Ocean Networks Canada's tenth annual expedition season began on 28 April 2016 with an 8-day cruise to maintain the ocean observing instrumentation and platforms in Saanich Inlet and the Strait of Georgia. The Canadian Coast Guard Ship John P. Tully and the ITB Subsea Remotely Operated Vehicle (ROV) Oceanic Explorer supported the recovery, repair and deployment of seafloor installations and experiments, complemented by scientific sampling.
Measuring Turbidity Currents in the Fraser Delta.
One highlight of this year’s inshore cruise was the repair and upgrade to the Fraser Delta Dynamics Laboratory site, originally deployed in 2008. The Fraser River delta is known for its dynamic nature: laden with thick sediment deposited on its journey from source to sea, subsea landslides occur regularly, particularly during the spring freshet season, when glacier snowmelt dramatically quickens the river flow. Our annual inshore expedition is timed to coincide with the spring freshet swells and Natural Resources Canada (NRC) leads several research programs studying these extreme events, known as turbidity currents.
On 5 June 2012, an avalanche of fluidized sediment displaced the delta dynamics instrument platform and severed it from its cable. Fortunately, data recorded as it tumbled down the slope enabled a reconstruction of the flow. Dr. Gwyn Lintern from Natural Resources Canada (NRCan) recently published a paper about this event.
“Turbidity currents are very important for moving sediment from deltas to offshore. Triggered by a number of mechanisms, these underwater avalanches cause millions of dollars in damage to subsea cables and pipelines every year. Historical turbidity current deposits can tell scientists about the frequency of past earthquakes and coastal underwater ground failures. Being both underwater and sporadic, they are notoriously difficult to monitor, and despite their importance only a handful of observations are available Worldwide.
Using Ocean Networks Canada, we were able to design experiments to capture turbidity current flows. We not only have the ability to measure them, but have gathered data to be able to determine what triggers the flows, and we have in fact successfully predicted when they will occur.
At the same time, the instruments we have on the delta front tell us how much sediment is deposited from the Fraser River, and which direction that sediment is being transported. The latest deployment is designed to also provide information about natural submarine discharge of groundwater from beneath the lower mainland.” Dr. Gwyn Lintern, Natural Resources Canada.
A number of instruments on the delta dynamics platform were overhauled and deployed, including the seismic liquefaction in-situ penetrometer. This instrument includes piezometers to measure pressure in the water column, at the seafloor and at two points below the seafloor. These measurements are captured in real-time and can indicate an imminent underwater landslide.
Also deployed this year was an autonomous Aquascat platform, which uses sound to determine the size and stability of the settling sediment. Monitoring the stability of this turbid slope, and understanding how and when it can fail supports marine infrastructure and safety.
The last of the pigs: the end of a forensic chapter in Saanich Inlet.
Our 10-year relationship with the forensic pigs came to a close during this expedition, as we picked up our final deployment. After a year decomposing at 300 m in the Salish Sea, all that is left are bones and the creatures that colonized the carcass.
Since 2006, Simon Fraser University forensic criminologist Dr. Gail Anderson has been using pigs to mimic the decomposition of human bodies in local seas. A total of 18 pigs were deployed in the Strait of Georgia over 10 years, helping to solve crimes and shed light on the mystery of the severed feet in British Columbia. There is a plan to deploy more pigs off the west coast of Vancouver Island, where Dr. Anderson will be able to monitor the carcasses at up to 2700 metre depths. Read more about Dr. Anderson’s experiments here.
Unusually anoxic waters measured by annual benthic hypoxia transect.
For the 11th time in 11 years, University of Victoria scientists Jackson Chu and Verena Tunnicliffe repeated their benthic hypoxia transect in Saanich Inlet. Using remotely operated vehicles (ROVs) equipped with oxygen sensors, they mapped the natural oxygen levels in the habitat of dozens of species of benthic fish and invertebrates. Their ecological approach aims to answer three questions: (1) what oxygen levels do benthic fish and invertebrates naturally occur? (2) Under what habitat conditions is oxygen ‘too low’ (i.e. hypoxic) for these animals? And (3) how do the animals respond when their habitat becomes hypoxic?
Conditions at the study site were unusual for early May: anoxic waters (containing zero oxygen) had already expanded to the middle of the transect line, ~100 m bottom depth (see Benthic oxygen profiles graph). Although seasonal expansion of anoxic and hypoxic waters is normal in Saanich inlet, this usually occurs much later in the summer season. A direct consequence of hypoxia expansion is the shrinkage of viable habitat for the fish and invertebrates in this ecosystem. Their habitat becomes compressed into shallower depths as the animals redistribute to avoid the expanding low oxygen waters.
During their transect experiments, Chu and Tunnicliffe tracked the distributions of the hypoxia (low oxygen) tolerant slender sole (Lyopsetta exilis) to provide a quantifiable indicator of the extent of habitat compression. Slender sole is the first fish species encountered when surveying from the deep to shallow depths of the survey. For example, during the May 2013 hypoxia transect (a relatively ‘normal year’), slender sole first appeared at 123 m depth. This year, slender sole did not appear until 96 m, indicating a shoaling of their habitat towards shallower waters by ~20%. Because most of the seasonal de-oxygenation has yet to occur this year, the severity of the hypoxia expansion may increase. An additional survey would be required to confirm the full extent of habitat compression during this outlier year. Read more about Jackson Chu’s research here.
Other instruments maintained and deployed during this inshore cruise include the bottom boundary layer experiment, a new high frequency hydrophone at Delta node, and calibration of the new codar station at Georgina Point, Mayne Island.
The offshore cruise Expedition 2016: Wiring the Abyss to service, upgrade and maintain the NEPTUNE array, runs from 12-30 May (Leg 1) and then 12-24 June 2016 (Leg 2).