The influence of bottom water intrusion events on the biogeochemistry of a coastal fjord

Lack of bottom water exchange in fjord-like estuaries can result in low oxygen conditions and creating sites of redox-sensitive biogeochemical processes, such as denitrification. In many of these systems, occasional intrusions of well-oxygenated bottom water may temporarily alter redox gradients and sediment-water biogeochemistry. Quantifying the magnitude and importance of these changes is a challenge due to the short timescales over which these events can occur. Here we present results from Bedford Basin, a 71 m deep coastal fjord in eastern Canada, where a 20-year, weekly timeseries of bottom water conditions indicates that autumn wind-driven intrusion events are a common, but infrequent, feature of its circulation. To examine the impact of these intrusions on biogeochemistry, we deployed a benthic instrument pod at 60 m depth to record high-resolution measurements of temperature, salinity, nitrate, oxygen, and fluorescence over a 4-month period during the fall of 2018. During this time we captured two intrusion events, one in mid-Oct and another in mid-Nov. Both intrusion events occurred on a timescale of hours and resulted in sharp changes in temperature, salinity, oxygen, and nitrate. We used these measurements to constrain a coupled sediment-water column reactive transport model to examine the immediate and annual impacts of these intrusion events on oxygen and nitrogen dynamics in the basin bottom waters and across the sediment-water interface.