Fall Pond Turnover and Seasonal Water Quality Changes
What happens to a pond when water temperature cools down?
One of the first things noticed is disappearance of duckweed and algae. Then water begins to clear up.
Is this a coincidence? NO.
A series of changes are occurring to the pond.
Free floating weeds like watermeal and duckweed begin to disappear from the surface. They've lost buoyancy and dropped to the bottom of the pond.
Water clarity seems to improve. Less activity is occurring in the pond. Fish metabolisms slow, so less foraging, eating, producing waste.
Leaves fall into pond. Shoreline vegetation dies back and settles within the pond. Dead vegetation begins to slowly decompose into bottom muck.
Ponds that aren't well aerated become separated into multiple layers. As water temperature drops, water layers mix. This act of layers mixing is referred to as turnover. During pond turnover the oxygenated surface water mixes with colder low oxygen water from the bottom. As this mixing happens, oxygen levels can plummet. Nutrient levels can also spike. If you heard of a fall fish kill, this is likely the cause.
Poor Water Quality
At a glance the pond seems to be getting better in fall. A water quality test may say otherwise. Looks can be deceiving.
Because there is an influx of waste breaking down, that also means an increase in nutrients. Waste breakdown consumes oxygen.
Most all-season bacteria treatments are effective to 38-50 degrees depending on brand. The colder the temperature, the slower the reaction and effectiveness of all-season bacterias. In addition, lower oxygen levels decrease overall effectiveness.
Combating Pond Turnover and Poor Water Quality
Bottom aeration is the easiest option to reduce seasonal pond turnover. When pond water is circulated from bottom to surface, there isn't a separation of water layers throughout the seasons. Even distribution of dissolved oxygen reduces seasonal fish kills from the mixing of layers.