Table 2. --Rate of denitri fication as measured by N 2 production in lagoon 



sediments incubated with 0.3 atm acetylene and 20 yM NO3. Rates are expressed 

 per cm 3 of sediment per hour. 



Depth Interval 

 (cm) 



N2O Production 

 (nmol N 2 cm -3 h" 1 ) 



- 5 



10 - 15 



20 - 25 



1.9 x 10- 



-0.3 x 10- 



1.2 x 10- 



-0.1 x 10- 



2.8 x 10- 



■0.02 x 10- 



was performed in the presence of acetylene, no new nitrification was occurring, 

 inasmuch as acetylene also blocks nitrification_(Bremner and Blackmer, 1979). 

 During that same period of rapid decrease in NO3 concentration (0-6 hr), there 

 was a rapid increase in N 2 concentration in samples incubated with acetylene 

 (fig. 3B). The highest rates of N 2 production during the first 6 hr was seen 

 in the 20-25 cm depth interval (table 2). The concentration of N 2 tended to 

 decrease during the next 16 hr interval. The rate of NO3 reduction increased 

 with increasing concentration of substrate (fig. 4). 



Concentrations of N 2 in sediments incubated with NO"^ but without acetylene 

 were negligible and tended to decrease slightly with time (not shown). Changes 

 were small compared to changes measured in the presence of acetylene. 



DISCUSSION 



of denitrification reported here indicate that 



sediments are potentially active sites for 

 rations inside dead coral heads have been 



and Risk and Muller (1983) to be higher than 

 doubtedly indicates that nitrification is 

 ow oxygen concentrations occur as organic 

 s providing the proper conditions for 



1983). Although nitrification and denitrification 

 of oxygen to proceed, both can occur simultaneously 

 butional variations in oxygen concentration 

 nd Muller, 1983). 



The prelimin 

 coral reefs and a 

 denitrification. 

 reported by Webb 

 the surrounding s 

 occurring in dead 

 matter decays in 

 denitrification ( 

 require different 

 in coral heads du 

 that certainly mu 



ary measurements 

 ssociated lagoon 



Nitrate concent 

 and Wiebe (1975) 

 eawater; this un 



coral heads. L 

 coral heads, thu 

 Risk and Mul ler, 



concentrations 

 e to microdistri 

 st occur (Risk a 



Sediments in the lagoon behind the reef also appear to be active areas for 

 denitrification as well. This was demonstrated by both the N 2 production 



204 



