A9 - Potamogeton Natans Community

Summary

Synonymy

Floating-leaf vegetation Pallis 1911, Butcher 1933, Tansley 1939, all p.p.; Shallow water association Matthews 1914 p.p.; Potamogeton natans consocies Pearsall 1920; Submerged/floating vegetation Godwin 1923, Butcher 1933 p.p.; Potamogeton natans-Juncus fluitans sociation Spence 1964 p.p.; Potamogeton natans Gesellschaft Oberdörfer 1977.

Constant species

Potamogeton natans.

Physiognomy

Potamogeton natans is one of only two widely distributed pondweeds in Britain with floating leaves but the other, P. polygonifolius, is generally speaking a fairly small plant, whereas here the leafy stems commonly trail up through the water to lengths of more than 1 m, and anything up to 5 m in really deep places. The plant figures as an occasional in various kinds of aquatic vegetation, and can persist at low frequencies in some more open swamps, but this community includes all those stands where P. natans is a clear dominant over the water surface, often to the total exclusion of other freefloating or floating-leaved species and with just very sparse submerged or emergent associates. Where these plants become a little more common, rather ill-defined sub-commanities can be recognised, but if the associated floras are consistently rich, the vegetation is best regarded as a mosaic of the P. natans community with other aquatic assemblages.

Sub-communities

Species-poor sub-community. Here, P. natans is often the sole plant, or very occasionally accompanied by a little Nymphaea alba, Nuphar lutea or Lemna minor.

Elodea canadensis sub-community. The associates above can also occur in this sub-community occasionally, but more distinctive is the fairly frequent occurrence of Elodea canadensis, rarely E. nuttallii, occasional Lemna trisulca and various starworts, including Callitriche stagnalis, C. platycarpa and C. obtusangula, often difficult to distinguish without fruiting material (Wiggington & Graham 1981). Rarely, too, there can be some water-crowfoots, such as Ranunculus trichophyllus, R. peltatus and R. penicillatus which can also present problems of identification (Holmes 1979). Myriophyllum spicatum is sometimes found and there are very occasional records for various pondweeds, such as Potamogeton crispus, P. pectinatus or P. filiformis. Emergents such as Alisma plantago-aquatica, Sparganium erectum, Typha latifolia or Glyceria fluitans are often found in close association with the vegetation and, in shallower water, may grow through it.