1. They are derived from the molecular weight difference and the ratio of 

 oxygen to carbon dioxide exchange (the metabolic quotient) for reefs (Marsh and 

 Smith, 1978). Higher GPP have been reported (Helfrich and Townsley, 1963; 

 Connor and Adey, 1977), but such high levels of productivity have been subject 

 to question. Because of this, great care was taken in data collection, and 

 every potentially error-producing factor has been carefuly considered (see 

 Adey, et al . , 1981). The patterns we observed are not surprising once the 

 processes affecting reef productivity are examined. 



Processes Affecting Reef Productivity 



Rates of carbon fixation on reefs reflect a complex interaction of 

 environmental factors. Slight variations in some of these factors (water depth, 

 biotic composition, substrate availability, or solar radiation) will affect 

 these values. In the remaining sections of this paper, we will describe the 

 results of our studies of biological, chemical, physical, and geological factors 

 as they affect overall reef productivity. 



Biological Components: The Role of Algal Turfs 



Benthic algae, as both free-living forms and endosymbionts in coral and 

 other invertebrates, are the primary producers on reefs. The focus of our 

 study was on free-living benthic algae because it was the most abundant biological 

 component of the reefs we studied (see table 2). Large-scale and small-scale 

 data were integrated to determine community structure. Large-scale cover was 

 based on chain transects and is presented as a measure of reef complexity (SAR) 

 for the fore and back reef of each transect in table 2. Small-scale cover is 

 based on point counts of prepared microscope slides (table 3). 



To test the hypothesis that algal turfs are the most important primary 

 producers of the St. Croix reefs, we compared rates of turf biomass production 

 with rates of flow respiratory production. To make such a comparison, several 

 conversions are necessary to obtain common units of measure. All conversions 

 involve values from either table 1 or 2, or from published literature for a 

 similar environment. Values for each back reef will be reported in east to 

 west order (i.e., Isaac, Robin, and Halfpenny Reefs, respectively). Algal NPP 

 was calculated from community GPP values (table 1) by multiplying by 0.24, 

 which is the mean NPP/GPP ratio derived from chamber studies of algae by Wanders 

 and Wanders-Faber (1974) and Rogers and Salesky (1981). The resulting algal 

 NPP values for back reefs are 11.2, 21.2, and 12.2 g 62/m /d. To equate our 

 projected m? of a complex reef to a surface rather than a projected area, we 

 divided by the SAR (table 2). This gave values of 4.1, 7.1, and 9.9 

 g OpAsurfaceJ/mVd. Assuming that all productivity was from algal turfs, we 

 divided the surface production values by the proportion of turf in the back 

 reef (table 2) (values of 15.8, 19.7, and 18.5 g 2 /(planar) nr/d. Oxygen 

 production was converted to carbon production by multiplying by the difference 

 in molecular weight (0.375) and dividing by the photosynthetic quotient for 

 reefs (1.1) (Smith and Marsh, 1978), giving values of 5.4, 6.7, and 6.3 

 g C/(surface) m^/d. The conversion of carbon to organic matter involves the 

 ratio of 2.2 dry wt/g C (Westlake, 1963, and confirmed by our carbon analysis of 



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