As autumn sets in, and the temperature falls, I experience a thrill of anticipation at the coming transformation of the water in the ponds. The mirror surface will hold dazzling calligraphies drawn by the bare branches. Greens and browns will be replaced by a wide range of metallic blues, greys, silver and pinks.
The properties of the water itself begin to change in a process called “seasonal mixing”.
During the summer warm water rises, allowing pond weed to thrive. Colder water, being more dense, sinks to the bottom. At 4 degrees C it reaches its maximum density. But when the water temperature falls below 4 degrees C, water begins to expand again, becomes lighter and floats to the surface. At 0 degrees C, ice forms. “This seasonal process causes a lake’s waters to mix. Wind also plays a role. Winds get stronger during fall and help to mix the whole water column, from top to bottom. This seasonal mixing, called turnover, also occurs in the spring.”
https://www.michiganseagrant.org/lessons/lessons/by-broad-concept/earth-science/water-quality/weather-and-water-temperature/
