Pond scum is a general term for various types of overgrowth, including algae, cyanobacteria, or floating plants. This growth is typically fueled by an excess of nutrients, specifically nitrogen and phosphorus, combined with ample sunlight. When organic matter like leaves, fish waste, or fertilizer runoff introduces these nutrients, the water body becomes fertile ground for rapid growth. Effective management involves both immediate physical removal and long-term environmental controls to address the root causes of the nutrient overload.
Identifying What Kind of Scum You Have
Determining the type of growth is the first step toward successful removal, as different organisms require distinct management strategies. The most recognizable form is filamentous algae, often called “string algae,” which appears as long, slimy, hair-like strands that clump together in dense, mat-like masses floating on the surface. Planktonic algae are microscopic, single-celled organisms suspended throughout the water column, giving the entire pond a uniform murky green or pea-soup appearance.
It is common to mistake certain small aquatic plants for true scum, such as duckweed and watermeal. Duckweed consists of tiny, bright green leaves, often clustered in groups of three to five, with small roots dangling beneath the surface. Watermeal is even smaller, resembling fine, gritty particles floating on the surface, and it lacks visible roots. Both floating plants can rapidly cover the water, but they are structurally different from algae and respond to different treatment methods.
Quick Fixes and Physical Removal Techniques
Manual removal provides the most immediate relief by reducing the current biomass and improving the pond’s appearance. For string algae, a simple pond net, garden rake, or algae brush can be used to twist and pull the dense mats from the water. Exercise caution when using tools with sharp edges, such as a metal rake, to avoid puncturing any flexible pond liner.
Floating plants like duckweed and watermeal can be skimmed off the surface using fine-mesh nets or booms. After removal, place the harvested material far away from the water’s edge. This debris is rich in nutrients, and allowing it to decompose near the pond will result in those nutrients leaching back into the water. Regular skimming is necessary, as these organisms can double their population in just a few days.
Long-Term Biological and Environmental Control
Sustainable management focuses on reducing the nutrient load and disrupting the conditions that favor rapid growth. Introducing beneficial bacteria treatments accelerates the decomposition of organic sludge, fish waste, and excess nutrients. These specialized microorganisms consume excess nitrogen and phosphorus, effectively starving the algae of its food source. For larger water bodies, applying barley straw is a slow-release method that acts as an algistat, inhibiting new algae growth as the straw decomposes.
Aeration and circulation improve overall water health by preventing thermal stratification. Bottom-diffused aeration systems mix the water column, preventing the formation of an oxygen-poor zone at the pond floor. This movement ensures high dissolved oxygen levels, which promotes aerobic bacteria activity and helps lock up phosphorus in the bottom sediment, making it unavailable for algae growth. Incorporating aquatic plants like hardy water lilies or floating plants provides natural shading, limiting the amount of sunlight reaching the water. Aiming for approximately 60% surface coverage can significantly reduce the light available to planktonic and filamentous algae.
When Chemical Treatments Are Necessary
Chemical treatments, typically algaecides, offer a rapid solution but should be reserved for severe outbreaks or when non-chemical options have failed. Copper-based compounds are common commercial options, but they must be applied with care, especially in water bodies containing fish or other aquatic life. Copper is toxic at certain concentrations, and the risk of harm increases in soft water with low alkalinity, making a water test prior to application necessary.
Using algaecides risks a massive algae die-off, which can deplete the pond’s dissolved oxygen levels. As a large volume of algae decomposes simultaneously, the process consumes oxygen, potentially suffocating fish and other organisms. To mitigate this risk, treat only one-third to one-half of the affected area at a time, allowing several days between applications. This partial treatment strategy ensures that the oxygen demand from the decaying biomass does not crash the aquatic environment.