Oxygen dynamics in a deep‐silled fjord: Tight coupling to the open shelf

Oxygen is declining globally in subsurface seawater. Fjords are particularly susceptible to hypoxia, because entrance sills restrict water circulation. We studied the oxygen dynamics of the Kitimat Fjord System on the west coast of Canada, using 3 yr of data from moored sensors and water column profiles. Subsurface oxygen within the fjord system is principally controlled by the connection to Hecate Strait on the outer shelf. Variations in oxygen concentration in Hecate Strait are reflected in the water that is drawn into the fjord system year‐round at mid‐depth (50–150 m). Deep water is renewed annually, beginning in May/June. Remineralization results in a seasonal decline in oxygen at depth. In the absence of deep‐water renewal, the bottom water in the system could become hypoxic in 1–2 yr. Mixing over a shallow inner sill (100 m) replenishes the oxygen in the deep water of Gardner Canal year‐round. The decadal trends in oxygen concentration show an increase from the 1950s to a maximum in the late 1970s, followed by a decline through 2016. The decline over the period 1977–2016 is 0.018 mL L−1 yr−1 (~ 0.83 μmol L−1 yr−1), which is consistent with the decline observed over the upper continental shelf over the same period.