Donnie - Note if you had a rain event near the time of the dark color moving through the pond. There could have been an infusion of high DO demand water from the cattle area. Ref: Ted's note: "I also see in my notes that your water source is runoff from a neighbor that has 25 head of cattle and also the underground drainage goes into your pond."
Things to remember.
1. Water that is warmer holds significantly less DO (100% saturation) than colder water. Example DO is 100% saturated at 11.0ppm and 50F. At 90F 100% saturation of DO is 7.4. Hutchinson (Treatsie on Limnology Vol 1) mentions that the dissolved oxygen loss rate during an algae crash can CONSUME DO at the rate of 1.27 ppm/hour (16.5 ppm over 13 hrs). If Donnie's pond was 100% saturated at 90F and DO was at 7.5, it would not take long of for the DO to be exhaused once the daylight diminished. If the previous day was cloudy Donnie's pond may not have had 100% saturation of DO the day before the fish kill.
2. If Donnie uses blue dye then the pond likely was not saturated with DO. Blue dye supresses photosynthesis in the deeper waters and production of DO is always less than "normal" throughout the illuminated zone pond.
3. There are charts or data that indicate how much DO a Vertex (small pore bubbler) diffuser will add to a specific volume of water per hour based on air flow to that difuser. We also should take into consideration that temperature of any test data may be differernt than what occurred in Donnie's pond during the fish kill. Oxygen's ability to dissolve into water changes somewhat with temperature.
Ted Lea may be able to supply some data for us regarding the DO infusion rate into water from a specific size of membrane diffuser receiving a specific amount of cfm of air. I can look some average values for this up if necessary. Ted or Carry Martin may be able to find it faster.
It is important for readers to REMEMBER that the main purpose of bottom diffusers is to mix large amounts of water and not cause oxygen to dissolve into water to a large degree or in large amounts from the bubbling action. The intense bubbling at the surface can be deceiving regarding addition of DO. Oxygen dissolves poorly and slowly into water compared to the gasses of carbon dioxide or hydrogen sulfide. Oxygen is not very soluable in water, AND air only contains abt 20% oxygen, thus it takes a lot of repetitious bubbling or agitation to infuse lots of oxygen into water. Bottom aerators are poor emergency aerators. Surface splashers are good emergency aerators for the area surounding or near the mechanical mixer. This is the reason for BarO's above comment of "I have set up an emergency horizontal aeration system to supplement the other in case of another DO crash in the future.". Bruce Condello, Cecil Baird and others also have emergency aeration available in addition to bottom aerators.
