- water
- water-quality
Dissolved oxygen is the concentration of oxygen dissolved in water, the supply that fish and aquatic insects breathe. It is a foundational water-quality criterion: a stream must hold dissolved oxygen above a numeric floor, and an NPDES permit's effluent limits are set partly to protect it.
Dissolved oxygen is where several loads converge. Oxygen-demanding organic matter and ammonia in a discharge consume it directly; nutrient loading consumes it indirectly, because the algal blooms that total phosphorus and total nitrogen feed die and decay, drawing the reach down into an oxygen sag. Low flow makes all of this worse — less water holds less oxygen and dilutes less load — which is why a discharge's oxygen impact is judged at the same design low flow as its dilution.