A2 - Lemna Minor Community Lemnetum Minoris Soo 1947

Summary

Synonymy

Lemnetum minoris (Oberdörfer 1957) Müll. & Gors 1970; Societie van Lemna minor Westhoff & den Held 1969.

Constant species Lemna minor.

Rare species

Azolla filiculoides, Lemna minuscula.

Physiognomy

The Lemnetum minoris consists essentially of floating mats of Lemna minor, the small thalli often very numerous and densely crowded, but usually forming a single layer on the water surface. There are no other constants and, indeed, associates of any kind are often very few, their abundance being local or ephemeral or indicating a shift to different kinds of vegetation. L. gibba, for example, is generally scarce, though it can oust L. minor and mixtures of these plants often presage a development of the Lemnetum gibbae. Then, at a few places in south-east England, the rare alien duckweed L. minuscula has been recorded as an aggressive invader of this vegetation: an introduction from the warmer parts of North and South America, distinguished from our native species by its smaller size and single vein, it seems able to overwhelm other duckweeds, both here and in those other parts of Europe where it has gained a hold (Leslie & Walters 1983). The introduced aquatic fern Azolla filiculoides, a native of the tropical Americas, can also be found in this community, its small branching shoots with their pinkish leaves increasing to enormous abundance from time to time and smothering other floating plants with a thick layer of growth.

A distinctive form of the community has locally prominent L. trisulca beneath the floating mat of duckweed and some stands are characterised by the periodic occurrence of the aquatic liverworts Riccia fluitans and Ricciocarpus natans. Apart from this very modest amount of variety, though, most of the floristic differences among the Lemnetum minoris reflect the range of submerged aquatic and emergent communities with which it is found.

Sub-communities

Typical sub-community. This includes the most speciespoor stands with a generally very abundant, sometimes continuous, cover of L. minor.

Lemna trisulca sub-community. The submerged duckweed L. trisulca can attain considerable abundance beneath rather more open mats of L. minor than usual, with occasional records for a variety of other aquatics such as Potamogeton lucens, P. pectinatus, Callitriche stagnalis, C. hamulata and Myriophyllum spicatum.