100 



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O "a 



Q eg 



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 cc 

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 S 

 cc 



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(X) annual mean - individual reef 

 • seasonal mean-all reefs 



200 



400 



600 



800 



1000 



TOTAL ALGAL BIOMASS IAREA x SAR. x BIOM.I 

 Igm dry/m 2 projected) 



Figure 7. --Back reef: Gross primary productivity as a function of total algal 



biomass and monthly solar radiation 



for all reefs reveals a strong seasonal pattern in productivity corresponding 

 to these seasonal differences in light intensity (fig. 5). Although the seasonal 

 decrease in primary productivity roughly corresponds to a 40% reduction in 

 light during that period, the relationship is not exact, since the GPP in October 

 is reduced to only 30% of that value in May. A better correspondence exists 

 between the seasonality of primary production and the combination of algal 

 biomass and solar radiation. Loss of light in autumn is exacerbated on St. 

 Croix by a higher frequency of rainfall and cloudiness. Thus, incoming light 

 in October and November is considerably below that of December and January 

 (fig. 6). 



The GPP data, when plotted directly against total algal biomass (per 

 projected m^, fig. 7), suggests that a weak relationship between productivity 

 and total biomass exists on these reefs (note that this is not surface biomass 

 alone, but includes the effects of SAR and percent cover of algae). Also, the 



181 



