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Data Source
OTN glider group, TReX
Organizer
Adam Comeau, Ocean Tracking Network, Will Nesbitt, CERC.OCEAN, and Dalhousie University


Background:

Underwater gliders are a relatively new technology and have been used by teams around the world to measure important ocean variables for over 25 years (Testor et al., 2019), such as water temperature, ocean currents, underwater sound, nutrients and oxygen to name a few. They are long endurance, remotely piloted, semi-autonomous platforms that collect data while moving through the water column. They are routinely deployed on missions that last several months and span 1,000’s of kilometers following a set of programmed waypoints. See section entitled Extra information on gliders below for additional details.

The Ocean Tracking Network has deployed gliders in several regions of the Gulf of Saint Lawrence since 2017. The primary purpose for these glider missions has been to detect and monitor North Atlantic right whales, but they were also equipped with environmental monitoring sensors to measure oxygen concentration, temperature and salinity of the ocean.

Oxygen concentration is of critical importance to fish and animal stocks, and in recent years oxygen concentration in deep waters of the Gulf of Saint Lawrence has been in decline (Jutras et al., 2023, https://bg.copernicus.org/articles/20/839/2023/). This case study will present public datasets of oxygen concentration, water temperature, salinity and density collected by gliders and ships that can be used to examine the temporal and spatial trends and variability of oxygen in the Gulf of Saint Lawrence. 

Research Question


The main goal of the case study will be to describe the spatial and temporal variations of oxygen. Knowing the variability in the system will help inform how to create better surveys designed to capture short and long term trends in oxygen concentration in the Gulf. You are encouraged to address the following challenges to achieve this goal.  

  • It is always important to verify the accuracy of data collected by in situ sensors that can be subject to drift due to sensor use and biological fouling. A dataset of high accuracy oxygen measurements obtained from ship-based water samples is provided. Glider measurements of oxygen will need to be binned and co-located with the ship-based measurements in order for the comparison to be performed.
  • Categorize data by depth (upper, mid, deep) and into broad geographic regions (scale of ~50 km) then identify trends in oxygen concentration, water temperature, salinity and density over time.
  • Examine any possible relationships between the 4 variables from the previous bullet.
  • Gliders provide a unique opportunity to collect high resolution data compared to traditional oceanographic ship surveys which may survey a couple times a year. 
    • Examine geospatial variations of oxygen on the scale of 1 to 10’s of kilometers.
    • Examine temporal variations of oxygen on the scale of days to weeks. 
Variables


Glider based data

  • Environmental data collected by these gliders will be made available (by Jan 3, 2025) at the following website: http://cove-files.ceotr.ca:55441/share/LU5RKwiY
  • These data can have gaps in data due to planned sensor shut down for power savings, computer problems or other glider issues, or changes in the gliders flight behaviour. 
  • A variety of oxygen sensors have been used across these data sets. Although efforts have been made to ensure these sensors are properly calibrated and intercomparable, there could be sensor drift across long missions and across sensors.
  • The sensors used to measure oxygen concentration are Aanderaa Optodes, model 4831 and 5013. These sensors use optical methods to measure oxygen concentration and have a known lag time of approximately 60 seconds. This lag becomes apparent when comparing oxygen data from a glider diving and climbing through a strong oxygen gradient. There are published methods on how to correct for this lag, but no one approach has been used in all cases. For the purpose of this study, variability between data collected between sequential glider dive/climbs can be ignored. 

Ship based data

  • Ship based measurements of oxygen concentration were determined by Winkler titration on discrete water samples recovered directly from sampling bottles and will be made available at the following website: http://cove-files.ceotr.ca:55441/share/LU5RKwiY.

Extra information about gliders:
Gliders are moving platforms that travel approximately 0.5 to 1 km/hr. They achieve movement by means of a buoyancy engine which acts to make the glider negatively and positively buoyant. When the glider sinks and floats it does so on an angle moving the glider forward as it changes depth. The gliders used in the Gulf of Saint Lawrence dove to water depths up to 350 m depth or approximately 20 meters from the seafloor, whichever is shallower. More general information on gliders can be found at the following links: 

References:

Jutras, M., Mucci, A., Chaillou, G., Nesbitt, W. A., and Wallace, D. W. R.: Temporal and spatial evolution of bottom-water hypoxia in the St Lawrence estuarine system, Biogeosciences, 20, 839–849, https://doi.org/10.5194/bg-20-839-2023, 2023.

Testor P, de Young B, Rudnick DL, Glenn S, Hayes D, Lee CM, Pattiaratchi C, Hill K, Heslop E, Turpin V, Alenius P, Barrera C, Barth JA, Beaird N, Bécu G, Bosse A, Bourrin F, Brearley JA, Chao Y, Chen S, Chiggiato J, Coppola L, Crout R, Cummings J, Curry B, Curry R, Davis R, Desai K, DiMarco S, Edwards C, Fielding S, Fer I, Frajka-Williams E, Gildor H, Goni G, Gutierrez D, Haugan P, Hebert D, Heiderich J, Henson S, Heywood K, Hogan P, Houpert L, Huh S, E. Inall M, Ishii M, Ito S-i, Itoh S, Jan S, Kaiser J, Karstensen J, Kirkpatrick B, Klymak J, Kohut J, Krahmann G, Krug M, McClatchie S, Marin F, Mauri E, Mehra A, Meredith MP, Meunier T, Miles T, Morell JM, Mortier L, Nicholson S, O’Callaghan J, O’Conchubhair D, Oke P, Pallàs-Sanz E, Palmer M, Park J, Perivoliotis L, Poulain PM, Perry R, Queste B, Rainville L, Rehm E, Roughan M, Rome N, Ross T, Ruiz S, Saba G, Schaeffer A, Schönau M, Schroeder K, Shimizu Y, Sloyan BM, Smeed D, Snowden D, Song Y, Swart S, Tenreiro M, Thompson A, Tintore J, Todd RE, Toro C, Venables H, Wagawa T, Waterman S, Watlington RA and Wilson D, (2019) OceanGliders: A Component of the Integrated GOOS. Front. Mar. Sci. 6:422.
 
Acknowledgment

This case study was prepared by Adam Comeau, Will Nesbitt, Anja Samardzic, Dr. Joanna Mills Flemming, with help and guidance from the Case Study Committee of the Statistical Society of Canada. Any concerns and questions can be directed to the chair, Dr. Chel Hee Lee, via email, chelhee.lee@ucalgary.ca.