Recently, the city of Punta Gorda announced that some canals were experiencing flips or turns and this was contributing to an unpleasant odor. Many readers reached out, asking what exacting these flips and turns are and why some of the canals are doing it.
The flips and turns are describing a condition known as turnover. Turnover refers to the mixing of surface and bottom waters. It occurs as a result of density differences between the bottom waters and the surface waters. There are several factors that may contribute to density differences in waterbodies.
When surface waters heat up, a thermocline — a physical barrier between the warm, less dense surface waters and colder, denser bottom waters — may develop. When thermoclines are present, no mixing between the surface and bottom waters takes place. This warm/cool layering effect is known as stratification. A difference as slight as one degree can result in stratification. As the difference in temperature increases, the stratification becomes stronger and more stable, making it more difficult for the water to mix.
Stratification may also occur as a result of density differences caused by dissolved salts. Salt water is heavier than fresh water. When rain is abundant, a fresh water layer may develop above the denser salt water, again creating a physical barrier to mixing.
Stratification plays a role in determining how much and where dissolved oxygen is available within the water column. In aquatic environments, virtually all organisms require dissolved oxygen for respiration. This gas is constantly entering water from two main sources: The atmosphere and photosynthesis.
The atmosphere continuously provides oxygen gas through a process known as diffusion, which occurs as tiny oxygen molecules are pushed into the water by pressure from the atmosphere above. Photosynthesis is thought to be the predominant source of oxygen in waterbodies where algae or aquatic plants are abundant. Algae and plants use sunlight and carbon dioxide for growth and release oxygen into the water as a byproduct.
When waters are stratified, no mixing between the surface and bottom waters take place. As a result, oxygen in the surface water layer remains there, and the bottom water layer becomes oxygen-depleted.
Turnovers occur when the thermocline barriers are broken. Turnovers most notably occur in northern lakes every spring and fall as the lake freezes and thaws, but they may also occur here in Florida due to heavy wind, cold rain and even boat wakes.
When the barrier is broken, oxygen-rich surface waters mix with oxygen-poor bottom waters, resulting in lower oxygen conditions throughout the entire water column. If the demand for oxygen is great enough, it will quickly be depleted and may result in a fish kill.
During a turnover, the clarity of water often declines because mixing brings up nutrient-rich water from the bottom, resulting in a cloudy appearance. Sometimes there is even a distinct sulfurous or decomposition smell as rotting plants, algae and other matter rise to the surface. This is what is happening in the canals that are experiencing turnover now.
In Florida, many of our ponds, lakes and canals turn over on a regular basis. Because they are shallow, even the slightest wind and wave action can mix the water column, from top to bottom throughout the year. If a Florida waterbody does happen to maintain stratification, a turnover will generally occur in the fall, but it can also occur during the summer given the correct environmental conditions.
Turnover in our saltwater canals occur most frequently at dead ends, where water movement is limited. The transition from stratification to turnover can occur within just a few hours, especially if accompanied by strong winds. Although it can be a bit smelly and suffocate fish and other aquatic animals in the immediate area, rest assured turnover is a natural and normal process.
Betty Staugler is the UF/IFAS Extension Charlotte County agent for the Florida Sea Grant Program. She is active in many areas relating to boating, fishing, and watershed/coastal living. Sea Grant supports research and education activities that help Florida’s shoreline communities, industries and citizens wisely use the state’s coastal and marine resources. Contact her at firstname.lastname@example.org or 941-764-4346.