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A good rule of thumb is at oxygen concentrations below 5 milligrams per liter (5 mg/L) many species become stressed, and at concentrations below 3 mg/L most species can die from oxygen deprivation.
These concentrations are also known as the daily mean and minimum dissolved oxygen criteria for high aquatic life use in the Texas Surface Water Quality Standards. Although oxygen levels may stay above the minimum level during the day, it only takes a few minutes below the minimum at night to be fatal to fish.
Cloudy weather in the Fairfield area began on Sept. 10. By the Sept. 12, solar radiation was only a third of the normal for that time of year, and fish began dying before dawn on Sept. 13.
We live in a very complex and balanced world - it seems like there is a delicate balance between solor radiation and oxygen levels that impact certain species of fish before others. I'm now curious to know what the weather and solar activity was on that day in Arkansas.... I'll poke around some more
By the time KAST personnel deployed the datasondes on the afternoon of the Sept. 14 the sun had begun to break through the clouds, and oxygen levels had begun to temporarily climb back above 5 mg/L.
This reprieve was short-lived; cloud cover reduced solar radiation on Sept. 15-17 and oxygen concentrations remained below the minimum level. By Sept. 18 the weather pattern had improved and oxygen production in the north cove remained above the minimum level.
Oxygen production in the south cove lagged several days behind due to its higher phytoplankton level, but by Sept. 20 abundant sunshine was once again allowing phytoplankton to produce high levels of oxygen in both areas. Another short period of cloudy weather from Sept. 22-25 reduced oxygen concentration again but was of short enough duration that no substantial fish kill occurred.
By combining oxygen data from the datasondes with solar radiation data from the weather station, TPWD biologists now had the critical information needed to understand the complex dynamics of the repeated kills at Lake Fairfield.