What is dissolved oxygen and why is it important?
Dissolved oxygen (DO), measured in mg/L, is the level of non-compound oxygen dissolved in water, it is measured using a dissolved oxygen sensor or dissolved oxygen probe. It represents the amount of oxygen available to organisms in the water. As such, is a key variable in understanding the health of a body of water.
The concentration of dissolved oxygen in a body of water depends on the physical, chemical, and biochemical activities taking place in and around the water. Just as electrical conductivity relates to free ions, dissolved oxygen relates to free non-compound oxygen i.e the oxygen must be ‘free’ and not the oxygen (O) which makes up water (H2O).
Solubility of water and dissolved oxygen.
Solubility is indicative of a body of water’s capacity for dissolved oxygen. The oxygen solubility of the water is related to salinity, temperature, and pressure.
Water temperature – Warmer water has higher kinetic energy, which increases gas molecules’ movement. In turn, the water ‘loses’ more gas to the surrounding atmosphere. As such, colder water will generally have more dissolved oxygen than warmer water if all other parameters are similar. Dissolved Oxygen reaches 14.6 mg/L at 0 °C and approximately 9.1, 8.3, and 7.0 mg/L at 20, 25, and 35 °C, respectively, at 1 atm pressure . The temperature fluctuations throughout the day and year play a large role in the measurements of dissolved oxygen sensors.
Pressure – Dissolved oxygen will increase as atmospheric or hydrostatic pressure increases. As such water at lower altitudes will be able to dissolve a great quantity of oxygen than a similar body at higher altitudes. For example, dissolved oxygen doubles when pressure increases from 1 bar to 2 bar. The solubility of dissolved oxygen will be higher with greater hydrostatic pressures too.
Salinity – Salinity represents the amount of dissolved in water. Dissolved salt reduces the capacity of water to dissolve oxygen. As a result, water with high salinity will have lower levels of dissolved oxygen compared with fresh water.
Factors affecting dissolved oxygen levels.
While temperature, salinity, and pressure alter water’s capacity to dissolve oxygen they do not in themselves directly increase or decrease the oxygen. Two factors primarily affect dissolved oxygen levels in water organisms, and the atmosphere.
Atmosphere – Oxygen molecules will naturally move from areas of high concentration to areas of low concentration. As the air is 100% saturated with oxygen (roughly 20.95% of air is oxygen) molecules move and dissolve into the water it interacts with. Wind, waves, and other aeration activities constantly mix water with the atmosphere. As a result, a gain and a loss of molecules constantly occur.
Organisms – Organisms both increase and decrease dissolved oxygen levels. For example, during photosynthesis along with sunlight plants take in water, carbon dioxide, and minerals and emit oxygen and organic compounds. Therefore, the presence of healthy plants will usually increase dissolved oxygen levels. Conversely, some organisms, such as fish and algae need to use oxygen to breathe and so will reduce oxygen levels.
Importance of dissolved oxygen levels.
Dissolved oxygen levels balance between oxygen produced by plants and oxygen consumed by organisms. In a healthy eco-system the balance will fluctuate daily and seasonally. However, production or consumption should not tip too far one way. In such cases, the eco-system can become irreversibly unbalanced. The mechanisms behind the eco-system are extremely complex and deserve their own articles. We will only briefly touch the dissolved oxygen balance.
The causes of an unbalanced system are numerous, changes in temperature or salinity are key factors along with inputs such as groundwater or runoff.
As previously mentioned, an increase in temperature would decrease the solubility of oxygen, but the greater effect would be on the organisms in the water. Organisms such as algae can thrive in warmer waters. Should a body of water heat up slightly this may cause an algal bloom. During these events, the algae grow and consume nutrients and sometimes kill other plants and fish with neurotoxins. The dead plants and fish produce nutrients for oxygen-consuming bacteria. As a result, the bacteria grow, resulting in less dissolved oxygen. This feedback loop then kills more fish who need higher levels of dissolved oxygen to breathe. An introduction of toxic chemicals either direct or through runoff would also have a similar effect by killing fish and plant life.
Dissolved oxgyen measurment methods
As dissolved oxygen is a critical measurement to understanding water quality using the correct tools for measurements is important.
There are three methods of measuring dissolved oxygen. Firstly, is the colourmetric method, a simple method but only provides approximate dissolved oxygen data. A more accurate method is the Winkler titration method, which measures the quantity of a neutralising agent used, as an indication of dissolved oxygen levels.
However, the gold standard is using a dissolved oxygen sensor/probe/sonde. These are electronic sensors that record continuously and will provide accurate data at set intervals. The accuracy of data provided makes dissolved oxygen sensors popular with hydrologists, along with the fact chemicals and glass containers do not need to be taken into the field for samples and measurements.
Dissolved oxygen sensor
The optical dissolved oxygen (ODO) sensor manufactured by Greenspan uses a robust, solid-state, fluorescence-based transducer. Fluorescence-based sensors are inherently reliable and low maintenance, with no need for replaceable membranes or electrolytes. The emitter sends light, at ~475 nm, to the backside of the sensing element. The wetted side of the sensing element consists of a thin layer of a hydrophobic sol-gel material.
A ruthenium complex is trapped in the sol-gel matrix, effectively immobilised and protected from water. Light from the LED excites the ruthenium complex immobilised in the sensing element.
The excited ruthenium complex fluoresces, emitting energy at ~600 nm. If the excited ruthenium complex encounters an oxygen molecule, the excess energy is transferred to the oxygen molecule in a non-radiative transfer, decreasing or quenching the fluorescence signal (see Fluorescence Quenching below). Consequently, the degree of quenching correlates to the level of oxygen concentration in contact with the sensing element.
Oxygen is able to efficiently quench the fluorescence and phosphorescence of certain luminophores. This effect (first described by Kautsky in 1939) is called “dynamic fluorescence quenching.” The collision of an oxygen molecule with a fluorophore in its excited state leads to a non-radiative transfer of energy. The degree of fluorescence quenching relates to the frequency of collisions, and therefore to the concentration of the oxygen-containing media.
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