7 Distribution and habitat characteristics of Vallisneria spiralis L. in Croatia Abstract Eight new localities of the rare, strictly protected macrophyte Vallisneria spiralis have been recorded during a comprehensive survey of water bodies in Croatia. One record is located in the Mediterranean Region, in Butoniga Reservoir, while the remaining records are from the Continental Region, the majority of them situated along the Kupa River. Vallisneria spiralis occurred in slightly basic and alkaline environments and in a wide range of nutrient availability. In Continental localities, the species was present within Myriophyllo-Nupharetum luteae of the alliance Nymphaeion-albae, while in Butoniga, it occurred within the vegetation of the alliance Potamogetonion. Prior to our research, the species was recorded in Croatia only twice, with the older record dating back to the 19 th century and the second from recent years, but neither one was confirmed in later surveys. The low number of records of V. spiralis, in spite of a wide-ranging search through hundreds of localities, confirmed the rare status of this species, although the reasons for such limited distribution remain unclear, especially bearing in mind that suitable aquatic habitats are quite widespread in the country. Iz vleček Med obširnimi raziskavami vodnih teles na Hrvaškem smo odkrili osem novih lokalitet redke, strogo zavarovane makrofitske vrste Vallisneria spiralis. Ena je v mediteranski regiji v umetnem jezeru Butoniga, preostale pa so v kontinentalni regiji, večina od njih se nahaja ob reki Kolpi. Vallisneria spiralis se pojavlja v zmerno bazičnih in kislih okoljih in v širokem razponu razpoložljivosti hranil. V kontinentalnem območju je vrsta prisotna predvsem v asociaciji Myriophyllo- Nupharetum luteae zveze Nymphaeion-albae, medtem ko v Butonigi uspeva v vegetaciji, ki jo uvrščamo v zvezo Potamogetonion. Pred našo raziskavo so vrsto zabeležili na Hrvaškem le dvakrat. Starejši podatek je iz 19. stoletja, novejša najdba pa je recentna, vendar v kasnejših pregledih ni bila nikoli potrjena. Majhno število najdb vrste V. spiralis, kljub intenzivnemu pregledu stotin lokalitet, potrjuje status redke vrste, čeprav razlogi za njeno redko pojavljanje ostajajo neznani, še posebej ker so primerni vodni habitati za njeno uspevanje v državi splošno razširjeni. Key words: Butoniga Reservoir, Kupa River, macrophytes, Potamogetonion, Nymphaeion albae, Southeastern Europe, river ecology. Ključne besede: umetno jezero Butoniga, reka Kolpa, makrofiti, Potamogetonion, Nymphaeion albae, jugovzhodna Evropa, rečna ekologija. Received: 1. 6. 2020 Revision received: 5. 7. 2020 Accepted: 9. 7. 2020 1 University of Zagreb, Faculty of Science, Department of Biology, Division of Botany, Marulićev trg 20/II, 10000 Zagreb, Croatia. 2 Hrvatske vode, Central Water Management Laboratory, Ulica grada Vukovara 220, 10000 Zagreb, Croatia. 3 University of Zagreb, Faculty of Agronomy, Department of Agricultural Botany, Svetošimunska cesta 25, 10000 Zagreb, Croatia. * Corresponding author. E-mail: antun.alegro@biol.pmf.hr Anja Rimac1  , Antun Alegro1 , *  , Vedran Šegota1  , Nikola Koletić1  , Igor Stanković2  , Sandro Bogdanović3  & Nina Vuković1  DOI: 10.2478/hacq-2020-0014 20/1 • 2021, 7–18 20/1 • 2021, 7–18 8 Anja Rimac, Antun Alegro, Vedran Šegota, Nikola Koletić, Igor Stanković, Sandro Bogdanović & Nina Vuković Distribution and habitat characteristics of Vallisneria spiralis L. in Croatia Introduction Vallisneria spiralis L., commonly known as eelweed, ee- lgrass or tape grass, is a dioecious perennial freshwater plant native to southern Europe, northern Africa, the Middle East and southwest Asia (Les et al. 2008, Huss- ner 2012). The species is a submerged hydrophyte with fibrous roots, horizontal runners, short stem and linear leaves arranged in a basal rosette. Leaves are up to 100 cm long and 1 cm wide, with parallel veins and finely toothed margins (Casper & Krausch 1980). Male plants bear numerous minute flowers enclosed in a dehiscing two-valved spathe. When mature, the flowers completely detach from the spathe and rise to the surface, where they are dispersed by wind and currents. Pistillate flowers are singular, enclosed in a spathe and carried to the water sur- face on a long stalk. Following fertilization, the stalks coil spirally and fruits are withdrawn underwater where they mature (Les et al. 2008). In addition to dispersal by seeds, V . spiralis efficiently spreads asexually by runners, and can colonize new areas through plant fragments (Hutorowicz & Hutorowicz 2008, Heidbüchel et al. 2016). Vallisneria spiralis is a typical component of lowland meso- to eutrophic aquatic habitats, both lotic and len- tic (Hussner & Lösch 2005, Mukhopadhyay & Dewanji 2005, Bolpagni et al. 2015, Anđelković et al. 2016). It was reported from oligotrophic conditions as well, how- ever, displaying lower growth rates (Bolpagni et al. 2015). The species shows high ecological plasticity, it promotes sediment stability and affects water column and sediment biogeochemistry, while its dense stands provide habitats for aquatic fauna (Pinardi et al. 2009, Ejsmont – Karabin 2011, Ribaudo et al. 2011). Vallisneria spiralis is a thermophilous species originating from warmer climates and as such has become widespread mainly in tropical and subtropical, as well as in other areas outside its native range with milder and thus favourable climatic conditions. Furthermore, its introduced range has expanded to colder regions, such as Central, Northern and Eastern Europe, as well as Siberia, where it has been reported mainly from artificially heated and geothermal water bodies (Hussner & Lösch 2005, Hutorowicz & Hutorowicz 2008, Katsman & Kuchkina 2010, Zarubina & Sokolova 2011, Wasowicz et al. 2014). Nevertheless, climate change and the associated increase in water tem- perature could lead to the further spread of the species in areas of higher latitudes and altitudes (Hussner & Lösch 2005, Wu & Ding 2019). Since the species is a widely used aquarium plant, ornamental trading and release from aquaria into the wild are recognized as the main ways by which it is introduced (Hussner & Lösch 2005, Thiébaut 2007, Martin & Coetzee 2011, Hussner 2012). The species is considered native to Croatia (Nikolić 2020) and is strictly protected under the Nature Protec- tion Act (Official Gazette 80/13, 15/18, 14/19, 127/19). However, its distribution is poorly known, with only a few localities reported. Therefore, it is classified as Data Deficient (DD) (Nikolić 2020) according to IUCN crite- ria, although Least Concern (LC) globally (Gupta 2017). Prior to our research, V . spiralis had been recorded in Cro- atia only twice. The first record from the Continental Re - gion dates back to the 19 th century (Herbarium specimen ID - ZA12062, ZA Herbarium Croaticum), while the second is relatively recent and reported from the Mediter- ranean Region (Glasnović et al. 2015). The aim of the present study was to determine the sta- tus of V. spiralis in Croatia. To accomplish this objective we (a) determined the current distribution of V. spiralis in Croatia, (b), identified its habitat preferences regard- ing the physico-chemical and chemical parameters, as well as water velocity and substrate type and (c) examined its preferences regarding the phytosociological characteristics. Materials and methods Study area Data on the distribution of Vallisneria spiralis, as well as physicochemical and phytosociological characteristics of its habitats were collected mostly within the national sys- tem for monitoring of surface waters, and partly through an independent project focusing on the aquatic and ripar- ian vegetation of the lowland watercourses. Over 600 lo- cations were surveyed during the vegetation seasons from 2010 to 2019, ultimately covering most of the Croatian territory, including 277 rivers and 46 lakes (including reservoirs) distributed in three biogeographical regions – Continental, Alpine and Mediterranean (European Com- munity 1992, European Environmental Agency 2020). Localities at which V. spiralis occurred are situated in the Continental Region (the Kupa and the Rečica rivers), as well as in the Mediterranean Region (Butoniga Reservoir) of Croatia (Figure 1). Butoniga is a reservoir in the Istrian Peninsula con- structed on the Butoniga River for the purpose of water supply and flood protection. The total reservoir area is 2.45 km 2 , with an average depth of 9.1 m and is situated at 41 m a.s.l. The water body is classified as heavily modi - fied (Vučković et al. 2019a), i.e. a body of surface water which is substantially changed in character as a result of physical alterations by human activity (European Com- munity 2000). The Kupa River is a right tributary of the Sava River, be- longing to the Black Sea Hydrological Catchment. It rises 20/1 • 2021, 7–18 9 Anja Rimac, Antun Alegro, Vedran Šegota, Nikola Koletić, Igor Stanković, Sandro Bogdanović & Nina Vuković Distribution and habitat characteristics of Vallisneria spiralis L. in Croatia in the mountains of the Gorski Kotar Region, flows a few kilometres eastwards before forming a natural, 118 km long border between north-west Croatia and southeast Slovenia and then continues to flow through Croatia, reaching its confluence in the town of Sisak. The total length of the river is 279.4 km, with a catchment area of 10.226 km 2 . In the upper and middle reaches, the river has a karstic character, while the lower reach, where V . spi- ralis was recorded, is classified as a large lowland river (Eu- ropean Community 2000, Mihaljević 2011). The Rečica River is a small lowland river with clayey-sandy substrate (European Community 2000, Mihaljević 2011), a right tributary of the Kupa River, with its mouth near the set- tlement Donja Rečica. Macrophyte survey Watercourses were surveyed for macrophytes along 100 m long transects, while multiple 6 m × 100 m transects were used when surveying macrophytes in lakes. In less-accessi- ble areas, the bottom was raked to reach the macrophytes, using either a long pole or a rope with a rake. Cover and abundance of each plant species were estimated using the expanded nine-degree Braun-Blanquet scale: r = one indi- vidual; + = up to 5 individuals; 1 = up to 50 individuals; 2 m = over 50 individuals, coverage < 5%; 2a = cover- age between 5 and 15%; 2b = coverage between 15 and 25%; 3 = coverage between 25 and 50%; 4 = coverage between 50 and 75%; 5 = coverage over 75% (Barkman et al. 1964, Braun-Blanquet 1964, Dierschke 1994). All macrophytes were identified to the species level. The identification of Vallisneria spiralis was based on the morphological characteristics with the use of several iden- tification keys (Casper & Krausch 1980, Lowden 1982, van de Weyer et al. 2011, Nikolić 2019). Collected speci- mens of V. spiralis were preserved in 50% ethanol with 10% of glycerol added, or desiccated in a herbarium press and deposited in herbarium ZA (Thiers 2020) (ID num- bers ZA56139 - ZA56159). The nomenclature follows EURO+MED (2020) for vascular plants and AlgaeBase (Guiry & Guiry 2020) for Charophytes, while the syntax- onomical system proposed by Mucina et al. (2016) and Škvorc et al. (2017) was applied for the vegetation types. The coordinates of the localities were recorded using an eTrex 30x GPS device and a distribution map was created using ArcGIS 10.5 software. Figure 1: Distribution map of Vallisneria spiralis in Croatia. Slika 1: Karta razširjenosti vrste Vallisneria spiralis na Hrvaškem. 20/1 • 2021, 7–18 10 Anja Rimac, Antun Alegro, Vedran Šegota, Nikola Koletić, Igor Stanković, Sandro Bogdanović & Nina Vuković Distribution and habitat characteristics of Vallisneria spiralis L. in Croatia Furthermore, all localities within the national surface water monitoring system were sampled for basic water physicochemical parameters and nutrients. Conductiv- ity, salinity and pH were measured in situ with the Sev- enMulti Modular Meter System (Mettler Toledo) under standard conditions. Dissolved oxygen was measured in situ with a Hach HQ40D Portable Multi Meter using an optical sensor. Water samples for alkalinity measurement were stored at 4–5 °C and alkalinity was determined by titration within 24 hours of sampling. Water samples for total phosphorus analysis were preserved with sul- phuric acid and analysed with a UV-VIS spectrometer (Perkin Elmer Lambda 25). Water samples for total ni- trogen were collected in dark bottles, filled to the top, stored at 4–8 °C and analysed within 24 hours using a Shimadzu TOC-VCPH equipped with an analyser for total nitrogen. Water samples for ammonium, nitrate and orthophosphate analysis were immediately filtered on glass filters with 0.45 µm pores, stored at 4–8 °C and analysed within 24 hours on ionic chromatographer (Di- onex 3000). The box-plot diagrams of physicochemical and chemical parameters were prepared using the pro- gram package Past 4.02 (Hammer et al. 2001). Water flow velocity was estimated according to Janauer (2003), who distinguishes four categories (1 = no flow, stagnant, 2 = low flow velocity, from just visible to ca. 30 cm/s, 3 = medium flow velocity, ca. 35–65 cm/s, 4 = high flow velocity, ca. > 65 cm/s). Results Eight new localities of Vallisneria spiralis were recorded during the research of aquatic vegetation in Croatia (Figure 1). The majority of the localities are situated in the Continental part of Croatia. Six are distributed along 96.05 km of the lower course of the Kupa River (Figure 2), while a single locality is recorded in the Rečica River, near its confluence with the Kupa. The single locality in the Mediterranean part is from Butoniga Reservoir in Istria. In the Kupa and Rečica rivers, V. spiralis was rooting in depths of up to 1.5 m, on muddy, sandy and gravelly sub- strate in medium and slow flowing water. During June and early July, flowering was not observed, while flow- ering individuals were recorded in early August. Male plants were found in Gradec Pokupski and Letovanić, while female plants were observed in Mala Gorica (Figure 2, Figure 3). In the Kupa River, dense stands of V. spiralis were ob- served in Letovanić, where aquatic vegetation was abun- dantly developed and dominated by Potamogeton nodo- sus, Myriophyllum spicatum and Najas marina (Figure 3). Vallisneria spiralis abundance was lower in other locali- ties in the Kupa, where it was associated with the more abundant M. spicatum, Ceratophyllum demersum and Nuphar lutea (Table 1). In the Rečica River, all macro- phyte species were represented with only a small number of individuals. Surveyed aquatic vegetation at both rivers belongs mostly to the alliance Nymphaeion-albae Oberd. 1957 (class Potamogetonetea Klika in Klika et Novák 1941), i.e. vegetation of rooted floating-leaf macrophytes of sheltered nutrient-rich freshwaters of Western and Central Europe. The communities can be assigned to the association Myriophyllo-Nupharetum luteae (W. Koch 1926) Hueck 1931. However, elements of the alliance Potamogetonion Libbert 1931 are also present, since the communities gradually change depending on the water depth and velocity. Figure 2: Vallisneria spiralis localities along the Kupa and Rečica rivers. Slika 2: Lokalitete vrste Vallisneria spiralis vzdolž rek Kolpa in Rečica. 20/1 • 2021, 7–18 11 Anja Rimac, Antun Alegro, Vedran Šegota, Nikola Koletić, Igor Stanković, Sandro Bogdanović & Nina Vuković Distribution and habitat characteristics of Vallisneria spiralis L. in Croatia The water in the Kupa River was slightly basic and alkaline, with conductivity not exceeding 428 µS/cm at any investigated locality. The measured concentration of reactive phosphorous was low, with annual average con- centration ranging from <0.005 in the most upstream lo- cality to 0.019 mgP/L in the most downstream locality. The measured concentration of ammonium was low at all investigated localities, while the concentration of nitrates was moderately elevated, but still indicating a good sta- tus regarding the eutrophication (Table 2, Figure 4). On the contrary, the measured concentration of nutrients in the Rečica River indicated eutrophication. In this locality, water was also slightly basic, alkaline, with conductivity ranging between 89.9 and 465 µS/cm (T able 2, Figure 4). In Butoniga Reservoir, flowering female plants (Fig- ure 3) were recorded in early August, rooted in the muddy substrate in up to 1 m deep, stagnant water. The species was here present within the vegetation of alliance Potamogetonion, codominant with Potamogeton perfola- tus and P. nodosus, as well as Chara vulgaris (Table 1), a member of alliance Charion vulgaris (W. Krause et Lang 1977) W. Krause 1981 (class Charetea intermediae F. Fukarek 1961). Water in Butoniga Reservoir was clear, slightly basic and alkaline, with conductivity ranging from 286 to 428 µS/cm. Regarding the measured nutri- ent concentrations, the reservoir is oligotrophic (Table 2, Figure 4). Figure 3: (A) Vallisneria spiralis habitus; (B, C) female plants at Butoniga Reservoir; (D) stand of V. spiralis in Letovanić; (E) Mala Gorica, typical habitat at the Kupa River. Slika 3: (A) habitus vrste Vallisneria spiralis; (B, C) ženske rastline v umetnem jezeru Butoniga; (D) sestoj vrste V. spiralis pri kraju Letovanić; (E) Mala Gorica, značilno rastišče ob reki Kolpi. 20/1 • 2021, 7–18 12 Anja Rimac, Antun Alegro, Vedran Šegota, Nikola Koletić, Igor Stanković, Sandro Bogdanović & Nina Vuković Distribution and habitat characteristics of Vallisneria spiralis L. in Croatia Figure 4: The box-plot diagrams of water physicochemical and chemical parameters at Vallisneria spiralis localities. * The values measured at Šišinec, situated between Pokupsko and Letovanić, were used as the measurements were not performed at the two latter sites. Slika 4: Grafikoni kvantilov fizikalno - kemijske in kemijske lastnosti vode na lokalitetah vrste Vallisneria spiralis. * Vrednosti, merjene na lokacijah pri kraju Šišinec, med krajema Pokupsko in Letovanić, smo uporabili, ker nismo naredili meritev na slednjih dveh lokacijah. 20/1 • 2021, 7–18 13 Anja Rimac, Antun Alegro, Vedran Šegota, Nikola Koletić, Igor Stanković, Sandro Bogdanović & Nina Vuković Distribution and habitat characteristics of Vallisneria spiralis L. in Croatia Finding site Butoniga Reservoir Kupa River Rečica River Donje Mekušje Donja Rečica Gradec Pokupski Letovanić Mala Gorica Year of sampling 2015 2016 2016 2016 2018 2016 2016 E 13.929641 15.597712 15.670385 15.857849 16.200952 16.240963 15.666359 N 45.326645 45.487189 45.481073 45.547518 45.496732 45.477213 45.481133 Vallisneria spiralis L. 2a 2m 2m 1 2a 2m 1 Nuphar lutea (L.) Sm. . 1 4 2m 2a 2a 1 Myriophyllum spicatum L. . 1 4 2m 2b 2a . Ceratophyllum demersum L. . + + 2m 2a 2a 1 Alisma plantago-aquatica L. . + + r + . 1 Potamogeton perfoliatus L. 2b . 1 1 . 1 . Potamogeton nodosus Poir. 2a . . 1 2b . 1 Stuckenia pectinata (L.) Börner . 2m . 1 2m . . Najas marina L. . . + 1 2b 2a . Najas minor All. . . . 1 2a + . Potamogeton berchtoldii Fieber . . . . 2m 2m . Nitellopsis obtusa (Desv.) J.Groves . . . . + 2m . Elodea canadensis Michx. . . + . 1 . . Sparganium emersum Rehmann . . 1 . . . 1 Potamogeton pusillus L. . . . + . . 1 Chara vulgaris L. 2b . . . . . . Nitella opaca (Bruz.) C.Agardh . + . . . . . Persicaria dubia (Stein) Fourr. . . . 1 . . . Rorippa sylvestris (L.) Besser . . . 1 . . . Alisma lanceolatum With. . . . r . . . Ranunculus fluitans Lam. . . . . 2m . . Butomus umbellatus L. . . . . . + . Lemna minor L. . . . . . 2m . Table 1: List of species at finding sites of Vallisneria spiralis, with the estimated cover and abundance according to expanded Braun-Blanquet scale and coordinates in WGS84 coordinate system, x = longitude, y = latitude. At locality Pokupsko (x =15.991128, y = 45.485593), visited in 2016, relevé was not made. Tabela 1: Seznam vrst na rastišču vrste Vallisneria spiralis z ocenjeno pokrovnostjo in pogostnostjo v skladu z razširjeno Braun- Blanquetovo skalo in koordinate v WGS84 koordinatnem sistemu, x = zemljepisna dolžina, y = zemljepisna širina. Na lokaciji Pokupsko (x =15,991128, y = 45,485593), ki smo jo obiskali leta 2016, nismo naredili vegetacijskega popisa. 20/1 • 2021, 7–18 14 Anja Rimac, Antun Alegro, Vedran Šegota, Nikola Koletić, Igor Stanković, Sandro Bogdanović & Nina Vuković Distribution and habitat characteristics of Vallisneria spiralis L. in Croatia Parameter Butoniga R eser v oir Kupa River Rečica River Donje Mekušje Gradec Pokupski Šišinec* Mala Gorica water temperature [°C] range 7–29.5 2.7–25.6 2.7–26.8 2.90–26.7 2.6–29 3.8–23.5 avg. 16.80 15.30 15.10 16.20 17.00 13.20 pH range 8–8.2 7.52–8.3 7.48–8.2 7.72–8.3 7.36 –8.3 6.8 –8.2 avg. 8.10 8.10 8.00 8.10 8.00 7.70 conductivity [µS/cm] range 286–428 321–391 325–410 316–405 230–409 89.9–465 avg. 359 350 358 357 354 297 alkalinity [mgCaCO₃/L] range 124 –195 164–200 165–210 160– 207 109–208 32–221 avg. 158 177 180 180 176 147 dissolved oxygen [mgO₂/L] range 8.1–13.2 7.8–13.6 8–14 7.2–13.7 9.2–13.2 5.77–14.2 avg. 9.70 10.70 10.70 10.40 10.40 9.20 ammonium [mgN/L] range — <0.008–0.033 <0.008–0.110 <0.008–0.076 <0.008–0.083 0.016–0.788 avg. — 0.017 0.031 0.023 0.023 0.352 nitrates [mgN/L] range 0.02–0.26 0.28–0.89 0.32–0.89 0.33–0.87 0.38–0.88 0.36–1.34 avg. 0.15 0.63 0.65 0.65 0.66 0.84 total nitrogen [mgN/L] range 0.23–0.52 0.52–1.20 0.56–1.31 0.65–1.17 0.62–1.39 0.41–2.38 avg. 0.37 0.83 0.92 0.88 0.96 1.42 orthophosphate [mgP/L] range <0.005–0.010 <0.005–0.010 <0.005–0.030 <0.005–0.020 <0.005–0.077 0.016–0.087 avg. 0.004 <0.005 0.012 0.011 0.019 0.052 total phosphorous [mgP/L] range 0.010–0.030 0.011–0.047 0.020–0.059 0.021–0.060 0.029–0.117 0.050–0.526 avg. 0.020 0.023 0.036 0.036 0.053 0.160 Table 2: Basic physicochemical and chemical parameters of water at Vallisneria spiralis localities, based on monthly measurements during the year of sampling. *The values measured at Šišinec, situated between Pokupsko and Letovanić, are given instead as the measurements were not performed at the two latter sites. Tabela 2: Osnovne fizikalno - kemijske in kemijske lastnosti vode na lokalitetah vrste Vallisneria spiralis na osnovi mesečnih mer- itev v letu vzorčenja. * Vrednosti, merjene na lokacijah pri kraju Šišinec, med krajema Pokupsko in Letovanić, smo uporabili, ker nismo naredili meritev na slednjih dveh lokacijah. Discussion Prior to our research, Vallisneria spiralis has been reported only from two localities in Croatia, first from the Lonja River, a record dating back into the 19 th century (Her- barium specimen ID - ZA12062, ZA Herbarium Croati- cum) and second from the Modro Oko Spring, reported in 2011 (Glasnović et al. 2015). The Lonja River is a me- dium-sized, eutrophic river, situated in the Continental part of Croatia. During our research, several localities in the river have been surveyed, including the stretches near Sovoj and Trebež, where the first record was made, but V. spiralis was not recorded. Extensive hydromorphologi- cal modifications in the 1970s, including the construc- tion of dykes and retention, significantly altered the river flow and certain parts of the river completely dried up. As a consequence of the abovementioned changes, it is pos- sible that V. spiralis no longer persists in this watercourse. The Modro Oko Spring is an oligohaline limnocrene karst spring situated in the Neretva River Delta in the Mediter- ranean part of Croatia, with clear water of high alkalinity and conductivity (Rimac et al. 2018). The aquatic vegeta- tion of the spring was surveyed several times, from 2014 to 2016 (Rimac et al. 2018) and in 2018 (Vuković et al. 2018, 2019), however, V. spiralis was never confirmed. In all localities surveyed within this study, measured values of pH and alkalinity indicate slightly basic, alka- 20/1 • 2021, 7–18 15 Anja Rimac, Antun Alegro, Vedran Šegota, Nikola Koletić, Igor Stanković, Sandro Bogdanović & Nina Vuković Distribution and habitat characteristics of Vallisneria spiralis L. in Croatia line conditions and along with measured water tempera- ture and electrical conductivity fall within the ranges previously reported for V. spiralis (Hussner & Lösch 2005, Mukhopadhyay & Dewanji 2005, Rai & Tripathi 2009, Ejsmont-Karabin & Hutorowicz 2011, Collas et al. 2012). The species was growing on various substrates, muddy, sandy and gravely, in up to 1.5 m deep water, as already observed in other studies (Hussner & Lösch 2005, Zarubina & Sokolova 2011). The species is a typi- cal component of lowland, meso- to eutrophic aquatic systems (Hutorowicz 2006, Pyka et al. 2007, Gecheva et al. 2011, Landucci et al. 2011), but known to occur in oligotrophic conditions as well (Bolpagni et al. 2015). Regarding the nutrient level, localities in Croatia cover considerable range, with the lowest level in oligotrophic Reservoir Butoniga, where V. spiralis was accompanied with Chara vulgaris and Potamogeton perfoliatus, both good indicators of low phosphorous concentration (Ste- fanidis et al. 2018). A study conducted in Greece showed that lake vegetation types characterized by the domi- nance of P. perfoliatus and V. spiralis appear to be associ- ated with high nitrate concentrations (Stefanidis et al. 2019), but this was not the case in Croatia. In the Rečica River, levels of nutrients were high and V. spiralis was growing with Ceratophyllum demersum and Potamogeton pusillus, species tolerant to eutrophication (Smolders et al. 2001). In this locality, aquatic vegetation was sparsely developed, with all species represented with only few individuals, presumably because of considerable hydro- morphological alterations, i.e. channelization and river- bed deepening. The majority of localities are situated along the Kupa River, which is in near-natural condition along its entire course (Urbanič et al. 2020). Measured levels of nutri- ents suggest satisfactory status for a lower course of a lowland river, with only moderately raised concentration of nitrates. Here, V. spiralis was associated with species characteristic for lowland medium- to slow-flowing wa- tercourses, such as Potamogeton nodosus, Myriophyllum spicatum, Najas marina, C. demersum and Nuphar lutea, within assemblages generally associated with more eutro- phic water. These communities, as well as the community from the Rečica River can be assigned to the association Myriophyllo-Nupharetum luteae (W. Koch 1926) Hueck 1931., where constant presence of C. demersum indicates nutrient enrichment, at least in the sediment. Such com- munities are known to develop near the banks, in slow flowing and shallow segments of the rivers. Regarding the hydromorphology, localities on the Kupa River were assessed as in near natural condition, with no or only minor alternations in hydrological regime, longitudinal connectivity and morphology (Vučković et al. 2019b). Eight newly recorded localities resulting from a com- prehensive study of the aquatic vegetation covering the majority of the country’s territory provide a valuable new insight into the distribution of V. spiralis in Croatia, con- firming its status of a rare species. The species is consid- ered native to Croatia and as a very rare is strictly pro- tected. However, the information on its origin in other Southeastern European countries is contradictory. Ac- cording to European Floras, the species is native to the whole Southeastern Europe (Hayek 1932–1933, Tutin et al. 1964–1980), but on the contrary, it is reported as an alien from Serbia (Lansdown et al. 2010, Anačkov et al. 2011, 2013), Romania (Ciocârlan 2009) and European Turkey (Lansdown et al. 2010). Furthermore, according to Soó (1964–1980), V . spiralis is usually considered native to the Mediterranean parts of the Balkans, while occur- rences in the Pannonian ecoregion are most likely alien. In the neighbouring Serbia, V . spiralis is a most frequently re- corded aquatic alien plant (Anđelković et al. 2016), listed as invasive in the Province of Vojvodina (Anačkov et al. 2011, 2013). It mostly inhabits tidal rivers, as well as per- manent non-tidal smooth flowing watercourses, with the majority of records from the waterbodies situated in the Danubian floodplain, belonging to the Pannonian ecore- gion (Radulović et al. 2010, Anđelković et al. 2016). Simi- larly, it is reported from Danube Delta and Banat Region (Ciocârlan 2011, Otves et al. 2014) in Romania, where the species is naturalized according to Anastasiu et al. (2007) and Lansdown et al. (2010) and potential threat to aquatic ecosystems (Sîrbu 2007, Otves et al. 2014). On the other hand, records from its native range in the Mediterranean, mostly include stagnant water bodies of different ecological characteristics. In Greece, V. spiralis is dominant or common macrophyte in shallow littoral zone of deep oligotrophic lakes, as well as in shallow meso- eutrophic and eutrophic lakes on calcareous bedrock, situ- ated both at low- and mid-altitudes (Koumpli-Sovantzi 1989, Danielidis 1996, Hollis & Stevenson 1997, Chalkia & Kehayias 2013a,b, Matzafleri et al. 2013, Stefanidis et al. 2018). Furthermore, it is present in both Macedonian and Greek part of Lake Megali Prespa, a large, deep, basic and mesotrophic lake, situated at 850 m a.s.l (Talevska et al. 2009, Stefanidis et al. 2018.). In Italy, the species was also recorded at mid-altitudes, in shallow and basic lakes – eutrophic Lake Chiusi (Stella 1988, Lastrucci et al. 2014) and meso- to eutrophic Lake Trasimeno (Landucci et al. 2011). In both lakes, it was present and dominant within association Potamo perfoliati-Vallisnerietum spiralis Losev et Golub in Gloub, Losev et Mirkin 1991 (Lastrucci et al. 2014), while in Lake Trasimeno, it occurred in sev- eral other communities of the classes Potamogetonetea and Charetea intermediae (Landucci et al. 2011). 20/1 • 2021, 7–18 16 Anja Rimac, Antun Alegro, Vedran Šegota, Nikola Koletić, Igor Stanković, Sandro Bogdanović & Nina Vuković Distribution and habitat characteristics of Vallisneria spiralis L. in Croatia Although the majority of populations of V. spiralis in Croatia are located in the Continental Region, which for the most part corresponds to the Pannonian ecoregion in terms of the Water Framework Directive (European Com- munity 2000), there is no evidence that the species has been introduced at some point. Having in mind its eco- logical preferences, and the fact that suitable aquatic habi- tats are quite widespread in Croatia, such limited distribu- tion is indeed interesting and unexpected, especially the absence from the waterbodies of the Danube floodplain in eastern Croatia, which were quite well investigated in re- cent times (e.g. Ozimec & Topić 2001, Kočić et al. 2008, Ozimec & Topić 2018, Rožac et al. 2018, Nikolić 2020). Acknowledgments The research was undertaken within the Project of Wa- ter Bodies Surveillance, financed by the State Institution for Water Management “Hrvatske vode”. We would like to thank Hrvatske vode for the provision of the data on physicochemical and chemical properties of water. We are also grateful to two anonymous reviewers for their constructive input and helpful suggestions and colleague Mladen Plantak, always willing to help and discuss geog- raphy and hydromorphology. Anja Rimac , https://orcid.org/0000-0002-8720-9493 Antun Alegro , https://orcid.org/0000-0001-6193-2395 Vedran Šegota , https://orcid.org/0000-0002-9233-7457 Nikola Koletić , https://orcid.org/0000-0002-0435-3060 Igor Stanković , https://orcid.org/0000-0001-7341-144X Sandro Bogdanović , https://orcid.org/0000-0002-1952-6059 Nina Vuković , https://orcid.org/0000-0001-8149-0782 References Anastasiu, P ., Negrean, G., Bașnou, C., Sîrbu, C. & Oprea, A. 2007: A preliminary study on the neophytes of wetlands in Romania. In: Rabitsch, W., Essl, F & Klingenstein, F . (eds.): Biological Invasions– from Ecology to Conservation. NeoBiota 7: 181–192. Anđelković, A. A., Živković, M. M., Cvijanović, D. Lj., Novković, M. Z., Marisavljević, D. P., Pavlović, D. M. & Radulović, S. B. 2016: The contemporary records of aquatic plants invasion through the Danubian floodplain corridor in Serbia. Aquatic Invasions 11(4): 381–395. Anačkov, G., Bjelić-Čabrilo, O., Karaman, I., Karaman, M., Radenković, S., Radulović, S., Vukov, D. & Boža, P . 2011: Lista invazivnih vrsta na području AP Vojvodine, 0.2 beta verzija. [List of invasive species in AP Vojvodina, 0.2beta version]. http://iasv.dbe.pmf. uns.ac.rs (accessed 14 April 2020) Anačkov, G. T., Rat, M. M., Radak, B. Dj., Igić, R. S., Vukov, D. M., Rućando, M. M., Krstivojević, M. M., Radulović, S. B., Cvijanović, D. Lj., Milić, D. M., Panjković, B. I., Szabados, K. L., Perić, R. D., Kiš, A. M., Stojšić, V. R. & Boža, P . P . 2013: Alien invasive neophytes of the Southeastern part of the Pannonian Plain. Central European Journal of Biology 8: 1032–104. DOI: https://doi.org/10.2478/ s11535-013-0225-6 Barkman, J., Doing, H. & Segal, S. 1964: Kritische Bemerkungen und Vorschläge zur quantitativen Vegetationsanalyse. Acta Botanica Neerlandica 13: 394–419. Bolpagni, R., Laini, A., Soana, E., Tomaselli, M. & Nascimbene, J. 2015: Growth performance of Vallisneria spiralis under oligotrophic conditions supports its potential invasiveness in mid‐elevation freshwaters. Weed Research 55(2): 185–194. DOI: https://doi. org/10.1111/wre.12128 Braun-Blanquet, J. 1964: Pflanzensoziologie, Grundzüge der Vegetationskunde. 3rd Edition. Springer-Verlag, Berlin, 631 pp. Casper, S. J. & Krausch, H. D. 1980: Süßwasserflora von Mitteleuropa, Pteridophyta und Anthophyta, 1. Teil, Band 23. Gustav Fischer Verlag, Stuttgart, 403 pp. Chalkia, E. & Kehayias, G. 2013a: Zooplankton and environmental factors of a recovering eutrophic lake (Lysimachia Lake, Western Greece). Biologia, Section Zoology 68(3): 459–469. DOI: https://doi. org/10.2478/s11756-013-0171-9 Chalkia, E. & Kehayias, G. 2013b: Zooplankton community dynamics and environmental factors in Lake Ozeros (Greece). Mediterranean Ma- rine Science 14(3): 32–41. DOI: https://doi.org/10.12681/mms.534 Ciocârlan, V. 2009: Flora Ilustrată a României. Pteridophyta et Spermatophyta. Editura Ceres, Bucureşti, 1138 pp. Ciocârlan, V. 2011: Vascular flora of the Danube Delta. Analele Ştiinţifice ale Universităţii „Al. I. Cuza” din Iaşi (serie nouă), Secţiunea II a. Biologie vegetală 57(1): 41–64. Collas, F . P . L., Beringen, R., Koopman, K. R, Matthews, J., Odé, B., Pot, R., Sparrius, L. B., van Valkenburg, J. L. C. H., Verbrugge, L. N. H. & Leuven, R. S. E. W. 2012: Knowledge document for risk analysis of non-native Tapegrass (Vallisneria spiralis) in the Netherlands. Department of Environmental Science, Faculty of Science, Institute for Water and Wetland Research, Radboud University Nijmegen, Nijmegen, 48 pp. Danielidis, D. B., Spartinou, M. & Economou-Amilli, A. 1996: Limnological survey of Lake Amvrakia, western Greece. Hydrobiologia 318: 207–218. DOI: https://doi.org/10.1007/BF00016682 Dierschke, H. 1994: Pflanzensoziologie. Grundlagen und Methoden. Eugen Ulmer Verlag, Stuttgart, 683 pp. Ejsmont-Karabin, J. 2011: Does invasion of Vallisneria spiralis L. promote appearance of rare and new rotifer (Rotifera) species in littoral of the lakes heated by power station (Konin Lakes, W. Poland)? Polish Journal of Ecology 59(1): 201–207. Ejsmont-Karabin, J. & Hutorowicz, A. 2011: Spatial distribution of rotifers (Rotifera) in monospecies beds of invasive Vallisneria spiralis L. in heated lakes. Oceanological and Hydrobiological Studies 40(4): 71–76. DOI: https://doi.org/10.2478/s13545-011-0043-2 EURO+MED 2019: Euro+Med PlantBase – the information resource for Euro-Mediterranean plant diversity. http://www.emplantbase.org (accessed 10 April 2020) European Community 1992: Council Directive 92/43/EEC of 21 May 1992 on the conservation of natural habitats and of wild fauna and flora. Official Journal of the European Communities lL206: 7–50. European Community 2000: Directive 2000/60/EC of the European Parliament and of the Council of 23 October 2000 establishing a framework for Community action in the field of water policy. Official Journal of the European Communities L327: 1–72. 20/1 • 2021, 7–18 17 Anja Rimac, Antun Alegro, Vedran Šegota, Nikola Koletić, Igor Stanković, Sandro Bogdanović & Nina Vuković Distribution and habitat characteristics of Vallisneria spiralis L. in Croatia European Environmental Agency 2020: Biogeographical regions. https://www.eea.europa.eu/data-and-maps/data/biogeographical- regions-europe-3 (accessed 1 July 2020) Gecheva, G. M., Cheshmedjiev, S.D. & Dimitrova-Dyulgerova, I. Zh. 2011: Macrophyte-Based Assessment of the Ecological Status of Lakes in Bulgaria. Ecologia Balkanica 3(2): 25–40. Glasnović, P ., Novak, Š., Behrič, S. & Fujs, N. 2015: Towards a Checklist of the Vascular Flora of the Neretva River Delta (Croatia). Natura Croatica 24 (2): 163–190. Guiry, M. D. & Guiry, G. M. 2020: AlgaeBase. World-wide electronic publication. https://www.algaebase.org (accessed 10 April 2020) Gupta, A. K. 2017: Vallisneria spiralis. In: The IUCN Red List of Threatened Species 2017: e.T164121A84301330. DOI: https://dx.doi. org/10.2305/IUCN.UK.2017-1.RLTS.T164121A84301330.en. (accessed 3 April 2020) Hammer, Ø., Harper, D.A.T. & Ryan, P .D. 2001: PAST: Paleonto- logical Statisctisc software package for education and data analysis. Paleontologica Electronica 4(1): 1–9. Hayek, A. 1932–1933: Prodromus florae peninsulae Balcanicae. Verlag des Repertorium Specierum novarum Regni Vegetabilis, Dahlem bei Berlin, 472 pp. Heidbüchel, P ., Kuntz, K. & Hussner, A. 2016: Alien aquatic plants do not have higher fragmentation rates than native species: a field study from the River Erft. Aquatic Sciences 78: 767–777. DOI: https://doi. org/10.1007/s00027-016-0468-1 Hollis, G.E. & Stevenson, A. C. 1997: The physical basis of the Lake Mikri Prespa systems: geology, climate, hydrology and water quality. Hydrobiologia 351: 1–19. DOI: https://doi. org/10.1023/A:1003067115862 Hussner, A. & Lösch, R. 2005: Alien aquatic plants in a thermally abnormal river and their assembly to neophyte - dominated macrophyte stands (River Erft, Northrhine – Westphalia). Limnologica 35(1-2): 18–30. DOI: https://doi.org/10.1016/j.limno.2005.01.001 Hussner, A. 2012: Alien aquatic plant species in European countries. Weed Research 52(4): 297–306. DOI: https://doi.org/10.1111/j.1365- 3180.2012.00926.x Hutorowicz, A. 2006: Vallisneria spiralis L. (Hydrocharitaceae) in Lakes in the vicinity of Konin (Kujawy Lakeland). Biodiversity Research and Conservation 1-2: 154–158. Hutorowicz, A. & Hutorowicz, J. 2008: Seasonal development of Vallisneria spiralis L. in a heated lake. Ecological Questions 9: 79–86. Janauer, G.A. 2003: Methods. Archiv für Hydrobiologie Supplement 147(1–2), Large Rivers 14(1-2): 9–16. Katsman, E. A. & Kuchkina, M.A. 2010: Introduction of Vallisneria spiralis into the Desnogorsk Reservoir. Russian Journal of Biological Invasions 1(3): 159–161. DOI: https://doi.org/10.1134/ S2075111710030033 Kočić, A., Hengl, T. & Horvatić, J. 2008: Water nutrient concentrations in channels in relation to occurrence of aquatic plants: a case study in eastern Croatia. Hydrobiologia 603(1): 253–266. DOI: https://doi.org/10.1007/s10750-007-9276-x Koumpli-Sovantzi, L. 1989: The Aquatic Flora of Aetoloakarnania (W Greece). Willdenowia 18(2): 377–385. Lastrucci, L., Bonari, G., Angiolini, C., Casini, F ., Giallonardo, T., Gigante, D., Landi, M., Landucci, F ., Venanzoni, R. & Viciani, D. 2014: Vegetation of Lakes Chiusi and Montepulciano (Siena, central Italy): updated knowledge and new discoveries. Plant Sociology 51(2): 29–55. DOI: https://doi.org/10.7338/pls2014512/03 Landucci, F ., Gigante, D. & Venanzoni, R. 2011: An application of the Cocktail method for the classification of the hydrophytic vegeta- tion at Lake T rasimeno (Central Italy). Fitosociologia 48(2): 3–22. Lansdown, R. V., Anastasiu, P ., Barina, Z., Bazos, I., Çakan, H., Cako- vić, D., Delipetrou, P ., Matevski, V., Mitić, B., Ruprecht, E., Tomo- vić, G., Tosheva, A. & Király, G. 2016: Review of Alien Fresh water Vascular Plants in South-east Europe. In: Rat, M., T richova, T., Scalera, R., Tomov, R., Uludag, A. (eds.): ESENIAS Scientific Reports 1. State of the Art of Alien Species in South - Eastern Europe. University of Novi Sad, IBER-BAS & ESENIAS, Novi Sad & Sofia, pp. 137–154. Les, D. H., Jacobs, S. W. L., Tippery, N. P ., Chen, L., Moody, M. L. & Wilstermann-Hildebrand, M. 2008: Systematics of Vallisneria (Hydrocharitaceae). Systematic Botany 33(1): 49–65. Lowden, R. M. 1982: An approach to the taxonomy of Vallisneria L. (Hydrocharitaceae). Aquatic Botany 13: 269–298. DOI: https://doi. org/10.1016/0304-3770(82)90064-X Martin, G. & Coetzee, J. 2011: Pet stores, aquarists and the internet trade as modes of introduction and spread of invasive macrophytes in South Africa. Water SA 37: 371–380. Matzafleri, N., Psilovikos, A., Neofytou, C. & Kagalou, I. 2013: Determination of the trophic status of Lake Kastoria, Western Macedonia, Greece. In: Zouboulis, A., Kungolos, A. & Samaras, P . (eds.): Proceedings of the Fourth international conference on small and decentralized water and wastewater treatment plants. University of Thessaly, Aristotle University of Thessaloniki, Thessaloniki Hellenic Union of Municipal Enterprises for Water Supply and Sewerage, Volos, pp. 163–169. Mihaljević, Z. 2011: Tipologija tekućica Hrvatske temeljem zajednice makrozoobentosa. Hrvatske vode 19(76): 111–116. Mucina, L., Bültmann, H., Dierßen, K., Theurillat, J.-P ., Raus, T., Čarni, A., Šumberová, K., Willner, W., Dengler, J., García, R. G., Chytrý, M., Hájek, M., Di Pietro, R., Iakushenko, D., Pallas, J., Daniëls, F . J. A., Bergmeier, E., Santos Guerra, A., Ermakov, N., Valachovič, M., Schaminée, J. H. J., Lysenko, T., Didukh, Y. P ., Pignatti, S., Rodwell, J. S., Capelo, J., Weber, H. E., Solomeshch, A., Dimopoulos, P ., Aguiar, C., Hennekens, S. M. & Tichý, Lj. 2016: Vegetation of Europe: Hierarchical floristic classification system of vascular plant, bryophyte, lichen, and algal communities. Applied Vegetation Science 19 (Suppl. 1): 3–264. DOI: https://doi. org/10.1111/avsc.12257 Mukhopadhyay, G. & Dewanji, A. 2005: Presence of tropical hydrophytes in relation to limnological parameters – a study of two freshwater ponds in Kolkata, India. Annales de Limnologie - International Journal of Limnology 41(4): 281–289. DOI: https://doi. org/10.1051/limn/2005019 Nikolić, T. 2019: Flora Croatica – vaskularna flora Republike Hrvatske. Volumen 4. Ekskurzijska flora. Alfa d.d., Zagreb, 664 pp. Nikolić, T. 2020: FCD – Flora Croatica Database. DOI: https://hirc. botanic.hr/fcd/ (accessed 3 April 2020) Official Gazette 80/13, 15/18, 14/19, 127/19: Zakon o zaštiti prirode (Nature Protection Act). Otves, C., Neacsu, A. G. & Arsene, G.-G. 2014: Invasive and potentially invasive plant species in wetlands area of Banat. Research Journal of Agricultural Science, 46(4): 146–161. Ozimec, S. & Topić, J. 2001: Typha laxmannii Lepechin (Typhaceae), a new species in the Croatian flora. Natura Croatica 10(1): 61–65. 20/1 • 2021, 7–18 18 Anja Rimac, Antun Alegro, Vedran Šegota, Nikola Koletić, Igor Stanković, Sandro Bogdanović & Nina Vuković Distribution and habitat characteristics of Vallisneria spiralis L. in Croatia Ozimec, S. & Topić, J. 2018: Macrophytes of the Croatian Danube reach. In: Georg, J., Gaberščik, A., Květ, J., Germ, M. & Exler, N. (eds.): Macrophytes of the River Danube Basin. Academia, Prague, pp. 252–267. Pinardi, M., Bartoli, M., Longhi, D., Marzocchi, U., Laini, A., Ribaudo, C. & Viaroli, P . 2009: Benthic metabolism and denitrification in a river reach: a comparison between vegetated and bare sediments. Journal of Limnology 68(1): 133–145. DOI: https:// doi.org/10.4081/jlimnol.2009.133 Pyka, J. P ., Stawecki, K. & Zdanowsk, B. 2007: Variation in the contents of nitrogen and phosphorus in the heated water ecosystem of the Konin lakes. Archives of Polish Fisheries 15: 259–271. Radulović, S., Cvijanović, D. & Vukov, D. 2010: A riverside tale: Assessment of altered habitat effects on macrophyte assemblage on the river Tamiš, Serbia. Archives of Biological Sciences 62(4): 1163–1174. DOI: https://doi.org/10.2298/ABS1004163R Rai, P . K. & T ripathi, B. D. 2009: Comparative assessment of Azolla pinnata and Vallisneria spiralis in Hg removal from G. B. Pant Sagar of Singrauli Industrial region, India. Environmental Monitoring and Assessment 148(1–4): 75–84. DOI: https://doi.org/10.1007/s10661- 007-0140-2 Ribaudo, C., Bartoli, M., Racchetti, E., Longhi, D. & Viaroli, P . 2011: Seasonal fluxes of O 2 , DIC and CH 4 in sediments with Vallisneria spiralis: indications for radial oxygen loss. Aquatic Botany 94(3): 134–142. DOI: https://doi.org/10.1016/j.aquabot.2011.01.003 Rimac, A., Stanković, I., Alegro, A., Gottstein, S., Koletić, N. Vuković, N., Šegota, V. & Žižić-Nakić, A. 2018: The Brazilian Elodea (Egeria densa Planch.) invasion reaches Southeast Europe. Bioinvasion records 7(4): 381–389. DOI: https://doi.org/10.3391/bir.2018.7.4.05 Rožac, V., Prlić, D. & Ozimec, S. 2018: The vascular flora of Kopački rit Nature Park (Croatia). Acta Botanica Slovenica 61(2): 47–70. Sîrbu, C. 2007: Considerations regarding the alien plants from Moldavian flora (Romania), deliberately introduced by man. Buletinul Grădinii Botanice Iaşi 14: 41–50. Smolders, A. J. P ., Lamers, L. P . M. & Roelofs J. G. M. 2001: Aquatic macrophytes in assessment and monitoring of ecological quality. In: Karttunen, K. (ed.): Monitoring and assessment of ecological status of aquatic environments. Nordic Council of Ministers, Copenhagen, pp. 23–32. Soó, R. 1964–1980: A magyar flóra és vegetáció rendszertani- növény földrajzi kézikönyve 1-6. Synopsis systematico-geobotanica Florae vegetationisque Hungariae I-VI. Akadémiai Kiadó, Budapest. [in Hungarian] Stefanidis, K., Sarika, M. & Papastegiadou, E. 2019: Exploring environmental predictors of aquatic macrophytes in water‐depen- dent Natura 2000 sites of high conservation value: Results from a long‐term study of macrophytes in Greek lakes. Aquatic Conservation Marine and Freshwater Ecosystems 29: 1133–1148. DOI: https://doi. org/10.1002/aqc.3036 Stella, E. 1988: Contribution to the taxonomy and distribution of Cyclops abyssorum Sars (Crustacea, Copepoda) in several lakes and ponds of central Italy. Hydrobiologia 167/168: 381–385. Škvorc, Ž., Jasprica, N., Alegro, A., Kovačić, S., Franjić, J., Krstonošić, D., Vraneša, A. & Čarni, A. 2017: Vegetation of Croatia: Phytosocio- logical classification of the high-rank syntaxa. Acta Botanica Croatica 76(2): 200–224. DOI: https://doi.org/10.1515/botcro-2017-0014 Talevska, M., Petrovic, D., Milosevic, D., Talevski, T., Maric, D. & Talevska, A. 2009: Biodiversity of macrophyte vegetation from Lake Prespa, Lake Ohrid and Lake Skadar. Biotechnology & Biotechno- logical Equipment, Sup1 23: 931–935. DOI: https://doi.org/10.1080/ 13102818.2009.10818575 Thiers, B. 2020 Index Herbariorum: A Global Directory of Public Herbaria and Associated Staff. New York Botanical Garden’s Virtual Herbarium. http://sweetgum.nybg.org/science/ih/ (accessed 3 April 2020) Thiébaut, G. 2007: Non-indigenous aquatic and semiaquatic plant species in France. In: Gherardi, F . (ed.): Biological invaders in inland waters: profiles, distribution and threats. Springer, Dordrecht, pp. 209–229. T utin, T. G., Heywood, V. H., Burges, N.A., Valentine, D. H., Walters, S.M. & Webb, D. A. 1964–1980: Flora Europaea 1-5. Cambridge University Press, Cambridge. Urbanič, G., Mihaljević, Z., Petkovska, V. & Pavlin Urbanić, M. 2020: Disentangling the effects of multiple stressors on large rivers using ben- thic invertebrates – A study of Southeastern European large rivers with implications for management. Water 12(3), 621: 1–25. DOI: https:// doi.org/10.3390/w12030621 Vučković, I., Čanjevac, I., Bočić, N., Buzjak, N., Orešić, D., Plantak, M., Srebočan, M., Vidaković, I, Valožić, L., Međan, D., Martinić, I., Kereković, A., Mišetić, S. 2019: Hidromorfološki monitoring stajaćica 2019a: Prirodoslovno-matematički fakultet Sveučilišta u Zagrebu, Elektroprojekt, projektiranje, konzalting i inženjering d.d., Zagreb pp. 11–13. Vučković, I., Čanjevac, I., Plantak, M., Martinić, I., Vidaković, I., Srebočan, M. & Mišetić, S. 2019b: Hidromorfološki monitoring tekućica – Nabava hidromorfoloških podataka za potrebe inter- kalibracije bioloških metoda ocjene ekološkog stanja na rijekama. Prirodoslovno-matematički fakultet Sveučilišta u Zagrebu, Elektropro- jekt, projektiranje, konzalting i inženjering d.d., Zagreb, pp. 57–69. Vuković, N., Šegota, V., Koletić, N., Vilović, T., Rimac, A. & Alegro, A. 2018: Searching for aliens - update on Egeria densa Planchon and Myriophyllum heterophyllum Michx. in the Neretva River Delta (South- ern Dalmatia). In: Jelaska, S. D. (ed.): Book of Abstracts, 3 rd Croatian Symposium on Invasive Species with International Participation. Croatian Ecological Society, Zagreb, pp. 44–44. Vuković, N., Šegota, V., Koletić, N., Vilović, T., Rimac, A. & Alegro, A. 2019: Re-discovering the aquatic flora of Neretva River Delta. In: Ivković, M., Stanković, I., Matoničkin Kepčija, R., Gračan, R.(eds.): Book of Abstracts of 3 rd Symposium of Freshwater Biology. Croatian Association of Freshwater Ecologists, Zagreb, pp. 36–36. Wasowicz, P ., Przedpelska-Wasowicz, E.M., Guðmundsdóttir, L. & Tamayo, M. 2014: Vallisneria spiralis and Egeria densa (Hydrochari- taceae) in arctic and subarctic Iceland. New Journal of Botany 4(2): 85–89. DOI: https://doi.org/10.1179/2042349714Y.0000000043 Weyer van de, K., Schmidt, C., Kreimeier, B. & Wassong, D. 2011: Bestimmungsschlüssel für die aquatischen Makrophyten (Gefäß- pflanzen, Armleuchteralgen und Moose) in Deutschland. Landesamt für Umwelt, Gesundheit und Verbraucherschutz (LUGV), Potsdam, 158 pp. Wu, H. & Ding, J. 2019: Global change sharpens the double-edged sword effect of aquatic alien plants in China and beyond. Frontiers in Plant Science 10 (Article 787): 1–11. DOI: https://doi.org/10.3389/ fpls.2019.00787 Zarubina, E. Y. & Sokolova, M.I. 2011: Long term changes of Vallis- neria spiralis L. population in the cooling pond of the Belovo Power Plant (Southwest Siberia). Russian Journal of Biological Invasions 2(1): 8–12. DOI: https://doi.org/10.1134/S2075111711010115