Research that demonstrates low oxygen levels in the depths of Saanich Inlet might seem like concerning news; after all, marine life needs oxygen to survive. But on a planet facing climate change, this is also good news: without oxygen, the organisms living on the seafloor, or benthos, can't decompose, thus reducing the amount of carbon that cycles back to the atmosphere as carbon dioxide, a powerful greenhouse gas.
Figure 1. Saanich Inlet is a 274 metre deep glacially carved fjord separated from the Salish Sea by a shallow sill that restricts water inflow, making it naturally low in oxygen.
Coastal fjords are known hot spots for organic carbon burial, storing 11 percent of the carbon buried in the oceans—an estimated 18 million metric tons a year. New research using Ocean Networks Canada’s (ONC) Saanich Inlet cabled ocean observing infrastructure provides further evidence as to why this deep Vancouver Island fjord is a natural place that stores carbon, also known as a carbon sink.
Just as fish respire underwater through their gills, organisms and bacteria that live near and within ocean sediments also need oxygen to survive. Their health depends on oxygen mixing downwards through the bottom boundary layer of the ocean into the sediment.
In coastal fjords such as Saanich Inlet, a major limitation on the health of the benthic organisms and the turnover of organic matter raining to the seafloor is the rate at which dissolved oxygen reaches the sea bottom. To quantify the downward flux of dissolved oxygen continuously over several months, researchers from Oregon State University, ONC and Victoria-based Rockland Scientific used a key oceanographic technique known as eddy-covariance. The researchers deployed sensors for measuring small variations of oxygen and temperature in close proximity to an instrument measuring small changes in water velocity on ONC’s Saanich Inlet cabled observatory (Figure 2).
Figure 2: One of the best-studied marine basins in the world, Saanich Inlet is the site of ONC's first cabled observatory VENUS, deployed in 2006. The combination of easy access and unusual features has attracted researchers to this glacially carved fjord near Victoria, BC since the 1930s.
Designed to measure both the smallest scales and the fastest variations, the sophisticated sensors generate a great amount of high-resolution data to determine the rate of gas exchange in a natural ocean ecosystem (Figure 3). The eddy covariance technique is well-suited for a cabled observatory’s high-speed real-time continuous Internet connectivity, where data rates are virtually unlimited.
Figure 3: To gather high-resolution data for the eddy covariance technique, Victoria-based Rockland Scientific’s sensors require precise synchronization and careful placement in a fixed, closely positioned configuration.
“The eddy covariance technique is challenging because it requires high sensitivity oceanographic sensors to operate with a very fast response rate and a low noise level,” comments Fabian Wolk, the President and Co-Founder of Rockland Scientific. “The ONC platform in Saanich Inlet is great to work with because it is well-connected, well-operated and easily reachable—only 20 minutes by boat. It’s like having a great big laboratory for technical and scientific experimentation right on our doorstep.”
“Cable observatories like those maintained by ONC (Figure 4) are invaluable for moving beyond snapshots of ocean processes to full characterization. The observatory teams are tremendous collaborators, providing a backbone of technical and data resources that few single scientists are able to support,” says project lead Clare Reimers, from the College of Earth, Ocean and Atmospheric Sciences, Oregon State University.
Figure 4: The Saanich Inlet camera frame supporting the Eddy Covariance “birdcage” instrumentation during deployment.
The research results suggest that Saanich Inlet’s slow currents and weak turbulence contribute to a relatively small flux of oxygen. The small amount of oxygen that does reach the bottom sediments is due to the Inlet’s annual deep water renewal events and periodic turbulence. This research also highlights the uniqueness of near bottom conditions in coastal fjords, explaining why Saanich Inlet is a natural carbon sink.
“This research teaches us that these coastal basins may best be characterized as regional sediment traps that promote the burial of marine and terrestrial sources of organic matter by restricting oxygen exposure, limiting oxygen demand and preventing the export of organic matter to outer shelf and slope habitats.” Read the full research paper here.
The complex ocean carbon cycle involves two different forms of carbon: inorganic carbon is a result of the anthropogenic CO2 emissions being dissolved into salt water, while biologically ‘fixed’ carbon is stored in marine organisms and sediments. The amount of biological carbon in the ocean varies spatially and over time, variously representing either a net sink or a net source of carbon.
Research is continuing to help us fully understand the processes that drive these changes.
