Algae, other microorganisms, and sediment are what determine how murky or cloudy lake water appears. It’s not the cold temperature that gives winter water greater clarity, but the effects that the cold water has on what lives in a lake’s ecosystem, such as algae. In the figure, the Secchi depth line follows the water and air temperature lines, with warmer months having less clear water.ĭoes this mean that colder water is more clear than warm water? Not necessarily!
In these monthly 2020 samples, Wawasee’s water clarity varied from 6.2 ft to 15.0 ft into the water. The summer has shallow water clarity (the “hill” in the middle of the black line). The depth of disappearance is recorded.įrom our sampling, we learned that the Wawasee Secchi depth is actually very deep during winter months (shown by the low black line on the figure above). Water clarity is measured by a Secchi disk, which is lowered into the water until the white and black are indistinguishable. Fresh oxygen gets mixed into lakes with wind, and that can only happen when there’s no ice! Spring turnover in March quickly restores DO in preparation for summertime.
That being said, if winter hangs around too long and the ice stays in a thick layer on the surface of a lake, dissolved oxygen will eventually begin to decrease at the bottom of the lake, like we see at the bottom of the February line on this figure. Plus, oxygen-using organisms tend to be less active in cooler temperatures! This allows DO to move throughout all parts of the lake. This depletion isn’t as extreme in the winter months because the colder temperatures reduce the intensity of the lake’s stratification. Remember that the effects of lake turnover restores the balance dissolved oxygen through the whole lake! In December and March, during and after turnover, all layers of lake Wawasee contained nearly equal amounts of DO.ĭuring summer stratification, however, DO cannot flow between layers.ĭO concentrations in the bottom layers plummet as normal consumption and decomposition happens, and by mid-summer, nearly all the DO in the bottom half of the lake is gone. Meanwhile, from late spring to mid-autumn, DO tends to drop. You’ll notice that the late fall to early spring show consistently higher amounts of DO across all lake layers. For reference, most fish need more than 2.0 mg/L of DO to survive! Take a look at the bar graph above. But DO at the lake bottom varies much more it ranged from 0.1 – 13.8 mg/L last year. That’s good news for fish and other aquatic critters who rely on oxygen!įreshwater organisms such as fish, bacteria or macroinvertebrates need dissolved oxygen (DO) in order to breathe.ĭO near the surface is abundant, ranging from 8.1 – 14.8 mg/L on Wawasee last year. This cycle acts as an ecological reset button, creating a healthier balance throughout the entire lake by refreshing dissolved oxygen levels in the bottom water. In 2020, turnover occurred in April and November. Lake turnover is the mixing of all three lake layers as surface temperatures become less extreme, and in Wawasee (as with all of our other lakes) it usually occurs twice a year: once in the spring and once in the autumn. The months where the temperature line starts to straighten out again show us lake turnover is taking place. This is when the lake is stratified: differences in temperature separate the lake into three non-mixing layers. You may have also noticed that the lines in the graph bend heavily in the summer months. Too much colder, and the water will start to expand and form crystals: ice! Much like syrup, it seems “thicker” in this very cold, dense state. You may be able to predict when the lake will freeze based on the water’s appearance as the water temperature gets closer to freezing, the surface can look heavier or syrupy. The lake froze over fully on Janu(also known as the ice-on date), and thawed on March 6 (ice-off). That’s colder than your fridge but not as cold as your freezer. This shows that the lake is the same temperature top to bottom, and that temperature can be pretty cold! Our January sampling, which occurred before the lake froze over, showed the lake was 36☏. If we analyze the temperature figure above, we see that the first and last few months of 2020 consist of straight lines.
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