Berry turning over?

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It is actually quite common to get a surge in algae growth when a lake turns over in the fall. Bacteria break down organic matter on the bottom of a lake all year long. Phosphate and ammonium (plant nutrients) are released during the process. During the warm part of the year when a lake is well stratified the nutrients build up below the thermocline. There is little or no mixing between the upper and lower water layers and there is little or no algae in the lower layer to consume the nutrients due to the lack of light and other limiting factors. The fall turnover mixes the banked nutrients into the upper water column and algae growth can really take off as a result.

Also, fall turnover actually begins when the surface water temperture reaches about 50°F. That's because the density difference between 39°F (4°C) and 50°F (10°C) water is not that great which allows the stratification to begin to break down. See the difference between shaded areas for water density at 5-10°C and 20-25°C in the figure below. A little wind at this time can really speed up the mixing. A lake is completely mixed when it's the same temperature top to bottom.

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