Our study examines the productivity of three reef environments in tropical, 

 oligotrophic seas. Large-scale temporal and spatial variations in their primary 

 productivity with respect to potentially important biological, chemical, 

 physical, and geological differences between the reefs are explored. 



MATERIALS, METHODS, AND STUDY SITES 



Our study was conducted on the southeastern bank barrier reef of St. Croix 

 from October 1977 to November 1978. It extends 35 km along most of the windward 

 shore of the island and is the most extensive reef system on the Puerto Rican- 

 Virgin Island shelf. St. Croix is a small island which is unaffected by 

 neighboring islands. Temperature, salinity, and wind direction and intensity 

 are all remarkably constant. The south shore is relatively unpopulated, and it 

 has no natural rivers affecting salinity or ambient nutrients. The south shore 

 of St. Croix is ideal for examining the factors that affect reef productivity. 

 The two major factors that change from east to west are water motion (decreasing) 

 and geological reef development (increasing). 



The western reefs of St. Croix lie on a shallower limestone basement than 

 do the eastern reefs; they were the first to grow to sea level (Adey, 1975; 

 1978a), and they developed broader, more continuous reef flats. Several of 

 these reefs have become so shallow that the abundance of live coral is reduced 

 due to the negative effects of intense sunlight (including ultraviolet), tempera- 

 ture fluctuations, and desiccation during periods of extraordinarily low tides. 



We studied three reefs on the south shore of St. Croix intensely and 

 several others to a lesser extent (fig. 1). The following is a description of 

 the morphology of the reefs from east to west. Isaac Bay Reef is a "young" 

 reef with a high coral cover, rapid reef growth rates, and a crest that is just 

 reaching sea level today. Mean depths are 6.3 m for the fore reef and 0.9 m for 

 the back reef. The reef structure is open (many channels and breaks in the reef 

 crest), and there is no continuous reef flat. Robin Bay Reef is more mature, 

 with a nearly continuous crest and a moderately deep and broad reef flat. Mean 

 depths are 5.1 m for the fore reef and 0.7 m for the back reef. Halfpenny Bay 

 Reef is mature, with a continuous, broad, and shallow reef flat. Mean depths 

 are 5.1 m for the fore reef and 0.3 m for the back reef. We selected these 

 reefs because they fall along a gradient of reef maturity and because they are 

 particularly well suited for the upstream/downstream method. All of these reefs 

 have a continuous unidirection water flow inward across the reef to the lagoon. 



A profile of Robin Reef along a permanent transect line established at the 

 beginning of our study is shown in figure 2. Comparisons with Isaacs and 

 Halfpenny Reefs are given in the accompanying table. The transects were parallel 

 to the predominant current flow and were established using drogues to indicate 

 the exact current flow. We omitted breaker zones due to logistic and scientific 

 problems created by the turbulence (accelerated diffusion). The outer (seaward) 

 limit to the fore reef was defined by the first appearance of the dominant reef- 

 building coral, Acropora palmata . The inner fore reef and outer back reef zone 

 were adjacent to the breaker zone. The inner (landward) limit of the back reef 

 was defined by the abrupt transition between the reef and sandy lagoon. Our fore 

 and back reef zones are consistent with other published studies (Adey, 1975; 

 Adey and Burke, 1976). 



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