Duckweed refers to a group of tiny, free-floating aquatic plants, including Lemna (lesser duckweed) and Spirodela (giant duckweed). These plants are among the fastest-growing on Earth, quickly forming a dense, green carpet on the surface of calm water bodies. The question of whether they enrich the water with oxygen is often raised. The answer is not a simple yes or no, as duckweed’s effect on dissolved oxygen levels shifts dramatically depending on the time of day and the total surface area it covers.
Oxygen Production Through Photosynthesis
Like all green plants, duckweed produces oxygen through photosynthesis during daylight hours. The plant uses solar energy to convert water and carbon dioxide into sugars, releasing oxygen as a byproduct. Since duckweed is in direct contact with the water column, much of the oxygen produced is released directly beneath the plant’s fronds.
In a small, controlled environment, such as an aquarium with partial surface coverage, this daytime oxygen production can be measurable and temporarily beneficial. The oxygen released contributes to the dissolved oxygen content of the water, supporting aquatic life. This effect is most pronounced under bright, direct light, which maximizes the plant’s photosynthetic rate.
The Critical Impact of Surface Coverage
Despite its ability to produce oxygen during the day, a thick mat of duckweed often leads to a net reduction in dissolved oxygen over a 24-hour cycle, especially in natural ponds. This shift from producer to consumer is primarily due to two factors that become prominent when the plant covers 50% or more of the water surface.
The first factor is nighttime respiration, a process all plants perform continuously. Duckweed consumes oxygen from the water 24 hours a day, but at night, photosynthesis ceases, making the plant a net oxygen consumer. When a large biomass of duckweed covers the surface, its collective nighttime respiration can rapidly draw down oxygen levels in the water column.
The second, and often more significant, factor is the physical blockade of the water surface. A dense duckweed layer acts as a physical barrier, preventing the diffusion of atmospheric oxygen directly into the water. This diffusion is the primary source of oxygen for many larger, non-aerated water bodies. This blockade compounds the problem by cutting off the water’s ability to replenish the oxygen consumed by organisms. Furthermore, the dense mat shades the water below, suppressing photosynthesis by submerged plants and algae, removing another important source of dissolved oxygen.
Duckweed’s Secondary Effects on Water Quality
Beyond its complex interaction with dissolved oxygen, duckweed significantly affects water quality through other mechanisms. One of its most notable benefits is its rapid uptake of excess nutrients, particularly nitrates and phosphates. Duckweed is highly efficient at this process, with removal efficiencies for phosphate and nitrate often exceeding 90% in wastewater treatment systems.
By aggressively absorbing these compounds, duckweed acts as a natural filter, which starves other types of floating algae. This action helps to prevent harmful algal blooms and can curb eutrophication, where excessive nutrients lead to poor water quality. Additionally, the dense surface layer creates significant shading, which helps regulate water temperature by preventing excessive solar heating.