Dissolved Oxygen

Scientific context

Spotted wolffish

Spotted wolffish are now rare in the Estuary and northern Gulf of St. Lawrence and are listed as a threatened species in Canada o ensure the species' recovery, the Department of Fisheries and Oceans wishes to better understand its key habitats. A team of scientists from the Maurice Lamontagne Institute (MLI) has been studying the relationships between wolffish, other demersal species and various aspects of their habitat including dissolved oxygen. In the deep troughs of the Estuary and Gulf of St. Lawrence, water is hypoxic (i.e. low oxygen levels). This can have a significant impact on the distribution and life cycle of wolffish.

Cottidae
Cottidae
Liparidae
Liparidae
Zoarcidae
Zoarcidae

Dissolved oxygen was measured with a newly designed apparatus based on an Aanderaa optode model 3830. The correlation between spotted wolffish, demersal fish communities and oxygen, a key feature of environmental conditions in the Gulf of St. Lawrence, can now be determined using this new tool avalaible to ocean sciences. This new instrument has been used successfully in several surveys, including the annual bottom trawl survey in the Estuary and northern Gulf of St. Lawrence at depths down to 500 m. It can also be combined with a towed camera system. The apparatus is fastened to the head rope of the trawl and records oxygen concentration, depth and temperature near the bottom during trawling.

Fish caught by the trawl, including many small species belonging to three poorly known families of the demersal community, Cottidae, Liparidae and Zoarcidae have been described.

Key habitats are thus determined by combining information from various sources, multibeam acoustic surveys, towed camera surveys, beam trawl catches, annual bottom trawl survey data, and oceanographic databases (salinity, temperature and dissolved oxygen).

 

 

Technical challenges and solutions

Three methods are available to measure dissolved oxygen levels in seawater.

The chemical method (Winkler 1888, modified in 1965) provides an absolute oxygen value. It is accurate, precise and remains the reference to which other methods are compared to. Although it has been partially automated, it has some inherent limitations. It requires water samples to be brought back to the surface for analysis. High-grade chemicals and great care are required during sampling and titration procedures, which are not always possible while at sea. The operator's skill level can also affect reproducibility. Furthermore, this method does not provide continuous results.

Optode Aanderaa
Optode Aanderaa

The most common method to measure oxygen continuously involves the use of galvanic or polarographic probes (Leland C. Clark, 1954). In these sensors, the presence of oxygen induces an electrical current in an electrolyte solution between an anode and a cathode isolated from the environment by a gas-permeable membrane. These probes have noticeable advantages such as providing continuous oxygen measurements and are easy to use. It is a well-established technology that has been enhanced over the years to reduce many of its shortcomings. For example, the use of pulsed current, smaller probes and forced water circulation have all been improvements over the basic design. However, certain problems have not been eliminated. These probes consume oxygen during the measuring process, the membrane and electrolyte degrade rapidly and many external factors such as salinity, temperature and pressure affect readings.

The most recent method available to measure dissolved oxygen involves the use of fluorosensors or optodes (Lubber & Opitz, 1975; Aanderaa, 2002). This technology is based on the quenching of a fluorescent complex in the presence of oxygen. Oxygen is not consumed in the process, so stirring is not required as is the case with electro-chemical probes. The effect of pressure is predictable and there is no membrane or solution that can degrade or become fouled over time. Calibration is required but not more than once a year. These sensors are extremely stable.

To facilitate oxygen data acquisition during trawling operations, a custom autonomous system had to be developed. It had to be maintenance-free and be as rugged and reliable as the common temperature loggers that are used on fishing gear. The system also had to include a pressure sensor to determine the exact moment when the trawl touches the bottom. Of the three existing methods, only the fluorosensor could be adapted for use on a trawl and withstand the difficult conditions it is subjected to.

Titration bottles (Winkler chemical method)
Titration bottles (Winkler chemical method)
Typical galvanic probe
Typical galvanic probe

A new instrument design

The prototype was named OPT (Oxygen, Pressure, Temperature) and includes an oxygen optode (Aanderaa 3830) and pressure sensor coupled to a data-logger. The data is recorded onto non-volative Compact-Flash memory. The system is housed inside a watertight acetal (Delrin) container. The system is fully autonomous, does not require to be connected to other instruments or to the surface and can record for more than 96 hours. The oxygen, temperature, and pressure probes are rugged and do not require frequent calibration. The unit has no moving parts and can withstand a static pressure in excess of 600 meters and the rugged conditions encountered by a benthic trawl.

Inside the housing

Inside the housing :

  • A protective cage with six openings providing water circulation.
  • Optode connection.
  • A micro-controller based programmable data logger that records onto removable Compact-Flash media.
  • Stainless steel pressure sensor.
  • A NiMH 2000 or 2200 mAh rechargeable battery pack that provides 24 to 94 hours of run time depending on sampling rate.
Housing

Housing. The system, ready for immersion, is a 31 cm long, 16cm diameter cylinder with attachment rings.

A beam trawl aboard the CCCS Calanus II A Campelen-type bottom trawl aboard the CCCS Teleost

Two typical uses for the OPT, a beam trawl aboard the CCCS Calanus II (left) and on a Campelen-type bottom trawl aboard the CCCS Teleost (right).

Validating the new instrument

Tests were carried out in water tanks to evaluate the optode's behaviour and to determine the time required to reach equilibrium in a steep oxygen gradient at constant temperature. Extreme oxygen values were chosen to cover all possible conditions found in the Gulf and Estuary. The sensor was subjected to a rapid transition from oxygen-saturated water to water at 5% oxygen saturation. The reverse operation was also carried out.

For both transitions, three equilibrium indices were calculated. The time required to reach 63% of the equilibrium value (e-folding time scale) and the times required to achieve 95% and 99% of this value. It was determined that the 63% value is approximately 50 seconds while 95% of equilibrium is reached after about 150 seconds.

Adaptation time for a low to high concentration transition

Adaptation time for a low to high concentration transition

While this adaptation time limits the optode's usefulness as a tool to measure oxygen in strong gradients, for example in a rapid vertical profile, a 5 minute maximum adaptation time is fully compatible with oxygen measurements on a bottom trawl.

Several tests were done both in a laboratory setting and at sea to compare results obtained with the optode with those from the chemical methods. These tests were carried out at various salinities, temperatures and depths. These have confirmed that once it is properly calibrated, the OPT provides reliable data that is both highly reproducible and accurate.

Methods comparison : optode vs. chemical method (Winkler)

Methods comparison : optode vs. chemical method (Winkler)

The sea trials of the prototype have been very positive. Considering the quality of the data and the sturdiness of the instrument, we can expect that the OPT will see a widespread use.