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More about phosphorus...

Why too much phosphorus in a lake is a bad thing

by Angela McLaughlin

     You’ve probably heard some things about the big ‘P’ before – phosphorus. But what exactly is it and how does it affect the lake?  

     Phosphorus is a naturally occurring element that exists all over – in minerals, soil, water, and living organisms. 

     According to the Water Research Center, “Phosphorus occurs naturally in rocks and other mineral deposits. During the natural process of weathering, the rocks gradually release the phosphorus as phosphate ions, which are soluble in water.”

     Phosphorus and phosphates exist naturally in a lake; however, certain things can cause an excess of phosphorus. This becomes more than the lake’s ecosystem can handle, causing unhealthy side effects, such as degradation of water quality.     


Too much phosphorus can cause excessive algae growth. Some of these algae blooms can be harmful to humans and even fatal to pets and native wildlife – we are seeing this in Minneapolis lakes right now. This algae damages the water quality, negatively impacting creatures that call the lake home, including populations of fish.

     It’s also certainly not conducive to a fun environment for swimmers, boaters, or fisherpersons. 

     The EPA says phosphorus is considered the “limiting nutrient” in aquatic ecosystems, meaning that the availability of it controls the pace at which algae is produced. 

     According to limnologists, one pound of phosphorus can produce up to 500 pounds of algae! 

     External sources of phosphorus can come from shore erosion, manure, organic waste, and fertilizers. It attaches to soil particles, which can then move into bodies of water as runoff. 

     And here’s the thing: once phosphorus gets dumped into a lake, it doesn’t necessarily go away again. This is what is referred to as an “internal load,” or phosphorus that is not a new source but has accumulated over many, many years and which moves between the sediments and water column.  

     It is estimated that about 60% of the total phosphorus load from all sources in Lake Independence is already in the lake!

     So, to break it down… Excess quantities of phosphorus = water quality problems and harmful algae growth.

     Keep an eye out for future articles, which will dive deeper into phosphorus in Lake Independence, what can be done about it, and what LICA is doing to help continue to make our lake even cleaner. 

Bald Eagle Lake.jpg

After all phosphorus inflows have been minimized, it may be possible to do a lake-wide alum treatment to 

contain existing phosphorus at the bottom of the lake.

There's been extensive research on alum treatments. The following article describes the alum treatment applied to Bald Eagle Lake:

Research on alum treatments

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.” 

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