Synonymy

Isoetes lacustris consocies Pearsall 1918, 1921; Isoetes lacustris society Spence 1964; Isoeto-Lobelietum (Koch 1926) Tx. 1937 sensu Schoof-van Pelt 1973 p.p.

Constant species

Isoetes lacustris or I. setacea.

Rare species

Isoetes lacustris, Subularia aquatica.

Physiognomy

The Isoetes community comprises submerged swards dominated by the aquatic pteridophytes, Isoetes lacustris and/or the more local I. setacea. Both species are gregarious perennials, I. lacustris in particular forming extensive dense stands, but sparser covers of scattered individuals occur, and I. setacea is often found in quite small, sporadically distributed patches. The growth habit is rosette-like, the plants bearing numerous quilllike leaves, stiff and brittle in I. lacustris, rather more spreading and flexible in /. setacea and, when the individuals are closely packed, the overall impression is of an uneven lawn. The size of the plants varies considerably with the depth and turbulence of the waters, and there is perhaps also some measure of genetic difference between what are often very isolated populations, but I. lacustris is generally the larger species, the leaves reaching up to 25 cm or more, with those of I. setacea usually less than 15 cm. Nonetheless, where the two quillworts occur together, as is locally the case, the smaller species, commonly the minor element in the sward, is easily overlooked (Page 1982a, b).

Very many stands of this kind of vegetation are pure or almost so, but a number of associates occur occasionally and some of these can be locally abundant, especially where the Isoetes swards extend into shallower waters. Littorella uniflora and Lobelia dortmanna, for example, often occur in the same lakes as the community and can sometimes be found sparsely distributed among the quillworts, when they are hard to distinguish from them. Then, there is quite frequently some Myriophyllum alterniflorum or free-floating Juncus bulbosus, with occasional patches of Potamogeton perfoliatus, P. berchtoldii, P. crispus, P. obtusifolius or, not recorded here but regarded as characteristic by Page (1982a), P. alpinus. Callitriche hamulata, C. hermaphroditica, Sparganium angustifolium, Scirpus fluitans, Utricularia vulgaris agg. and the rare Subularia aquatica can also be seen, with Elodea canadensis or E. nuttallii locally prominent in some more fertile waters. Nitella spp. or, less commonly, Chara spp. are sometimes abundant too.

Habitat

Isoetes vegetation is characteristic of barren, stony substrates in the clear, deep waters of less fertile lakes through north-west Britain.