slurries that are enriched with nitrate and incubated under anaerobic conditions 

 (Billen, 1978; Koike and Hattori , 1978, 1979; Oren and Blackburn, 1979; Iizumi, 

 et al . , 1980) to be good qualitative but not necessarily good quantitative 

 tools--such experimental conditions destroy sediment microenvi ronmental structure 

 (e.g., nitrate, oxygen, and organic matter concentrations) that cannot be 

 simulated exactly in the laboratory. This is particularly problematic in the 

 Thalassia bed sediments with their complex root structure. The use of the 

 acetylene blockage technique also incurs problems. For example, acetylene blockage 

 of N?0 reduction may be inhibited by the presence of sulfide (Tarn and Knowles, 

 1979), and acetylene itself may be oxidized anaerobical ly (Culbertson, et al . , 

 1981) or may inhibit nitrification (Bremner and Blackmer, 1979; Blackmer, et 

 al . , 1980), methanogenesis (Oremland and Taylor, 1975), methane oxidation 

 (deBont and Mulder, 1976), and the growth of sulfate-respi ring bacteria (Payne 

 and Grant, 1982). Considering these limitations, it is likely that the 

 denitrification rates reported here are underestimates of the field rates. 



The amount of fixed N being lost through denitrification in the lagoon 

 behind the coral reef conceivably could be as high as that fixed by N2 fixation 

 on the reef itself. No measurements of N2 fixation are available for Mayaguana. 

 However, measurements of N2 fixation have been made on a number of other coral 

 reefs. Maximum rates found at Enewetak, for example, were 600 pmol N2 m"^ h" 1 

 during daylight (Wiebe, et al . , 1975; Webb, et al . , 1975). For an average rate 

 over 24 hr, this rate should be decreased by approximately 50% to 300 ymol m~2 

 h _ l, since such high rates only occur during daylight. The above methodological 

 limitations aside, we can make a rough approximation of denitrification in the 

 lagoon. The results from the acetylene inhibition experiments indicated that 

 denitrification rates were approximately 0.2 nmol cm _ 3 h _ l throughout the top 

 25 cm of lagoon sediment. The integrated value for the top 25 cm is then 

 50 umol m - 2 h~l. The area of the lagoon is estimated to be 10 times the area 

 of the reef. Therefore, in the reef-lagoon ecosystem, denitrification in the 

 lagoon would more than balance the N2 fixation on the reef. While these measure- 

 ments and calculations are only preliminary, they do point to the probable 

 importance of denitrification in reef ecosystems and to the need for further 

 measurements. 



ACKNOWLEDGMENTS 



We thank Walter Adey and the crew of the R/V MARSYS RESOLUTE for giving us 

 the opportunity to make these measurements. We also thank S. V. Smith and K. 

 L. Webb for their critical comments and L. Woodbury, D. Engle, and G. Canaday 

 for technical help. 



REFERENCES 



Balderston, W. L., B. Sherr, and W. J. Payne. 1976. "Blockage by Acetylene 

 of Nitrous Oxide Reduction by Pseudomonas perfectomari nus . " Appl . Envi ron. 

 Microbiol . 31:504-508. 



Billen, G. 1978. "A Budget of Nitrogen Recycling in North Sea Sediments Off 

 the Belgian Coast." Estuarine Coastal Mar. Sci . 7:127-146. 



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