Synonymy

Vegetation of nearly stagnant waters Tansley 1911 P-P-

Constant species Ranunculus aquatilis.

Physiognomy

The Ranunculetum aquatilis is dominated by clumps or patches of Ranunculus aquatilis, a very variable annual or perennial crowfoot, able to grow submerged, when the shoots are spreading to erect, as a delicate floating plant or terrestrially on moist ground, when it is loosely cespitose (Haslam 1978, Rich & Rich 1988). Cover is very variable and many stands are small, but in suitable conditions this species can make luxuriant growth, becoming most abundant in early summer.

The vegetation often grows in intimate association with various other aquatics and amphibious plants, including stands of Ranunculus peltatus, which is morphologically very similar to and readily confused with R. aquatilis (Holmes 1979, Rich & Rich 1988). But the most frequent companions in denser stands are various stoneworts, particularly Callitriche stagnalis and C. obtusangula, patches of which can become prominent with the crowfoot early in the season, and Glycerio-Sparganion herbs, which increase their cover later. Commonest among these latter are smaller Glyceria spp., especially G. fluitans, and Apium nodiflorum, but Nasturtium officinale, Veronica beccabunga and Berula erecta can also be found, and each of these can grow in vigorous patches among the submerged plants or as floating or emergent shoots. Myriophyllum spicatum, Ceratophyllum demersum and Elodea canadensis are sometimes seen, and there can be floating-leaved mats of Potamogeton natans with patches of duckweed thalli caught among the vegetation on the water surface or wet mud.

Habitat

The Ranunculetum aquatilis is typically found in and around the margins of mesotrophic to fairly nutrient

Rich waters, sometimes quite fast-moving, in other cases standing or sluggish. It probably occurs through much of southern Britain outside the highland zone, extending into streams around the upland fringes, but becoming much more common in streams, dykes, canals and pools in the lowlands. It tolerates periodic or seasonal drying and will colonise disturbed or ephemeral water-margin habitats.

The difficulty of identifying R. aquatilis with certainty and in particular the problem of separation from R. peltatus (Holmes 1979, Rich & Rich 1988), makes it hard to define precisely the geographical or environmental limits of this vegetation. It seems to occur through most of the lowlands and the upland fringes, and is concentrated in shallower waters, often much less than 1 m deep and sometimes fluctuating, with fertility that avoids the extremes of impoverishment and marked eutrophication.