Potamogeton compressus

Tracheophyta MagnoliopsidaPotamogetonaceaePotamogetonPotamogeton compressus

Ecology

P. compressus, a rhizomatous perennial, has been recorded from a wide range of habitats: lakes, sluggish rivers, ditches, canals and flooded mineral workings. Its sites share a tendency to be still or slowly flowing, mesotrophic and slightly to moderately base-rich. Lowland.

Status

Native

World Distribution

Eurasian Boreo-temperate element.

Broad Habitats

Standing water and canals

Light (Ellenberg): 7

Moisture (Ellenberg): 12

Reaction (Ellenberg): 7

Nitrogen (Ellenberg): 4

Salt Tolerance (Ellenberg): 0

January Mean Temperature (Celsius): 3.4

July Mean Temperature (Celsius): 15.9

Annual Precipitation (mm): 697

Length: 90

Perennation - primary

Perennial

Life Form - primary

Perennial hydrophyte (perennial water plant)

Woodiness

Herbaceous

Clonality - primary

Detaching ramets above ground (often axillary)

Count of 10km squares in Great Britain: 134

Count of 10km squares in Ireland: 0

Count of 10km squares in the Channel Isles: 0

Atlas Change Index: -1.68

JNCC Designations

NBNSYS0000002142

External Species Accounts

Scarce Atlas Account

Scarce Atlas Account: 

Potamogeton compressus L.

Grass-wrack pondweed

Status: scarce

 

The natural habitat of this aquatic species is apparently slowly-flowing lowland rivers and nearby ox-bows, and calcareous lowland lakes. It has, however, also colonised canals and drainage ditches. Most of the records from the English Midlands and the Welsh Borders are from canals. In the Norfolk Broads it grows in mesotrophic grazing marsh ditches in a diverse community which includes other scarce aquatics such as Hydrocharis morsus-ranae and Stratiotes aloides.

P. compressus sometimes sets seed, but vegetative reproduction is probably more frequent than regeneration by seed. This is accomplished by specialised turions, which also act as agents of dispersal.

As the map shows, P. compressus has decreased markedly in Britain. It is extinct in the Cambridgeshire fenland, where it was last collected in 1912, and in the Thames valley, where it was last recorded from Oxfordshire in 1947. The reasons for these disappearances are not fully understood; in Cambridgeshire it became extinct before other scarce aquatics declined. Eutrophication was certainly responsible for its extinction at other sites such as Balgavies Loch and Rescobie Loch, and in the Broads. Its decline in canals can be attributed to some falling into disuse and drying out, whereas the flora of others has deteriorated as they have become increasingly used for pleasure boating. It may, however, still survive in some canals for which we have no recent record. The mesotrophic drainage ditches in Broadland in which it survives are very vulnerable to eutrophication; many have become floristically impoverished in the last seventeen years in non-intensive grazing marshes, even in areas where management had been aimed at maintaining their botanical interest (Doarks 1990).

P. compressus is widespread in temperate Eurasia. In North America it is replaced by P. zosteriformis Fernald, which is regarded by some authorities as a subspecies of P. compressus

A similar species, P. acutifolius, is a plant of grazing marsh ditches which has also declined in Britain and now qualifies for inclusion in the Red Data Book.

 

C. D. Preston