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

High-precision measurements of N-2 in benthic chamber waters indicated that denitrification occurs within the major sedimentary facies in Port Phillip Bay. The integrated fluxes of biogenic N-2, 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 similar to 1.3 mmol N-2 m(-2) day(-1)) at organic carbon loadings of similar to 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 N-2. 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 (NO3)-N-15, 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.