Dissolved oxygen (DO) is the amount of oxygen in water. The main sources of DO in water are the atmosphere and aquatic vegetation. The oxygen is then lost through oxidation of sediment, by respiration of aquatic organisms, and oxidation of organic matter. The input and consumption of DO in an aquatic system are influenced by temperature, water depth and water movement.
Figure 1: Dissolved Oxygen cycle in freshwater [Source: http://www.nanoos.org/education/learning_tools/hypoxia/oxygen_underwater.php]
If the amount of oxygen consumed exceeds the input amount, the oxygen becomes depleted in the system. If DO levels become too low then stress is put on aquatic systems, and most aquatic species cannot survive without oxygen. The water quality guidelines for the protection of aquatic life are found in Table 1, and are dependent on water temperature and life stage. Adult fish have the ability to survive at lower values of DO depending on the species and water temperature, but the limited oxygen will negate their abilities to reproduce along with the survival of juvenile fish. If an aquatic system begins to have lower DO levels than in the past, this could change the biodiversity of the ecosystem as this will attract species which have tolerances to lower DO, or anaerobic organisms (which don’t need oxygen for survival) while ones with less tolerance will leave.
Waste water from washing fruits and vegetables tends to have low DO levels. It is important to try and increase the DO in this water through different systems such as settling ponds with aerators and other technologies to reduce the environmental impacts. The Lake Simcoe Protection Plan indicated a goal of 7 mg/L DO in deep water in 2008; in 2012 the DO levels had improved over the values from 2008 but were not yet at the goal.
References
References
- Canadian Council of Ministers of the Environment. 1999. Canadian water quality guidelines for the protection of aquatic life: Dissolved oxygen (freshwater). In: Canadian environmental quality guidelines, 1999, Canadian Council of Ministers of the Environment, Winnipeg.
- Government of Ontario. 2013. Minister’s Annual Report on Lake Simcoe 2011-2012. Queen’s Printer of Ontario. Toronto, Ontario
- Oram, B. 2014. Dissolved Oxygen in Water. Water Research Watershed Center. http://www.water-research.net/index.php/dissovled-oxygen-in-water
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