Diurnal measurements of changes in oxygen concentration and water flow 

 were taken in all seasons of the year and were used to derive rates of community 

 oxygen production (which reflect net or apparent productivity) and consumption 

 (respiration). Accurate measurements of discharge rates (rate of water volume 

 flow) are crucial to this upstream/downstream method. For this, we used a 

 Marsh-McBirney model 527 electromagnetic, x-y current meter. Water samples 

 were taken at each transect every 2 hr over a 2- to 3-day period during each 

 season. Particular care was taken to maximize the level of precision at every 

 step of analysis (see Adey, et al . , 1981). A plot of a typical diurnal cycle 

 of oxygen concentration at the Robin Bay site is shown in figure 3. Standard 

 methods of calculating production in upstream-downstream methods were employed 

 (Marsh and Smith, 1978). Computer plotting and integration of the area under 

 the curve above the zero oxygen production level provided net daytime primary 

 productivity values. The area below the zero line provided respiration values. 



The biotic composition (percent cover) and surface area of these reefs 

 were determined with chain transects (methods detailed in Rogers, et a! . , 

 1982). We measured all surfaces along each transect that we could reach, except 

 for truly cryptic habitats (nearly devoid of light). This method also gave a 

 measure of surface area per projected square meter of bottom [m 2 /m 2 , called a 

 surface area ratio (SAR)]. 



To examine community structure of algal covered surfaces, random Acropora 

 palmata substrate samples were collected from both fore and back reef zones. 

 Algal cover and distribution on the surface were mapped using grids with 1 cm 

 squares placed over the top and bottom surfaces. Algal turfs were subsampled 

 by scraping from 3-12 visually representative, but distinct, 1 cm 2 microquadrats, 

 Each sample included the upper millimeter of calcium carbonate in order to 

 sample endolithic, crustose, and tightly adherent algae. Solutions of 5% HC1 

 and 5% formalin were used to decalcify and fix the samples. A representative 

 subsample of the scraped algal turf samples was spread in a monolayer over a 

 microscope slide. Species composition of algal turf communities was quantified 

 using a "point count" technique. This involved counting the number of times 

 identifiable algae crossed points of intersection on an ocular grid placed 

 within a compound microscope. 



At each of the reef study sites, water samples were collected and analyzed 

 for nutrients at the beginning and end of each field session, between 1000 and 

 1400 hr. Standard precision handling techniques were employed at every stage, 

 and analyses for nitrate, nitrite, phosphate, and total dissolved phosphorus 

 were accomplished with a Beckman DU spectrophotometer. Extensive descriptions 

 of methods and data taken during the course of this study appear in Adey, et_ 

 al. (1981). 



RESULTS AND DISCUSSION 

 Reef Productivity 



The reefs studied were highly productive. Community metabolism, as shown 

 by mean diel oxygen production at each station, and seasonal metabolism at all 

 stations is given in figures 4 and 5. Gross primary productivity (GPP), net 



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