Every lake is unique. Specific strategies to address a lake's nutrient enrichment problems must focus on activities in the watershed and, if needed, in-lake restoration techniques. Lake management approaches fall into two categories, the "quick-fix" and long-term management. The quick-fix offers a short-term solution such as the application of algaecides to kill unwanted algae. This approach treats the biological symptoms of a lake problem but does not address the underlying causes of these symptoms.
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.
Phosphorus generally limits the growth of freshwater algae in most Washington lakes, although nitrogen is also an important nutrient. When phosphorus is the limiting nutrient, there is a direct relationship between the amount of phosphorus in a lake and the amount of algae growing in the lake. As phosphorus levels increase, the amount of algae increases too. At very high levels of phosphorus, other nutrients or light may limit the growth of algae. Long-term management of excessive algae requires the removal of phosphorus sources to the water body. Reducing phosphorus inputs to lakes can affect the amount of algae in the lake by removing a key nutrient.
External nutrient sources such as fertilizer use, pet wastes, stormwater runoff, septic system effluents, waterfowl, agriculture, and even rainfall can contribute nutrients to a lake. Lake management removes or modifies as many of these nutrient sources as possible, especially those sources shown to be contributing the greatest nutrient load to the water body. If in-lake restoration techniques are necessary, they should be followed by, or occur simultaneously with 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.
Controlling nutrient sources will not improve lake water quality immediately in many cases. Years may pass before lakes cleanse themselves of accumulated nutrient loads. For this reason, in-lake restoration techniques have been developed to accelerate recovery. These techniques may not be suitable for all lakes and all conditions. Consider using these techniques only after a lake specialist has evaluated the lake and recommended one or more of these options.
This website provides a quick overview of some common in-lake techniques. Please refer to the third edition of Restoration and Management of Lakes and Reservoirs, authored by G. Dennis Cooke, Eugene B. Welch, Spencer A. Peterson, and Stanley A. Nichols, 2005, for a comprehensive and scientific discussion of these and other lake management methods.
Oxygen (or air) is pumped into the deep, often nutrient-enriched, oxygen-depleted layer that forms in deeper lakes called the hypolimnion (see the illustration of the cross section of lake water layers to the right). The goal of hypolimnetic aeration is to maintain oxygen in this layer to limit phosphorus release from sediments without causing the water layers to mix (destratify).
Hypolimnetic aeration increases habitat and food supply by providing more oxygenated waters. On the down-side, hypolimnetic aerators are expensive to operate. It may be difficult to supply adequate oxygen to the hypolimnion without destratification and subsequent algal blooms. This technique is suitable for deep lakes with an oxygen-deficient hypolimnion.
Some lake managers use siphons to remove nutrient rich water from the hypolimnium. This reduces nutrients and eliminates some of the low oxygen water. Hypolimnetic withdrawal is suitable for small, deep lakes with oxygen-poor or nutrient-rich bottom water. This technique can have severe repercussions on downstream receiving waters which receive nutrient-enriched waters.
Artificial circulation provides increased aeration and oxygen to a lake by circulating the water to expose more of it to the atmosphere. Aeration systems are generally used in shallow water bodies. A number of artificial circulation systems can provide aeration including surface spray (fountains), paddlewheels, and air diffusers. Artificial circulation disrupts or prevents stratification and increases aerobic habitat. The effect of aeration on algae varies. Aeration does not necessarily decrease algal biomass, but may lead to less cyanobacteria (blue-green algae). Some cyanobacteria have gas vacuoles which allow them to regulate their position in the water column. By circulating the water, cyanobacteria may spend more of their time in the dark, reducing their competitive advantage over other kinds of algae. Internal loading of phosphorous may also decline if sediments remain oxygenated. When lake sediments lack oxygen, conditions exist to release phosphorus into the water.
Dilution projects direct a low-nutrient water source into and through a lake as a means of diluting and flushing nutrients from the higher-nutrient lake water. Flushing may wash out surface algae and replace higher-nutrient lake water with lower-nutrient dilution water. Lower-nutrient water may lead to fewer problem algae in the water. On the downside, dilution requires large volumes of low-nutrient water (which may be scarce or expensive) and does not eliminate sources of phosphorous from the sediments or the watershed. Green Lake in Seattle and Moses Lake in Grant County are examples of Washington lakes where dilution has been successfully used.
Drainage channels or pipes are used to divert nutrient-rich waters to the downstream side of lakes. In some lakes, like Lake Washington in Seattle, nutrient diversion meant diverting sewage from this lake. Depending on the project, major engineering may be required at great expense and other receiving waters may be affected by the nutrient-rich water. Diverting streams also eliminates a water supply to the lake and may interfere with fish runs.
Heavy equipment or specialized hydraulic dredges can remove accumulated lake sediments to increase depth and to eliminate nutrient-rich sediments. Dredging may control rooted aquatic vegetation, deepen the water body, and increase lake volume. By removing nutrient-rich sediment, dredging may improve water quality. Some dredging drawbacks include resuspension of sediments during the dredging operation and the temporary destruction of habitat. Large-scale dredging is extremely expensive due to equipment costs, permitting issues, and spoils disposal. Because of costs, dredging is typically done on a limited scale. Vancouver Lake in Washington was dredged in the early 1980's to help improve water quality. Although many shallow lakes may benefit from this method, dredging's great expense limits its widespread use in most water bodies.
Aluminum, iron, or calcium salts can inactivate phosphorus in lake sediments. Lake projects typically use aluminum sulfate (alum) to inactivate phosphorus. Alum may also be applied in small doses for precipitation of water column phosphorus. 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 (precipitation). 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 (inactivation). Algal levels decline after alum treatment because phosphorus levels in the water are reduced.
The length of treatment effectiveness varies with the amount of alum applied and the depth of the lake. Alum treatment in shallow lakes for phosphorus inactivation may last for eight or more years. In deeper lakes, alum treatment may last far longer. Long Lake in Kitsap County and Green Lake in Seattle provide examples of recent successful use. Click here to read an alum report from Green Lake.
Some lake managers use alum to precipitate phosphorus from the water column by continuously injecting small amounts of alum during the summer months (micro-floc alum injection). The hypolimnetic aerator in Newman Lake, Washington, injects small amounts of alum into the water as it operates to provide additional management of phosphorus. Read more about beautiful Newman Lake (seen in the photograph) at this website. The photograph was used with the permission of the Newman Lake Flood Control Zone District.
Check with state and local agencies to determine which permits are required before proceeding with any of these activities.
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