Constant species

Glyceria fluitans.

Physiognomy

The Glycerietum fluitantis is dominated by a low mat or floating carpet of Glyceria fluitans, sometimes continuous and very species-poor, in other cases as a more open cover with a variety of associates, some of which may attain local prominence. No other species reaches even occasional frequency throughout but the most usual associates are plants of shallow water margins such as Alisma plantago-aquatica, Myosotis scorpioides, Apium nodiflorum and Eleocharis palustris.

Sub-communities

Glyceria fluitans sub-community. This sub-community includes pure or very species-poor stands in which G. fluitans forms an often thick and lush mat. Lemna minor is the only frequent associate.

Sparganium erectum-Mentha aquatica sub-community. Sparganio-Glycerietum fluitantis Br.Bl. 1925. Here, there is a usually more open cover of the dominant among clumps of Typha latifolia, Sparganium erectum, Nasturtium officinale and Carex otrubae with scattered plants of Mentha aquatica, Myosotis scorpioides, Alisma plantago-aquatica and Rumex crispus.

Alopecurus geniculatus sub-community: Washlands and wet alluvial meadows Ratcliffe 1977 p.p.. Alopecurus geniculatus is often abundant and sometimes co-dominant in this sub-community and there is occasionally a little Poa trivialis. Rumex crispus is preferentially frequent here.

Habitat

The community is characteristic of shallow, standing or sluggish, mesotrophic waters and fine mineral sub

strates and is commonly found around ponds and wet depressions in fens and pastures and on the margins of dykes and small streams. The Glyceria sub-community includes stands where a quaking mass of G. fluitans extends out into small areas of sometimes deeper open water in fens or in other undisturbed situations where the dominant can pre-empt a niche and spread. The Alopecurus sub-community is more restricted to shallower waters and occurs around the gently sloping silty margins of ponds and pasture hollows. The Sparganium-Mentha sub-community is an often narrow and fragmentary community where G. fluitans gains a hold among more robust swamp species on stream sides and around pond edges.

G. fluitans is a succulent and palatable grass which is avidly eaten by stock and herbivorous wildfowl. On the Fen washlands, this community formed part of the swamp/grassland mosaics traditionally maintained by winter flooding and summer grazing and now surviving largely on the Ouse Washes where they provide valuable grazing for a variety of bird populations. Poaching by stock can break up the cover of the community and perhaps aid its spread by the trampling in of broken stems which readily re-root in moist ground.