Boy has time flown by this summer. I can’t believe it is September already and the last update was way back in January when the lake was refilling from the drawdown. Sorry about that.
The lake has greened up over the last week. I started this update last week saying the lake looked pretty good, but conditions have changed so that by September 18 we were seeing cyanobacteria films on the water surface, as explained below.
What a difference a week makes. These two charts show how the lake starts to become isothermal this time of year. On September 9 the temperature line (red) looks pretty vertical, but at six meters it slightly cools. This is enough to keep oxygenate water (blue line) from circulating deeper in the lake. Contrast this to September 16 where the lake has become isothermal from the surface to 10 meters. This in turn allows oxygenated water to circulate down to 10 meters as well.
This has importance for overall lake health. As the lake becomes isothermal the phosphorus in the hypolimnion (deep water) starts to mix with the epilimnion (shallow water), which can lead to a late-season algae or cyanobacteria bloom. However, oxygen circulating towards the bottom means more of the sediment is oxygenated and can start to adsorb phosphorus and take it out of the water column. The question is which reaction happens more quickly; will the phosphorus concentration increase enough to produce a bloom, or will phosphorus be quickly adsorbed to a point where the epilimnetic phosphorus concentration does not increase much?
It looks like we are experiencing elevated phosphorus and a late season bloom this week as a result of this increased mixing. Phosphorus results from September 9 are higher than the August 26 sample, and cyanobacteria biovolume has increased as well. This is particularly noticeable during the early morning on the west and east ends of the lake where cyanobacteria surfaces at night and wind concentrates it. This hopefully will not persist because even though the phosphorus concentration is higher than in late August, it only increased from 0.019mg/L to 0.023mg/L. Our next sample date is this coming Monday (Sept. 23) so we will know more once we get results from that trip.
Weather
As mentioned above, the cool weather the past week has mixed the lake to 10 meters, leaving only a small amount of the deeper sediment anoxic. The weather is going to warm this weekend into Tuesday, but the days are shorter and nighttime temperatures are getting down to mid to upper 50’s so the lake will continue to cool. The concern will be the sunlight continuing to feed the existing cyanobacteria bloom, so residents and easement members should monitor the lake condition before engaging in water contact sports. As the water cools the cyanobacteria will not be as active and the lake will shift to diatoms again, but that typically happens in mid to late October and corresponds with cloudy and rainy weather which slows down all phytoplankton activity.
Secchi
Water transparency at Main Lake has decreased as we experience the late season bloom. Water clarity in the lake center is better than in the margins due to the wind shifting the cyanobacteria to the ends, plus the deeper water allows the cyanobacteria to disperse deeper in the water column instead of being concentrated in shallow water at the east and west ends.
Temperature
The lake temperature continues to cool, with the surface at 72 degrees and the temperature near the bottom at 58 degrees. The lake will be isothermal when these two lines meet, which was mid-October last year.The lake
Lake temperature at 1 meter (red line) and 14 meters (blue line). The light red and blue lines are temperatures from 2023.
Phosphorus is what we watch most closely since that is what feeds our cyanobacteria blooms. Our alum program does a good job of reducing the concentration in the lake, but it takes vigilance to make sure all our systems are operating properly and removing as much phosphorus as possible.
Phosphorus
As of September 9 phosphorus has increased in Lakewood Bay, Main Lake, and West Bay. While we have had some rain that could be contributing to nutrients, in the main lake that is exacerbated by deeper water mixing to the surface.
Lakewood Bay
Lakewood Bay has a large population of cyanobacteria at the moment, which is showing up in the phosphorus results. These are benthic species that typically spend their time near the bottom, but if they move up in the water column they are collected in our samples, resulting in higher phosphorus.
Main Lake
Main Lake phosphorus has increased over the past two sample trips. This is a result of rainfall flushing the watershed and the lake starting to mix to deeper depths, which brings phosphorus to the surface.
Main Lake Phosphorus in ug/L. Grey lines are total phosphorus since 2020, and blue line is total phosphorus in 2024. The orange bar is when alum was applied.
West Bay
Our phosphorus results continue to bounce around a bit, but the concentration has been lower than last few years. We are going to focus on reducing the phosphorus in West Bay because it seems to be growing more cyanobacteria than in prior years.
West Bay Phosphorus in ug/L. Grey lines are total phosphorus since 2020 and blue line is total phosphorus so far in 2024.
Phytoplankton
Phytoplankton is a category that includes both algae and cyanobacteria. We are not really concerned about algae growth; except they can get too numerous in Blue Heron Canal and West Bay. However, if we treat to reduce the population it may be replaced by cyanobacteria so it is a careful dance. As fall approaches and we see cloudier weather the growth will diminish. However, we may see some areas where films collect as the cyanobacteria cells die off and float to the surface.
Main Lake
The overall phytoplankton population in the Main Lake is higher than last year but cyanobacteria is low. As of August 26 only 30% of the biovolume is cyanobacteria, with Aphanizomenon and Dolichospermum dominating. That shifted towards cyanobacteria in our September 9 sample which comprised over 90% Aphanizomenon and Dolichospermum. These are potentially toxin producing cyanobacteria, but the concentration is low enough that the risk is minimal. With low phosphorus in the lake the hot weather the next few days should not cause a large bloom. However, these have been accumulating at the surface in some locations so be cautious when you are using the lake.
Main Lake phytoplankton biovolume. Green bars are cyanobacteria and blue bars are diatoms.
Lakewood Bay
Our alum application on June 14 reduced phosphorus enough to remove most of the cyanobacteria. The population increased again in mid-July, decreased at the end of the month, and increased again in mid-August. While earlier data showed diatom were the dominant population the August 26 and September 9 samples were overwhelmingly cyanobacteria, specifically Limnothrix. This is similar to a spike we saw in West Bay earlier in the year. This particular cyanobacteria grows on the bottom so maybe they are surfacing temporarily or some disturbance brings them closer to the surface.
Phytoplankton population in Lakewood Bay, with diatoms represented by blue bars and cyanobcteria by green bars. Total biovolume in 2024 is the orange line, compared to 2023 total biovolume represented by a grey line.
West Bay
West Bay is a real diatom nursery with a biovolume an order of magnitude higher than the Main Lake. There was a spike in cyanobacteria (Limnothrix) in mid-July which caused concern, but that quickly receded by the end of the month. Our September 9 sample showed 57% cyanobacteria, with Aphanizomenon and Dolichospermum possibly blowing in from the main lake.
West Bay phytoplankton. Orange line is total biomass for 2024 and grey line is total biomass last year. Total biovolume of diatoms has really taken off this spring. The total population is typically high this time of year but seems to be much greater than last year. Note the scale is two times the scale used in the MLC and LBC charts.
Aquatic Plants
One treatment on June 6 targeted curlyleaf pondweed. The applications were in the Lost Dog creek delta and what we call Frantzen cove near 1160 North Shore road. We treated springbrook Creek delta on September 19 to control coontail (Ceratophyllum demersum). Coontail is also growing around Schaefer Island on the west end of the lake, however it is sporadic and has only topped out in a couple spots on the south shoreline. A rake drag did not pull up much biomass so it should not pose a problem.
