A4 - Hydrocharis Morsus-Ranae-Stratiotes Aloides Community Azolla Filiculoides in Aquatic Vegetation

Summary

Synonymy

Stratiotes aloides vegetation Pallis 1911, Ellis 1939, Lambert & Jennings 1951, Lambert 1965, all p.p.; Hydro charité tum morsus-ranae van Langendonck 1935 p.p.’, Hydrocharito-Stratiotetum (van Langendonck 1935) Westhoff 1946 p.p.', Ceratophyllum-Stratiotes nodum Wheeler & Giller 1982.

Constant species

Callitriche platycarpa, Ceratophyllum demersum, Hydrocharis morsus-ranae, Lemna minor, L. trisulca, Polygonum amphibium, Stratiotes aloides, Utricularia vulgaris.

Rare species

Myriophyllum verticillatum, Stratiotes aloides.

Physiognomy

The Hydrocharis morsus-ranae-Stratiotes aloides community has a usually abundant submerged element comprising luxuriant masses of Ceratophyllum demersum with Utricularia vulgaris and quite often some of the rare Myriophyllum verticillatum. Callitriche platycarpa occurs very frequently in small amounts and there is commonly a little Elodea canadensis and occasionally some Potamogeton obtusifolius or the rare Ceratophyllum submersum. Another rarity, Stratiotes aloides, is now particularly associated with this kind of vegetation within its much contracted range, and its striking large rosettes of rigid leaves can be quite abundant here. They remain submerged for much of the time, rising to the surface at flowering, but reproduction is by offsets in this country, male plants being very scarce and fruit never set (Clapham et al. 1962).

Floating above this layer is a surface mat of Hydrocharis morsus-ranae, Lemna minor, L. trisulca, Polygonum amphibium and Nuphar lutea, often mixed in with an abundant algal scum, mainly Rhizoclonium. Occasionally there are some emergent shoots of Poten-tilla palustris, Hottonia palustris, Berula erecta, Oenanthe aquatica, Sparganium erectum, Nasturtium officinale, Sium latifolium, Alisma plantago-aquatica or Sagittaria sagittifolia.

Habitat

The Hydrocharis-Stratiotes community is a very local vegetation type of mesotrophic, calcareous standing waters. It seems now to be restricted to a few localities, mostly in Broadland, though similar vegetation appears to have been more common there are one time, and perhaps occurred scattered through eastern England in the past.

In their study of the Catfield and Irstead fens in the Ant valley in Norfolk, Wheeler & Giller ( 1982Z?) showed that this community was most characteristic of those parts of the closed internal dyke system nearer to the fen margins, where there was direct or indirect influx of land-drainage waters and a shift from the deep peat of the fen basin to a mineral substrate. Chemical analysis of both waters and sediments there revealed higher levels of inorganic nitrogen (mainly nitrate) than in the more central areas of the fens, and slightly, but significantly, greater concentrations of calcium and magnesium throughout the year.