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Turnover, which occurs twice yearly in most lakes, is a big ecological event and is incredibly important for water quality and ecosystem health. Up at the lake office, it also signals the wrap up of lake sampling season (though water quality testing continues at the intake, on tributaries, and at many other points throughout the treatment process). This year's fall turnover occurred just the other day -- Thursday, November 6th! What is lake turnover, and why does it matter? Read on to find out.
Like all seasonal shifts, fall turnover is directly related to the amount of sunlight our hemisphere receives. During the warm summer months, lakes in the temperate zone (which includes Maine) tend to stratify. The upper layer of water (the epilimnion) warms from the heat of the sun, while the lower depths (the hypolimnion) stay cool. Because warm water is less dense, it remains at the surface while the cool water stays below, and there is a distinct temperature difference between the two layers called the thermocline. You may have felt this stratification yourself -- dive into a deep lake on the hottest day of the summer, and you'll still feel a chill as you reach the cooler water below the surface.
As Earth orbits the Sun and our tilt causes less sunlight to reach the northern hemisphere, many changes occur in the lake. As the days shorten, the epilimnion receives less sunlight and begins to cool down. As it cools the density of the surface water increases, and eventually temperatures and densities reach equilibrium across the water column. This allows the water to freely mix throughout the lake -- a process helped along by the windy days we often get around this time of year.
Turnover is very important for the health of lake ecosystems. During the stratified months, dissolved oxygen, which is generated from wind mixing and photosynthetic activity, collects in the epilimnion. The oxygen in the hypolimnion, on the other hand, is steadily depleted, sometimes to the extent that this layer becomes anoxic and largely unlivable for some oxygen-loving species. Fall turnover redistributes dissolved oxygen across the entire water column, creating habitable conditions for species throughout the lake.
In the graph above, we've plotted the water temperature measured at three different depths throughout the 2024 sampling season. The temperature of the water at the surface and at a depth of 10 meters changes over the course of the season, while the water at 30 meters remains the same temperature. The surface temperature reaches its peak and begins to drop in July, and by October the water at 10 meters is beginning to cool as well. We can see from the graph that the temperature of the water at all three depths is equivalent on November 14th, a clear signal that fall turnover has occurred.
This year's turnover date is earlier than last, but not out of the ordinary -- there is variability in the timing as with many other lake events (like ice-in and -out). Scientists are studying long-term trends in turnover in relation to climate change, however, and while it is clear that warmer water temperatures will impact lake stratification, it's not exactly clear how. One projection is that summer stratification will persist for longer, which could lead to more oxygen depletion in the hypolimnion, but more long-term data collection is needed before we know for sure how our lakes will be impacted in future years.
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