String algae, also known as filamentous algae, is a common issue for owners of ponds and water features, often signaling an imbalance in the aquatic environment. This organism appears as long, hair-like strands or thick, slimy mats that typically attach to rocks, plants, or the sides of the water feature. When left unchecked, this algae can form dense blankets that are unsightly and can interfere with the function of pumps and filtration systems. An overgrowth suggests the water is supplying an abundance of nutrients and light. Addressing this problem requires a strategic approach combining immediate physical removal with targeted treatments and long-term environmental management.
Immediate Physical Removal
The initial and most direct approach to managing an outbreak involves manually removing the existing algae biomass. Physical extraction immediately lowers the overall nutrient load in the water, which would otherwise be released back into the system if the algae were chemically treated and allowed to decompose. This step provides quick visual improvement and prepares the water for subsequent treatments.
Techniques for physical removal include using specialized algae nets, a pond rake, or winding the strands around a stick or a long-handled brush. Perform this removal slowly and deliberately to minimize the fragmentation of the algae strands. Breaking the algae into small pieces releases spores and fragments into the water column, potentially leading to new growth. Once removed, the algae biomass should be disposed of far away from the water source, ideally in a compost heap, to ensure the captured nutrients do not drain back into the pond.
Targeted Chemical and Biological Treatments
Interventions using external agents, whether chemical compounds or living organisms, can effectively treat the remaining algae after the bulk has been physically removed. These treatments focus on killing or consuming the existing algae population. Since these methods introduce agents into the water, they require careful application to maintain the health of the broader ecosystem.
Chemical Treatments
Algaecides offer a rapid solution to algae outbreaks, generally falling into copper-based or peroxide-based formulations. Copper algaecides are widely used and effective against filamentous algae, but they must be dosed with caution. Copper can be toxic to sensitive aquatic life, particularly koi, goldfish, and trout, if concentrations exceed safe limits.
Peroxide-based treatments, often in granular form, work by oxidizing the algae on contact and are useful for spot-treating algae on rocks or waterfalls. These products break down into water and oxygen, making them safer for fish and plants than copper compounds. However, any rapid die-off of a large algae bloom can severely deplete the dissolved oxygen in the water as the dead organic material decomposes. This oxygen depletion can stress or kill fish, requiring additional aeration during the treatment period.
Biological Controls
Introducing beneficial organisms is a natural way to control algae by either consuming it directly or outcompeting it for resources. Certain fish species, such as grass carp, graze on algae and other aquatic weeds. Snails can also assist in cleaning up small amounts of algae from submerged surfaces.
A comprehensive biological strategy involves the consistent application of concentrated beneficial bacteria. These commercial bacteria mixtures, often called sludge removers, break down organic waste like fish excretions, dead leaves, and uneaten food. By consuming these materials, the bacteria reduce the concentration of dissolved nutrients, specifically nitrates and phosphates, effectively starving the algae. For these bacteria to thrive and work efficiently, the water needs adequate dissolved oxygen, often supplied through an aeration system.
Long-Term Environmental Management
Achieving lasting freedom from string algae requires changing the underlying environmental conditions that fuel its growth. Algae proliferation is driven by two primary factors: excess nutrients and abundant sunlight. Sustainable control focuses on reducing these inputs to shift the ecological balance in favor of desirable aquatic life.
Nutrient Control
Algae flourishes when there is an overabundance of nitrogen and phosphorus compounds in the water. These nutrients originate from sources like decomposing organic debris, fertilizer runoff, and fish waste. A fundamental management practice is reducing fish feeding to only what they consume within five minutes, as uneaten food quickly breaks down and releases phosphates and nitrates.
Removing the accumulated organic sediment, or muck, from the pond bottom is another effective strategy, as this material is highly concentrated with nutrients. Specialized filter media and treatments, such as those containing lanthanum-modified clay, can bind free-floating phosphate molecules, converting them into an inert form that algae cannot utilize. Natural products like barley straw, when decomposing, release compounds that inhibit algae growth, including a small amount of hydrogen peroxide.
Light Regulation
Algae requires sunlight to grow, and reducing light penetration can limit its proliferation. The most common method of light regulation is the introduction of floating aquatic plants, such as water lilies, water lettuce, and hyacinths. These plants act as natural shade covers. Covering approximately 60 to 70 percent of the water surface is recommended to suppress algae growth while still allowing for gas exchange.
In situations where plants are impractical, pond dyes can be used to tint the water a blue or black color. This coloration effectively blocks specific wavelengths of light necessary for photosynthesis, inhibiting the growth of submerged algae. Shade cloths or pergolas placed over the water feature can also provide mechanical shade during peak sunlight hours.
Water Quality and Aeration
Stagnant water and low dissolved oxygen levels create an environment that favors algae and hinders the decomposition of organic waste. Enhancing water quality through circulation and aeration is an important component of long-term management. Installing devices like aerators, fountains, or bottom diffusers increases the dissolved oxygen content throughout the water column.
Increased oxygen levels support the beneficial aerobic bacteria that break down organic matter more efficiently, preventing the buildup of nutrient-rich sludge. Aeration also prevents thermal stratification, which can occur in deeper water features, and helps release gases like carbon dioxide. Consistent water movement prevents algae from settling and proliferating in still, warm areas, contributing to a less hospitable environment for future blooms.