Synonymy

Moderately swift current vegetation Butcher 1933 p.p.

Constant species

Ranunculus penicillatus ssp. pseudofluitans.

Physiognomy

This community comprises stands of submerged aquatic vegetation dominated by the crowfoot now known as Ranunculus penicillatus ssp. pseudofluitans, a taxon previously designated as R. penicillatus ssp. calcareus (Butcher 1960, Cook 1966, Holmes 1980, Webster 1988). It is a perennial plant with shoots up to 3 m long, growing in clumps and trailing downstream in the flowing water that provides the usual habitat. Maximum cover is attained early in the season, with luxuriant growth of the fine tasselled leaves occurring where conditions are congenial, and clumps can be very numerous such that whole streams of considerable size become choked with this vegetation. The var. vertumnus, sometimes separated from a var. pseudofluitans (Holmes 1980, Webster 1988), was not distinguished in the sampling.

The community is frequently found in close association with other crowfoot vegetation, like that dominated by R. aquatilis and R. peltatus, and various other aquatic assemblages, but among denser stands associates are usually few and sparse. Callitriche stagnalis and Potamogeton pectinatus are occasionally seen, and there are sometimes small patches of Lemna minor caught among floating shoots where the flow is slacker. Then, there are quite often some trailing or emergent shoots of plants such as Nasturtium officinale, Veronica beccabunga and Berula erecta, all of which tend to become more luxuriant rather later in the season, as the abundance of the crowfoot is fading.

Habitat

The community is confined to base-rich but generally only moderately fertile waters, of moderate to quite fast flow and with sandy to gravelly beds, mostly in limestone catchments in lowland England and Wales, and especially towards the south.

As diagnosed by Holmes (1979, 1980) and Webster (1988), R. penicillatus ssp. pseudofluitans is centred in southern England, particularly in waters on the Chalk and Oolite, with some occurrences also on London and Oxford Clays. Further west and north, it can be found on Carboniferous Limestone, Devonian and Silurian rocks, which provide most of the substrates in Wales, but it scarcely penetrates into Scotland, being found in just a very few localities on Old Red Sandstone.