In situ O2 and N2 measurements detect deep-water renewal dynamics in seasonally-anoxic Saanich Inlet

Abstract

We interpret 2.5 years of dissolved oxygen and N2 excess concentrations from in situ observations at mid-depth in Saanich Inlet, a seasonally anoxic fjord. N2 excess was derived by combining measurements of total dissolved gas pressure from a gas tension device (GTD) with dissolved oxygen measurements and comparing to expected equilibrium concentrations. We present an important and previously ignored correction for the effects of hydrostatic pressure on GTD deployments at depth. Calibration of the dissolved oxygen measurements was the largest uncertainty for our data. During late summer/early fall deep-water renewal events, we observed rapidly declining oxygen and increasing N2 excess for several days, which we interpret as deep anoxic waters being pushed to mid-depth, where we observed them, by the entering dense waters that flow to the bottom. High rates of denitrification in the bottom waters over most of the year provided the N2 excess. We describe a recovery period lasting a few days following each active renewal event during which oxygen increased and N2 fell again. The recovery period appears to be caused by lateral mixing with new waters entering at shallower depths on the western side of the inlet.