Potamogeton Species Research Articles

Identification of the Polar Constituents of Potamogeton Species by HPLC‐UV with Post‐Column Derivatization, HPLC‐MSn and HPLC‐NMR, and Isolation of a New ent‐Labdane Diglycoside.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Molecular phylogeny of Japanese Potamogeton species in light of noncoding chloroplast sequences

In the genus Potamogeton, high morphologic and ecologic diversification and difficulty in evaluating systematic usefulness of various characteristics have led to the definition of many small groups of species corresponding to sections or subsections. The 18 species of Japanese Potamogeton that have high variation represent 11 of the 14 species groups proposed in a recent treatment of the genus. We performed phylogenetic analyses of these 18 species of Japanese Potamogeton with three allied genera by using the noncoding region of chloroplast DNA between trnT (UGU) and trnL (UAA). Our phylogenetic tree revealed that Japanese Potamogeton were divided into two main groups (I and II). In general, each group was characterized by the shape of the submerged leaves and anatomic features of the stem. Group I comprised eight species with broad submerged leaves, and these species lacked sub-epidermal bundles in the stem. Group II comprisesd 10 species characterized by linear submerged leaves and the presence of sub-epidermal bundles. Group II was further divided into two subgroups (IIa and IIb). Subgroup IIa taxa did not have a dormant period or physiologically specialized turions, and the stele was of the trio or proto-type. In comparison, subgroup IIb members have dormant period in the form of axillary or apical turions, and stele types were oblong or circular. Potamogeton crispus and P. maackianus, each of which has distinct morphology and ecology, are included in group I but differed from the other Potamogeton in having unique length mutations and substitutions in the trnT– trnL sequence.

Effects of sediment nutrients and depth on small-scale spatial heterogeneity of submersed macrophyte communities in Lake Pleasant, Pennsylvania

Sediment concentrations of total and available nitrogen (N), phosphorus (P), and potassium (K) and organic matter from the littoral zone of Lake Pleasant, Pennsylvania, were highly variable. Only organic matter and total N were correlated with depth, however. This result suggests the existence of more complex environmental gradients than the prevailing paradigm of monotonic changes in sediment characteristics with increasing depth. The spatial heterogeneity of submersed aquatic plant communities was significantly correlated with depth, and available N and P. Canonical correspondence analysis demonstrated that these three factors explained 38% of the variance in community structure. Other sediment characteristics (available K, organic matter, and total N, P and K) were not significant by themselves, but all variables combined explained 63% of community-structure variance. Cluster analysis identified species or groups of species typical of endpoints on the depth versus nutrient axes. Myriophyllum exalbescens was typical of deep sites with relatively nutrient-rich sediments, whereas deep nutrient-poor sites were dominated by Vallisneria americana and Megalodonta beckii. Shallow nutrient-rich sites were dominated by several species of Potamogeton and Elodea canadensis, and shallow nutrient-poor sites were dominated by Heteranthera dubia and Najas flexilis. These results demonstrate the importance of sediment characteristics in determining macrophytes' community structure within lakes.

Identification of the polar constituents of Potamogeton species by HPLC-UV with post-column derivatization, HPLC-MS n and HPLC-NMR, and isolation of a new ent-labdane diglycoside

The polar extracts of Potamogeton pectinatus, P. lucens, P. perfoliatus and P. crispus (Potamogetonaceae) were analyzed by HPLC-UV-MS and their chromatographic profiles were very similar. The polar constituents of P. pectinatus were more exhaustively investigated by HPLC-UV with post-column derivatization, HPLC-MS n and HPLC-NMR, which allowed the on-line identification of various known flavones (dereplication). One of these compounds, luteolin 3′- O-glucoside, has never been characterized in the Potamogeton genus. The HPLC-UV-MS and HPLC-NMR analyses revealed also the presence of ent-labdane diterpene glycosides in the polar extracts of P. pectinatus and P. lucens and led to the isolation of a new ent-labdane diglycoside from P. pectinatus, β- d-glucopyranosyl-(1 → 2)-β- d-glucopyranosyl-15,16-epoxy-12-oxo-8(17),13(16),14- ent-labdatrien-19-oate.

A Century of Change in Macrophyte Abundance and Composition in Response to Agricultural Eutrophication

Clear Lake, Iowa, USA is a shallow, agriculturally eutrophic lake that has changed drastically over the past century. Eight macrophyte surveys since 1896 were pooled and examined to characterize long-term impacts of eutrophication on macrophyte community composition and relative abundance. Surveys in 1981 and 2000 revealed few submergent and floating-leaved species and a dominance in emergent species (Scirpus, Typha). Over the past century, however, species richness has declined from a high of 30 species in 1951 to 12 found today, while the community composition has shifted from submergent-(99%) to emergent-dominated floras (84%). Potamogeton praelongus was the first emergent species to disappear but was followed by several other clear water Potamogeton species. Several floating leaved and emergent genera increased in relative abundance with eutrophication, notably Nuphar, Nymphaea, Phragmites, Polygonum, Sagittaria, Scirpus, and Typha. P. pectinatus was present over the entire century due to its tolerance of eutrophic conditions. Macrophyte growth was generally light-limited, with 93% of the variance in relative abundance of submergent species explained by changes in water transparency. Clear Lake exhibits signs of alternative stable states, oscillating between clear and turbid water, coupled with high and low submerged species relative abundance. The maximum macrophyte richness occurred as the lake oscillated between submergent- and emergent-dominated states. Changes in the water level have also impacted macrophyte growth since the area of the lake occupied by emergent macrophytes was negatively correlated with water level. Strongest correlations indicated that macrophytes respond to water level variations with a 2-year time-lag.

Ent-Labdane glycosides from the aquatic plant Potamogeton lucens and analytical evaluation of the lipophilic extract constituents of various Potamogeton species

Two new ent-labdane glycosides, one known furano- ent-labdane and a new hydroxylated fatty acid were isolated from the dichloromethane extract of the freshwater aquatic plant Potamogeton lucens. The new compounds were assigned the structures of β- d-glucopyranosyl-8(17),13 -ent-labdadien-16,15-olid-18-oate, 18-β- d-glucopyranosyloxy-8(17),13 -ent-labdadien-16,15-olide and 13( R)-hydroxy-octadeca-(9 Z,11 E,15 Z)-trien-oic acid by spectroscopic means. The algicidal activity of these compounds was tested against Raphidocelis subcapitata. Based on our previous study of Potamogeton pectinatus, other constituents were identified in P. lucens by LC–UV–MS, LC–NMR and GC–MS. The lipophilic extract profiles of both species are presented. Two other species, Potamogeton perfoliatus and P. crispus, were also investigated by analytical comparison of their non-polar extracts. The distribution of ent-labdanes characterized in Potamogeton is summarized.

Long-term changes of submerged macrophytes in the Lower Danube Wetland System

Transition towards hypertrophy has affected biodiversity and productivity of most aquatic and wetland systems in the Lower Danube Wetland System (LDWS) over the last two decades. The aquatic macrophytes have been deeply involved in ecosystem reorganization in these circumstances. Representative shallow lakes and channels located in the river floodplain and delta were studied in this period in terms of space distribution, diversity, species composition, primary production and main hydrogeomorphic features (morphometrical and physico-chemical parameters). Changes in submerged vegetation dynamics along two successive decades after 1980 included diminishing areas of about 50% in the Danube Delta, restructuring primary producers by suppressing aquatic weed in some lakes or parts of lakes and decreasing species richness to few populations with upright and floating growth strategy. Biomass production showed different trends, from severe reduction in some lakes and periods to marked increase in other ones, provided by a changed and simplified species structure, generally dominated by Ceratophyllum demersum L. and Potamogeton species. It was concluded that the eutrophication effects are maintaining, modulated by diverse response mechanisms developed by submerged macrophytes. Finally, the main lessons to be learned for the management of aquatic macrophyte-dominated systems in the framework of redesigning the LDWS structures are emphasized on the basis of a large scale and long-term prospect of the Danube River and Black Sea Basins.

Effects of Elevated CO2 Concentration on Phenotypic, Physiological and Biochemical Characteristics of Submersed Plant Potamogeton crispus Leaf

The effects of elevated CO 2 concentration on growth and development of terrestrial plants have been studied intensively, but less scientific interest h as been raised on aquatic plants. In this study, a greenhouse experiment with su bmersed species Potamogeton crispus, one of dominant species in lakes along Yangtze River Basin, was studied comparatively in higher (1 000±50 μmol·mo l -1 ) and ambient (about 380 μmol·mol -1 ) CO 2 concentration to t e st whether CO 2 has significant impacts on phenotypic and some physiological an d biochemical characteristics as main effect. In addition, allometric analysis w as applied to distinguish whether phenotypic changes such as total leaf area are induced by environment (plastic) or plant size dependent (non_plastic). The res ults showed that phenotypic characteristics of P. crispus leaf in elevated C O 2 concentration changed significantly compared with that in ambient CO 2 concent ration. Leaf shape was more stubby, but leaf width, single leaf area, total leaf area per plant and leaf area ratio increased significantly in elevated CO 2 co ncentr ation. However, specific leaf area and leaf length did not differ between treatm ents. Total leaf area per plant was closely correlated with plant weight, indica ting that change in this phenotypic characteristic may depend on plant size. The result of allometric analysis certified that this change was a non_plastic resp onse, but plant size dependent. In addition, higher CO 2 concentration led to t he changes in pigment concentration. Chlorophyl a, chlorophyl b, total chlorophyl, carotenoid concentration and the ratio of chlorophyl a/b decreased in elevated CO 2 concentration. The soluble sugar concentration increased, but protein, N a nd P content decreased under higher CO 2 condition. It is concluded that elevated a tmospheric CO 2 concentration results in significant changes in some physiologi ca l and biochemical characteristics, especially increase of soluble sugar concentr ation, but not necessary in leaf phenotype. Maybe true adjustment in leaf ph enotype is under strong genetic rather than CO 2 control.

Aquatic macrophytes of the mountain lake Krnsko jezero, Slovenia

The macrophyte vegetation in the mountain lake Kmsko jezero has been monitored since I 991. Five species of aquatic macrophytes,occupying different depths, were present in the lake. Changes in depth distribution were detected due to accelerated eutrophication caused by inputs of nutrients from the watershed. In the period between 1997 and 1998 repeated earthquakes additionally influenced the processes in the lake Kmsko jezero, by increased input of matter. The occasional phytoplankton blooms reduced water transparency and consequently disturbance to growth, development and distribution of anchored macrophytes occurred. In 1998 macrophytes spread down to 7.0 m, reaching the maximum at 5.8 kg DW m2 at 2 m depth. The total primary production in the lake was estimated at 4991 kg organic matter per year out of which Chara and Potamogeton species presented 95.5 % and 0.6 %, respectively.

Ecdysteroid content and distribution in plants of genus Potamogeton

Species of the genus Potamogeton L. growing in European North-East Russia were studied for the presence of ecdysteroids. Coupled HPLC/RIA showed that all Potamogeton species studied ( P. alpinus Balb., P. berchtoldii Fieb., P. compressus L., P. gramineus L., P. lucens L., P. natans L., P. pectinatus L. and P. perfoliatus L.) contain 20-hydroxyecdysone, ecdysone and some unidentified minor compounds in the concentration range 0.2–4.0 μg ecdysone equivalents/g dry wt, depending on the species, growth stage and plant part studied. Detailed study of 20-hydroxyecdysone distribution in individual plants of P. pectinatus L. revealed that highest content of ecdysteroids is associated with points of active growth (170–250 μg/g dry wt in apical leaves of main and side-shoots). The presence of ecdysteroids in pondweeds and the pattern of their distribution within individual plants indicate a possible role in the interaction between plants and herbivorous arthropods in aquatic ecosystems.

Phenotypic plasticity inPotamogeton (Potamogetonaceae)

Sources of the extensive morphological variation of the species and hybrids ofPotamogeton were studied, especially from the viewpoint of the stability of the morphological characters used inPotamogeton taxonomy. Transplant experiments, the cultivation of clones under different values of environmental factors, and the cultivation of different clones under uniform conditions were performed to assess the proportion of phenotypic plasticity in the total morphological variation. Samples from 184 populations of 41Potamogeton taxa were grown. The immense range of phenotypic plasticity, which is possible for a single clone, is documented in detail in 14 well-described examples. The differences among distinct populations of a single species observed in the field were mostly not maintained when grown together under the same environmental conditions. Clonal material cultivated under different values of environmental factors produced distinct phenotypes, and in a few cases a single genotype was able to demonstrate almost the entire range of morphological variation in an observed trait known for that species. Several characters by recent literature claimed to be suitable for distinguishing varieties or even species were proven to be dependent on environmental conditions and to be highly unreliable markers for the delimiation of taxa. The unsatisfactory taxonomy that results when such classification of phenotypes is adopted is illustrated by three examples from recent literature. Phenotypic plasticity was found to be the main source of morphological variation within the species ofPotamogeton, having much more influence than morphological differences caused by different genotypes.

Selenium Accumulation in Submerged Aquatic Macrophytes Potamogeton pectinatus L. and Ruppia maritima L. from Water with Elevated Chloride and Sulfate Salinity

Submerged aquatic macrophyte species Potamogeton crispus L. (curlyleaf pondweed) and Ruppia maritima L. (widegeongrass) were examined for selenium accumulation from agricultural drainage water in the field and under laboratory conditions. High concentrations of chloride and sulfate salts were found in the drainage water of the constructed wetland at Tulare Lake Drainage District, Corcorn, California. Samples of P. crispus and R. maritima collected from the field, had similar plant-tissue Se concentrations, but the rhizomes accumulated significantly greater amounts of Se than the shoot tissues. When the plants were grown in culture solution supplemented with either chloride or sulfate salt, R. maritima was found to be more salt tolerant than P. crispus, and P. crispus accumulated more Se than R. maritima. Free seleno-amino acids were detected in the plant tissue, and organic Se was detected in the culture solution after 10 days of growth. The release of organic Se into the water by the plants may have a negative impact on the wetland environment.

Applying the seedling-emergence method under waterlogged conditions to detect the seed bank of aquatic plants in submerged sediments

Seed bank studies focused on submerged aquatic plants are generally performed under submerged conditions, using the seedling-emergence method. However, if a study targets at both submerged species and helophytes, submerged conditions are generally not suitable. We tested the emergence of seedlings from sediment samples from a freshwater canal, comparing a waterlogged treatment with a submerged treatment. From the sediment samples 31 species emerged. Eight species germinated in significantly larger numbers from the waterlogged treatment, one being a submerged species and four helophytes. No species germinated better under submerged conditions. The total number of species germinating under waterlogged conditions was significantly higher than under submerged conditions. Germination experiments with Potamogeton species showed no significant differences in germination percentage between a waterlogged and submerged treatment for at least two species. In the study of seed banks of submerged sediments, aimed at both submerged aquatic plants and helophytes, a waterlogged treatment is therefore suitable to detect both groups of plants.

Factors Affecting Foraging Patterns of Juvenile Bluegill (Lepomis macrochirus) in Vegetated Habitats of a Wisconsin Lake

ABSTRACT We collected behavioral data of juvenile bluegill in vegetated habitats with underwater videography in a Wisconsin lake to gain a better understanding of how juvenile bluegill foraging patterns are influenced by environmental and/or biotic variables characteristic of vegetated habitats. Relationships among foraging patterns and invertebrate density, aquatic plant stem densities, water quality, water depth, and light were determined. Three plant species (Potamogeton illinoensis, P. robbimii and Ceratophyllum demersum), as well as invertebrate density, water temperature, and percent of light transfer in the water column were associated with foraging duration of blued. We suggest these data are useful in determining the habitat required for enhancement of bluegill populations and recruitment of young fishes.

Development of submerged macrophytes in shallow Lake Müggelsee (Berlin, Germany) before and after its switch to the phytoplankton-dominated state

The occurrence of macrophyte- and phytoplankton-dominated states has been studied in shallow Lake Muggelsee, which, due to its rather high average depth and naturally eutrophic condition, differs from most typical shallow lakes. At the beginning of the 20 th century, a diverse vegetation covered one third of the lake with a maximum colonisation depth of 4 m. Secchi depths were high although cyanobacteria blooms, dominated by Aphanizomenon flos-aquae and Microcystis spec., occurred in late summer. Increasing eutrophication finally led to disappearance of submerged plant stands around 1970 and a switch to phytoplankton dominance. Cyanobacteria were now mainly represented by Planktothrix agardhii and Limnothrix redekei, preventing a clear-water phase in spring. Turbidity increased significantly after disappearance of submerged plants Since 1990, external nutrient loads have been reduced. Cyanobacteria structure reversed to dominance of Aphanizomenon and Microcystis resulting in a clear-water phase in spring and re-appearance of submerged macrophytes. Nevertheless, submerged plant stands still covered only 3 % of the total lake area in 1999. Average colonisation depth, biomass and colonisation densities of the dominant species Potamogeton pectinatus are low. Hitherto, re-colonisation by other species formerly present in the lake has not taken place. Exclosure experiments revealed that grazing by herbivorous birds or fish hamper re-colonisation.

Development of multimetric indices of biotic integrity for riverine and palustrine wetland plant communities along Southern Lake Michigan

Riverine and palustrine wetland plant communities were examined in order to propose a multimetric plant index of biotic integrity. The objectives were to determine the structural and functional attributes of these wetland plant communities, calibrate reference conditions in assessing aquatic plant communities, provide methods for further development and testing of the index, and present a case study. The index is based on a rapid assessment method using the information collected from a species list and cover estimates. Sampling was done using a modified relevé sampling approach with a modified Braun-Blanquet Cover Abundance Scale Method for estimating percent cover. More than 20 characteristics of aquatic plant communities were evaluated and 12 metrics in five categories were developed. Structural metrics focused on community composition, key indicator species such as number of Carex and Potamogeton species, and guild type. Functional metrics included sensitivity and tolerance measures; percent emergent, pioneer, and obligate wetland species; and the number of weed species as a substitute metric. Abundance was estimated based on evenness of average cover densities. Individual condition was suggested as a measure of the lowest extremes of biotic integrity. Palustrine study sites ranged across a disturbance gradient from ‘least-impacted’ to ‘poor'; riverine study sites ranged from high quality to some of the most degraded riverine sites in the Great Lakes region. Ninety-five species of aquatic vascular plants were found in 42 families. The most common families were Cyperaceae (15 species), Polygonaceae (9 species), and Juncaceae (6 species). Fourteen submergent, four floating, two woody and 75 emergent aquatic plant taxa were found. Five species were on the endangered, threatened, or rare list for the State of Indiana. Sites receiving the highest index scores included several of the a priori least-impacted sites while the lowest scores were located near-field to a large industrial landfill. The index will need to be further validated and tested but shows potential as a rapid index of biotic integrity using aquatic plant assemblages.

Ebullitive release of lacunar gases from floral spikes of Potamogeton angustifolius and Potamogeton amplifolius: effects on plant aeration and sediment CH 4 flux

Field and laboratory studies examined two Potamogeton species ( P. angustifolius Berchtold Presl. and P. amplifolius Tuckerm.) for effects of ebullition from submersed floral inflorescences, or spikes, upon plant aeration and sediment CH 4 release. Results indicated that daytime release of lacunar gas via ebullition from submersed floral spikes of mature clones was light-dependent and comparable to post-emergence flux. Analysis of tracer movement through plants and measurements of lacunar pressure revealed that daytime convection was driven by a sustained pressure difference (3–6 kPa at midday) between non-ebullient and ebullient clones, and enhanced exchange of lacunar gas between clones connected via rhizome. In comparison to non-ebullient clones, stem lacunae of ebullient clones showed higher concentrations of CH 4 and CO 2 and lower concentrations of O 2. In situ measurements of diffusive and ebullitive CH 4 flux from plants and sediment showed that ebullitive release from submersed floral spikes of P. angustifolius represented 19–29% of total areal CH 4 flux. This indicates that, in addition to effects on plant aeration, ebullitive release of lacunar gases from submersed rhizomatous angiosperms may represent a heretofore-unquantified source of CH 4 emission from aquatic systems.

A comparison of growth responses between two species of Potamogeton with contrasting canopy architecture

This study examines the response of two species of Potamogeton (Family: Potamogetonaceae), with differing canopy architectures, to an artificial light gradient. Potamogeton ochreatus Raoul and P. tricarinatus F. Meull. and A. Bennett were grown in water with an attenuation coefficient of 8.8 m −1 at various depths (10–81 cm) to give initial instantaneous irradiances between 0.4 and 460 μmol m −2 s −1. The average daily water column irradiances ( I ̄ ave ) between the planting depth and the water surface, over 15 daylight hours, ranged from 3.8 to 18.4 mol m −2. After about 80 days all P. tricarinatus plantings, except those at 81 cm, formed dense surface canopies which could access atmospheric CO 2 and had a maximum relative growth rate (70±4 mg g −1 per day ) and net assimilation rates (0.1–0.9 mg cm −2 day −1) significantly above those of P. ochreatus ( 57±3 mg g −1 day −1 and 0.1–0.5 mg cm −2 day −1 , respectively). P. ochreatus, which had a more diffuse and fully submersed habit, had a lower specific absorption coefficient (0.1 m −2 g −1) and average daily light compensation point (37 μmol m −2 s −1) than P. tricarinatus ( 0.9–1.2 m −2 g −1 and 57 μmol m −2 s −1 , respectively), but had a relative growth rate of approximately 25 mg g −1 per day even at an initial instantaneous irradiance of 0.4 μmol m −2 s −1. In addition, P. ochreatus allocated about 80% of its biomass to leaves and stems irrespective of the light climate, whereas only small P. tricarinatus plants preferentially allocated biomass above ground. As energy levels increased, P. tricarinatus allocated a greater proportion of biomass to tissues capturing the limiting resource, light. As the light climate became more favourable, P. tricarinatus allocated more biomass to the rhizome. However, when compared to a wider range of submerged macrophytes, the two species optimised their respective growth rates by reacting to varying I ̄ ave in a similar way. Both responded to lower than optimal I ̄ ave by increasing photosynthetic area and to above optimal values of I ̄ ave by decreasing photosynthetic area.

Historical changes in species composition and richness accompanying perturbation and eutrophication of Danish lowland streams over 100 years

1. European lowland streams have experienced increased perturbation and eutrophication during the past 100 years. We use archive information from 27 Danish stream sites around 1896 and new data from 208 stream sites in 1996 to evaluate the accompanying changes in stream vegetation. Among the stream sites, 13 were both studied in 1896 and 1996. 2. The species richness of submerged plants has declined profoundly over the 100‐year period, particularly among the large group of Potamogeton species. This is evident both from the direct comparison of the 13 stream reaches included in both studies, and from the general comparison of all stream reaches included in the two studies. 3. The Potamogeton vegetation has become less diverse, and is now dominated by species resistant to frequent disturbance through a high dispersal capacity. Potamogeton species, adapted to eutrophic conditions, have also increased relative to species more typical of oligotrophic conditions over this period. The dominant submerged species in the contemporary stream vegetation generally show a high capacity for dispersal and regrowth of detached shoots. 4. The decline of species richness in Danish streams can be partly explained by a decline in the species richness in lakes in the stream system. The rich vegetation observed downstream of lakes in the past has mostly disappeared due to loss of the vegetation in the now eutrophic lakes and increasing turbidity downstream. 5. The overall decline in richness, and the directional change in dominance patterns among stream species, can thus be explained by the loss of suitable habitats and the strong anthropogenic impacts, which have driven several European aquatic species close to extinction.

Macrophyte decline in Danish lakes and streams over the past 100 years

Summary 1 Freshwater habitats in cultivated and densely populated lowland regions of Europe have experienced profound changes during the last 100 years. We take advantage of the long interest in aquatic plants in Denmark to compare the submerged flora in lakes and streams in 1896 and 1996. 2 Most of the lakes which contained a diverse submerged vegetation 100 years ago now have the high phytoplankton biomasses and summer transparencies below 2.0 m characteristic of eutrophication. The majority of 17 lakes included in both old and recent studies have lost all or most of their submerged species. Species richness for those lakes that were vegetated did not, however, differ significantly between old and recent studies. 3 Species richness declined markedly in the 13 streams included in both studies. Over all sites, there was also a significant decline of species richness per locality. Potamogeton species declined from 16 to 9, despite an 8‐fold increase in the number of sites surveyed. 4 Similar compositions and rank‐abundances of Potamogeton species in lakes and streams studied 100 years ago reflect suitable growth conditions and mutual exchange of propagules. Today, low habitat diversity and frequent disturbance in streams and low recruitment from lakes favours only robust, fast‐growing species capable of regrowth following weed cutting and dredging. 5 A positive interspecific relationship observed in the contemporary stream vegetation between mean local abundance and number of occupied sites was probably promoted by redistribution of plants as a result of disturbance and efficient dispersal in the interconnected stream network. 6 The freshwater macrophyte flora in north‐west Europe presently includes a high proportion of rare species which are threatened by extinction. Both species typical for oligotrophic conditions (e.g. P. filiformis and P. polygonifolius) and another group of large, slow‐growing species (e.g. P. alpinus, P. lucens, P. praelongus and P. zosterifolius), were once common but are now infrequent, while other transient species have remained rare (e.g. P. acutifolius, P. colouratus, P. densus and P. rutilus). The presence of many species that barely survive in small and distant populations will make re‐assembly of submerged aquatic communities difficult.