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.
This species appears to have been in gradual decline for over a hundred and fifty years. It is almost extinct in lakes and rivers, and is declining in grazing marsh ditches. In Broadland populations have been lost since 1970. Some of the most vigorous surviving populations are in canals, especially the Montgomery branch of the Shropshire Union Canal.
Eurasian Boreo-temperate element.
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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
Perennation - primary
Life Form - primary
Clonality - primary
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
External Species Accounts
Scarce Atlas Account
Potamogeton compressus L.
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