved downstream by 

 lagoonward currents, where it is more subject to herbivory; 

 "benthic algal fragments constituted by far the largest por- 

 tion of the net plankton on the windward interisland reef at 

 Enewetak, and Calothrix constituted 20 to 60% (by 

 volume) of these fragments" (Wiebe, Johannes, and Webb, 

 1975); (3) Calothrix may release much of its fixed nitrogen 

 into solution; in culture as much as 40 to 60% was 

 released as peptides and amino acids. 



Concentration of reactive and organic phosphorus does 

 not change as water passes over tr II; at tr III, 

 reactive P decreased and organic P increased in concentra- 

 tion, both slightly but consistently (Pilson and Betzer, 

 1973). Samples of reef rock dominated by Schizothrix 

 actively take up phosphorus at a mean rate of 0.27 nmole 

 P cm h~' during the day but also continually lose 

 labeled P; mats dominated by Jania showed very little net 

 uptake or loss (Pomeroy, Pilson, and Wiebe, 1974). These 

 authors did not detect any special mechanism for retention 

 of phosphorus by the windward reef platform community. 

 Phosphorus incorporated by Schizothrix may be cycled 

 through the food web by herbivorous fishes and inver- 

 tebrates, which excrete it as phosphate. 



Calcium Transport and Calcification 



In addition to monitoring organic carbon production. 

 Smith (1973) was able to use the CO2 system of the 

 interisland windward reef platform to determine gross cal- 

 cification rates. Both a transect dominated by algae (tr II) 

 and one with both algae and corals (tr III) added CaC03 at 

 a rate of about 4 kg m^ yr \ comparable to similar 

 habitats elsewhere (Smith and Kinsey, 1976). Smith (1973) 

 estimated the erosion rate at less than 1 kg m~^ yr~\ suf- 

 ficiently close to the standard error of the calcification rate 

 to be ignored. Net calcification on the interisland reef plat- 

 form thus probably approximates gross calcification. 

 Locally production may be much more rapid: coralline 

 algal pavement at the reef rim produces CaCOs at a rate 

 of 8 to 16 kg m"^ yr"MSmith and Harrison, 1977). 



Bacteria 



DiSalvo (1973) noted the occurrence of a re- 

 duced layer of sand, suggesting bacterial decay of plant 

 material, at the foot of beaches on the windward sides of 

 Enewetak and Parry Islands. 



Benthic Flora 



Studies of the algae of the windward platforms subse- 

 quent to the initial work of Odum and Odum (1955) have 

 noted some differences but have been neither thorough 

 nor frequent enough to determine whether they indicate 

 spatial or temporal patchiness, or both, or long-term 

 trends. At the algal ridge crest and for about 200 m 

 lagoonward on interisland platforms, the crustose 

 Porolithon on/codes and the turf-forming Jania capillacea 

 continue to dominate the surface of the lithified reef rock 

 described above (Buddemeier, Smith, and Kinzie, 1975). 

 Bailey-Brock, White, and Ward (1980) characterized four 

 macroalgal zones inshore of the algal ridge crest on the 

 island reef platform at Enewetak Island (Table 4) where 

 turf-forming algae dominate. The algal turf is much 

 thicker — about eight times as much biomass — and holds 

 much more water at low tide in their zones 3 and 4 than 

 in the inshore zones. Such turfs are the characteristic algal 

 growth form in physically stressed tropical environments. 

 They arc more resistant to desiccation at low tide and to 

 herbivory than separate individual plants, which are more 

 productive and better competitors but are less resistant to 

 harsh physical conditions (Hay, 1981). 



The importance of the widespread blue-green alga 

 Calothrix Crustacea has been mentioned above. This 

 diaphanous, yellow-brown film covers large areas of the 

 windward flat. "Along the upper intertidal bench zone 

 another growth form of the same species occurs as a 

 black, feltlike mat up to 5 mm thick. At low tide, most of 

 this mat dries out. It is not heavily grazed by fish owing to 

 the shallowness of the water in which it grows. In areas of 

 the windward reef flat, dominated by other algae, C. Crus- 

 tacea is found ubiquitously as an epiphyte" (Wiebe, 

 Johannes, and Webb, 1975). 



In their initial study, Odum and Odum (1955) dis- 

 tinguished several ecological groups of primary producers 

 in the intertidal region of the interisland transect: phyto- 

 plankton; zooxanthellae in coral F)olyps, sea anemones, and 

 Tridacna; filamentous algae within skeletons of living 

 corals; encrusting filamentous, crustose coralline, and 

 fleshy green algae affixed to smooth and rough surfaces; 

 and algae associated with dead coral heads. They 

 estimated, admittedly crudely, dry biomass of primary pro- 

 ducers. In the coral-algal ridge zone, algae in corals and 

 encrusting and free-living algae contributed about equally 

 to the total estimate of 635 g m~^. In the encrusting 

 zone, encrusting algae and algae boring within the rock 

 substrate accounted for most of the biomass. The esti- 

 mates of Bailey-Brock, White, and Ward (1980) for com- 

 parable zones on an island platform are similar; the latter 

 researchers reported order-of-magnitude lower biomass 



