Synonymy

Myriophyllum alterniflorum consocies Pearsall 1918; Chara-Myriophyllum alterniflorum sociation Spence 1964.

Constant species

Myriophyllum alterniflorum.

Rare species

Potamogeton filiformis.

Physiognomy

The Myriophylletum alterniflori comprises species-poor vegetation in which Myriophyllum alterniflorum is the obvious dominant, growing thickly in congenial situations, though often sparse and battered in more turbulent waters. No other species occurs frequently, but occasional include Juncus bulbosus, Littorella uniflora, Lobelia dortmanna, Callitriche hamulata, C. stagnalis, Potamogeton natans, P. gramineus, Chara and Nitella spp. and the moss Fontinalis antipyretica. The rare P. filiformis is found among this vegetation at a few sites around the Scottish seaboard.

Habitat

The Myriophylletum is characteristic of lime-poor and less fertile waters, standing to quite swiftly flowing or spatey, in lakes, pools and streams, predominantly in the north and west of Britain.

M. alterniflorum grows best in base-poor waters of low conductivity, draining catchments of acidic rocks, like harder arenaceous sedimentaries or lime-free igneous and metamorphic rocks (Haslam 1978). Stands of this vegetation can be found in the lowland south and east of the country, but the community is very local there, being strongly confined to streams and pools on deposits like the Tertiary sands of the New Forest, and it becomes widespread and common only in the upland fringes of the north and west, where suitable rocks make up the bulk of the landscape. The plant will tolerate quite impoverished conditions, so it can thrive even where the substrates of the catchment are very resistant to erosion, though it tends to occur in peaty waters, such as those draining the extensive tracts of upland blanket mire, only where their dystrophic character is ameliorated a little by the deposition of some mineral material.

M. alterniflorum has a fairly shallow rooting system but, in the sandy to stony beds favoured here, it can gain quite extensive and firm anchorage and its thin, finelycut foliage offers but slight resistance to faster flow or wave-wash (Pearsall 1918, Haslam 1978). The shoots will break with moderate force, but remaining fragments readily regrow, so plants can persist in spatey waters (Haslam 1978).