Denitrification and denitrifying efficiencies in sediments of Port Phillip Bay: direct determinations of biogenic N2 and N-metabolite fluxes with implications for water quality

Abstract

High-precision measurements of N₂ in benthic chamber waters indicated that denitrification occurs within the major sedimentary facies in Port Phillip Bay. The integrated fluxes of biogenic N₂ , ammonia, nitrate and nitrite showed that the stoichiometric relationship between organic C and N in the muddy sediments, occupying about 70% of the seafloor, was 5.7, this being similar to the Redfield ratio of 6.6. High denitrifying efficiencies (75–85%; denitrification rates ~1.3 mmol N₂ m^–2 day^–1) at organic carbon loadings of ~15–25 mmol m^–2 day^–1 indicate that most N processed through the sediments was returned to the overlying waters as biologically (generally) unavailable N₂. At sites of high organic carbon loadings to the sediments (>100 mmol m^–2 day^–1) denitrification rates and denitrifying efficiencies were near zero and most N is returned to the Bay waters as biologically available ammonium. In chambers ‘spiked’ with ¹⁵NO₃ , denitrifyers used nitrate produced in the sediments in situ, rather than the exogenous nitrate in overlying waters. The sedimentary microbial processes of ammonification, nitrification and denitrification are therefore tightly coupled.