Algae, Phosphorus, and How Alum Can Help

What are Algae?

Algae are microscopic, free-floating organisms that are commonly found in lakes. They are similar to terrestrial plants in that they also require sunlight and nutrients to grow. Algae are the important base of the aquatic food web that ultimately supports juvenile and adult fish. However, the excessive growth of undesirable types of algae (Cyanobacteria) causes severe water quality problems. Cyanobacteria (a.k.a blue-green algae) thrive when nutrients are high and are capable of producing toxins. They grow rapidly to cause blooms and scums and usually associated with nearly all lake water quality problems.

What are the Causes and Symptoms of Poor Water Quality?

The nutrient of concern in lakes is phosphorus. Just like terrestrial plants, algae need phosphorus to grow and it is scarce in lakes. That means increases in phosphorus leads to more algae and more undesirable kinds of algae (Cyanobacteria). This uncontrolled growth directly or indirectly causes nearly every common water quality problem in lakes. The algae produce blooms and surface scums that cloud the water and reduce clarity. They smell and are visually unpleasant. They can produce toxins that cause the lake to be closed due to health concerns to humans, pets and livestock. The algae eventually die and oxygen is removed from the water as it decays. This can cause severely stressed fish or kill fish. Recreation opportunities are limited and lakeshore property values decline as the quality of the lakes get worse. Fortunately, conditions can be improved by lowering the amount of phosphorus in the lake water. Just as more phosphorus increases the amount of algae, less phosphorus lowers algae and improves water quality.

How are Property Values affected by Water Quality?


Lake property is obviously aesthetically and economically desirable due to the lake and it is well-established that the quality of the lake is tightly linked to the property’s value. For example, Bemidji State University in Minnesota conducted a comprehensive study of 2000 lake home sales between 1966 and 2001. The value of each home sale was compared to the clarity of the lake at the time of sale and they found a very close relationship. A 3 foot increase in clarity resulted in a $427 increase in the sale price for every foot of lake shoreline. Even in 2001 dollars, that equates to a $17,000 gain on a relatively small lot with 40 feet of shoreline. Interestingly, there was a greater economic penalty for declines in water quality. A 3 feet decrease in clarity caused a $500 decrease in the sale price per foot of shoreline. This calculates to a $24,000 decline in the value of the same-sized 40-foot lot. Good lake management practices and water quality improvements not only ensure enhanced and safe recreational opportunities, but also protect and grow the lake owner’s investment.

Where Does the Phosphorus Come From?

Internal and External loading.png

Phosphorus enters lakes from two sources. Phosphorus that is brought into the lake from outside sources are called external sources. These sources originate in the watershed and are either directly rinsed into the lake or flow to a stream that enters the lake. Common external sources include lawn fertilizers, septic systems, agricultural practices, stormwater, soil erosion and geese…anything that causes phosphorus to enter the lake from the watershed.

Once the external source of phosphorus enters the lake, it is deposited in the lakebed and is recycled back into the water column. This is the second source of phosphorus and it originates from within the lake itself. This is called an internal source and these inputs are most common during the summer and winter when lakewater oxygen concentrations are low or zero near the bottom. This causes changes in the chemistry of the lakebed that leads to the phosphorus leaching out of the sediment and into the water.

Both external and internal sources of phosphorus are important in managing lake water quality. The relative contribution of each source must be understood and measured to design the best solution for any given lake. External sources can be measured in the water entering the lake or estimated from models of the surrounding watershed. Internal sources are determined by collecting lakebed sediment samples (cores) and measuring the amount phosphorus available to leach out into the water.

What is Alum and How does it Work?

Alum (aluminum sulfate) is a nontoxic liquid that is commonly used in water treatment plants to clarify drinking water. It’s use in lakes began in the early 1970’s and is used to reduce the amount of phosphorus in the water. Lower amounts of phosphorus leads to lower amounts of algae and associated symptoms of poor water quality. Alum is most often used to control phosphorus release from the lake bottom sediments (internal loading). Research has shown that even when external sources of phosphorus from the surrounding watershed are lowered, the internal cycling can continue to support significant nuisance algal blooms.

Alum is applied to lakes using specialized equipment and barges that ensure the precise placement of the material in the lake. On contact with the water, the liquid alum forms a fluffy aluminum hydroxide precipitate called floc. Aluminum hydroxide (the principle ingredient of common antacids such as Maalox) binds with the phosphorus to form an aluminum phosphate compound. The compound is insoluble in water and the bound phosphorus can no longer be used as food by the algae. As the floc settles, phosphorus and particles are removed from the water column and leaves the lake noticeably clearer. The floc then forms a thin layer on the bottom that binds the phosphorus that leaches out of the bottom sediments during internal loading events. This keeps the phosphorus from entering the overlying water and makes it unavailable to the algae.

4 Comments Add yours

  1. NickR says:

    Is the lake safe to use for any activity where your body could be in the water such as swimming, water skiing, kayaking, or simply hanging out at the sandbar During the treatment?


    1. habaquatics says:

      Yes, the alum is drinking water quality and there are no recreational restrictions for lake users. We only ask that boaters maintain a safe distance from the barge. For more information see


  2. bhwaterplant says:

    What happens eventually to the phosphorus that is locked into the alum and the floc settled to bottom? Does it need to be dredged up ? Is this a yearly application or how often would we need to reapply?


    1. habaquatics says:

      The alum application results in a very thin layer of alum floc (about 1 inch) that quickly becomes mixed into the upper layer of sediments in the lakebed and isn’t visible after a month or so. The alum floc has unfilled binding sites that get filled with phosphorus as the floc intercepts the phosphorus that leaches out of the lakebed over time. Once the phosphorus is attached to the floc, it will not dissolve into the water and the it is not longer available for the algae to use. The floc does not need to be removed. In most cases, the alum application is designed to last around 20 years. Thank you for your interest and feel free to contact us for more detailed information!


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