Algae are a natural and necessary component of any lake ecosystem, forming the base of the aquatic food web. These microscopic organisms perform photosynthesis, contributing dissolved oxygen and supporting other life forms. A problem arises when factors lead to an algae bloom, transforming clear water into a murky, green soup. Excessive algae growth diminishes the aesthetic value of the waterbody and compromises its ecological health.
Why Algae Blooms Happen
The most significant factor driving nuisance algae blooms is an overload of nutrients. Phosphorus and nitrogen act as fertilizers, and their excessive availability allows for uncontrolled growth. These nutrients enter the lake system from the surrounding land, a process known as external loading.
Runoff from the watershed is the primary pathway for nutrient entry, carrying phosphorus and nitrogen from sources like fertilizers, septic systems, and eroded soil. Even small increases in these nutrients can trigger a substantial bloom, particularly for cyanobacteria (blue-green algae). Warm water temperatures and extended periods of sunlight accelerate the reproductive rate of algae.
Immediate Methods for Algae Removal
Addressing an existing, dense algae bloom requires immediate action to reduce the biomass and restore water quality. These methods target the symptom—the present algae—rather than the root cause of the nutrient problem.
Chemical Treatments
Chemical treatments, utilizing algaecides like copper sulfate, kill large quantities of algae quickly. Copper-based products are effective, but their application must be carefully managed and often requires a permit. Applying the chemical at approximately 5.4 pounds per acre for the top two feet of water is often sufficient in high-alkalinity lakes.
The rapid die-off of a massive bloom risks dissolved oxygen depletion, as the decomposing algae consume oxygen, potentially causing fish kills. Treating harmful algae can cause cells to burst, releasing toxins dangerous for aquatic life and humans. Algaecides are often applied only to a third of the waterbody at a time to mitigate the adverse effects of a large-scale decomposition event.
Mechanical Removal
Mechanical removal offers a non-chemical solution for surface algae mats and accumulated organic sediment. Simple skimming or netting can remove floating filamentous algae, but this approach is labor-intensive and only practical for small areas. Hydro-raking is a more robust method, using a floating barge equipped with a hydraulic rake attachment.
The hydro-rake physically removes nutrient-rich organic muck and rooted vegetation from the lake bottom, operating in depths from 18 inches to 10 feet. This process permanently removes accumulated sediment, which can hold up to 500 pounds of phosphorus per scoop. Extracting this nutrient-laden material directly reduces the internal nutrient supply available for future blooms.
Biological Controls
Biological controls manage existing or recurring algae. Methods include introducing sterile triploid grass carp, which feed on certain submerged aquatic plants and algae. Another method uses barley straw, which inhibits new growth rather than killing existing algae.
As the barley straw decomposes, it releases compounds like hydrogen peroxide, which suppresses the growth of new algal cells. The straw is typically applied early in the season before the bloom starts, at a rate of 100 to 300 pounds per surface acre, and remains effective for about six months. The straw must remain well-oxygenated near the surface for anti-algal activity, as overdosing can lead to oxygen depletion when the material decays.
Preventing Future Algae Growth
Long-term control requires limiting the available nutrient supply within the lake and preventing new nutrients from entering. These preventative measures address the underlying cause of the problem for a more sustainable solution.
Nutrient Inactivation
Nutrient inactivation is an in-lake restoration technique focused on reducing available phosphorus. The most common form is the application of aluminum sulfate (alum), carefully injected into the water column. When alum contacts the water, it forms a light, fluffy material called a floc, which binds with dissolved phosphorus.
This floc settles to the lake bottom, forming an insoluble aluminum phosphate compound and creating a thin, protective “blanket” over the sediment. This blanket seals the lake bottom, preventing the release of phosphorus accumulated in the sediment over many years, a process known as internal loading. A single, properly executed alum treatment can maintain water quality improvements for five to twenty years.
Aeration Systems
Installing a bottom-diffused aeration system prevents internal phosphorus loading. In deeper lakes, water separates into distinct temperature layers, leading to low-oxygen (anoxic) conditions near the sediment. This lack of oxygen causes the sediment to chemically release stored phosphorus back into the water column, fueling algae blooms.
Diffused aeration systems use a shore-mounted compressor to pump air through weighted lines to diffusers resting on the lake bottom. The fine bubbles create a vertical circulation pattern, lifting the anoxic bottom water to the surface where it absorbs oxygen from the atmosphere. This vertical mixing eliminates the oxygen-depleted zone, preventing the chemical release of phosphorus and disrupting thermal stratification.
Watershed Management
Watershed management practices are important for controlling the external nutrient supply from the land surrounding the lake. Key practices include:
- Establishing vegetated buffer zones along the shoreline, where deep roots of native plants filter and absorb nutrients from runoff.
- Diverting nutrient-rich runoff away from the lake.
- Regular maintenance of septic systems, as failing systems can leach phosphorus and nitrogen into the groundwater and surface water.