The oxygen dissolved in water is critical to the survival of fish and shellfish living in it. All living creatures in the water need oxygen to survive, but as dissolved oxygen levels decrease, it becomes harder for animals to get the oxygen they need to survive. Low dissolved oxygen is often the result of eutrophication, which occurs when there are too many nutrients (such as nitrogen and phosphorus) in the water, causing dense algal blooms to grow. When the algae die and decompose, the decomposition process uses up dissolved oxygen in the water, reducing the oxygen available for fish and other organisms, which may then become stressed or even die.
How is it measured?
Dissolved oxygen is measured bi-monthly to monthly from June through September at 74 stations. The minimum dissolved oxygen concentration for each station was scored according to the threshold categories and represented. Scores for each sampling station were then averaged by reporting region.
Threshold levels
| Threshold (mg L-1) | Score |
|---|---|
| ≥7.0 | 100 |
| 6.0–<7.0 | 80 |
| 5.0–<6.0 | 60 |
| 3.0–<5.0 | 40 |
| 1.0–<3.0 | 20 |
| <1.0 | 0 |