We are four days from the start of our drawdown and the weather of late has improved the lake condition. Cool, cloudy weather has reduced cyanobacteria growth and the rain has helped break up any that are still on the surface. The lake is 66 degrees as of today, down from a high of 78 in mid-August, but the drawdown will pull off most of the warm surface water which will accelerate cooling of the remaining water.
Looking at the chart below you can see how the oxygen level at the deeper layers is increasing (dark blue line). This can be attributed to the lake mixing from the surface to nearly 10 meters, compared to only mixing to six meters back in August. In a non-drawdown year the surface temperature (dark red line) will cool and the upper portion of the temperature line will shift left until it is straight from top to bottom. This indicates the lake is the same temperature throughout and oxygen can mix freely to the bottom.
Oxygen at the bottom of the lake is important because it allows phosphorus to become trapped in the sediment again and not available to feed cyanobacteria. As mentioned in the last update, there are occasions where a late season bloom is initiated when phosphorus at depth is brought to the surface during these cooling periods, but with our drawdown that will be reduced since high nutrient water will be flushed through the hydro plant.
Another way of looking at the lake temperature is how the surface and bottom temperatures change over time. The graph below shows the water temperature at one meter (red line) and 14 meters (blue line), and how that changes throughout the year. In early spring the top and bottom are the same temperature, but as the lake warms the two start to diverge. Once the days get shorter in August the lake starts to cool and eventually the temperatures become equal again, indicating the fall “turnover”.
Note that I included the temperature from last year in this graph (light blue and red lines). I wanted to show how the lake temperature differs depending on if we use aeration. Without aeration the lake bottom stays approximately the same temperature throughout the summer, which means the surface has to cool to around 53 degrees before turnover happens. Our hypolimnetic aeration system circulates water between about 8 meters to the bottom, which gradually warms water at the bottom during summer. In fall the water at the bottom warms to about 63 degrees so the surface water does not have to cool as much and turnover happens about a month earlier.
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. Clarity has decreased again, back down to just over five feet. As algal productivity slows this fall the clarity typically increases. However, with the drawdown this year there will be a lot of sediment exposed which will lead to more turbidity in the lake. Clarity is expected to be low as a result.
Phytoplankton biovolume in Lakewood Bay increased again and cyanobacteria seems to be the primary contributor. Last year the biovolume was almost twice this year (light grey line). September 25 will be the last sample event before drawdown so there will be no data until the lake refills in spring.
Cyanobacteria in Main Lake showed the expected decline due to the second alum treatment. Although this seemed like a bad summer for cyanobacteria it was quite a bit better than last year. The total biovolume this year is represented by an orange line but I added last year as a light grey line. As you can see the biomass in 2022 was much higher throughout the summer, but we still had persistent surface films this year. Unfortunately since cyanobacteria can regulate their buoyancy it does not take a very large population to create a surface film, and since most folks’ recreational contact is with the surface they have to be careful and monitor conditions prior to entering the lake.
West Bay continues to be dominated by diatoms, but green algae have increased in population. They are not shown on the graph below, but make up most of the gap between the diatoms and total biomass. Some cyanobacteria were present in the sample, but it does not seem to actively grow in West Bay but is blown in from the main lake. West Bay will be one of the first areas to dewater during our drawdown so it is likely to be dry by the end of the first week of October.