Impact of deep-water renewal events on fixed nitrogen loss from seasonally-anoxic Saanich Inlet

Abstract

We interpreted profiles of N 2/Ar ratios, δ 15 N – N 2 , and O 2 concentration collected in Saanich Inlet, British Columbia, Canada over an annual cycle. Our measurements and data from a regional cabled observatory indicated that four deep- or bottom-water renewal events occurred over our study period. Each event was correlated with a period of weak tidal currents, such that very low tidal mixing allowed inflowing water to retain its high density as it moved across the sill and into the deeper basin. By quantifying the concentration of excess N 2 in each month and the vertical diffusion rate, we determined that the N 2 production rate ranged from 1.7 ± 0.3 mmol N 2 m − 2 d − 1 in summer to 8.1 ± 2.8 mmol N 2 m − 2 d − 1 in winter. This depth-integrated estimate accounts for all pathways resulting in fixed (bioavailable) nitrogen loss as N 2 gas, including denitrification and anammox, and incorporates any benthic production of N 2 that diffuses into the overlying water column. In spring and summer, the maximum N 2 excess corresponded to the maximum δ 15 N – N 2 , indicating that denitrification approached completion. In these months, the average isotopic composition of the fixed N consumed was 7.5 ± 1.2‰. Following bottom-water renewal in fall, which brought in nutrient-rich, low-N 2 water, the N 2 concentration increased and became progressively more enriched in 15N. The high rates of N 2 production in Saanich Inlet likely exist in other anoxic basins that undergo periodic deep-water renewal by nitrate-rich waters.