As expected, the lake is cooling rapidly with these overnight lows in the 50’s (refer to image below). As the weeks progress we will see the red line indicating water temperature near the surface approach the blue line which is water temperature near the bottom. When they meet the lake has “turned over” meaning water that has been trapped at the bottom of the lake all summer due to thermal stratification can now circulate throughout the entire water column.
This turnover period has benefits and risks. The benefit is the lake will have oxygen from top to bottom, contrary to now when the bottom of the lake lacks oxygen due to decomposition of organic matter. This lack of oxygen is bad for fish and other organisms, but it also means phosphorus is being released from the sediment. When oxygenated water circulates to the lake bottom the phosphorus will adsorb to sediment again and not feed algae.
Which brings us to the risky bit. If the water cools too rapidly the phosphorus at the bottom of the lake may not entirely bind to sediment and can move to the surface. This occasionally causes a late season bloom as algae takes advantage of sunny weather to feed on the increased supply of phosphorus. However, this time of year the days are shorter and water cooler so the blooms should be short lived. In addition, we will be entering the rainy season soon so that will further suppress the growth.
Water clarity increased dramatically from July 24 to August 2, and from August 28 to September 5, which reflects the improvement brought on by the alum treatments. However, some locations are experiencing persistent surface cyanobacteria despite the Main Lake treatment. It may take a bit for these areas to clear now that we have reduced phosphorus in the larger body of water.
Phytoplankton biovolume in Lakewood Bay increased a bit on August 28, but most of the increase is due to diatoms. Cyanobacteria is down to 31% of the biovolume, with the dominant species Aphanizomenon flos aquae. The dominant diatom species is Melosira ambigua, which is a colonial diatom that forms long branches.
Cyanobacteria in Main Lake has indeed increased since August 14. Although the total biovolume has not increased, the percentage of cyanobacteria has. The lake is up to 61% cyanobacteria, a considerable increase from the 11% on August 14. The dominant species is Dolichospermum circinalis comprising about 18,000 cells/mL.
On August 28 there was no cyanobacteria in the West Bay sample, with 91% of the biomass being diatoms, with Melosira ambigua the most dominant. You may be surprised that no cyanobacteria has shown up considering what the surface looks like at the moment. This confirms that the cyanobacteria in West Bay was not generated there but had blown in from the Main Lake. Since we sample about three feet deep we do not pick up anything floating on the surface. The cyanobacteria in West Bay should diminish after our second alum application since the Main Lake will not be seeding so much floating material.
