A13 - Potamogeton Perfoliatus-Myriophyllum Alterniflorum Community

Summary

Synonymy

Submerged vegetation West 1910; Shallow water association Matthews 1914 p.p.;, Linear-leaved associes Pearsall 1918 p.p.; Chara-Myriophyllum alterniflorum sociation Spence 1964 p.p.; Potamogeton gramineus society Spence 1964.

Constant species

Littorella uniflora, Myriophyllum alterniflorum, Potamogeton gramineus, P. perfoliatus.

Rare species

Callitriche hermaphroditica, Elatine hexandra, E. hydropiper, Isoetes setacea, Najas flexilis, Potamogeton filiformis, P. rutilus, Subularia aquatica, Utricularia intermedia.

Physiognomy

The Potamogeton perfoliatus-Myriophyllum alterniflorum community subsumes the bulk of the richer and more varied pondweed assemblages in which the broader-leaved Potamogeton perfoliatus and P. gramineus, together with Myriophyllum alterniflorum, provide the most consistent floristic element, with never more than very occasional occurrences of either P. pectinatus or M. spicatum. Each of these constants can be abundant and luxuriant, although in more turbulent waters even their total cover is often insubstantial, and one or other species may be locally eclipsed by the dominance of some less frequent associate, or be altogether absent.

Only a few other plants are found frequently throughout the community, but there is almost always some Littorella uniflora, thickening up in places to form a fairly dense sward of submerged shoots among the pondweeds, and Chara spp. occur commonly, quite often in great abundance, with Nitella spp. becoming frequent too in one of the sub-communities. Then, there is generally an open floating-leaved element, with Potamogeton natans the most usual contributor, P. polygonifolius figuring in peatier places. Polygonum amphibium is occasional, too, where this kind of vegetation extends into shallower marginal waters, the Nymphaea alba, Nuphar lutea and the rare N. pumila are sometimes found further out. Floating masses of Juncus bulbosus are occasionally seen and the community can provide a locus for the uncommon Callitriche hermaphroditica. The moss Fontinalis antipyretica is fairly frequent and sometimes luxuriant.

A range of other submerged pondweeds occurs in the community and differences in the frequency and abundance of these provide much of the local diversity in this vegetation. Potamogeton pusillus, P. berchtoldii, P. obtusifolius and, in shallower waters, the rare P. filiformis, are the commonest of these. Less frequent, though also occurring occasionally with some prominence, are P. praelongus, P. crispus, P. alpinus, P. lucens, P. x zizii (P. lucens x P. gramineus), P. x nitens, P.friesii and the rare P. rutilus. As with the Potamogeton-M. spicatum vegetation, locally dense growth of Elodea canadensis sometimes overwhelms the diversity among these plants, but this is not such a common feature here.