Long-stalked Pondweed (P. praelongus) distribution is classified as circumpolar and suboceanic. In Europe, it grows in its northern part, it is completely missing in the Mediterranean; in similar latitudes it grows in Asia and North America. It mainly grows in the boreal climatic zone, it hardly extends beyond the polar circle.
In the past, this species grew in several localities from lowlands to highlands. In thermophyticum, it was found in the Vltava River in Prague: the Vltava near Holešovice, Podolí, Zlíchov, Braník. The species was observed there in 1983 for the last time. In phytochorion Hradecké Polabí, it grew in oxbows of the Orlice river in Malšovice and Malšova Lhota. The locality in Malšova Lhota is called Jezuitské jezero lake and it was the penultimate Czech native locality of P. praelongus. In 1987, there was about 100 000 stems of P. praelongus; unfortunately, during the next two years this population became extinct due to eutrophication.
In mesophyticum, this species was known in Ralsko-bezdězská tabule, i.e. in the Ploužnický stream and the Ploužnický pond near Mimoň (before 1878), in the Českolipská kotlina, i.e. in the Ploučnice river near Česká Lípa (1894) and Zákupy – Veselí (1921) and Mimoň (around 1848). In east Bohemia, P. praelongus grew in the phytochorion Týnišťský úval in the oxbow lake of the Orlice river between Blešno and Nepasice (1982), and in the oxbow lake near Albrechtice nad Orlicí (1909). Records of this species presence in the Třesický pond near Kosičky in the Bydžovská pánev and in the Strašovský pond near Strašovice in the Pardubické Polabí are unreliable.
In southern Bohemia, the species was observed in the Budějovická pánev in Putim (1871), in the Střední Povltaví in the Otava river and in ponds (in the Jistec pond it was observed before 1887, in Martínka settlement near Písek it was observed in 1924 and in Písek in 1923).
Many records about the species occurrence are incorrect: the Ohře and Teplá rivers by Karlovy Vary, the Ohře river by Cheb, Podrudohoří, Běleč nad Orlicí in east Bohemia, Žďár nad Sázavou, and Frýdlant in the Frýdlant highlands. In most of these cases, it was confused with Potamogeton alpinus.
Potamogeton praelongus is currently only found in one native locality (the Orlice river oxbow near Hradec Králové – transient protected area Rameno u Stříbrného rybníka). As a part of the rescue programme, micorpopulations were introduced into new localities (another Orlice river oxbow called Kašparovo jezero and the Ploučnice river). There are two recent rescue cultures of P. praelongus in the Kokořínsko region in central Bohemia and in the Institute of Botany in Třeboň (IB, CAT). In 2007, it was found in the Orlice river, too, but later this occurrence was not confirmed.
The last native locality of the species is the Oxbow at the Stříbrný rybník Pond near Hradec Králové in the Orlice floodplain. P. praelongus grew on the right side of the oxbow outfall into the Orlice River in 1996, and its cover of about 20 m2was spreading into the Orlice River. Another cover of about 5 m2 existed on the left side of the oxbow in about 30 m distance from its outfall. Individual shoots of P. praelongus were disseminated in covers of other aquatic macrophytes, mainly of P. alpinus and Nuphar lutea. A similar condition of this locality was found in 2010–2012, when P. praelongus population was at its maximum concerning the density and the degree of coverage. Apart from coherent polycormons, there were individuals disseminated among Nuphar lutea plants near the first hook of the oxbow. In 2000, P. praelongus population size was about 30 shoots. In the winter of 2001–2002, dry mud excavation was carried out below the outfall of the Stříbrný potok stream and in 2003, another mud excavation was done between the first hook and the outfall into the Orlice River, where several P. praelongus individuals had been outplanted. Before starting the excavation, those individuals were taken out and thereafter they were returned. In 2004, their surviving was checked and confirmed.
Development of the population density after the mud excavation was positive until 2012, with a sharp increase in 2008–2009 from fewer than 100 shoots to almost 500 ones and in the next year to 1461 shoots, thereof 99 fertile. Yet in 2013, the density dropped to 357 shoots and this decreasing process has been going on until today. In 2014, the density was about the same as in 2000, when preparation of the rescue program had begun. At present, individual, not vigorous shoots are recorded. The reason is considered inconvenient long-term conditions in the TPA (insufficient water transparency of 20–55 cm) and intensive damage to plants by ducks and other organisms. High content of nutrients in the water and sediment is caused by clogging with organic material,coming from dead aquatic and wetland plants and leaf fall from riparian vegetation. Although P. praelongus tolerates high content of nutrients in the environment, it is damaged by anoxic conditions and toxic gases released by muddy sediment (hydrogen sulphide, methane). Another threat arises from overpopulation of plankton decreasing the water transparency and overpopulation of filamentous algae (due to synergic effect of high temperature and high content of nutrients) covering the shoots of P. praelongus. The recent critical condition of the population is connected not only to gradual clogging, but also to increasing shading by riparian vegetation, and to fluctuation of water regime caused by climatic changes; floods and persisting turbidity of water prevent light from access to the plants, which is especially important at the time of intensive growth and creating generative organs.
Outside of the rescue program, P. praelongus was outplanted into several revitalised pools in the PLA Kokořínsko: a pool above Harasov Pond (2001), a pool in the floodplain of the Liběchovka (2001), pools below Plešivec (2002), a backwater pool at Štampach (2003), the pool at Medonosy (2010), and a pool at Tupadly (2013). Long-term monitoring of the populations in Kokořínsko revealed that their condition depends on the succession stage of the pools. In most of pools with sufficient depth (0.5–1.2 m), the population density was highest in the 5th–7th year after outplanting. Then the population decreased to tens of shoots and further to individual shoots. In small and shallow pools (at Medonosy, possibly also at Tupadly), P. praelongus reached its maximum in the 3rd year, but then disappeared later, weakened by warmer water and outcompeted by competitively stronger species.
The Kašparovo jezero Lake is a parapotamon-type right-side oxbow of the Orlice River, connected to the river in its lower part. In its morphology, it is similar to the TPA, where P. praelongus has survived so far. Although the upper part of the Kašparovo jezero Lake is considerably clogged and shaded, the lower part is influenced by streaming water. These conditions, comparable with those in the TPA, enabled rooting to 10 shoots outplanted in 2008. They developed successfully there during the following two years. The population decreased in 2011 due to heavy browsing by grass carp and inconvenient interventions (probably including pulling the plants out). Only 3 shoots were found in 2015, and only one in 2016.
Suitable localities for P. praelongus were searched for along the stream of the Ploučnice, where the species formerly grew. In 2010, ten shoots were outplanted into an oxbow near a road bridge at Heřmaničky. This population thrived well, and thanks to reinforcing it in the following years, it reached around 100 shoots in 2016. Outplantings right into the Ploučnice River were not successful because of strong stream and floods. This oxbow is more convenient for the species in spite of other limiting factors, e.g. browsing by ducks and swans, damage by canoeists pulling canoes onto the bank, and interests of fishermen.
In Europe, it occurs from Iceland, Scandinavia and northern part of European Russia southwards to the British Isles, France, northern Italy and Slovenia, northern Ukraine and central part of European Russia up to the 54th parallel.
The species occurs abundantly in Norway, Sweden, Denmark, Finland, in the border area between Estonia and Russia in the Chudskoe Lake (Peipsi). It also occurs in Netherlands, in streams of northwest Germany, in lakes in Spanish Pyrenees and in Slovenia. P. praelongus is of least concern as a critically endangered species in the whole central Europe.
In Poland, slightly over 100 sites with P. praelongus were noticed in total (during 1945–2001). The species occurs mainly in the northern part of Poland (Pomerania, Warmia – Mazury Wielkopolska, Podlasie and Wielkopolskie regions), especially near Gdaňsk and in the northwest part near Białystok, it is quite abundant also in the Lublin region in the East near the border with Ukraine and Belorussia. A rare occurrence was recorded in the central and southern parts of the country. It is regarded as critically endangered in the Opole province. It is disappearing also from the Lower and Upper Silesia. This species was recorded in the Wroclaw glacial valley in Lower Silesia and in the northern part of Silesia (region Wyżyna Śląska).
In the Czech Republic, there is the south border of the world area of this species and recently there exists only 1 native population in the Orlice river floodplain. There are less convenient site conditions in the Czech Republic and thus P. praelongus occurrence there probably has a relict character. Presence of this species was confirmed in sediments of the Holocene lake Šúr in west Slovakia. It grew in the lake, which was oligo-mezotrophic, together with other macrophytes (Chara sp., Batrachium sp., Potamogeton filiformis, Myriophyllum spicatum).
In Asia, it grows between the 52nd parallel and the polarcircle, especially in Siberia, eastwards to the Chukchi Peninsula, Kamchatka Peninsula, Kuril Islands, in the southeast up to Hokkaido. Its isolated localities exist in southwest Asia in northeast Turkey and Armenia. The species is known from the Baikal Lake and the Erhai Lake in China, too.
In North America, it occurs from Aleutian Islands and Alaska to central California, in the southwest to Utah and in the east to Newfoundland and New Jersey, Minnesota, USA and Alberta in Canada. In America, the species abundanceis declining, too. Isolated localities exist in central Mexico.
In Greenland, recent occurrence is known in its west and southwest part. Former occurrence in other lakes of the island was confirmed by analysis of macrofossils in the lake sediments.
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