Something interesting and important is about to happen to the water in Caz and many other area lakes – “fall turnover.” To understand the process, it might be best to start with the second ice-out of 2017 (remember ice melting in February and then refreezing in March?).
At ice-out, water temperatures were consistently about 36°F, just above freezing, from the surface to the bottom. By late April, the longer and warmer days had increased water temperatures to the upper-40s from top to bottom. In the next several weeks into May, the lake started to stratify, forming different layers based on differences in water temperature and density.
Because water becomes less dense as it warms above 39°F, the relatively warm water started to “float” on top of the colder, heavier water. This eventually results in a distinctly warm layer – the epilimnion – at the top of the water column separated from a cold, dense layer – the hypolimnion – at the bottom with these two layers separated from one another by the thermocline, a relatively thin layer of rapid temperature decrease.
By the end of June, the epilimnion was a swimmable 71°F from the surface all the way down to 20 feet beyond which the temperatures steadily dropped to the mid-50s (about the temperature of groundwater) at the bottom. The difference in density between the epilimnion and hypolimnion is generally strong enough to keep these layers from intermixing, so the hypolimnion starts to get “cut off” from the oxygen above and progressively becomes anaerobic (lacking dissolved oxygen), and the waste products, including sulfurous gasses, of anaerobic bacteria start to accumulate, and some of the phosphorus previously “locked” in the bottom muck may begin to be released under these anaerobic conditions – we’ll come back to that…
By the end of July, the epilimnion was just a little warmer at 73°F but had thickened to about 25 feet – more than half the depth of the lake – with the hypolimnion holding steady in the mid-50s (still similar to groundwater because it’s stuck at the bottom of the lake).
Because much of our summer was cooler than average, the temperature profile changed very little into late August. In the last month, water temperatures and the thickness of the epilimnion started to decrease overall (despite the late September “10 days of summer” that did warm the surface a bit).
In the coming weeks, shorter and cooler days will continue to decrease water temperatures, eventually eliminating the density difference between the epilimnion and hypolimnion. As this happens, wind will start to push the surface water, and the entire water column – top to bottom – will start to circulate resulting in fall turnover.
Remember those anaerobic conditions at the bottom, and the fact that the hypolimnion’s water was stuck at the bottom for several months starting in mid-summer? During turnover, those sulfurous gases and potentially some phosphorus start to get circulated to the surface which is what can give fall turnover an unpleasant odor and sometimes lead to late-season algal blooms.
After turnover, the entire water column continues cooling until the surface gets cold enough to form ice, and it’s time to dust off the ice-fishing gear.
