The mornings have been getting a bit chillier as we move deeper into fall, and one of the seasonal changes that can occur at this time of year is stratification, also known as turnover. A certain telltale sign of turnover can be a peculiar smell in waterbodies of all sizes including ponds, tanks and reservoirs. Gases present in the water – especially hydrogen sulfide – cause what is sometimes referred to as a rotten egg-like smell.
“Lakes start to stratify as water surface temperatures change,” said Jeremy Nickolai, environmental programs coordinator for the BRA. “Three layers are formed: the epilimnion, the thermocline and the hypolimnion. The epilimnion is the warm surface layer, the thermocline is the middle layer characterized by a rapid decrease in temperature, the hypolimnion is the cold dense bottom layer with low levels of dissolved oxygen (anoxia).
“In this anoxic environment biological processes, decomposition for example, are still occurring, one of the byproducts being hydrogen sulfide (rotten egg smelling stuff).”
While most of the reservoirs in the Brazos River basin experience turnover, Nickolai said one that does not is Lake Granger because it is shallow.
A report by the Texas Parks and Wildlife Department notes that the turnover process can happen quickly when weather changes are dramatic.
“The mixing process referred to as fall turnover, can be relatively sudden, with an entire reservoir turning over in less than a week during windy conditions.”
The AgriLife report indicates that turnover can be accelerated by cold rain and winds.
“Usually this happens during thunderstorms,” the report noted. “Many people confuse turnover with an algae die-off; externally they can look very similar but without a cold rain and wind there can be no (fall) turnover.”
The process of stratification repeats in reverse during the spring months, when air temperatures begin to warm and water temperatures follow suit.
One of the factors that affect the timing of lake turnover is the intensity of the wind, according to the Texas Parks and Wildlife Department. Another factor in the timing of turnover is the depth of the body of water experiencing stratification. Deeper lakes usually turn over later in the fall because deeper water is cooler and it takes longer for temperatures to mix.
As air and water temps begin to decrease in the fall, the temperature difference between the stratified layers decreases, increasing the ability for them to mix or the surface layer cools to a temp that it becomes denser than the bottom layer,” Nickolai said. “When this happens it is referred to as turnover. When that bottom layer is exposed to the surface, it releases that hydrogen sulfide to the atmosphere.”
Turnover is a natural and common occurrence, and it can certainly take a toll on aquatic life.
“The problem arises when this stratification is broken down quickly, causing the two layers to mix or ‘turnover,’” according to Texas AgriLife Extension. “The turnover mixes the oxygen-rich surface water with the deep oxygen-depleted water. The dissolved oxygen concentration in the mix can be too low to support life in the pond. Both fish and plankton can die from low dissolved oxygen following a turnover.”
Nickolai said isolated fish kills are sometimes reported during turnover because oxygen levels in the water can decrease.
Turnover can cause an immediate lull in fishing opportunities, but once the water is more uniformly cooled, the fish perk up again, the TPWD notes.