A3 - Spirodela Polyrhiza-Hydrocharis Morsus-Ranae Community

Summary

Synonymy

Hydrocharitetum morsus-ranae van Langendonck 1935.

Constant species

Berula erecta, Elodea canadensis, Glyceria fluitans, Hydrocharis morsus-ranae, Lemna gibba, L. minor, L. trisulca, Spirodela polyrhiza.

Rare species

Azolla filiculoides, Wolffia arrhiza.

Physiognomy

The major element of the Spirodela polyrhiza-Hydrocharis morsus-ranae community consists of a floating mat of various mixtures of the duckweeds Lemna minor, L. gibba and, particularly distinctive here, Spirodela polyrhiza, together with Hydrocharis morsus-ranae. The rare Wolffia arrhiza is also occasionally found and the alien fern Azolla filiculoides sometimes occurs. Beneath this, there is a closely associated layer of submerged plants, with Elodea canadensis and Ceratophyllum demersum very common and sometimes abundant, and entangled among them the thalli of L. trisulca. Other more occasional associates are Callitriche platycarpa, C. obtusangula, Ranunculus circinatus, Zannichellia palustris, Potamogeton crispus, P. pectinatus and P. berchtoldii. Then, there are generally some emergents, not taller shading species, but ones with their shoots often semisubmerged among the aquatics or sometimes totally underwater. Most frequent among the available samples were Berula erecta, which can grow well even fully submerged, when it produces shorter, stockier plants than in swamps (Haslam 1978), and Glyceria fluitans, whose lush masses of shoots trail out into open water, but Apium nodiflorum and Agrostis stolonifera have also been recorded growing among this kind of vegetation (Wolseley et al. 1984).

Habitat

The Spirodela-Hydrocharis community is confined to unpolluted and unshaded, clear mesotrophic and eutrophic standing waters, sometimes quite calcareous or slightly saline, in the warmer lowlands of southern and eastern Britain. It is especially characteristic of more open dykes and ponds, disused and little-used canals, and can return fairly quickly after physical cleaning but, with widespread pollution and eutrophication, it is becoming increasingly local.

The two most distinctive species of this community, Spirodela and Hydrocharis, as well as the associated C. demersum, L. gibba and, to a lesser extent, L. trisulca, are largely confined to the English lowlands, south of a line from the Mersey to the Humber, where the mean annual maximum temperature is more than 28 °C (Conolly & Dahl 1970). Within this zone, this kind of vegetation is restricted to more mesotrophic standing waters, often of artificial origin, such as small ponds and dykes, sometimes cut through mineral substrates, sometimes through peat. In certain localities, the waters are fairly calcareous, and the community can also occur in the moderately brackish sections of drainage systems in reclaimed coastal marshes (Charman 1981).