Duckweed (Lemna minor)

Duckweed is the plant every aquarist eventually has a love-hate relationship with: the smallest and fastest-growing floating vascular plant in fresh water, and the one most likely to carpet a tank if you turn your back on it. Each plant is a flat green disc only a millimetre or two across — the whole body is a single photosynthetic thallus, with no real stem or leaf, trailing one or two fine rootlets for nutrient uptake. That tiny size is its superpower: where the larger fronds of Salvinia need open surface, duckweed slips into the gaps between bigger fronds and grows up into the light, which is how it eventually takes a mixed mat over.

The fastest floater there is

Speed is duckweed's whole strategy. In good light and nutrient-replete water it doubles in about two days — roughly twice as fast as Salvinia — well within the one-and-a-half-to-three-day range measured in well-managed cultures (Hillman 1961; Skillicorn et al. 1993). Like all floating plants it sits in the brightest, most CO₂-rich part of the tank, drawing carbon dioxide straight from the air, so carbon rarely limits it; it makes modest use of bicarbonate on top, a little more than Salvinia does. Its tissue is even more protein-rich than Salvinia's and carries no woody lignin (Skillicorn et al. 1993) — fast, soft, and nitrogen-hungry. It is a touch cooler-loving than Salvinia and noticeably more cold-hardy: it can ride out near-freezing spells by forming dormant overwintering buds (turions) that sink and wait. It is strictly freshwater and tolerates a broad pH range.

The most aggressive ammonia scavenger in the tank

Duckweed carries the highest-affinity ammonium uptake of any floating plant in the model, characterised directly for Lemna minor (Cedergreen & Madsen 2002). It runs at near-full speed even when ammonium is very dilute, which makes it an even more aggressive scavenger than Salvinia — and especially useful in the tail of a cycle, once the initial ammonia spike has passed and what remains is a thin trickle that slower plants would barely register.

How duckweed takes over a mixed mat

Put duckweed and Salvinia in the same tank and, given a few weeks to months, duckweed wins the surface. Three things combine to produce that takeover, and all three are biology rather than bookkeeping:

• It grows in the gaps. Because duckweed's fronds are millimetres across, they occupy the interstices between Salvinia's centimetre-scale pads. The model lets duckweed judge its own crowding by its own coverage rather than the combined mat, so it can establish and keep growing even on a surface Salvinia has already filled.
• It blocks more light per unit mass. Tiny fronds intercept more light per unit of carbon than large ones, so a given amount of duckweed casts a deeper shadow than the same mass of Salvinia — and the shadow falls on the Salvinia beneath it.
• It pushes the slower plant under. Once the combined mat is dense, the slower-growing species accumulates extra mortality from being shoved below the surface into the dark, while the fastest grower — duckweed — takes no such penalty. The balance tips gradually toward duckweed over months. This Lemna-over-Salvinia displacement is exactly what is seen in field and aquarium comparisons (Landolt 1986; Skillicorn et al. 1993).

The suffocation risk

The flip side of a thriving duckweed mat is the reason "duckweed took over my tank" is so often followed by "and then my fish gasped at the surface." Small flat fronds in tight contact with the water form a near-continuous lid, and that lid badly throttles gas exchange across the surface — studies of choked tanks measure oxygen transfer at only a small fraction of open-water rates under a dense Lemna mat (Pokorný & Rejmánková 1983; Morris & Barker 1977). At night, when photosynthesis stops and everything in the tank is consuming oxygen, the usual rescue — oxygen diffusing in from the air — is largely cut off, because the mat's own daytime oxygen was released upward into the atmosphere, not into the water. Carbon dioxide from respiration builds up under the lid instead of venting, pulling pH down with it. A runaway duckweed mat is genuinely capable of suffocating a tank overnight.

Salvinia versus duckweed at a glance

The two floating plants in the model scavenge ammonia and shade the water alike, but at different tempos and with very different mats:

The exact growth rates, ammonia and light half-saturations, temperature and pH thresholds, and the coefficients behind the competitive displacement are tabulated in the Parameter Reference.

Further reading

• Salvinia — the larger floating fern duckweed competes with and eventually displaces
• Macrophytes: Aquatic Plants — floating-mat mechanics, including how a mat throttles gas exchange across all the air-water channels
• Producers — how all the algae and plants fit together
• Nitrogen Cycle — the dilute ammonia duckweed is so good at scavenging
• Parameter Reference — every rate, half-saturation, and threshold behind this page, with citations

Key references

• Cedergreen, N. & Madsen, T.V. (2002). Nitrogen uptake by the floating macrophyte Lemna minor. New Phytologist 155, 285–292.
• Hillman, W.S. (1961). The Lemnaceae, or duckweeds. Botanical Review 27, 221–287.
• Landolt, E. (1986). The family of Lemnaceae — a monographic study. Veröffentlichungen des Geobotanischen Institutes ETH, Zürich 71, 1–566.
• Morris, P.F. & Barker, W.G. (1977). Oxygen transport rates through mats of Lemna minor and Wolffia sp. Canadian Journal of Botany 55, 1926–1932.
• Pokorný, J. & Rejmánková, E. (1983). Oxygen regime in a fishpond with duckweeds and Ceratophyllum. Aquatic Botany 17, 125–137.
• Skillicorn, P., Spira, W. & Journey, W. (1993). Duckweed Aquaculture. World Bank Technical Paper No. 239.
• Walstad, D.L. (1999). Ecology of the Planted Aquarium. Echinodorus Publishing.