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Potential Sediment Denitrification Rates in Estuaries of Northern Gulf of Mexico

U.S. Environmental Protection Agency, Gulf Ecology Div., 1 Sabine Island Dr., Gulf Breeze, FL 32561;

Charles M. Bundrick

Dep. of Mathematics and Statistics, Univ. of West Florida, Pensacola, FL 32514.

^* Corresponding author (flemer-david{at}epamail.epa.gov).

ABSTRACT

The three-season average of sediment potential denitrification rates (PDRs) (i.e., NO^–₃ saturated; acetylene blockage method) for five study areas within urban bayous and bays in the Pensacola Bay area, Florida, ranged between 43 and 223 nmol of N g^–1 h^–1. Average PDRs extrapolated to a unit area basis approximated 500 to 1000 µmol of N m^–2 h^–1 that are relatively high values but comparable to those where conditions for denitrification are favorable. A regression model, based on a larger number of measured environmental factors for the spring than fall and winter indicated that NO^–₂ + NO^–₃ concentrations explained most of the total variability (R²: 27%; P < 0.003) in PDRs. The NO^–₂ + NO^–₃ concentrations were also predictive of PDRs (R² ranged from 0.56–0.98; all P-values <0.05) on four separate occasions for comparisons made within five study areas and three seasons. Sediment trace metal concentrations (e.g., Ni), based on published values, were high enough to cause reduction in PDRs through direct toxicity to denitrifiers at several stations. Sediment metals toxicities, based on published sediment quality guidelines, could occasionally cause a reduction in macrobenthic infaunal bioturbation and irrigation. Such a reduction could attenuate the flux of dissolved oxygen into sediments and cause a reduction in denitrification rates by limiting the coupled processes of nitrification and denitrification. Also, a reduction in the flux of NO^–₂ or NO^–₃, a substrate for denitrification, into sediments can directly limit denitrification rates.