Filamentous algae, often called “pond moss,” “blanket weed,” or “string algae,” is characterized by long, slimy strands that form dense mats. The presence of this organism signals an imbalance in the pond’s ecosystem, driven by an overabundance of nutrients. While some algae is natural and supports the aquatic food web, unchecked growth diminishes aesthetic appeal and can threaten the health of fish and other pond life. Understanding the root cause of this growth is the first step toward restoring clarity and ecological balance.
Identifying the Growth and Underlying Causes
Filamentous algae is easily recognized as a mass of interwoven threads that appear like wet cotton or wool, initially growing on submerged surfaces in shallow areas. As the algae photosynthesizes, it produces oxygen bubbles trapped within the mat, causing sections to detach and float to the surface, creating unsightly scum. This rapid proliferation is a direct symptom of two primary environmental factors: excessive sunlight and an overload of nutrients.
The main culprits for this nutrient overload are phosphorus and nitrogen, which act as fertilizer for the algae. These compounds enter the pond system from various sources, including runoff from fertilized lawns and gardens, decaying organic matter like fallen leaves and grass clippings, and fish waste. Ponds with poor circulation or those situated in high-sunlight areas provide the ideal environment for these nutrients to be quickly metabolized into large algal blooms.
Accumulation of organic debris on the pond bottom breaks down, continuously releasing nitrates and phosphates into the water column. This internal nutrient recycling sustains the algae’s growth cycle, making long-term control difficult without addressing the underlying sediment buildup.
Immediate Removal Strategies
The first intervention for an active algae bloom involves physically removing the bulk of the existing material to immediately reduce the overall biomass and export trapped nutrients. Manual removal can be accomplished using a pond rake, a specialized algae brush, or a simple net. It is important to remove the algal mats slowly and deliberately to prevent stirring up the bottom sediment, which would release more nutrients.
After removing the algae, dispose of it away from the pond’s edge, as decaying matter will leach stored nutrients back into the water. For heavy accumulations, pond vacuums can gently remove sludge and algae from the bottom without sediment disturbance. While manual removal offers immediate results, it is a short-term fix that will likely need repetition during peak growing seasons.
For rapid reduction of severe blooms, specific chemical treatments, known as algaecides, can be employed. Options include copper-based compounds or sodium carbonate peroxyhydrate, which releases hydrogen peroxide upon contact with water. Copper algaecides are effective but must be used with caution, as high concentrations can be toxic to fish like koi and trout. Hydrogen peroxide-based treatments break down into water and oxygen, leaving no chemical residue.
Precise adherence to the manufacturer’s dosage instructions is mandatory. A substantial algae die-off can cause a rapid drop in dissolved oxygen levels as the dead material decomposes, posing a threat of fish mortality. To mitigate this risk, treat no more than one-third to one-half of the pond at a time and increase aeration immediately following application.
Preventing Regrowth Through Water Quality Management
Long-term control focuses on creating an inhospitable environment for filamentous algae through sustained water quality management. This approach requires reducing available nutrients, often accomplished by introducing beneficial bacteria treatments. These microorganisms consume excess nitrogen and phosphorus, starving the algae of its primary food source.
The bacteria accelerate the decomposition of organic sludge and fish waste, reducing the buildup of nutrient-rich sediment. For best results, apply these bacteria regularly, particularly during warmer months when water temperatures exceed 50°F. Consistent physical removal of decaying leaves, excess fish food, and other debris further limits the nutrient supply.
Proper aeration and water circulation are powerful preventative tools. Installing devices like submersed aerators, fountains, or waterfalls ensures the pond is well-oxygenated, promoting the growth of beneficial aerobic bacteria. Increased dissolved oxygen levels also sequester phosphorus, encouraging it to bind to the bottom sediment where it is unavailable to the algae.
Limiting the amount of sunlight penetrating the water is another effective strategy, as algae requires light for photosynthesis. This can be achieved by introducing aquatic plants, such as water lilies, which provide surface coverage and shade. These plants also compete directly with the algae for nutrients. Non-toxic pond dyes can also tint the water, blocking sunlight and slowing the rate of algal growth.