generated by degradation of the reef structure by borers, fecal material from fishes and 

 invertebrates that use these holes as shelters or encrust the walls of the holes, and non- 

 reefal material (including terrestrial and pelagic) that is trapped inside the reef as seawater 

 percolates through the porous structure. Organic materials in these sediments are 

 metabolized by microorganisms, and in the process, nutrients are regenerated. Elevated 

 concentrations of nutrients have been measured in waters from reef cavities (Di Salvo, 1969; 

 Andrews and Muller, 1983; Szmant-Froelich, 1983). 



I suggest that 'burps' of nutrient enriched water exit these holes and provide benthic 

 primary producers with short episodes of exposure to higher nutrient concentrations. During 

 Hydrolab Mission 83-10, whose objective was to study the role of herbivorous fish in nutrient 

 regeneration, we measured nutrient (Nrty, N03 and total dissolved N) concentrations of 

 surface and bottom waters at two sites, one near shore ("Habitat" site), the other less 

 protected and more offshore (East Slope site), as well as waters from about 400 m offshore 

 of the reef, and from inside caves at the East Slope site (Table 1). The results of these 

 measurements show that reef water is 3 to 4 times higher in NO3 and slightly higher in NH4 

 and organic N concentrations than oceanic waters. The most dramatic difference in 

 nutrient concentration, however, can be seen between the offshore water and the cave 

 water. Cave water concentrations are 13 times higher in NO3, 2 times higher in NH4 and 3 

 times higher in organic N than offshore waters. These enrichments in the caves represent a 

 signficiant increase in nutrients for any primary producers that might have access to them. 

 Dye injections into caves showed that there was rapid outwelling of cave waters onto the 

 reef, and, importantly, that these cave waters flowed within 1 m of the bottom for 10-15 

 min or longer before mixing upwards. This indicates that there is a process that restricts 

 vertical mixing and dilution of nutrient enriched outwelled cave waters, such that benthic 

 primary producers would have sufficient time to strip the nutrients from these waters 

 before they mix upwards. Corollary evidence that 'burps' of enriched water exist near the 

 reef bottom were obtained from fish excretion experiments during which fishes were 

 incubated inside PVC pipes. Concentrations of NO, were always constant during each 

 incubation but varied considerably between incubations (Table 1). Since these were short 

 incubations (30 min ) replicated repeatedly with new fish during 4 to 5 hour periods, and 

 conducted at various locations over the reef, the differences in NO, concentration reflect 

 true spatial and temporal differences in NO, concentrations of reel bottom waters. The 

 rapid outwelling of the dye from the caves further indicates that rates of nutrient 

 regeneration in cave sediments must be fairly high in order to maintain the observed 

 enrichments in spite of the high water flow. 



Table 1. Mean nutrient concentrations at various sites in Salt River Canyon, St. Croix 

 (Hydrolab Mission 83-10) during August 4-10, 1983 (mean + 1 std. dev.). 



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