Synonymy

Ceratophyllum demersum vegetation Pallis 1911, Ellis 1939, Lambert & Jennings 1951, Lambert 1965, ail p.p.; Ceratophylletum demersi (Pop 1962) den Hartog & Segal 1964.

Constant species

Ceratophyllum demersum, Elodea canadensis.

Rare species

Azolla filiculoides, Potamogeton coloratus.

Physiognomy

The Ceratophylletum demersi is characterised by often dense free-floating masses of Ceratophyllum demersum, usually with at least a little, sometimes much, Elodea canadensis. Some stands have few other plants, apart from a patchy floating mat of Lemna minor, but others are richer, with frequent records for Ranunculus circinatus, Callitriche stagnalis and one of the other introduced Canadian pondweeds, E. nuttallii, with L. gibba and Nuphar lutea among the floating element.

Sub-communities

Ranunculus circinatus sub-community. Richer stands of the Ceratophylletum are included here with, in our data, preferentially frequent R. circinatus, one of the easier water-crowfoots to distinguish, with its leaf segments all held rigidly in a single plane (Holmes 1979). Callitriche stagnalis is also very common and this is one of the aquatic communities in which the presence of E. nuttallii has been increasingly recognised (Simpson 1984). Also among the submerged plants is occasional Myriophyllum spicatum, Potamogeton pectinatus, the nationally rare P. coloratus, a species found only very locally and mostly in eastern England, and Sparganium emersum. Lemna minor is scarce but L. gibba and L. trisulca occur, together with occasional Nuphar lutea and Azolla filiculoides.

Lemna minor sub-community. Apart from C. demersum and E. canadensis, L. minor is the only frequent plant in this sub-community.

Habitat

The Ceratophylletum demersi is typical of still or slowmoving, eutrophic waters, mainly in the warmer, southeastern lowlands of Britain. It is a widespread community of sluggish streams, and pools, dykes and canals, and is perhaps becoming increasingly common in such waters because of eutrophication with agricultural run-off.

C. demersum is largely confined to those parts of the country (Perring & Walters 1962) where there is a mean annual maximum temperature in excess of 28 °C (Conolly & Dahl 1970), but it has a widespread range through Europe, except the Arctic regions, and its association with this part of the country may be as much a reflection of its nutrient requirements as any warmth-dependence.