Constant species

The vegetation included here is characteristically heterogeneous but the most frequent species are Apium nodiflorum, Nasturtium officinale (including N. microphyllum) and Veronica beccabunga. Some authorities have also erected vegetation types on the constancy of Catabrosa aquatica or Glyceria plicata.

Physiognomy

Though floristically varied, this vegetation has a highly distinctive structure. Most of the species are small perennials with shoots that are procumbent and rooted below and ascending (or sometimes floating) above. These form a bushy and often patchy canopy of somewhat uneven height, though generally less than 30 cm. Each of the most frequent species can be locally dominant, forming quite large pure clumps which monopolise long stretches of water margin or alternate with patches of other species. In other cases, there is a more intimate mixture of smaller individuals of two or more of these most frequent species with a variety of associates, the commonest of which are Myosotis scorpioides, Mentha aquatica, Alisma plantago-aquatica, Berula erecta (in the south and east) and small Glyceria spp. (G. fluitans, G. plicata, their hybrid G. x pedicellata or G. declinata). Each of these may also assume local prominence, too, and there may be patches of Ranunculus sceleratus, Juncus bufonius or Callitriche stagnalis on bare mud. Then, there are often occasional records for species from a variety of vegetation types which are frequently in close juxtaposition at water margins, most notably swamps, tell-herb fens and some mesotrophic grasslands. Added to this spatial variation, there is often an element of seasonal change in the vegetation in relation to natural periodicity in growth patterns (Apium nodiflorum, for example, often spreading markedly in late summer and Berula erecta in early spring: Haslam 1978) or in response to disturbance such as dredging or cutting.

Habitat

This vegetation is most typical of the unshaded margins of mesotrophic to eutrophic waters where there is some accumulation of mediumto fine-textured mineral sediments. It may be a temporary feature fronting emergents as they colonise shallow open waters but, where there is some measure of marginal disturbance by turbulence in running waters or from dredging, cutting or moderate trampling or where there is seasonal drying up of standing or running waters, it often forms a permanent, though temporally variable, fringe. It is very common around small standing waters like field ponds, along canal and dyke margins and in the shallows of low-order streams.