142 



KOHN 



gala were tha dominant cover organisnns of the algal ridge, 

 and corals were very sparse. For about 160 m lagoon- 

 ward, Porolithon and the turfy red coralline alga Jania 

 capillacea dominated; some zoanthids and holothurians 

 were also common. Over the next 180 m, the corals 

 Pontes. Acropora, and Heliopora increased in abundance 

 on the limestone pavement, and Porolithon continued. 

 Transect III (tr III; 270 m) was characterized by cover of 

 Porolithon at the algal ridge and Jania turf throughout. 

 Foraminifera and gastropod molluscs were common, but 

 there were virtually no corals on the transect. Water depth 

 over these transects at low tide was less than 1 m. 



As Table 2 indicates, the transect with the greater 

 dominance of algae throughout (tr 111) was the more pro- 

 ductive. And as shown in Table 3, only the predominantly 

 algal turf areas were highly autotrophic; the other regions 

 produced about as much organic matter as they consumed. 

 Smith and Marsh (1973) also demonstrated that the C:02 

 metabolic quotient for the platform was very close to 1.0, 

 as prior authors had assumed but had not tested. 



TABLE 2 



Gross and Net Primary Organic 



Productivity of an Interisland 



Windward Reef Platform at Enewetak Atoll* 



"Data from Smith and Marsh (1973) and Marsh 

 (1970). Upper figures: g C m~ h~'; lower figures: 

 gCm-2y-' 



Primary productivity of algal ridge crest areas dom- 

 inated by Porolithon is much lower than the transect as a 

 whole (Tables 2 and 3). Marsh (1970) concluded that this 

 zone contributes much less to overall reef productivity than 

 the others and that the reef-building activities of the crus- 

 tose coralline algae are more important than their energy 

 fixing. 



Bakus (1967) estimated net primary productivity of the 

 inner portion of the Enewetak Island platform where the 

 blue-green algae Calothrix Crustacea and Schizothrix cal- 

 cicola are the dominant plants. Because his method mea- 

 sured increased standing crop in cages that excluded large 

 herbivorous fishes but not small fishes or benthic inver- 

 tebrates, the result, about 440 g C m~ yr~', is undoubt- 

 edly an underestimate. 



Nutrient Cycling 



Because nitrogen in forms assimilable by photosyn- 

 thetic organisms is an important limiting factor of primary 

 productivity in the sea, the recent discovery that nitrogen 

 fixation occurs at high levels on Enewetak's windward reef 

 platforms (Webb and Wiebe, 1975; Webb, DuPaul, Wiebe, 

 Sottile, and Johannes, 1975) is an important contribution 

 to understanding coral reef-associated ecosystems. Studies 

 at tr II and tr III described above indicated increasing con- 

 centrations of nitrate and ammonium as water crosses an 

 island reef platform. The highest rates of production (about 

 1.5 nmoles cm~ h~' each of NO3 and NH4 ) occurred 

 on rock surfaces supporting a mixed algal turf dominated 

 by Calothrix Crustacea (about 80%) and Schizothrix cal- 

 cicola (about 20%). This rate of nitrogen fixation is com- 

 parable to those in managed agriculture (Wiebe, Johannes, 

 and Webb, 1975). Calothrix covered the surface as a thin, 

 yellow-brown film over large areas of the platform and 

 penetrated the limestone to a depth of several millimeters 

 (Webb and Wiebe, 1975). Although the nitrogen fixing, 

 chemoautotrophic bacterium Mtrobacter agilis colonized 

 slides placed on the substratum and after 4 weeks attained 

 densities high enough to fix NO3" at the observed rates 



TABLE 3 



Ratio of Productivity and Respiration on an Interisland 

 Windward Reef Platform at Enewetak Atoll 



*g C m ^ h 



