Turnover
You might know about lake turnover – but just in case, here’s a quick review. Differences in the temperature of lake water keep the water column from mixing for most of the year. In the summer, the warm, oxygenated water floats on top of cold, dense, and anoxic water. The lack of oxygen in the deep water allows nutrients and other compounds to dissolve in that water. At some point in the spring and fall, the temperature will be the same throughout the lake from top to bottom, which allows the lake water to mix, called turnover. Waters carrying oxygen, nutrients, and sediment mingle. This mixing turns the lake water murky, and algae can take advantage of all the nutrients that are carried up into the light.
Our three lakes mix at different times
While we still collect physical data, at this time of year we focus on water chemistry and continue to send samples to be analyzed at a certified lab. When we sampled on November 11, 2018, our three lakes were at different stages of turnover. Rippowam, our smallest and shallowest lake, mixes first because the small volume of water changes temperature most rapidly. Rippowam appeared to have turned over in October, the previous time we sampled. In November, Rippowam had obviously mixed and had the highest levels of dissolved oxygen, showing that algae have had a chance to take advantage of the nutrients that turnover brings. Oscaleta’s water had the same temperature from top to bottom, but turnover appears to have happened recently, as the dissolved oxygen levels are lower, indicating less algae activity. Waccabuc, our largest lake, has not yet mixed totally, although the process is starting. In past years Waccabuc has not mixed completely until December.
The lakes will stratify again in the winter as the coldest water comes to the surface, just like ice cubes float on warmer water, and then they will mix again in spring turnover.
The timing of turnover and the chemical composition of the water can affect the lake water quality in subsequent seasons, so we continue to sample until fall lake turnover is complete, and we also try to collect samples in the spring during turnover.
Turnover image thanks to Lake Candlewood.
More information on turnover is on our water and light page.