Spring turnover is a big deal here in the Lakes and Ponds Program. Our staff and volunteers conduct an early season sampling blitz (Spring P) to capture water conditions while lakes and ponds are fully mixed. Teams strive to reach dozens of lakes in the few weeks before summer stratification sets in. In contrast, fall turnover signals the end of the field season for most of us. We pack our gear away and turn our thoughts to understanding what all those samples mean.
Fall turnover is just as important to a lake as the spring event. Cool weather causes the water temperature at the surface to gradually match that of the deeper waters. Add a little wind and the water will eventually be well mixed from top to bottom. To measure a constant temperature in the middle of main Lake Champlain from the surface to the bottom at 300 ft is still a benchmark moment for me after all my seasons of fieldwork.
It’s a pivotal moment too for the living creatures in the lake as well. Nutrients locked into the deep waters by summer stratification are released to the entire lake during fall turnover. Just like spring turnover, there is an increase of phytoplankton in response, followed by an increase in the zooplankton which eat them. Lack of sunlight and short days means the growth is less intense than what follows spring turnover, but you may notice a period of increased water turbidity and possibly a cyanobacteria bloom following fall turnover. The increased growth provides a good feeding opportunity for the entire food chain before the winter fully sets in.
Though we don’t talk about it much, lakes do stratify in winter. Ice fisherman are well aware of this. (Perhaps swimmers in the Polar Bear events are too, but rest of us are usually keeping clear of the water once the temperatures drop.) The temperature profile, however, is opposite that of summer. Due to physical properties unique to water, the coldest water is the least dense and ‘floats’ to the top. That means ice forms at the surface and slightly warmer waters below provide habitat for living creatures during the winter. If ice didn’t float, lakes would freeze solid to the bottom and fish would have nowhere to go. It’s hard to imagine what life in a lake would be like if ice were heavier than water.
In celebration of fall turnover and winter ice that floats, here are links to some limnology videos on lake stratification. The last one is older and of mediocre quality but shows a fun experiment you can try at home.