What causes the mixing? Surprisingly the air temperature is the largest factor, not the intensity of individual storms. This has been a particularly cold winter, causing the lake to mix deeper and weeks earlier compared to most other years.
Is deep mixing good or bad? It is extremely good for the lake as it is renews the water at the lake bottom with “fresh” oxygen-rich water from the surface. Oxygen is constantly being lost from the lake bottom, so it requires replenishment. Mixing also helps cool the bottom of the lake, which slowly warms due to geothermal heating.
Are there any other impacts? The deepest waters of the lake are also the clearest waters, so when they are mixed with the overlying water there is a short period of high clarity. This year, two days after mixing, the Secchi depth was an astounding 115 feet, almost 33 feet deeper than it had been a week earlier.
The mixing also redistributes nutrients. Algae and organic material in the lake eventually ends up at the bottom, and through decomposition nutrients are released. These nutrients can build up over many years, so when deep mixing takes place, the bottom nutrients are carried all throughout the lake. In some years this is the largest source of nutrients to the lake surface and can lead to increased algal growth as well as a decline in lake clarity. In the coming months, we expect clarity to decrease as algae grow and fine particles begin entering the lake with the snowmelt.
A big shout out to the TERC field team who are out in all seasons collecting measurements and deploying instruments in all kinds of weather, and to our data impresario Dr. Shohei Watanabe who waits eagerly for each piece of new data. The temperature data collection was in part funded by the Tahoe Regional Planning Agency. The temperature buoys are operated in collaboration with the NASA Jet Propulsion Laboratory.