Determination of denitrification in the Chesapeake Bay from measurements of N2 accumulation in bottom water

This study demonstrates the feasibility of using direct N₂ measurements in an estuary for determination of denitrification. High precision measurements of dinitrogen: argon ratios (N₂∶Ar) were made by membrane inlet mass spectrometry on water samples taken along the length of the Chesapeake Bay in July and October 2004. The N₂∶Ar ratio in low salinity surface water was elevated relative to air saturation by 0.3–0.5% with no systematic change along the length of the Bay. N₂∶Ar in high salinity bottom water exhibited a linear increase in the landward direction along a 144-km longitudinal section. In this section of the Bay covering 20% of the main stem, the bottom water salinity was statistically uniform and the increase in N₂∶Ar was in the direction of net residual current flow. The system was analyzed as a capped river with the assumption that N₂ entered the water from the underlying sediment where denitrification is known to take place. The rate of denitrification needed to support the measured increase in N₂ was calculated using an average residual current velocity and water column depth. The increase in N₂ with distance (0.046μmol N l⁻¹ km⁻¹) equated to an average denitrification flux of 73 μmol N m⁻² h⁻¹. N₂ fluxes determined on sediment cores taken from the source and terminus regions of the delineated water mass were 45±23 and 83±39 μmol N m⁻² hr⁻¹, respectively, which were not statistically different from the whole system estimate. The measured change in oxygen concentration within the bottom water was used to estimate nitrogen remineralization and the efficiency of denitrification. Denitrification efficiency (nitrogen denitrified/nitrogen remineralized) was estimated to be in the range of 22–28% for the bottom water sediment system and 30–37% considering the sediment zone alone.