Differences in Potamogeton praelongus Morphology and Habitats in Europe

One of the most southern European occurrences of Potamogeton praelongus is in the Czech Republic (CR), with only one native population in the Orlice River floodplain in Eastern Bohemia, the only surviving site from 10 Czech localities known 45 years ago. is species is critically endangered in the CR and needs to be actively protected with a rescue program. e number of P. praelongus sites increases along a latitudinal gradient, from Central to North Europe (CR, Poland, Sweden, and Norway), and correlates with improving conditions (water transparency and nutrient content in water) for this species along this gradient. Although differences in site conditions between Central and North Europe were caused by changes in landscape geomorphology and vegetation during the glacial and postglacial eras, presently, anthropogenic impact is primarily observed. e Czech sites for P. praelongus have distinctly lower water depth and transparency, and conversely, higher conductivity, temperature, and shade levels than the Nordic ones. ese extreme conditions in most biotopes of this light-demanding and mesotrophic species in the CR result in lower fitness and different morphological parameters in the Czech populations. ese changes lead to decreased competitive ability against filamentous algae and more competitive aquatic plant species and increase the impact of animals. e Nordic nonintensively managed landscape provides numerous large, deep lakes, which are optimal for this aquatic species. In the CR, P. praelongus can currently only survive in rivers and their oxbows, but these are heavily influenced by intensive landscape management and fishing. Comparison of the populations and their site conditions in the CR and other European countries showed that all Czech native and artificially established sites are exposed to eutrophication and its consequences, which are the most important factors affecting the survival of P. praelongus.


Introduction
Long-stalked pondweed (Potamogeton praelongus) is a submerged perennial aquatic plant with a long perennial rhizome (Kaplan, 2010). e shoot length varies depending on water depth and transparency ; the seeds have a physiological dormancy and germinate aer stratification . Genetic analyses have revealed a very low genetic diversity in existing micropopulations of the Czech Republic (CR) (Kitner et al., 2013). e distribution of P. praelongus is classified as circumpolar and suboceanic. e species mainly grows in the boreal climatic zone, rarely extending beyond the polar circle. In Asia, it is found between the fiy-second parallel and the polar circle (Hultén, 1964). In America, it occurs in several parts of the US and Canada (Egertson et al., 2004). Some isolated localities exist in central Mexico (Hultén, 1964). e species was recently found in the western (Bennike & Anderson, 1998) and southwestern part of Greenland (Vöge, 2002). In Europe, the species prefers slightly alkaline water in Norway, Sweden, Denmark, Finland, and Chudskoe Lake (Peipsi) on the Estonia-Russia border. It also occurs in the Netherlands, northwestern Germany, the Spanish Pyrenees, and Slovenia. More than 100 sites have been recorded in Poland (Gąsiorowski & Kupryjanowicz, 2009).
Potamogeton praelongus occurs in lakes, river floodplains (river oxbows, pools), moderately flowing water, ponds, and reservoirs. It grows especially in unpolluted, mesotrophic, 0.2-2.0-m deep water on humic or sandy soils, and clayish, muddy, or peaty beds (Casper & Krausch, 1981). In Nordic countries, it grows in streams and lakes with high water transparency and mainly gravel bottoms in water up to several meters (ca. 4 m, own observation). In the CR, it usually grows in river oxbows or backwater pools in 0.2-0.7-m deep water, where it requires a moderate shade to prevent overheating . In Slovenia (Gaberščik et al., 2005), the species occurs in well-vegetated or slowly running, 2-3-m deep waters, even in flooded alluvial plains. It does not grow well in shallow, heated, and illuminated waters and prefers a 1-2-m water column (Hough & Wetzel, 1977). Ellenberg et al. (1992) mentioned that the species is light-demanding, only rarely growing in conditions with relative lighting less than 40%.
e response of aquatic plants to environmental variability has been intensively discussed during the last few decades (Bornette & Puijalon, 2011;Lacoul & Freedman, 2006). Different life forms of vascular plants use resources (e.g., light and nutrients) in specific ways and differ in their response to environmental changes (Akasaka & Takamura, 2011;Alahuhta & Heino, 2013;Netten et al., 2011;Vöge 2002). Ecological preferences, life forms, and the dispersal of particular plant species are used to investigate the use of functional traits of aquatic plants, as well as their morphology (such as leaf area and morphological index), to assess eutrophication in freshwater lakes (Stefanidis & Papastergiadou, 2019). Eutrophication, which results in high phytoplankton biomass and low summer water transparency, has led to problems not only in densely populated Central Europe but also in the Nordic part of Europe (Mäemets et al., 2010;Rintanen, 1996). Increasing eutrophication also correlates with the decline in aquatic plant species richness; e.g., in 100 Danish lakes and streams, the number of Potamogeton species has declined from 16 to nine (Sand-Jensen et al., 2000). Fast-growing species capable of regeneration aer disturbances (e.g., weed-cutting and dredging) prevail instead of the once-common Potamogeton species, which are typical of oligotrophic conditions (Sand-Jensen et al., 2000). As P. praelongus has a relatively high tolerance range, it is able to spread to localities that were once oligotrophic while disappearing from highly eutrophic waters (Mäemets et al., 2010). Underwater light availability is one of the most important environmental factors affecting the growth and morphology of aquatic plants ( Barko et al., 1986). In the case of low light availability in the water column, submerged macrophytes might adopt one of two distinct strategies: elongation of shoot length towards the water surface to alleviate low-light stress or enhancing low-light tolerance by photosynthetic adjustments (Chen et al., 2016).
Although underwater light availability in shallow waters is favorable, the high temperature of water during the growing season supports the expansion of hygrophytes, algae, plankton, and aquatic mosses instead of the less competitive P. praelongus (Prausová, 2016;. Strong eutrophication of shallow water bodies results in a large decline in macrophytes, caused by the intense shading from phytoplankton and filamentous algae (Lacoul & Freedman, 2006). Parts of P. praelongus shoots covered with filamentous algae necrotize, forming a smelly and muddy black layer of organic material on the bottom (Prausová, Kozelková, et al., 2015). e washing away of P. praelongus shoots and increased disturbance of the populations during high water flow and floods have also been recorded (Grinberga, 2010;Husák & Kaplan, 1997). e influence of animals is another factor limiting the growth of P. praelongus (Egertson et al., 2004;Prausová, Kozelková, Tomášová, Brodský, Havelka, Dvořák, et al. 2017;Prausová, Kozelková, et al., 2015).
Potamogeton praelongus is a critically endangered species in the CR (Čeřovský et al., 2001;Grulich, 2012). About 90% of the historic Czech localities have disappeared due to eutrophication, successional extinction, destruction of the natural habitats, the introduction of herbivorous fish, and intensive fishing, as well as water pollution from chemicals used on the surrounding agricultural landscapes (forests, gardens, and banks of water bodies) . erefore, only one native population currently exists, located in the oxbow of the Orlice River near Hradec Králové in Eastern Bohemia. According to Law No. 114/1992 Coll., on nature conservation and landscape protection, the species is listed as a critically threatened species in the CR. is species is included in e IUCN (International Union for Conservation of Nature and Natural Resources) Red List of reatened Species in the least concern (LC) category of species protection. According to the List, it is a widespread species with stable populations that are not in danger. However, the IUCN List also states that local loss of the species requires attention, and it is necessary to gather new information about the level of risk throughout its distribution range (Lansdown, 2014). In the European red list, the species is also in the LC category (Bilz et al., 2011) but is considered a critically endangered species in Central Europe (Pott, 1995;Schubert et al., 1995). Within the framework of a rescue program for P. praelongus, reintroductions in the CR have been currently restricted to the Orlice and Ploučnice basins. e species has been experimentally grown in several revitalized pools in the Protected Landscape Area of Kokorinsko in Northern Bohemia.
e first aim of this study was to find differences between site conditions in the optimal biotopes of P. praelongus in North-European countries (Norway, southern Sweden), Poland (also glaciated during the Last Glacial Maximum), and Czech sites with a decreasing population. Sweden, Norway, and Poland were selected for this study because of the high density of natural P. praelongus sites in these countries. e second aim was to compare morphometric characteristics of shoots and leaves of the species, indicating its adaptation to different stress factors, and links to several bioindicators of water trophy, such as the species composition of vascular plants, desmids, and cyanobacteria. As P. praelongus grows in extreme ecological conditions for submerged mesotrophic macrophytes in the CR, the third aim of this study was to provide recommendations on how to restore its last native site in the Orlice River floodplain.

Field Data Collection
Potamogeton praelongus sites were studied in 2014-2016 in four European countries ranging between 50-67°of latitude and 10-23°of longitude ( Figure 1). ese were localities in the CR (five pools and three river oxbows; observation for the whole growing seasons 2014-2016), southern Sweden (four lakes; September 16-18, 2014), Norway (14 lakes; July 18-26, 2015), and Poland (five localities: three lakes, one lido, and one river lagoon; August 26-27, 2016). Sweden, Norway, and Poland were selected for this study because of the high density of natural P. praelongus sites in these countries.
Water parameters (electrical conductivity, pH, dissolved oxygen content) were measured in sites with healthy P. praelongus populations by portable devices (Hach HQ 30d) at a water depth of 20-30 cm, water transparency by Secchi desk, and illumination by luxmeter (Voltcra LX-1108). Shade level (%) was calculated as the difference between 100% and the ratio (%) of illumination (lx) of the P. praelongus site to the illumination (lx) of an open, fully lighted place. In Sweden, Norway, and Poland, only one measurement was done at each site, while parameters were measured repetitively and subsequently averaged in the Czech localities.  Annual precipitation reflects the fluctuating water regime typical of the (mostly 2-month) hot and dry period in summer, observed in the last few years in the CR.
Potamogeton praelongus and other species of vascular plants were observed and determined in open water (from a boat) and the littoral zones. Undetermined species were gathered and identified by specialists (A. Hájek, M. Hroneš, and Z. Kaplan). e nomenclature of vascular plant species followed Danihelka et al. (2012) and e Plant List (2013). e depth of P. praelongus sampling for morphological measurements was site-specific. Morphological parameters of 15-20 individuals of P. praelongus at each locality were examined in situ and in vivo by measuring the internode length and the length and width of three leaves in the central part of a shoot. Leaf length was measured from the point of leaf intersection with the shoot to the leaf apex. e widest part of the leaf was measured for leaf width. All measurements were done on fresh biological material to an accuracy of 1 mm. All these parameters were selected because of their ability to indicate environmental variability and availability to be measured in all the studied sites (fertile individuals were not found at all localities).
To verify a preference of vegetative to generative reproduction, we also counted the number of germinating plants of P. praelongus, the determination of which was based on our knowledge of seed germination stages and plantlet growth from laboratory germination tests Prausová, Sikorová, & Šafářová, 2015).
Site conditions recorded in all localities (Table S1) were used for comparison between the Czech and the other localities. Since five Czech localities were monitored in 2014-2016, the mean values of the measured site condition were used.
Acta Societatis Botanicorum Poloniae / 2021 / Volume 90 / Article 901 Publisher: Polish Botanical Society 2.2. Statistical Analysis e environmental data collected to explain the differences between Potamogeton praelongus sites in North and Central Europe were statistically evaluated by principal component analysis (PCA). e environmental conditions were characterized by mean monthly temperature, annual temperature range, and annual precipitation [taken from the database WorldClim version 2 (Fick & Hijmans, 2017)], as well as pH, water transparency, shade level, electrical conductivity, water depth, and content of dissolved oxygen. Environmental conditions were used as response data aer logarithmic transforms due to nonnormal distributions. Data on the environment variables were centered; thus, no standardization was performed. Data locations were not centered or standardized.
e floristic data of vascular plants were coded as 1 or 0 for presence or absence, respectively. A preliminary survey of the data revealed the length of the longest gradient exceeded 4. e unimodal detrended correspondence analysis (DCA) method without any data transformation was selected as the most suitable to evaluate site similarities based on vascular plant species composition. TWINSPAN in JUICE 7 (Roleček et al., 2009;Tichý, 2020) was used to compare the vascular plant species composition of the localities according to the Sorensen index of dissimilarity (Maximum Dissimilarity Index 0.657) and created three clusters. A synoptic table was arranged according to the class of species frequency in the localities of each cluster, and only species occurring in frequency classes II-V (21%-100%) are shown in Table 1. e mutual similarity of localities based on species composition correlated with environmental data was tested by constrained multivariate analysis. Binary coded floristic data were projected on a graph based on environmental data, which included air temperature, water temperature, pH, water transparency, shade level, electrical conductivity, water depth, and dissolved oxygen content. When choosing a suitable method, the longest gradient length in the biological data, which exceeded 4 (exactly 4.63), had to be taken into account. Data were evaluated by canonical correspondence analysis (CCA) with a permutation test (999 permutation). Subsequently, forward selection was used to estimate the significance of individual environmental characteristics. All multivariate data were tested in CANOCO 5 (Ter Braak & Šmilauer, 2012).
On the basis of specific morphological parameters, the differences between localities were evaluated by one-way analysis of variance or a nonparametric version of this test (Kruskal-Wallis test). Several localities had to be excluded because of missing data for the morphological parameters (Kvitblikvatnet, Langen, Harasov, Hermanicky, Kasparovo Lake, Medonosy, and Tupadly).
e selection of the test was based on the analysis of the normality data distribution in each morphological parameter. If a nonnormal distribution of the data set was detected, a logarithmic transformation was performed. In case the transformation Acta Societatis Botanicorum Poloniae / 2021 / Volume 90 / Article 901 Publisher: Polish Botanical Society Table 1 Synoptic table of the most frequent species of aquatic plants in three groups of localities (see Table 3). Classes of species frequency, i.e., span of species frequency (in %) in particular classes (I-V) in brackets. Class I (0%-20%); Class II (21%-40%); Class III (41%-60%); Class IV (61%-80%); Class V (81%-100%).  described by the regression model was given by the coefficient of determination (R 2 ), while the statistical significance of the regression was tested by analysis of variance. All analyses of the morphological parameters were performed in NCSS (Hintze, 2001).

Site Conditions and Vegetation Characteristics
e PCA analysis revealed four well-separated groups of localities that differed in environmental conditions: (i) the first contained most of the localities from Norway and was characterized by a high content of dissolved oxygen and high pH; (ii) the second included localities from Sweden and the Norwegian locality of Dovatnet;  this group had a high water depth, and good water transparency; (iii) the Czech localities create a relatively compact group with high shade level, electrical conductivity, and water-and air temperature; (iv) the two Polish localities Jastrząb and Iłżanka, which are located between groups (i) and (iii). According to the species composition and site conditions, the last two groups can be merged into one group, having the highest trophy of the whole set. e first two axes of the PCA explained 75% of all variability of the data (Figure 2).
TWINSPAN analysis divided all 31 localities into three groups according to their similarity in species composition (Table 3, Table 1). e synoptic table (Table 1) shows the frequency of the aquatic plant species in each of these groups. Species that occurred in all three clusters included Potamogeton praelongus, P. natans, P. perfoliatus, Nuphar lutea, Elodea canadensis, and Stuckenia pectinata.
According to the DCA (Figure 3), the grouping of the localities based on particular site conditions slightly changes if species composition in the localities is considered. e first two axes in Figure 3  bodies. ese lakes with water depths of more than 1.5 m are characterized by a high content of dissolved oxygen and high pH (7.98-8.76), low conductivity (22-138.5 μS cm −1 ), and water transparency of 1.1-3.6 m. e mean monthly temperature median for all sites is 1.6°C, with the lowest from Arvillingen (−0.3°C) and the highest from Spanen (6.2°C). e most frequent aquatic plant species were Potamogeton alpinus, P. perfoliatus, Potamogeton ×nitens, and Myriophyllum alternifolium (Table 1). Potamogeton praelongus grows in Spanen Lake together with P. lucens, P. natans, Nymphaea alba, and Nuphar lutea. Several localities in Sweden (except for Spanen), Poland (Rogoźno, Uściewierz), and Norway (Yttre Svea and Byavatnet) belong to the second group, which is characterized by higher mean monthly temperature (median 6.2°C), lower pH (7.4-8.05), higher conductivity (54.7-144.7 μS cm −1 ), and shade level of 8%-43% during observations in July. ese localities are typified by having deep water (1.5-4.5 m) and excellent water transparency (up to 3.6 m). Some P. praelongus lake localities are more shaded by woody vegetation around the lakes (Langen 35%, Trummen 43%). e most frequent aquatic plant species were Potamogeton natans, P. obtusifolius, and Nuphar lutea (Table 1). Potamogeton praelongus grows with Nymphaea candida, Hydrocharis morsus-ranae, Myriophyllum spicatum, Utricularia australis, and Stratiotes aloides in the Polish localities of Rogoźno and Uściewierz. All these localities are affected by fishing and recreation. e P. praelongus population in Rogoźno Lake is influenced by ducks, while the population in Trummen Lake in Sweden is threatened mainly by white fish and crayfish. Trummen Lake is located in the surroundings of Vaxjo, and its current eutrophication is caused by past dumping of local waste from a hospital and several factories. On the other hand, oligotrophic species like Isoëtes lacustris, Littorella uniflora, and Lobelia dortmanna were observed in the locality of Sodra Vixen.
e Czech locations (black points in Figure 2) are obviously distinct from the others; they were more shaded and warmer (mean monthly temperature median of 8.2°C, observed water temperature in July 16.9-21.8°C) with very high electrical conductivity (275-410 μS cm −1 ) during the observations in July. High values of both these water characteristics (temperature 16-26°C, conductivity 192-333.5 μS cm −1 ) were also found in three Polish locations, Iłżanka, Jastrząb, and Rotcze, in July. Only one Norwegian location (Dagsjoen) belongs to this group because of its high conductivity (303 μS cm −1 ). e highest content of dissolved oxygen and the highest pH value was detected in the lido of Jastrząb, which is the most exploited for recreation among the Polish localities in the study. All the locations in this group are the most affected by anthropogenic factors in the whole set and are dominated by natant or unrooted macrophytes (Potamogeton natans, P. berchtoldii, Stuckenia pectinata, Elodea canadensis, Ceratophyllum demersum, Lemna minor, and Persicaria amphibia; Table 1). Potamogeton praelongus grows there also together with Nuphar lutea, as well as Potamogeton crispus and Callitriche spp. in the Czech localities. We observed animal grazing of P. praelongus in all Czech and Polish study locations of this group.
Site conditions significantly impacted the species diversity of vascular plants in particular localities (Figure 4). e first two axes explained 16.14% of the total variation, and these axes also explained 45% of the variability described by the explanatory variables. e explanatory variables affect 35.8% of total data variability. e CCA analysis was statistically significant, with the permutation test of the first axis F = 1.9, p = 0.003 and all axes F = 1.3, p = 0.002. e most shaded populations (average 60%) impacted by higher water temperature (average 18°C) and electrical conductivity (average 360 μS cm −1 ) are in the CR. By contrast, the Norwegian localities are less shaded (average 5%), have lower nutrients (average conductivity 114 μS cm −1 ), and are cooler (average temperature 15°C). Conductivity, shade level, and their interaction are significant factors, while the least significant factors are pH and the content of dissolved oxygen in the water (forward selection). All considered variables together explain 35.8% of the total variation. Of this variation, conductivity contributes 22.8% (F = 2.3, p = 0.001), shade 18.5% (F = 18.5, p = 0.002), air temperature 17.3% (F = 1.7, p = 0.002), water temperature 16.8% (F = 1.7, p = 0.008), and water transparency 15.9% (F = 1.6, p = 0.13). Meanwhile, water depth contributes 15.5% (F = 1.5, p = 0.054), pH 12.1% (F = 1.2, p = 0.219), and oxygen 8.6% (F = 0.8, p = 0.699).

Morphological Parameters of Potamogeton praelongus
Plants in different localities differed significantly in all measured parameters (Table 2, Figure 5). e Czech individuals of Potamogeton praelongus had the widest leaves and the largest leaf area. On the other hand, the length of their shoot internodes was the shortest, as they grow in shallow waters. In the Nordic localities,  P. praelongus individuals have longer leaves and longer shoot internodes; the longest ones were found in the Norwegian Lake Tolkvatnet, where individuals were collected from water at depths of 3-4 m.
Linear regression revealed that morphometric parameters of P. praelongus individuals were slightly influenced by some water site conditions ( Figure 6). e regression model was significant in the case of water conductivity and transparency. Conductivity affected both leaf length (R 2 = 0.256, F = 7.569, p = 0.012) and width (R 2 = 0.173, F = 4.603, p = 0.043), while water transparency was correlated with leaf length (R 2 = 0.175, F = 4.658, p = 0.042).

Sensitivity to Changes in Site Conditions and Impact of Other Organisms
While suitable localities for Potamogeton praelongus have nearly disappeared in the CR, this species survives in Northern Europe in large populations that are noticeably different from those in the CR. In the mid-Holocene, conditions in Central Europe were like the Nordic sites, as confirmed by achenes found in sediments of a Holocene lake near Trebon in South Bohemia (Jankovská, 1980), in the Holocene lake of Sur in western Slovakia (Petr et al., 2013;Potůčková, 2015), in Jezioro Lake in south Poland (Fajer et al., 2012), and in lakes in Greenland (Eisner et al., 1995). Bennike and Anderson (1998) suggested that temperature changes with increased extremes were the factors that caused P. praelongus to disappear aer the early and Acta Societatis Botanicorum Poloniae / 2021 / Volume 90 / Article 901 Publisher: Polish Botanical Society Figure 5 Differences in morphological parameters of Potamogeton praelongus in particular groups. Letters a, b, and c characterize group differences based on multiple comparison tests (Kruskal-Wallis Z test with Bonferroni correction). Grouping of study sites follows that shown in Table 3. mid-Holocene in the central part of East Greenland because of subsequent changes in water chemistry.
Seasonal temperature stratification during the growing season is known to occur in large ponds and lakes (Sculthorpe, 1967) while, in small water bodies, water temperature is influenced by many factors like their size, water input, springs, and groundwater fluxes (Sakura, 1993).
In our study (July 2014(July , 2015, water temperature in the Potamogeton sites ranged from 16 to 26°C in the Czech and several Polish localities, and from 12.9 to 17.2°C in the Nordic ones. For example, the summer median water temperature was 14.8°C in Danish lakes (Lauridsen et al., 2015). e highest water temperature in the Czech localities was detected in shallow backwater pools (temperatures >25°C continuing for several days), where damage to shoots by overheating was visible (own observations). Vital and undamaged shoots were observed only in the deepest part (more than 1 m) of the backwater pool at Stampach in the Protected Landscape Area (PLA) Kokorinsko (Prausová, Kozelková, Tomášová, Brodský, Havelka, Dvořák, et al., 2017). We observed declining water depth during the summer at all Czech localities, which was reflected in higher water temperature and lower transparency and related to the quantitative increase of phytoplankton and filamentous algae. Increased temperature may lead to prolonged and more intense anoxia phases (Portnoy, 1991), which may stimulate microbial processes (Mulholland et al., 1997) Acta Societatis Botanicorum Poloniae / 2021 / Volume 90 / Article 901 Publisher: Polish Botanical Society Figure 6 Linear regression between site conditions (x axis) and morphometric parameters (y axis), only cases where the regression model was statistically significant are shown. and increase nutrient availability (Pant, 2007), which is related to macrophyte stress (Weltzin et al., 2003). is situation may result in a rapid succession in small backwater pools over a short time, as we observed in PLA Kokorinsko (Harasov, Plesivec, Stampach, Tupadly).
Shading of the water surface was one of the most statistically significant factors in this study. Our measurements showed that P. praelongus prefers sites with more than 40% light. Extensive shading of the water surface decreases the probability of aquatic plant occurrence. On the other hand, submerged macrophytes in shallow lakes can form dense vegetation that controls the phytoplankton biomass through several mechanisms, including allelopathic interactions (Gross, 2003), nutrient limitation (van Donk et al., 1993), and shading, as well as by sheltering the zooplankton that can graze on and reduce the phytoplankton (Burks et al., 2002;Schriver et al., 1995;Timms & Moss, 1984).
In most localities, P. praelongus grows in neutral to slightly alkaline waters (the CR: pH 7.1-8.4; Poland: pH 8.2-9.8; Norway and Sweden: pH 7.4-9.4). e chemical reaction of water (pH) and water movement are related to the availability of carbon dioxide in water by maintaining the equilibrium between inorganic carbon and carbon dioxide (Bornette & Puijalon, 2011). Potamogeton praelongus is able to use HCO 3 − ions efficiently as a source of carbon for photosynthesis, which gives it an advantage when growing in hard water (Husák & Adamec, 1998). In all Czech localities, calcium concentration ranges from 50 to 60 mg L −1 . Total alkalinity higher than 1.2 mekv L −1 is beneficial for this species (Husák & Adamec, 1998).
A high ion content may signify not only hard but also eutrophicated water. e highest conductivity was measured in the Czech localities (275-401 μS cm −1 ) and the Polish Iłżanka lagoon (333 μS cm −1 ), while values of 192-281 μS cm −1 were measured in other Polish localities. While eutrophication of the Czech localities and the Polish Iłżanka lagoon comes from the surrounding agricultural landscape, the Polish locality lido of Jastrząb is affected by recreation activities. Eutrophic water in the Czech P. praelongus localities is determined by N and P concentrations, with (NO 3 − ) N concentration between 0 and 14 mg L −1 , and (PO 4 3 − ) P concentration between 30 and 1,000 μg L −1 . Conductivity values are significantly lower in the Nordic sites, e.g., 99.4 μS cm −1 in Finland (Rintanen, 1996), and 70.5-137 μS cm −1 in Greenland (Vöge, 2002). In our study, lakes with the highest trophy in Norway and Sweden were distinguished by having an electrical conductivity higher than 100 μS cm −1 (10 lakes), while it was less than 100 μS cm −1 in lakes with the lowest trophic level (eight lakes).
We observed the negative influence of animals (mainly grazing) on P. praelongus in several of the study locations. It is known (Husák & Kaplan, 1997) that the last but one native population of P. praelongus in the CR (Jezuitske Lake) was destroyed by Ctenopharyngodon idella. e current Czech and several Polish locations for P. praelongus are endangered by aquatic birds (especially ducks), while the population in Trummen Lake in Sweden is threatened mainly by white fish and crayfish (Strand, 2017). Although ducks grazed P. praelongus in other Polish localities (e.g., Rogoźno Lake), only the upper parts of shoots were damaged, owing to the depth of the water.

Species Composition and Biodiversity of Plant Communities
A low number of vascular plant species is a typical feature of the association of Potametum praelongi Hild 1959 in the frame of the Potamion Miljan 1933 alliance (Šumberová, 2011). Based on the similarity of vascular plant species composition in the 31 studied localities, we determined three groups that differed in site conditions, mainly electrical conductivity, water depth, and transparency. In nutrient-rich conditions, competition for light between phytoplankton and macrophytes results in reduced cover and even the disappearance of macrophytes (Declerck et al., 2005;Muylaert et al., 2010;Scheffer et al., 1993). Potamogeton praelongus grows in water bodies with water depths of 0.26-1.05 m in the Czech localities, while water depths of 1.5-4.5 m were found in most of the observed Nordic ones. Species typical of shallow waters can be found in most of the Czech localities, e.g., Callitriche sp., Calliergonella cuspidata, Batrachium sp., Lemna sp., Spirodela sp., while Potamogeton species, Myriophyllum alterniflorum, and Nuphar pumila grow in deep waters in the Norwegian and Swedish lakes. Czech natural localities, e.g., river oxbows, have similar site conditions and macrophyte plant communities to the most eutrophicated Polish localities (Iłżanka lagoon and the lido of Jastrząb), and one part of the Swedish restored site in Trummen Lake (Strand, 2017). e decline of P. praelongus in eutrophic sites is connected to the spread of competitive macrophytes, like Nuphar lutea (Prausová, 2016;Prausová, Kozelková, Tomášová, Brodský, Havelka, Dvořák, et al., 2017) and littoral species like Alopecurus aequalis, Mentha aquatica, Ranunculus flammula, Veronica beccabunga, or Phragmites australis in small and shallow pools, as is known from Poland (Nowak & Nowak, 2004), the CR (Prausová et al., 2011;Prausová, Kozelková, Tomášová, Brodský, Havelka, Dvořák, et al., 2017), Finland (Rintanen, 1996), Estonia (Mäemets et al., 2010), and Lithuania (Balevičienė & Balevičius, 2006). ere has been a continual expansion of the broadleaved Nuphar lutea in the last native Czech locality of P. praelongus, spreading over more than 70% of the water surface. e same situation was found in Kasparovo Lake with a reintroduced population of P. praelongus in the Orlice River floodplain (Prausová, 2016;Prausová, Kozelková, Tomášová, Brodský, Havelka, Dvořák, et al., 2017). In artificial backwater pools planted with a P. praelongus outdoor rescue culture in PLA Kokorinsko, the succession was considerably faster than in river oxbows or lakes, with P. praelongus, P. natans, or P. lucens being suppressed by Calliergonella cuspidata in 7-10-year-old backwater pools (Prausová, Kozelková, Tomášová, Brodský, Havelka, Dvořák, et al., 2017).

Morphological Adaptations and Probability of Generative Reproduction in Relation to Site Conditions
Elongated shoot height and increased specific leaf area are morphological adaptations of macrophytes to increasing water depth (Fu et al., 2012(Fu et al., , 2014Maberly, 1993). Leaf area could be an indicator of light-limiting conditions favoring plants with a large leaf area, such as large-leaved or dense-leaved plants, over plants with a small leaf area (Stefanidis & Papastergiadou, 2019). In our study, Central European and Nordic P. praelongus individuals significantly differed in their shoot length, the shape, width, and length of leaves, and length of shoot internodes. Czech individuals had the widest leaves and the largest leaf area probably because of low water transparency and the shortest shoot internodes due to the low water depth in all localities. Exposure to currents or waves strongly influences plant growth, development, reproduction, and clonal growth (Doyle, 2001;Strand & Weisner, 2001). e minimum light requirement of submerged macrophytes is linked to their growth form (Middelboe & Markager, 1997). e length of submerged shoots is limited by water depth; thus, the internodes and whole shoots are considerably shorter in Czech than in Nordic individuals. Leaf length was significantly influenced by conductivity and water transparency, while leaf width was affected by electrical conductivity and the level of shading of the surface. High conductivity probably indicates better plant nutrition and related intensive growth, which can be reflected in leaf size. Wider leaves and larger leaf areas are important to ensure enough insolation for photosynthesis in the Czech localities.
Although the reproduction of P. praelongus was not a focus of our study, we observed that the Nordic populations rarely reproduced with achenes. Although achenes of P. praelongus gathered in Czech localities can germinate aer interruption of their dormancy Prausová, Sikorová, & Šafářová, 2015), seed germination success and subsequent growth of the plantlets in natural sites in Central Europe is expected to be relatively low because of low water transparency, threats by herbivores and competitive organisms (algae, mosses, vascular plants, ducks, and fish), and disturbances (such as floods). However, small plantlets that had germinated from seeds were observed in natural localities in Norway (Prausová, Kozelková, Tomášová, Brodský, Havelka, Pitelková, & Hašler, 2017).

Recommendations for Nature Conservation in the CR
In the CR, rescue programs to support the most endangered species, like P. praelongus, are on-going (Prausová, Kozelková, Tomášová, Brodský, Havelka, Dvořák, et al., 2017). e effectiveness of the programs depends heavily on better knowledge of the life-history traits of the species, which will be of great importance for environmentally friendly waterways engineering (Bornette & Puijalon, 2011). Activities implemented as part of the rescue program include not only site-and population monitoring and selection of potential localities for planting P. praelongus shoots from in-vitro culture and rescue cultivation but also protection of the last native site of this species in the CR. Temporarily Protected Area has been incorporated into the Natura 2000 network as the Natural Monument Orlice since 2017. Revitalization of this site was realized in winter 2018/2019. e aim of the revitalization was to decrease the trophic level by the excavation of mud and efficient protection against pollution from various sources (e.g., wastewater treatment plants), to differentiate water depth, eliminate incoming sediments, and reduce the cover of competitive macrophytes, riparian vegetation, and shading trees.

Supplementary Material
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