No Potamogeton species is more characteristic of deep water than P. praelongus. It is a rhizomatous perennial that usually grows at depths greater than 1 m in clear, mesotrophic water in lakes, rivers, canals and major drains. It has only rarely been recorded from shallow water. 0-800 m (Loch Coire Cheap, Mid Perth).
P. praelongus appears to have been lost from many waters in the southern half of its range since 1930. The most likely cause of this decline is eutrophication. As a species of deeper water it can be inconspicuous, and it may be under-recorded in Scotland and Ireland.
Circumpolar Boreal-montane element.
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Light (Ellenberg): 7
Moisture (Ellenberg): 12
Reaction (Ellenberg): 7
Nitrogen (Ellenberg): 5
Salt Tolerance (Ellenberg): 1
January Mean Temperature (Celsius): 3.2
July Mean Temperature (Celsius): 14.1
Annual Precipitation (mm): 1178
Perennation - primary
Life Form - primary
Clonality - primary
Clonality - secondary
Count of 10km squares in Great Britain: 252
Count of 10km squares in Ireland: 90
Count of 10km squares in the Channel Isles: 0
Atlas Change Index: -0.26
Scarce Atlas Account
Potamogeton praelongus Wulfen
Status: not scarce
This species usually grows in water at least 1 metre deep, and can occur as a dominant below 1.5-3 metres. It is most frequently found in mesotrophic lakes, both in limestone areas and in lakes which are in predominantly acidic catchments but receive drainage from outcrops of base-rich rocks such as basalt or limestone. In Scotland and northern England it also occurs in slowly-flowing rivers and flooded quarries. Species which characteristically grow in the same water bodies include Potamogeton gramineus, P. perfoliatus and P. x zizii. In southern England and Wales P. praelongus is recorded from rivers, canals, the Broads and larger fenland drains. It grows in water bodies from sea-level up to 790 metres at Lochan an Tairbh-uisge.
It is a rhizomatous perennial which overwinters as leafy shoots. In some habitats the water is too deep for plants to flower but if plants do flower they fruit freely. Little information is available about its germination requirements.
P. praelongus has decreased markedly in the lowlands. Much of this decline has taken place since 1950: this species was frequent in the Cambridgeshire fenland in the 1950s (Perring, Sell & Walters 1964) but is now known from only two localities. It seems likely that eutrophication, perhaps coupled with unsympathetic management, is responsible for this dramatic decline, but there is only circumstantial evidence for this. As a deep-water species it does not grow in habitats such as grazing marsh ditches, in which other pollution-sensitive species have persisted after they disappeared from larger water bodies. It is still frequent in parts of Scotland, where it is probably somewhat under-recorded.
P. praelongus is widespread in northern and central Europe, extending north to Iceland and northern Scandinavia and south to France and Bulgaria. It has declined in some other areas of lowland Europe, including the Netherlands (Ploeg 1990). It is a circumboreal species, found in Asia and North America with an isolated occurrence in Mexico.
C. D. Preston