Contribution to the knowledge of Mediterranean wetland vegetation: Lemnetea and Potamogetonetea classes in Western Sicily

The freshwater aquatic vegetation of the Lemnetea and Potamogetonetea classes in Western Sicily was surveyed and analyzed. 85 lakes and small pools were investigated collecting 147 phytosociological unpublished relevés to integrate the very scarce available data (only 3 relevés). By applying statistical analyses on abundances data and on the bases of physiognomy and dominant species, two alliances (Lemnion minoris and Stratiotion) and four different coenoses have been identify for the Lemnetea class; while 11 associations, assigned to two orders (Potamogetonetalia pectinati and Callitricho hamulatae-Ranunculetalia aquatilis) and four alliances (Potamogetonion, Nymphaeion albae, Ranunculion aquatilis and Ranunculion omiophyllo-hederacei) have been recognized for the Potamogetonetea class. A new association (Junco bufonii-Ranunculetum omiophylli ass. nov.) and a new subassociation (Ranunculetum peltati ranunculetosum rionii subass. nov.) have been proposed, whereas other six vegetation units were found to be new for the study area (Potamogetono-Ceratophylletum submersi, Potamogetonetum pusilli, Ranunculetum aquatilis, Ranunculetum peltati and Ranunculetum rionii, Lemno-Callitrichetum obtusangulae). For all the coenoses recognized, new insights on the floristic composition, syntaxonomy, synphiognomy, synecology and synchorology are reported, offering a reasoned overview of the aquatic vegetation of the western sector of the main Mediterranean island.


Introduction
Inland small standing-water ecosystem act as preminent hotspots for biodiversity although they are globally threatened (Bolpagni et al. 2019). Reclamation, eutrophication, fragmentation, and alien species invasion along with the effects of climate change are just some of key threats affecting freshwaters and wetlands (Benavent-González et al. 2014;Angiolini et al. 2017;Reid et al. 2019). Indeed, freshwater biota, and macrophytes in particular, are counted among the biological components that are most at risk of extinction under Anthropocene conditions (Bolpagni et al. 2018;Dudgeon 2019). This is especially true for the Mediterranean islands which are the backbone of one of the most diverse biodiversity-rich areas worldwide (Cañadas et al. 2014).
To this respect, Sicily -the largest Mediterranean island -is a key case study due to the intrinsic rarity of humid habitats and macrophytes (Troìa & Lansdown 2016;Minissale et al. 2017;Romanov et al. 2019). Inland small standing-water ecosystem are normally found in small depressions from the low hills to the mountainous areas (Caldarella 2014), interspersed within different land use types (e.g., agricultural areas, pastures, prairies, garrigue, forest clearings, wooded environments). Furthermore, they are under the control of a multiplicity of local environmental (e.g., exposure, altitude, shading, surrounding vegetation, hydroperiod, water supply) and anthropogenic drivers (e.g., trampling, water withdrawals, reshaping of banks).
To fill this knowledge gap, the present work concerns the phytosociological survey on the aquatic coenoses of the Lemnetea and Potamogetonetea classes, carried out through numerous unpublished relevés, in addition to the scarce available data (only 3 relevés). The survey involved the most representative biotopes of the Western Sicily (85 different lakes and small freshwater ponds and pools), distributed from the coastal area to the mountain belt, spread over various geological formations and with different hydroperiods.

Study area
The study area comprises the western sector of Sicily, including the provinces of Trapani, Palermo, Agrigento, and Caltanissetta ( Figure 1). The target sites (85) embrace water bodies with different hydroperiods (permanent vs temporary), and origin (natural, semi-natural and artificial) (Suppl. material 1, Table S1).
Most of studied biotopes fall within the Tyrrhenian orographic ridge, characterized by limestone, limestone-dolomite, and dolomite substrates whose formation dates back from the Mesozoic to Tertiary periods. Moving from the Madonie Chain towards west, altitudes tend to decrease up to the Trapani Mts (from 1979 m a.s.l. of the Pizzo Carbonara to the 751 m a.s.l. of the Monte Erice), with internal peaks including the Rocca Busambra (1613 m a.s.l) and the Sicani Mts with the Monte Cammarata (1578 m a.s.l) (Figure 1). The Agrigento and Nisseno sectors are instead part of an evaporite complex which includes soils of the Chalky-sulphurous Formation of the upper Miocene (Musarella et al. 2018).

Phytosociological, synecological and syndinamical analyses
The aquatic vegetation was studied following the phytosociological method of the Zurich-Montpellier school (Braun-Blanquet 1964), as later modified by other authors (e.g., Rivas-Martínez 2005;Biondi 2011). 147 unpublished relevés were carried out in the period between 2012-2020, subsequently compared with the few phytosociological relevés available in literature. The relevés matrix, transformed according to the Van der Maarel (1979) scale, was converted into a distance matrix by the vegdist function of the vegan package (Oksanen et al. 2020) using the Euclidean distance as an index of distance. Subsequently, the distance matrix was subjected to cluster analysis using the hclus function of R (R Core Team, 2021) applying the Ward method. Subsequent arrangement of phytosociological tables (see Tables 1-14) has allowed the identification of the plant communities that have been classified according to the phytosociological syntaxonomic system, and the "International Code of Phytosociological Nomenclature" (Theurillat et al. 2020).
The nomenclature of the syntaxa of higher rank (alliances, orders, and classes) is in accordance with Mucina et al. (2016), with some references also to the "Vegetation Prodrome of Italy" (Biondi et al. 2014). The attribution of the coenoses at the association level o follows main-ly  and Chytrý (2011), as well as other specific works duly cited in the text. The identification, nomenclature and biological forms of the plant species follow Pignatti (2017Pignatti ( -2019. Biological forms are abbreviated as follows: I nat = free-floating hydrophyte; I rad = rooted hydrophyte; G rhiz = rhizomatous geophyte; H scap/caesp/bienn = scapose/caespitose/ biennial hemicryptophyte; T scap/caesp/rept = scapose/ caespitose/reptant hemicryptophyte. For each investigated community, the list of Diagnostic taxa (% constancy), Syntaxonomic notes, a Short description with information on Syndynamism and Bioclimate in Sicily, Synchorology, and Local distribution were provided.

Results and Discussion
The dendrogram obtained from the cluster analysis (Figure 2) allowed the identification of 4 plant communities belonging to the Lemnetea class (Table 15) and 11 to the Potamogetonetea class (Table 16), respectively, as detailed in the syntaxonomical scheme.
The clusters of associations are closely overlapped each other at the upper levels, as many species are largely shared between relevés of the various communities investigated. This is mainly due to the strong spatial overlap of coenoses that tend to colonize thin concentric strips along the shoreline of the studied water bodies. Despite this, cluster fidelity is 97%, as only 4 relevés belonging to two clusters (10, Ranunculetum aquatilis, and 12 Lemno-Callitrichetum obtusangulae) have been subjectively classified exclusively based on physiognomy and dominant species.
Syntaxonomic note -Concerning the communities dominated by L. minor L., several authors agree in identifying a basic unit with an "association value" (Géhu and Pedrotti 1992;Schratt 1993;). However, other authors, including Müller (1977), Scoppola (1982), and Pott (1995), do not find the ground for the definition of a specific syntaxon due to the wide ecological and biogeographical values of this species (Sburlino et al. 2004). The Lemnetum minoris is rather common in Europe including the Mediterranean area (Ninot et al. 2000;Brullo et al. 2002;Šumberová 2011a;Felzines 2012;Zervas et al. 2020). The dominant species develops in several habitat types, often forming mono-paucispecific stands (Maiorca et al. 2007;Sburlino et al. 2004;Šumberová 2011a;Bolpagni and Piotti 2015;Spampinato et al. 2019). The association can be found on marginal zone of still and nutrient-rich freshwater bodies, in very shallow sectors (Zervas et al. 2020).
Short description -Acropleustophytic aquatic community dominated by L. minor, able to form more or less dense free-floating stands in temporary or permanent ponds, which hardly exceed 1.5 meters in depth. This community is typical of sunny, stagnant water bodies, sheltered from the wind, and tends to interpenetrate with other aquatic coenoses. In the studied area, its growth optimum is in late spring, whereas it tends to regress in the presence of summer drying events, and stationing of livestock.
Synchorology -L. minor is a subcosmopolitan species, with an oceanic climatic optimum, which is present in North America, Europe, Africa, and Western Asia (Landolt 1986). L. minor communities have a wide distribution and are quite widespread in continental Italy (Brullo et al. 2001;Sburlino et al. 2004) and large islands (Biondi and Bagella 2005). In Sicily, they were recognized on the Nebrodi and Sicani Mts (Brullo et al. 1994;Marino et al. 2005), and in the Trapani (Guarino and Pasta 2017) and Palermo (at Bosco Granza; Gristina and Marcenò 2009) surroundings.
Short description -Acropleustophytic aquatic association dominated by L. gibba. It forms dense free-floating populations, which tend to cover the entire surface of small ponds. This association is typical of weakly flowing environments, artificial basins, and ditches used for watering livestock, up to 150 cm deep, sunny and/or partially shaded. The association's growth optimum is in late spring and it tolerates summer water stress conditions. Based on the present relevés, two distinct aspects have been described: one represented by monospecific or strictly paucispecific stands; and a second one characterized by a richer presence of hydrophytes, even if with very low cover values.
Syndynamism -This association is a pioneering assemblage, sometimes able to act as invasive. Due to the progressive eutrophication of waters with the advance of season (e.g., progressive concentration of solutes), in conjunction with higher summer water temperatures, it tends to locally replace the Lemnetum minoris of which can be considered as a seasonal vicariant.
Local distribution -It is a rather rare association. It was found at the Bosco Ficuzza area (Case Cuttitta, Gorgo Cerro and Gorgo Lungo), at the Monte Palmeto near Carini (PA), and at the mouth of the Mandrarossa Stream along the Agrigento coast (Menfi, AG).
Syntaxonomic note -L. minuta Kunth is an alien species native to the temperate and subtropical America (Banfi and Galasso 2010), locally considered as invasive as in many other Italian regions . From a phytosociological point of view, in Italy this species forms communities attributed at different associations such as Azollo filiculoidis-Lemnetum minuscolae Felzines & Loiseau 1991 or Lemnetum minuto-gibbae Liberman Cruz, Pedrotti & Venanzoni 1988, even if some authors consider these assemblages as a basal phytocoenon (Sburlino et al. 2004;Viciani et al. 2020 and references therein).
Short description -Acropleustophytic aquatic community dominated by L. minuta. It isable to form dense free-floating populations that can spread rapidly and cover the entire surface of the colonized water bodies, with a growth optimum in spring. It occurs in shaded environments, even if it is well suited to conditions of high insolation, and hypereutrophy. Indeed, this association is Syndynamism -L. minuta acts as a pioneering taxon, often becoming invasive.
Synchorology -In Northern and Central Italy (Sburlino et al. 2004;Ceschin et al. 2016), the species has shown a recently rapid spread. In southern Italy it has also started to spread by colonising lakes with high naturalness (Spampinato et al. 2019). Before the present work, the only reports for Sicily concerned two small ponds at the Monte Pellegrino, near Palermo (Marrone and Naselli Flores 2011).
Local distribution -It is a rather rare community; it was confirmed for the Monte Pellegrino at the Gorgo of Santa Rosalia (Suppl. material 2, Figure S1b) and Cozzo della Grattalora, and in the Trapani surroundings, near the city of Mazara del Vallo. Syntaxonomic note -C. submersum L. is a very rare species, both in Sicily and in Italy in general (Lastrucci et al. 2019a). Due to the rarity of this species, only scarce phytosociological data are available, at least for Italy. According to the phytosociological databases LISY (Bracco et al. 2007) and VegItaly , communities dominated by C. submersum have been reported only for few wetlands in Northern Italy (Gerdol et al. 1979;Piccoli 1998;Pedrotti 2003). According to Šumberová (2011a), C. submersum-dominated communities can be attributed to the association Potamogetono-Ceratophylletum submersi Pop 1962 typical of shallow, well-isolated, and warm in summer ponds, with remarkable water level variations (see also Hrivnák 2002). The association can also include the presence of lemnids.
Short description -Submerged community with ceratophyllid habit dominated by C. submersum. It is typical of shallow ponds, where the species tends to occupy the entire water body. It tolerates some degree of shading and can be favoured by the presence of a thin layer of floating pleustophytes (e.g., Lemna minor and L. gibba), as well as by a progressive seasonal eutrophication of waters and high summer temperatures. Its growth optimum is in late spring.
Syndynamism -In the study area, the association is in close contact with the Lemnetum minoris, which it replac-es in the deeper sectors of the Gorgo Lungo pond, where this species tends to spread into the wettest sectors of the helophytic belts dominated by Sparganium erectum L. and Schoenoplectus lacustris (L.) Palla.
Synchorology -In Sicily, C. submersum is quite rare: the Gorgo Lungo near Godrano (PA) is the only ascertained population for the western sector of Sicily so far (Lastrucci et al. 2019a).
Local distribution -Gorgo Lungo (Godrano, PA). Syntaxonomic note -In Sicily, and in the Nebrodi Mountains in particular, P. natans L. was considered a characteristic species of two different associations: the Utriculario-Potametum natantis Raimondo, Marino & Schicchi 2011, that belongs to the order Utricularietalia Den Hartog & Segal 1964 (Raimondo et al. 2011), and the Polygono-Potametum natantis Soò (1927) 1964, as reported by Brullo et al. (1994). This latter appears as a rather complex vegetation type, with two different as- pects: a first one dominated by Persicaria amphibia (L.) Delarbre, and a second one dominated by P. natans. In the study area, however, P. amphibia is rather rare and never plays a clear physiognomic role. Based on this, we prefer to refer the recorded P. natans coenoses to a new association for Sicily, the Potamogetonetum natantis Hild 1959. This community is typical of mostly mesotrophic, but also oligotrophic to eutrophic shallow water bodies (in the range 20-100 cm), with still or slowly moving waters (Šumberová 2011b). Some criticisms remain, in any case, respect to the attribution of this association to the alliance level. Several authors (e.g., Šumberová 2011b) ascribed this association to the Potamogetonion alliance, while others to the Nymphaeion alliance (e.g., Venanzoni & Gigante 2000;Jarolímek et al. 2008;Ferrez et al. 2011). Since P. natans contributes only marginally to form submerged stands, we are led to classify this association into the Nymphaeion alliance, that includes communities dominated by floating-leaved hydrophytes. In the study area, the association shows both monophytic and more diversified aspects. Short description -Rhizophytic community dominated by P. natans, often monospecific, in which other hydrophytes belonging to the Potamogetonetea are occasionally present. The coenosis is typical of permanent habitats (including artificial ones) with depths greater than 80 cm (and up to 4 m), with oligo-mesotrophic to mesotrophic waters and subjected to wide summer water variations. P. natans tends to form quite dense stands (> 80% in coverage), colonizing the innermost, permanent sectors of water bodies.
Local distribution -This association was found in both natural and artificial ponds lied in the hilly-mountain sector, scattered in several reservoirs of the Bosco Ficuz-  Syntaxonomic note -P. pusillus L. often shows a certain ecological amplitude being able to establish in freshwater habitats with both standing and weakly flowing waters (Preston 1995). In Italy, the communities of P. pusillus are generally ascribed to a basal phytocoenon (Biondi et al. 1997(Biondi et al. , 2004Piccoli 1998;Bernardo 2006, Lastrucci et al. 2008;Sburlino et al. 2008). However, in lentic habitats, such as small pools (Lastrucci et al. 2012) or natural lakes (Landucci et al. 2011), the P. pusillus communities show a greater compositional constancy that justifies the attribution to an association (Potamogetonetum pusilli).
Short description -The association is typical of mesotrophic to eutrophic water bodies. It occupies a wide range of depths (0.5-3.0 m), often colonizing disturbed habitats, or representing early successional stages of pond evolution (Šumberová 2011b). In the study area, P. pusillus is often associated to other sporadic hydrophytes, such as Ranunculus aquatilis L. and Callitriche brutia Petagna. The Potamogetonetum pusilli stands are rather dense (> 90% in coverage) and colonize depths of about 80-100 cm, occupaying the intermediate vegetation belt between central, deeper sectors and shorelines. It has its growth optimum in spring.
Syndynamism -In the study area, this association is mainly represented by paucispecific stands and replace the Ranunculion aquatilis at deeper waters. In the innermost sectors of ponds, it spreads into others Potamogetonetea class units and algal assemblages (Chara spp.). Along the littorals, it is in close contact with the helophytic consortia of the Phragmito-Magnocaricetea class.
Local distribution -This association has been found only in a few locations, both on flyschyoid and carbon-ate substrates. At the Rocca Busambra area, it is very rare, found only in the small lake locally called "r'u zù Rusulìnu" (Godrano, PA). Conversely, this association is much widespread at the Pizzo Cane Reserve, at the Pizzo Selva a Mare, as well as at Cozzo Valdaro and Contrada Randino.
Short description -Rhizophytic community dominated by P. pectinatus, sporadically associated with other rooted hydrophytes. It is typical of estuary environments with weakly flowing waters, deep water reservoirs (even over 3 m of depth), and semi-permanent, natural habitats where the dominant taxon tends to form dense stands (> 80% coverage). The growth optimum is in the spring-summer period. In the study area, we noted the coexistence of monospecific stands of P. pectinatus alongside richer assemblages in species, including algae of the genus Chara. The presence of a non-negligible contribution of macroalgae to P. pectinatus stands was previously highlighted by Gradstein and Smittenberg (1977) for Crete Islands.
Syndynamism -It colonizes the deepest sectors of the colonized water bodies, coming into contact with other coenoses of the Potamogetonetea class and algal assemblages (dominated by Chara spp.). Along littorals, this association tends to spread into the helophytic communities of the Phragmito-Magnocaricetea class.
Synchorology -P. pectinatus is a cosmopolitan species, typical of fresh-and brackish waters on all continents, except Antarctica. The Potamogetonetum pectinati was described for Germany (Carstensen 1955), and it is reported for other various European countries (Hilbig 1971;Schubert et al. 1995;Matuszkiewicz 2005;Šumberová 2011a), as well as for continental Italy (Sburlino et al. 2008), and Italian largest islands (Brullo et al. 2001;Biondi and Bagella 2005). In Sicily, it is widespread both in coastal areas, in aquatic habitats with low salinity, and mainland (Bartolo et al. 1982;Spampinato and Sciandrello 2013). It is mentioned for some river mouths, and lentic habitats in the Hyblean sector (Bartolo et al. 1982;Spampinato and Sciandrello 2013), in the Trapani (Brullo and Ronsisvalle 1975) and in the Nisseno areas Brullo and Sciandrello 2006;Sciandrello 2009).
Local distribution -In the study area, it was found in different artificial reservoirs, near Rocca Busambra (Contrada Casale, Contrada Cicio, and Contrada Alpe Cucco), on the Sicani Mountains (at the Gorgo di Sant' Andrea; Figure S3b) and at the mouth of the Carboj River along the Agrigento coast. Syntaxonomic note -Community of oligo-mesotrophic water bodies, and limestone substrates (Ferrez et al. 2011), distributed in Central and Southern Europe (Brullo et al. 1994). Several G. densa-associations have been described for the Iberian Peninsula (Loidi et al. 1997 (Melendo et al. 2003).
Short description -Rhizophytic community dominated by G. densa, only sporadically associated with other hydrophytes, such as Callitriche obtusangula Le Gall., occurs predominantly in sunny, permanent habitats, where develops medium coverage stands (up to 80%). This species prefers groundwater-dependent ecosystems, and spring waters which contribute to mitigate the excessive heating of water in summer. It preferably colonises littorals (up to Syndynamism -G. densa communities are closely associated to several aquatic plant associations colonizing the permanent sectors of water bodies (cl. Potamogetonetea), as well as algal assemblages (Chara ssp.), as recorded in the Nebrodi Mts (Guarino et al 2019), and helophytic belts (cl. Phragmito-Magnocaricetea).
Local distribution -A small population has been recorded in a reservoir located along the south side of the Monte Carcaci (Castronovo di Sicilia, PA); a second location is in a small wetland at the Bosco Granza (Sclafani Bagni, PA). Syntaxonomic note -The association is typical of shallow, eutrophic, and often salt-rich, warm ponds (Sumberová 2011b). Our stands can be attributed to the Ranunculetum rionii, an association reported for the first time in Italy by Lastrucci et al. (2019b) for the Acquato Lake, in Tuscany. R. rionii Lagger was recorded as new species for Sicily by Bartolucci et al. (2021). The populations of R. rionii of Western Sicily show a certain morphological affinity with R. trichophyllus Chaix, from which they differ for the presence of smaller and hairless achenes, and smaller petals.

RANUNCULETUM RIONII
Short description -Paucispecific aquatic community with a distinct myriophyllid habit, dominated by R. rionii, which tends to form compact and continuous belts along the outer edges of semi-permanent or ephemeral ponds characterized by strong seasonal water level variations. In the study area, this association was found in small ponds, even temporary ones, situated both on flyscioid substrates waterproofed by sub-alkali clay deposits, but also on limestone and calcarenites. The association's growth optimum is in late spring.
Syndynamism -This association is often in contact with other Potamogetonetea and algal assemblages (Chara spp.), whereas along littorals it spreads into helophytic belts, as testified by the presence of several helophytes with high frequency values in the collected relevés.
Local distribution -It has been recognized in several small ponds (both natural and artificial), distributed around the Sicani Mts, Rocca Busambra, Palermo Mts (at the Gorgo di Rebuttone; Altofonte -Suppl. material 2, Figure S1a), on the south side of the Monte Palmeto (Carini), and further east on the limestone platforms of Mazara del Vallo (at the Garufi plain; Suppl. material 2, Figure S2e). Géhu 1961 (Tables  10 and 16; Suppl. material 2, Figure S2b)
Taxonomic and Syntaxonomic note -The presence of R. aquatilis in Italy is rather debated. The species was reported with several regional lacunae for the flora of Italy (Pignatti 1982;Conti et al. 2005), until Desfayes (2008Desfayes ( , 2011 proposed to exclude it from the entire Italian territory. This proposal was accepted by Bartolucci et al. (2018) but not by the editors of the second edition of the "Italian Flora" by Pignatti (2017Pignatti ( -2019 that lists this species as present in Italy. Even if the R. aquatilis group is a very critical group, in our opinion, R. aquatilis is recognizable in the study area and distinguished from Ranunculus peltatus Schrank for various characters such as the length of peduncles, the leaf margin shape, length of the petals and the morphology of nectar pits, as reported by many European determination keys (Pignatti 1982;Cook 1986;Pizarro 1995;Wiegleb et al. 2017). From a phytosociological point of view, R. aquatilis-assemblages are attributed to the association Ranunculetum aquatilis Géhu 1961, growing on clear, stagnant or slow-running, sunny, neutro-basophilous and nutrient-rich waters, on silty-clayey  bottom, occasionally subject to desiccation or eutrophication (Buchwald 1994; Passarge1992). Short description -Batrachid community, with a clear predominance of R. aquatilis, which is associated with Gliceria notata Chevall. and other sporadic taxa of the Potamogetonetea class (Callitriche stagnalis Scop., C. brutia and C. obtusangula). It is typical of sunny ponds, characterized by strong seasonal water variations, situated on flyschoid substrates, but in some cases also on calcareous substrates. This association contributes to form a continuous belt along the external edges of colonized water bodies, at depths between about 50 cm and shores, therefore well adapted to the rapid drying of temporary Mediterranean ponds. The association's growth optimum is in late spring.

Bioclimate in Sicily -Mediterranean pluviseasonal-oceanic (meso-Mediterranean subhumid/humid).
Syndynamism -This association is in close contact with submerged algal assemblages (Chara spp.) or with coenoses of the Potamogetonetea and Lemnetea classes. Often, the association forms actual transitional communities towards deeper vegetation belts (Rels 13-14, table 10). Along the outer edges of the colonized water bodies, this association is progressively replaced by helophytic belts of the Phragmito-Magnocaricetea class, and sometimes it spreads into the Isoëto-Nanojuncetea Br.-Bl. & Tüxen ex Westhoff, Dijk & Passchier 1946 communities in habitats subjected to rapid seasonal water variations and the progressive drying of sediments.
Synchorology -R. aquatilis is a subcosmopolitan species, with a wide Euro-Asian distribution, as well as in North Africa and North America. In Italy, associations dominated by R. aquatilis have been reported mainly for Central Italy (e.g., Biondi et al. 1999). As put in evidence by several authors (Géhu and Meriaux 1983;Passarge 1992), one of the main issues related to the correct recognition of this association depends on the fact that many authors do not separate the assemblages dominated by R. aquatilis and R. peltatus, respectively. Consequently, the Ranunculetum aquatilis is often considered as a macro-association to which populations of both species can be attributed (see also Sumberová 2011b). Nevertheless, Géhu and Meriaux (1983) highlighted how in France the two species differentiate two distinct associations, ecologically and geographically vicariant. In particular, the Ranunculetum aquatilis shows a wide ecological amplitude, and a greater preference for organic and nutrient rich waters. In the study area, a clear distinction between the typical habitats of these two coenoses has been recognized. R. aquatilis prefers water bodies located at higher altitudes (hilly and sub-mountain horizons) and shows a more mesophilic character. Conversely, R. peltatus is also found at low-altitude locations (200-250 m s.l.m), even in coastal areas of North-Western Sicily.
Local distribution -The association has been detected in several hilly and submontane water bodies, both of natural and artificial origin, permanent and semi-permanent (with a hydroperiod of 9-10 months). It is distributed in the foothills north of the Rocca Busambra (e.g., Gorgo  Figure S2c) Diagnostic taxa (% constancy) -Ranunculus peltatus Schrank (100%, dominant).
Syntaxonomic note -R. peltatus (Suppl. material 2, Figure S2d) shows a high adaptability to different ecological conditions (Garbey et al. 2004), implying that this species may differentiated more than one community (see also ). The association Ranunculetum peltati is typical of small, clear lentic water bodies, with depths not exceeding 80-100 cm. These habitats can be moderately rich in nutrients, subjected to summer desiccation and livestock impacts, however the Ranunculetum peltati is particularly sensitive to pollution and shading (Passarge 1992). Two subassociations, the subass. typicum and the subass. potamogetonetosum were identified and typified by Passarge (1992). The Ranunculetum peltati was also reported for the Mediterranean area by Gradstein and Smittenberg (1977). In addition, for the oligotrophic, neutro-acidophilous deep standing and cold waters of temperate and Mediterranean siliceous areas of the Iberian Peninsula, the association Callitricho brutiae-Ranunculetum peltati Pizarro & Rivas Martínez 2004 was described. This latter association is considered as a geosynvicariant of the Ranunculetum peltati, and it was recently reported for some wetlands in Tuscany by  due the presence of some characteristic species of the association, such as C. brutia and Myriophyllum alterniflorum DC. In the study area, however, C. brutia is rather rare and only rarely found in the coenoses dominated by R. peltatus. Therefore, it seemed more appropriate to assign the Sicilian assemblages to the Ranunculetum peltati. Furthermore, the ecological conditions observed in Sicily seem rather far from those recorded for Spain. Here, the studied water bodies are not characterized by cold waters, and their depth is rather low. Based on this, we identified a new subassociation differentiated from R. rionii and species indicating high water fluctuations, such as Oenanthe fistulosa L. and Alisma lanceolatum With.
Short description -Batrachid community with a clear predominance of R. peltatus, only occasionally associated with other hydrophytes. It is typical of ponds characterized by strong seasonal water level variations, situated on carbonate substrates, with oligo-mesotrophic waters. This coenosis prefers the sunniest sectors of colonized habitats, sometimes extending over the entire surface of reservoirs/ ponds, developing in these conditions submerged stems up to depths greater than 2 m. With the progressive and rapid lowering of the water level, R. peltatus tends to go up to the emerged banks tolerating the initial phases of seasonal desiccation. Its growth optimum is in the late spring.
Syndynamism -This association is continuous with other coenoses of the Potamogetonetea class and sub- merged algal assemblages (Chara spp.); along littorals, it tends to be replaced by amphibian vegetation often dominated by G. notata. Synchorology -R. peltatus is a European species restricted to the temperate and southern areas of Europe, North Africa, and West Asia (Wiegleb et al. 2017).
Short description -This subassociation differs from the typical aspect of the Ranunculetum peltati mainly because it colonizes littorals (with depths up to 50-60 cm) and muddy sediments. The community is characterized by the constant presence of R. rionii and O. fistulosa, accompanied with a certain frequency by A. lanceolatum. It has the growth optimum in spring, and it prefers higher altitude habitats than the association typicum.
Local distribution -It is present in some ponds distributed in the core sector of the study area, at altitudes between 850 and 950 m a.s.l., along the southern side of the Rocca Busambra (contrade Marosa and Guddemi), and at the Sicani Mts (at the Gorgo di Carcaci). (Philippi 1978) Passarge 1992 (Tables 12 and 16; Suppl. material 2, Figure S1d)
Syntaxonomic note -C. obtusangula may form stands both in running and in stagnant waters. According to several authors (e.g., Meriaux and Verdevoye 1983;Brullo et al. 1994;Sburlino et al. 2008), in running waters its typical association is the Callitrichetum obtusangulae Seibert 1962, belonging to the Batrachion fluitantis Neuhäusl 1959 alliance. For several wetlands of Nebrodi Mts, Brullo et al. (1994) described a very peculiar and complex community ascribed to the Glycerio-Callitrichetum obtusangulae Brullo, Minissale & Spampinato 1994. This association is differentiated by C. obtusangula, C. brutia var. hamulata (Kütz. ex W.D.J. Kock) Lansdown and C. stagnalis, as well as by the presence with rather high frequencies of amphibian taxa such as Glyceria spicata Guss., Peplis portula L., and Alopecurus aequalis Sobol., besides a large group of aquatic species. The coenoses dominated by Callitriche genus in the study area exhibit some ecological similarities with those described by Brullo et al. (1994), however they differ strongly in the floristic assemblage. Here, only C. obstusangula has been found, whereas G. spicata is absent and replaced by G. notata, which cannot be considered as a good differential taxon because it is present in almost all the coenoses here reported, besides being the most frequent species in the study area (55% of frequency in the relevés matrix). Furthermore, given the importance of L. minor in differentiating the communities of C. obstusangula in the study area, it seems most appropriate to attribute these coenosis to the Lemno-Callitrichetum obtusangulae, also reported for the northern Italy by Sburlino et al. (2008). Passarge (1992) placed this association in the alliance Lemno-Callitrichion Passarge 1992, that currently has been placed in synonim with the Ranunculion aquatilis (Mucina et al. 2016). The Lemno-Callitrichetum obtusangulae is typical of summer-warm waters, rich in nutrient, tolerating drying periods, eutrophication, and slight salinity, but fearing cold winters and anthropogenic pollution (Passarge 1992). Short description -Aquatic vegetation dominated by C. obtusangula, with the frequent presence of L. minor. This community is typical of small, semi-permanent water bodies lied on quartzarenitic substrates, with high levels of clay or sometimes with sandy sediments. It prefers muddy bottoms, and conditions of partial sunshine, growing at depths not exceeding 60 cm, and tolerating slightly eutrophic conditions. The association's growth optimum is in late spring.
Syndynamism -Towards the deepest part of the water body sectors, this association is in contact with other kinds of Potamogetonetea and Lemnetea vegetation, while along littoral it spreads into the Phragmito-Magnocaricetea belts.
Synchorology -The association is distributed in the temperate-sub-Atlantic regions of Europe (Passarge 1992), while in Italy it has been previously reported only for the Veneto region (Sburlino et al. 2008).
Local distribution -Ponds located in the northern foothills of the Rocca Busambra, at altitudes between 500 and 650 m a.s.l., in the surroundings of Godrano (Gorgo Lungo, Gorgo Pizzo Campana, Case Franco, Vallone Frattina and Coda di Riccio), Monreale (Zotta Frascino and Vallone Arcere), and Marineo (Cozzo Bileo and Sovarita). Further east, it is found in the Sclafani Bagni area (at Bosco Granza and Lake Bomes; PA), as well as at the Madonie Mts (at Pietra Giordano; Geraci Siculo, PA).
Syntaxonomic note -In Italy C. stagnalis differentiates communities belonging both to the Ranunculion fluitantis (e.g., Buchwald 1994;Ceschin and Salerno 2008) and the Ranunculion aquatilis (e.g., Baldoni and Biondi 1993;Brullo et al. 1994;Brullo et al. 2001;Lastrucci and Becattini 2008;Lastrucci et al. 2015) alliances. This confirms the ecological amplitude of this species, able to form stands in both running and stagnant waters. From a syntaxonomic point of view, the C. stagnalis-stands are frequently ascribed to a Callitriche stagnalis phytocoenon, less frequently to the Callitrichetum stagnalis Segal 1965 (Ceschin and Salerno 2008). However, for South Italy, peculiar coenoses dominated by this species have been described. Brullo et al. (1994) classified the C. stagnalis stands recorded at the Nebrodi Mts as a new subassociation (subass. callitrichetosum stagnalis) of the Glycerio spicatae-Callitrichetum obtusangulae Brullo, Minissale & Spampinato 1994. At the Aspromonte Mt (Calabria), instead, a peculiar association, ascribed to the Ranunculion aquatilis alliance, dominated by C. stagnalis, Ranunculus ophioglossifolius Vill., P. portula, Juncus articulatus L. and P. natans has been described as Ranunculo ophioglossifolii-Callitrichetum stagnalis Brullo, Scelsi, Spampinato 2001(Brullo et al. 2001). Short description -Aquatic vegetation dominated by C. stagnalis, in which Ranunculus omiophyllus Ten. and G. notata are frequently found, often showing high cover-abundance values. This kind of vegetation colonizes the margins of small, permanent water bodies, fed by groundwater, or semi-permanent ponds with depths not exceeding 10-15 cm, with short summer drying phases. The community seems to prefer wet habitats with muddy bottoms, sunlight, and oligotrophic to slightly eutrophic waters/sediments (in ponds subject to livestock grazing/ trampling), situated on quartz-sand substrates. It has the growth optimum in spring.
In a small pond of the study area (Gorgo Cerro along the northern side of the Rocca Busambra; table 12, rel. 5), an extremely impoverished aspect of the Ranunculo ophioglossifolii-Callitrichetum stagnalis Brullo, Scelsi, Spampinato 2001 has been recorded. Accordingly, the presence of this association in Sicily could be hypothesized. Further investigations will be necessary to verify this preliminary insight.
Syndynamism -C. stagnalis forms stands in contact with other Potamogetonetea coenoses, in particular with the Junco bufonii-Ranunculetum omiophylli, compared to which tends to occupy more inundated zones. It represents a transitional phase towards wet vegetation of highly, marginal flooded sectors of ponds.
Local distribution -Small wet habitats on the northern side of the Rocca Busambra near the Bosco Ficuzza.
Syntaxonomic note -R. omiophyllus is mainly distributed in the Atlantic Europe, from North-Spain to British Islands, but with an extension in the Mediterranean, from North Africa to Sicily, and Southern Italy (Pizarro 1995;Pignatti 2017Pignatti -2019. According to Rivas-Martinéz et al. (2001, R. omiophyllus is a characteristic taxon of the Ranunculion omiophyllo-hederacei Rivas-Martinéz et al. 2002, a peculiar alliance characterized by rooted stoloniferous helophytic ranunculids with floating leaves, as Ranunculus hederaceus L. or R. omiophyllus, (Rivas-Martinéz et al. 2002). For Spain, two associations dominated by R. omiophyllus have been described: the Ranunculetum omiophylli Braun-Blanquet & Tüxen (1943)   Both these associations, therefore, appear quite different from the Sicilian assemblages. In Western Sicily, the assemblages dominated by R. omiophyllus host several differential species belonging the class Isoëto-Nanojuncetea (see Brullo and Minissale 1998) leading us to propose the institution of a new association, the Junco bufonii-Ranunculetum omiophylli. Its typical composition is made of species from both the Callitricho-Ranunculetalia and the Isoëto-Nanojuncetea classes. Short description -Slightly sciophilous, batrachid community with a clear dominance of R. omiophyllus, frequently associated with annual micro-helophytes of the Isoëto-Nanojuncetea class such as Juncus bufonius L. and Poa infirma Kunth. The coenosis develops on muddy bottoms of shallow depressions (up to 10-20 cm) on clayey substrates. Its typical water bodies are small and have prolonged flooding phases, that last until the beginning of the summer season (June-July), and are also subjected to intense trampling disturb. The growth optimum of this new association is in late spring.
Syndynamism -This association is in contact with several aquatic rooted coenoses (Potamogetonetea class) and in particular with Callitriche stagnalis communities, while it spread into the communities of the Isoëto-Nanojuncetea and Molinio-Arrhenatheretea Tüxen 1937 classes along the littorals of colonized wet depressions.
Local distribution -Frequent in small ponds located in the central-northern sector of the Bosco Ficuzza, within the municipalities of Godrano, Marineo, Mezzojuso, and Monreale.

Conclusions
New insights on the aquatic communities of the Lemnetea (Table 15) and Potamogetonetea (Table 16) classes, characterized by the predominance of threatened and rare macrophytes, have been offered for western Sicily. The study puts in evidence the presence of a rich diversified aquatic and wet vegetation, namely four free-floating communities (ascribed to two different alliances, Lemnion minoris and Stratiotion -Tables 1-4), and 11 rooted plant assemblages (belonging to four alliances, Potamogetonion, Nymphaeion albae, Ranunculion aquatilis, and Ranunculion omiophyllo-hederacei - Tables 5-14).
This relative high vegetation diversity is probably mainly to be attributed to the wide ecological range of small-standing water ecosystems investigated, that includes both natural and artificial ecosystems, as well as permanent and temporary ponds, reservoirs, situated in turn on a variety of geo-lithological substrates (limestones, quartzarenites, clays, gypsum of the evaporitic series of the Sicilian interior) and altitudes, from the few meters a.s.l. of the mouth of the Carboj River (AG) to the 1,326 m a.s.l. of the Gorgo di Pietra Giordano (PA) (see Suppl. material 1, Table S1). Indeed, origin and hydroperiod of small water ecosistems are pivotal features in driving the complexity of vegetation in wetlands and small water bodies, especially in semi-natural or artificial contexts (Bolpagni and Piotti 2016;Bolpagni 2020).
The present data greatly broaden the floristic-phytosociological knowledge of the aquatic communities of the largest Mediterranean island, as well as their syntaxonomic, synecological, and distributive features. Our survey allowed almost punctual evaluation of scattered, isolated aquatic habitats that have a relevant conservation value considering the huge pressures exerted by climate change and human activities in the Mediterranean region (Gianguzzi et al. 2013(Gianguzzi et al. , 2017Benavent-González et al. 2014;Angiolini et al. 2017;Tomaselli et al. 2020). Thus, the described vegetation includes very peculiar phytocoenoses, although the floristic originality of the aquatic communities monitored is generally low. Despite this, due to their rarity and to the fragmented distribution, vegetation dom-  inated by lemnids or species of the genera Potamogeton or Ranunculus (sect. Batrachium) can be listed among the most endangered plant communities at national and European scale (Bolpagni et al. 2017;Maiorca et al. 2020). However, the study also highlights some critical issues including the presence of the invasive species Lemna minuta. We confirm its strongly invasive character, as shown elsewhere in Italy, especially in eutrophic environments (Ceschin et al. 2016;Lastrucci et al. 2016 and references therein).
To sum up, the present work offers a preliminary overview of the spatial representativeness of macrophyte communities in a rich set of small wetlands and ponds in western Sicily. Despite their very small size, which makes them very sensitive environments, they represent a fundamental component of semi-natural and natural ecosystems, which require adequate protection and monitoring actions in Sicily, and more generally in the Mediterranean region and in the world.