Correct Option: A

Oxygen is one of several dissolved gases important to aquatic systems. Dissolved oxygen is necessary to maintain aerobic conditions in surface waters and is considered a primary indicator when assessing the suitability of surface waters to support aquatic life. For maintenance of aquatic health, dissolved oxygen concentrations should approach saturation – that concentration which is in equilibrium with the partial pressure of atmospheric oxygen. Dissolved oxygen can be measured in two ways: either in milligrams per liter (mg/L) or percent saturation (% sat). Water with an oxygen concentration of less than 3 mg/l will generally not support fish. When concentrations fall to about 3-4 mg/L, fish start gasping for air at the surface or huddle around the water fall (higher concentration points). Bio-converter bacteria may start to die off dumping toxins into the water compounding the lack of oxygen to the fish. Levels between 3 and 5 mg/l can normally be tolerated for short periods. Young Koi are less tolerant of low oxygen than the older, larger ones. Above 5 mg/l, almost all aquatic organisms can survive indefinitely, provided other environmental parameters are within allowable limits. Colder water fish (such as trout) need levels above 6 mg/L, and may require levels above 7 mg/L for spawning. Warm water fish can usually tolerate levels as low as 4 mg/L. If oxygen levels dip down to 1-2 mg/L, even only for a few hours, it can result in large fish kills.