plot of biomass as a function of seasonal productivity (line with closed circles, 

 fig. 7) indicates a cyclic pattern. The sharp reduction in algal biomass 

 recorded in the fall of 1978 (45% of that recorded in May) cannot be attributed 

 to wave action, since that is generally a quiet season and no unusual conditions 

 were experienced during the period of study. Data for nutrients (table 4) and 

 suspended matter (Adey, et al . , 1981) suggest very minimal effects of runoff on 

 the reefs. We observed, - b~ut did not quantify, changes in the abundance of 

 micrograzers (amphipods and worms). It is possible that during the spring and 

 summer algal biomass and the resident micrograzers increase with higher levels 

 of solar radiation. As light levels fall in autumn, grazing by the resident 

 micrograzers exceeds rates of algal growth. Such overgrazing could result in a 

 "crash" in both algal biomass and productivity. 



Geological Control of Reef Productivity: Synergy of Multiple Factors 



It is to be expected that those reefs which are less complex (lower SAR), 

 deeper, with lower epibenthic water currents, and have a larger percentage of 

 living stony coral (relative to algae) will be less productive than the shallow, 

 complex, algae-dominated reefs of the south shore of St. Croix. All of these 

 factors, however, are ultimately the result of the reef's geological history, 

 and thus the relationship between reef morphogenesis and primary production 

 should be considered. 



The south shore bank barrier reef system of St. Croix "matured" (i.e., 

 reached sea level) from west to east (see Adey, 1975; Adey and Burke, 1976, for 

 a summary of the geological history of the reefs of St. Croix). Isaac Reef is 

 relatively young and is just reaching sea level today. Its live Acropora 

 palmata cover is high (table 2), but its reef flats are narrow and the crest is 

 broken (forming numerous channels). Robin Reef is more mature, as evidenced by 

 its nearly continuous crest and wide, relatively shallow back reef. While 

 living Acropora palmata is still an important constituent, the percent cover of 

 A. palmata on Robin back reef is less than a third of that found on Isaac back 

 reef (table 2). Halfpenny Reef is older and has a continuous crest and a wery 

 shallow reef flat with very few living acroporids. The age difference between 

 Isaacs and Halfpenny Reefs is about 500-1,000 yr. 



Productivity on the young Isaac Reef is low, even though its SAR is quite 

 high. This is because coral is less productive than algae per unit of surface 

 area (Rogers and Salesky, 1981). At the other extreme, coral cover is low at 

 Halfpenny Reef, and this shallow reef flat is dominated by turf algae. However, 

 this reef is nearly planar (the SAR is half that of the other reefs), and thus 

 the surface area available for turf photosynthesis is proportionately reduced. 



Because carnivorous predators are so abundant on reefs, herbivores do not 

 stray far from their refuge in the spaces within the reef (e.g., Ogden, et al . , 

 1973; Talbot and Goldman, 1972). Thus, reefs with a high SAR are more heavily 

 grazed because they have more habitat space for reef-dwelling herbivores. In 

 this way, geological events that contribute to a high SAR on reefs also maintain 

 herbivore populations that intensely graze the dead coral substrate, thereby 

 maintaining the turf communities. 



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