c 

 o 



c 



CD 



o 



c 

 o 

 O 



8 12 



Elapsed Time, hrs 



16 



Figure 3. --(A) NO3 depletion in lagoon sediments spiked to 20 yM NO3; (B) N2O 

 production by lagoon sediments incubated with a 20 yM NO3 solution and in the 

 presence of acetylene. Open circles = 0-5 cm depth; triangles = 10-15 cm 

 depth; and closed circles = 20-25 cm depth. 



The concentrations of NO2 + NO3 in the sediments of the Thalassia bed were 

 higher than the overlying seawater (0.5 yM NO2 + NO3), ranging from 3.5 to 9 yM 

 with a peak concentration at about 11 cm depth (fig. 2). 



When sediments were incubated in seawater enriched to a concentration of 20 

 yM NO3, NO3 concentrations decreased rapidly in sediments from all depths to 

 below concentrations found in the pore waters (fig. 3A). Since this experiment 



203 



