ESTIMATING DENITRIFICATION RATES IN ESTUARINE SEDIMENTS - A COMPARISON OF STOICHIOMETRIC AND ACETYLENE BASED METHODS

We compared denitrification rates obtained using an adaptation of the acetylene block technique to rates estimated from benthic flux nutrien t stoichiometry in the subtidal sediments of Tomales Bay, California ( USA). By amending whole cores with acetylene and saturating nitrate co ncentrations, we obtained potential denitrification rates, which range d between 4 and 30 mmol N m(-2) d(-1). We determined the apparent Mich aelis constant (K-app) and the maximum potential rate (V-mp) of the de nitrifying community and used these constants in a rectangular hyperbo la to estimate in situ denitrification rates. Both the K-app and V-mp of the denitrifying community exhibited significant variation over bot h depth in the sediment column and time of sampling. Estimates of in s itu denitrification obtained using our 'kinetic-fix' adaptation of the acetylene block ranged between 1.8 (March) and 9 (Sept.) mmol N m(-2) d(-1). Denitrification rates obtained using benthic flux stoichiometr y ranged between 0.7 and 4.1 mmol N m(-2) d(-1). Average denitrificati on rates obtained using the 'kinetic-fix' acetylene block approach exc eeded those obtained from net benthic flux stoichiometry; however, the se differences were not significant. We conclude that our 'kinetic-fix ' adaptation of the acetylene block technique provides realistic estim ates of denitrification in sediments, even when pore water nitrate con centrations are low and nitrification and denitrification are closely coupled.