A widely-cited minimum desirable DO for the protection of aquatic life in a receiving water is approximately 4–5 mg/L (CCME Canadian Water Quality Guidelines specify a range depending on water type and life stage — typically not less than 6.5–9.5 mg/L for cold-water biota during early life stages, and not less than about 5.5–6.5 mg/L otherwise; a commonly used generic minimum floor is 4–5 mg/L to avoid measurable stress).
Below the minimum, aquatic organisms are stressed — sport and commercial fish species show reduced growth, impaired reproduction and avoidance behaviour once DO drops below roughly 4–5 mg/L.
Sensitive life stages are the limiting case — fish eggs and juveniles need more DO than adults, which is why guideline minimums are set higher during spawning/incubation periods.
Below about 2 mg/L, most game fish die and the water body becomes limited to pollution-tolerant organisms.
DO is also an indicator of overall water-body health — nitrifying bacteria, natural self-purification and re-aeration capacity all depend on there being an adequate DO reserve.
