Algae - Red Algae
At the upper end of the lake, red algae began to grow as the water receded.
Plants and animals release organic compounds as they decompose, and these compounds lessen the surface tension of water. That allows the bubbles to form.
The Fall of 2020, when the lake level was extremely low, was the first time I had seen the Red Algae in what is the deepest area of the lake.