Synonymy

Wolffio-Lemnetum gibbae Bennema (1943) 1946.

Constant species

Lemna gibba.

Rare species

Wolffia arrhiza.

Physiognomy

The Lemnetum gibbae comprises floating mats of the small thalli of Lemna gibba, often very numerous and densely crowded, sometimes forming a virtually continuous cover over many square metres, though generally disposed just one layer thick, except where the wind pushes up the thalli one upon another. No other species is constant, but L. minor is fairly common, occasionally occurring in some abundance, and there can be great difficulty in distinguishing this from non-gibbous forms of L. gibba which are found quite widely. Spirodela polyrhiza also occurs with locally high frequency and extensive cover and, where such mixtures have a little Hydrocharis morsus-ranae, the vegetation begins to grade to the more species-rich kind of duckweed mat. The tiny rootless thalli of Wolffia arrhiza are very occasionally found, but this plant is much scarcer with us than in some other parts of Europe, where it can be a constant in the Lemnetum gibbae, as in The Netherlands (Westhoff & den Held 1969).

Beneath the mat, submerged aquatics are generally few, but free-floating Lemna triscula and Elodea canadensis, and rooted shoots of Callitriche stagnalis, C. obtusangula, Potamogeton pectinatus and Zannichellia palustris have been recorded in the samples. And the Lemnetum gibbae often occurs sheltered among the shoots of emergents, a variety of which can be recorded among the mats: Nasturtium officinale and Apium nodiflorum were the commonest here, but the community is frequently found in close association with vegetation dominated by Glyceriafluitans, G. maxima, Sparganium erectum, Typha latifolia and, in more brackish situations, Scirpus maritimus (Charman 1981, Wolseley et al. 1984).

Habitat

The Lemnetum gibbae is typical of standing or sluggish, eutrophic and base-rich waters throughout south-east Britain, often in situations which are too recently disturbed or unstable to support much other aquatic vegetation. It occurs widely in sheltered stretches of pond and lake margins, and along the edges of very slow-moving lowland streams, but is especially common in canals and ditches, where it can make a quick recovery from cleaning and dredging.