Aquatic Eddy Covariance Measurements from an Ocean Observatory: Oxygen and Heat Transport near the Seafloor of a Hypoxic Fjord

Eddy covariance (EC) is a powerful in situ technique for measuring areal-averaged benthic fluxes together with boundary layer dynamics in aquatic environments. This study describes an integrated system of underwater EC sensors that was developed for operations through a cabled ocean observatory. The system was first demonstrated on Ocean Network Canada’s Victoria Experimental Network Under the Sea (VENUS) observatory that has a primary seafloor node located near the 100-m isobath in Patricia Bay off the eastern shore of Saanich Inlet, British Columbia. Separate fast-responding oxygen and temperature sensors were used with an Acoustic Doppler Velocimeter, and measurements were recovered over 222 days until the oxygen sensor was broken. Current velocities and turbulence (characterized by turbulent kinetic energy and dissipation rate estimates) were observed to be generally low with the highest rates occurring within winter months. EC fluxes that represented turbulent eddy transport in balance with the seafloor source/sink were derived through conditional criteria that excluded measurements occurring when the benthic boundary layer was not fully turbulent or when transient shifts in bottom water properties were observed.The accepted fluxes were converted to 5-day weighted averages spanning from late May through December. These fluxes of oxygen (-0.87 ±0.60 mmol m^-2 d^-1) and heat (0.21 ±0.23 W m^-2) vary predictably with the seasonal hypoxia-bottom water renewal cycle in Saanich Inlet. Wider adoption of EC flux instrumentation on ocean observatories is deemed feasible and recommended to address a suite of biogeochemical research questions linked to the seasonality of benthic metabolism.