Dissolved Oxygen (DO) simply refers to the amount of oxygen dissolved into the water sample, reported as either a percentage of mg/L.
While water molecules have their own oxygen atom, dissolved oxygen refers to the free oxygen that is dissolved in the water.
Oxygen dissolves from the atmosphere into surface waters much the same as sugar would dissolve into a cup of tea, through a form of movement for example, wind on the water surface or the turbulence of a river/stream.
Oxygen can also dissolve into water from the photosynthetic action of aquatic plants or from groundwater, which is colder and typically holds more oxygen than warmer surface water.
Dissolved oxygen is crucial in maintaining aquatic life as all depend on it for respiration. Aquatic systems will be structured to function on the normal level of dissolved oxygen level and changes to this level can be catastrophic.
There are minimum levels of DO required not just to sustain life but also to sustain biological processes, such as breeding and habitat selection.
DO changes can happen both from a change in natural conditions and through human interference. Snow cover and ice can be particularly problematic in closed systems such as lakes as it reduces the DO level for an extended period and this can cause ecosystem destabilisation and death.
Open systems such as rivers tend to be a bit more robust as they have the ability to regenerate the level of DO however this does not stop them being affected by a change in DO level such as that caused by eutrophication; when a nutrient increases causes an increasing plant & algae growth to the point at which all DO is used and the area becomes either hypoxic or anoxic.
Typically, DO measurements in the field have used a permeable or semi-permeable membrane. The oxygen will diffuse across this membrane and into the probe itself. Once inside a reduction reaction will occur that produces an electrochemical signal, which can be detected by the sensor.
The Proteus uses an optical dissolved oxygen sensor which uses an oxygen sensitive luminophore. The sensor emits a blue light which will then excite the luminophore to a higher energy level. As the luminophore returns to its ground state, it emits a red light. When oxygen is present, some of the energy will be transferred directly to the oxygen molecule. By comparing the blue light with the red light, a measurement on the amount of oxygen present can be determined.
The main advantage of an optical approach is they are insensitive to CO² fouling and removes the need of a chemical reagent, which always carries the potential of contamination. Optical DO sensing streamlines the process, providing high accuracy with minimal fuss.
