Dissolved oxygen (DO) is a relative measure of the amount of oxygen (O2) dissolved in water.
Oxygen gets into the water by diffusion from the atmosphere, aeration of the water as it tumbles over rocks and waterfalls, and as a product of photosynthesis. The oxygen content of water will decrease when there is an increase in nutrients and organic materials from industrial wastewater, sewage discharges, and runoff from the land. (Intensive land uses such as farming produce more nutrients in runoff than native forest.) Excessive plant and algae growth and decay in response to increasing nutrients in waterways can significantly affect the amount of dissolved oxygen available.
A wastewater indicator such as biochemical oxygen demand (BOD) is a laboratory test that measures the relative oxygen-depletion effect of a waste contaminant when the contaminant reacts (through biochemical reactions) with nutrients and bacteria. The negative effect wastewater has on mahinga kai and aquatic plant life, by reducing the amount of available oxygen, is indicated by an elevated BOD reading.
Potential impacts of low dissolved oxygen (DO) on water quality and mahinga kai
- Increased stress on aquatic life - mahinga kai and invertebrates require adequate oxygen levels, an essential for all aquatic life. Fish ‘breathe’ oxygen through their gills, and are able to absorb oxygen directly from the water into their bloodstream. A concentration of 5 mg/L DO is recommended for optimum fish health. Sensitivity to low levels of dissolved oxygen is species specific; however, most species of fish are distressed when DO falls between 2 and 4 mg/L. Death usually occurs at concentrations less than 2 mg/L. Larger fish are affected by low DO before smaller fish are. The number of fish that die during an oxygen depletion event is determined by how low the DO gets and how long it stays down.
- Reduced available habitat limiting where species can live and grow, i.e., fish avoid areas with low oxygen.