What does seasonal turnover mean for Lake Independence?
by Angela McLaughlin
Have you ever noticed how green the lake gets in autumn and wondered why?
Yeah, me too.
Lakes are complex ecosystems, and that’s saying it very simply.
As the Freshwater Society puts it, “A lake is a body of water, but it is also much more. A lake is an ecosystem, a biological community of interaction among animals, plants, and microorganisms, as well as the physical and chemical environment in which they live.”
A lot happens within the complex ecosystem of a lake, and Lake Independence is no different. In temperate climates like Minnesota, lakes form layers within themselves. These layers are based off of water temperature.
Water is densest at 39ºF or 4ºC, and the density changes at warmer or colder temperatures. During the summer months, temperature and density between the different layers become more distinct. Deep enough lakes will form three separate layers – the upper layer, or epilimnion; the middle layer, thermocline; and the bottom layer, hypolimnion.
Twice a year, the lake goes through a “turnover,” which is when the different layers within the lake mix. Within the next several weeks, Lake Independence will experience its fall turnover.
What does this mean?
The upper layer is what most of us are familiar with: that zone that is filled with warmer water, more light, fish, and plants. Below that, in the thermocline, the temperature drops drastically. It creates a kind of buffer between the top and bottom layers.
The very bottom layer, the hypolimnion, is an area where most of the plant matter accumulates or decays.
As fall approaches and brings cooler weather, the water temperature in the upper layer also begins to cool, creating a more uniform temperature throughout the lake. This uniformity dissolves the “barriers” between the layers that exist in the summer months (or winter months, in the case of spring turnover).
Having more uniform water temperatures means the density of the water is also more uniform; this means the water can mix easier.
Add a little wind into the equation, and the water that was trapped at the bottom of the lake all summer now mixes easily with the upper water – and you have fall turnover.
What does this turnover mean for the lake?
It all comes back to the big ‘P’ mentioned in the previous article: phosphorus.
Phosphorus in the lake is chemically attached to the sediment at the bottom as long as oxygen is present in the water. But during the summer, when the deep waters (that bottom layer) become depleted of dissolved oxygen, the chemical bond is no longer present, and phosphorus is released into the isolated bottom waters. This released phosphorus is then distributed throughout the lake during spring and fall turnover, providing extra nutrients for algae.
This mixing of the lake water and resulting increase in phosphorus explains some of the algae blooms we see during fall and spring. This turnover is a prime example of “internal loading,” discussed in the previous article about phosphorus. An internal load is phosphorus that is not a new source but has accumulated over many years and which moves between the sediments and water column.
And there you have it, folks! Fall turnover. Keep an eye out for changes in the lake during the upcoming season – see if you can spot what’s going on.
This article was compiled by Angela McLaughlin with the help of former LICA board member Lynne Bisagno and her article, “The Link Between Lake Turn Over & Algae Blooms” and the Freshwater Society’s “Guide to Lake Protection and Management.”