studied, but particularly on fore reef and sand substratum habitats of Isaac 

 Reef. Results of nutrient analyses for nitrate plus nitrite and phosphate are 

 listed in table 4. The nitrite and nitrate concentration strongly increases in 

 the water as it flows over the three reefs studied. Ten of the twelve sets of 

 measurements showed a highly significant yearly increase of 0.35-0.57 microgram- 

 at/1 across the reef. With a mean yearly flow rate of 6.83 m^/min/m for all 

 three transects (discussed below), this translates to a net "fertilization 

 rate" of 0.82 kg N/hectare/d (or 0.08 g N/m 2 /d). While this is roughly half the 

 rate estimated by Wieb, et al . (1975) for Eniwetak, the maximum rates we 

 encountered on the south shore reef of St. Croix were three times the mean. 



We assume that this fertilization is due to nitrogen fixation by cyanobacteria 

 (Wilkinson and Sammarco, 1983), but other possibilities exist. Regeneration of 

 nutrients from the internal cavity of a reef was demonstrated by Andrews and 

 Muller (1983). However, this is unlikely to occur on St. Croix reefs, since 

 the coral of these reefs grows on a pavement, below which the reef structure is 

 remarkably tight, with cavities usually sand filled (Adey, 1975; Adey and Burke, 

 1976). Empty cavities for nutrient regeneration are rare in St. Croix, which 

 is arid, devoid of rivers, and lacking in upwelling aquifers. Thus, nutrient 

 enrichment from ground water, such as that described in Jamaica (D'Elia, et 

 al . , 1981) is unlikely. Nitrogen fixing algae may achieve a competitive advan- 

 tage in more oligotrophic waters, and thus nutrient availability probably has 

 some significance in determining community structure. Nevertheless, the 

 fact that this "excess" nitrogen escapes from the reef and flows over the highly 

 productive back reef suggests that primary production on the reef proper is 

 not nitrogen limited. 



Phosphorus is more problematic, since an atmospheric source is not possible. 

 Reactive phosphorus as phosphate can be difficult to measure and, as briefly 

 discussed above, older work did not generally show any consistent pattern of 

 change in reef environments. A recent paper by Atkinson (1982) does show 

 significant upstream-downstream depletion of phosphate across 1000 m of reef 

 flat in Hawaii. However, in our St. Croix situation, with a mean incoming 

 concentration of 0.084 microgram-at/1 and a mean current flow of 6.83 m^/min/m, 

 phosphorus as phosphate is delivered to a meter width transect of reef at a 

 rate of about 25 g/d. Even if all net primary productivity were to be exported 

 from these reefs, using a C/P of 430 (from Atkinson and Smith, 1983, for turfs), 

 only about 6.4 g P/d/meter width would be removed. Thus, phosphorus is present 

 in large quantities, though at low concentrations, from the constantly inflowing 

 waters of the North Equatorial Current as it impinges on St. Croix. 



Epibenthic Water Flow and Discharge Rates 



Nutrient availability, gas exchange, and metabolite release for aquatic 

 primary producers is facilitated by water flow past algal thalli. Water flow 

 over reef substrata (epibenthic water flow) is dependent upon the discharge 

 rate (movement rate per volume of water), water depth, and on the wave surge or 

 oscillation rate of the overflowing water. In St. Croix, water movement over 

 the reefs results primarily from wind-driven wave energy. Thus, measured rates 

 of epibenthic water flow have geologic (depth) and geographic (wind) components. 



Epibenthic flow rates over reefs are variable in space and time. These 

 variations can affect both the productivity of the reef and the measurements of 



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