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Taxonomy
The correct name for this species is Grasswrack Pondweed, Potamogeton compressus L., but the name P. compressus has, in the past, sometimes been used to refer to what is now called P. friesii, so care should be taken with literature records prior to about 1950. Older books often used the name P. zosterifolius Schumacher for P. compressus (Clapham, Tutin & Moore 1989).
Identification
Confusions about the identification of P. compressus were largely resolved by J.E. Dandy & Sir George Taylor in the mid-20th century, and definitive descriptions are given by Preston (1995). The keys in older works are unreliable. It is easy to confuse P. compressus with either P. friesii or P. acutifolius, and specimens should be collected whenever it is legal and responsible to do so.
Distribution
Grasswrack Pondweed occurs throughout Europe and Asia as far east as Japan, according to reports on the internet, but not in North America (Preston & Croft 1997).
In Britain it occurs in numerous sites in England but in just one lake in Scotland and one canal in Wales. A report compiled from the Threatened Plants Database, listing its known sites in 2008 is available for downloading here (pdf 246kb).
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Ecology
Preston & Croft (1997) describe its habitat as still or slowly flowing, calcareous, mesotrophic water. It is found in rivers, canals, ditches and lakes in the lowlands. Rodwell (1995) lists it as occurring in A11 Potamogeton pectinatus- Myriophyllum spicatum community.
Status
In the New Atlas (Preston, Pearman & Dines 2002) it is described as having declined over the last 150 years. It is Nationally Scarce in Britain (present in fewer than 100 hectads) (Stewart, Pearman & Preston 1994) and currently listed as Endangered (Cheffings & Farrell 2005). However, analysis of the data on the TPDB (see chart below) shows that the decline is less severe than previously thought. It turns out that P. compressus moves about, creating the impression of a decline. It was slightly more abundant in the mid-20th century than before or afterwards, due to the abandonment of canals, but it has never been a common plant. It is a Priority Species on the UK Biodiversity Action Plan.
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Go to the Maps Scheme map
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Further work
Because of its ability to colonise new sites, it is important to continue to search for it and record it carefully. Studies into its ecology are needed, especially for the sites where it has persisted for many years, as these appear to be exceptionally rich and important sites for many other aquatic species.
As a BAP species, it often receives ‘interventions’ of various sorts, including introductions and translocations. These should be documented on Conservation Evidence (see Markwell & Halls 2008 for an example).
References
Markwell H.J. & Halls J.M. (2008) Translocation of a nationally scarce aquatic plant, grass-wrack pondweed Potamogeton compressus, at South Walsham Marshes, Norfolk, England. Conservation Evidence 5, 69-73.
Preston, C.D. 1995. Pondweeds of Great Britain and Ireland. Botanical Society of the British Isles, London.
Preston, C.D. & Croft, J.M. 1997. Aquatic Plants in Britain and Ireland. Harley Books, Colchester.
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Above: the number of new sites found (top line) and the number of sites lost (middle line) each decade for the last 110 years. The bottom line shows the number of known extant sites in any one decade.
The graph shows a dip towards the end, but this may be partly due to the fact that not all sites have been resurveyed in the last decade.
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Send us your comments and Feedback
Chris John (2/2/2008): The Rochdale Canal Restoration aimed to address one of the old UK BAP targets i.e. ‘Facilitate Colonisation of New Site.’ The results of the translocation work have been reported directly to Natural England through the Rochdale Canal Review Reports, and have also been fed back to the steering group. It was a moderate success, with good populations present during the first 2 to 3 years, with populations tailing off as new habitats were colonised. This supports the view that the plant is a very mobile primary coloniser. In addition ECUS have recently recorded P. compressus from the Rochdale Canal in Failsworth.
The Montgomery Canal has been operated within levels agreed with Natural England, and this has never been exceeded. In addition, purpose-built reserves were constructed both in channel and off line, and since the restoration there has been a maintained improvement in the macrophyte assemblage.
The Grantham Canal has not been developed for boats, and is still non-navigable, although it was used by trailboats for a single weekend in 2007. A section was dredged at Harlaxton in 2006/07 and further dredging is planned for the future. This dredging programme is being undertaken as positive management for the site with agreement from Natural England, with the aim of improving the available habitat; it has also been identified as on one of the remedies for the SSSI length.
Natural England recently commissioned a research report on the effectiveness of translocation techniques that should help improve the chances of successful translocation projects. Information on project successes or failures can be reported through the steering group and reports such as that commissioned by Natural England. Negative results are equally valid when developing and considering the effectiveness of management and conservation options, and can help highlight where further information is needed.
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