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Every lake is unique. Long-term management considers the environmental, cultural, and biological factors affecting the lake and sets a priority on finding lasting solutions.

Lake management is complicated and requires a coordinated effort of community groups, individuals, landowners, and government. To be effective, lake managers must commit to long-term strategies and investment. Specific strategies to address a lake's nutrient enrichment problems must focus on activities in the watershed and, if needed, in-lake restoration techniques.

Guide to lake protection and management

This second edition of this guide included updated material on the chemistry of lakes, watershed information, exotic species, altering runoff and lake use practices, development of a lake management plan, and nonstructural best management practices.

  • What is a lake? How does it work? This section offers the basic information, including an explanation of the physical, biological, and chemical properties of a lake, and how it interacts with other bodies of water in its watershed.
  • How do eutrophication, sedimentation, acidification, toxic contamination, and invasive exotic species affect lakes.
  • Learn about lake restoration strategies such as hypolimnetic withdrawal, artificial circulation (aeration), nutrient diversion, dredging, and in-lake treatment (nutrient inactivation).

The guide was a collaboration between MPCA and the Freshwater Society (Freshwater).

Tools for lake management

State agencies reviewed available techniques to reduce internal phosphorus loading. This guide provides an overview of the most common practices in Minnesota and is intended to help practitioners determine which practice may be best suited for their lake.

Frequently asked questions about in-lake treatment

Long-term management of excessive algae requires the removal of phosphorus sources to the water body, which in effect reduces the algae in the lake. If in-lake restoration techniques are necessary, they should be proceeded by or occur alongside appropriate long-term management actions to control sediments, nutrients, and toxic inputs. A successful lake restoration program should strive to manage both external and internal nutrient sources.

What is in-lake treatment?

Phosphorus enters the water either externally (from run-off or groundwater), or internally from the nutrient rich sediments on the bottom of the lake. Phosphorus is released from the sediments under anoxic conditions that occur when the lake stratifies and oxygen is depleted from the lower layer. Even when external sources of phosphorus have been curtailed by best management practices, the internal recycling of phosphorus can continue to support explosive algal growth. In-lake treatments are used to control this internal recycling of phosphorus from the sediments of the lake bottom.

How does it work?

Lake projects typically use aluminum sulfate (alum) or iron to inactivate phosphorus.

  • Alum: When applied to water, alum forms a fluffy aluminum hydroxide precipitate called a floc. As the floc settles, it removes phosphorus and particulates (including algae) from the water column. The floc settles on the sediment where it forms a layer that acts as barrier to phosphorus. Phosphorus, released from the sediments, combines with the alum and is not released into the water to fuel algae blooms. The length of treatment effectiveness varies with the amount of alum applied and the depth of the lake. Eugene Welch and Dennis Cooke (1995) evaluated the effectiveness and longevity of treatments on 21 lakes across the United States. They concluded that the treatments were effective in six of the nine shallow lakes, controlling phosphorus for at least eight years on average. Applications in stratified lakes were highly effective and long lasting; reduction of internal phosphorus loading has been continuously above 80%.
  • Iron absorbs phosphorus from the water column and forms a barrier over the sediment surface, providing high sediment phosphorus retention. During summer and winter, deep lake water is dark and isolated from mixing, and pH and dissolved oxygen concentrations decline in the water. In low dissolved oxygen, iron is used by the microbial community as an alternative electron acceptor to oxygen and phosphorus is released. Iron’s reaction to low oxygen and pH conditions means that aeration or artificial circulation may have to accompany application to prevent the breakdown of the oxidized barrier or a photosynthetically caused increase in pH.

Why treat a lake?

Though external sources such as stormwater run-off, are often the main contributors to excessive phosphorus to lakes, researchers have learned that lakes are very slow to recover after external sources have been eliminated. Lake sediments become phosphorus rich and can deliver phosphorus to the overlying water without external sources. When dissolved oxygen levels decrease in a lake's bottom waters, large amounts of phosphorus trapped in the sediments are released into the overlying water.

Is chemical treatment toxic to aquatic life?

Freeman and Everhart (1971) used constant flow bioassays to determine that concentrations of dissolved aluminum below 52 ug Al/L had no obvious effect on rainbow trout. Similar results have been observed for salmon. Cooke, et al (1978) adopted 50 mg Al/L as a safe upper limit for post-treatment dissolved aluminum concentrations. Kennedy and Cooke (1982) indicate that based on solubility, dissolved aluminum concentrations, regardless of dose, would remain below 50 ug Al/L in the pH range 5.5 to 9.0. MPCA guidelines for alum application require that the pH remain within the 6.0-9.0 range.

The treatment of lakes with alkalinities above 75 mg/L as CaCO3 are not expected to have chronic or acute effects to biota. Fish-related problems associated with alum treatments have been primarily documented in soft water lakes. However, many soft-water lakes have been successfully treated with alum, when the treatments are pH buffered.

Are there health concerns for people?

Concerns about a connection between aluminum and Alzheimer’s have been debated for some time. More recent research points to a gene rather than aluminum as the cause. In addition, aluminum is found naturally in the environment. Some foods are high in aluminum, including tea, spinach, and other leafy green vegetables.

How much does an alum treatment cost?

Costs of alum application are primarily dependent on the form of alum used (wet or dry), dosage rate, area treated, equipment rental or purchase, and labor. The cost is dependent on dosage requirements and costs to mobilize equipment. Treatment costs range from $280/acre to $700/acre. Liquid alum has been used when large alum doses were needed, and averages $1.75 per gallon.

How does the MPCA regulate in-lake treatment?

When MPCA receives a request for in-lake treatment, agency staff address external load reductions that are either occurring or planned. The MPCA works with Department of Natural Resources (DNR) Fisheries staff to discuss impacts to aquatic plants and fish. Many of the known chemicals, applied in a watershed with robust external load reductions, can be approved more quickly than lesser known chemicals in watersheds that have not identified external loading opportunities. If approved, the proposer will be sent a letter with guidelines from MPCA on notifications to local partners (DNR), monitoring during and after the event, and the acceptable pH range.

If the project is for continuous treatment of inflow to a lake, MPCA requires a National Pollutant Discharge Elimination System/State Disposal System permit. The permit requires monitoring and reporting of the inflow and outflow (if applicable) for phosphorus and the chemical used to reduce the nutrient. Permits last for five years before expiring, and application and annual permit fees apply.