BioInvasions Records (2018) Volume 7, Issue 4: 381–389 DOI: https://doi.org/10.3391/bir.2018.7.4.05 © 2018 The Author(s). Journal compilation © 2018 REABIC This paper is published under terms of the Creative Commons Attribution License (Attribution 4.0 International - CC BY 4.0)

Research Article

The Brazilian ( densa Planch.) invasion reaches Southeast

Anja Rimac1, Igor Stanković2, Antun Alegro1,*, Sanja Gottstein3, Nikola Koletić1, Nina Vuković1, Vedran Šegota1 and Antonija Žižić-Nakić2 1Division of Botany, Department of Biology, Faculty of Science, University of Zagreb, Marulićev trg 20/II, 10000 Zagreb, Croatia 2Hrvatske vode, Central Water Management Laboratory, Ulica grada Vukovara 220, 10000 Zagreb, Croatia 3Division of Zoology, Department of Biology, Faculty of Science, University of Zagreb, Rooseveltov trg 6, 10000 Zagreb, Croatia Author e-mails: [email protected] (AR), [email protected] (IS), [email protected] (AA), [email protected] (SG), [email protected] (VŠ), [email protected] (NK), [email protected] (AZ) *Corresponding author

Received: 12 April 2018 / Accepted: 1 August 2018 / Published online: 15 October 2018 Handling editor: Carla Lambertini

Abstract

Egeria densa is a South American aquatic considered highly invasive outside of its original range, especially in temperate and warm climates and artificially heated waters in colder regions. We report the first occurrence and the spread of E. densa in Southeast Europe, along with physicochemical and phytosociological characteristics of its . Flowering male populations were observed and monitored in limnocrene springs and rivers in the Mediterranean part of Croatia from 2013 to 2017. The populations inhabited clear, slow flowing, oligohaline water with high alkalinity and conductivity. Over the course of our research, the species displayed its invasive potential forming monospecific stands and surpassing the native flora in particular localities, while being obviously codominant with indigenous species in others. Although we cannot specify the exact path of introduction, we presume two possible scenarios: release from aquaria or epizoochory through water birds. The current situation in SE Europe, including Croatia, seems satisfactory regarding the number of aquatic alien species; nevertheless a careful and systematic approach to research into the management of invasive alien species is needed. Key words: invasive alien species, macrophytes, Mediterranean, Croatia, Neretva Delta, monitoring, Water Framework Directive

Introduction Nowadays, this species is considered a threat to aquatic outside of its original range. It Planch. is a submerged, freshwater has shown major invasive potential in countries perennial plant of the family . It is where it has been introduced, mainly through the native to , specifically , , ornamental trade, as it is very practical and attractive and . The original distribution range of the plant (Hussner 2012). This species has species includes SE Brazil, extending from the regions been proved to be a highly efficient invader due to its Minas Gerais and Espirito Santo, following the coast high biomass production, fast vegetative reproduc- southwards through Uruguay and reaching Buenos tion and efficient dispersal (Yarrow et al. 2009). Aires. In Argentina, the species is also present along Egeria densa is considered invasive or a pest in the Parana and Uruguay Rivers almost to the border South America (Yarrow et al. 2009), North America with Paraguay and occupies the region around Cordoba (Les and Mehrhoff 1999), (Haramoto and (Cook and Urmi-König 1984). In its native range, it Ikusima 1988; Alfasane et al. 2010), the Pacific is commonly found in still, 1–2 m deep water and is islands (Wagner et al. 1999), (Roberts et less common in shallower and flowing water (Cook al. 1999), New Zealand (Mccullough 1997), and Urmi-König 1984). (Coetzee et al. 2011) and Europe (Hussner 2012).

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The species is included in the List of Invasive Alien reclamation, construction of irrigation channels, river proposed by the European and Mediterranean bed regulation and hydrological regulation in general Plant Protection Organization (EPPO) and is reported started to alter the landscape. The marshes, lagoons from 17 European countries (Seregin 2008; Kikodze and lakes that once marked the lower course of the et al. 2010; Hussner 2012; Medvecká et al. 2012; Neretva River have been drastically reduced or have Wasowicz et al. 2014; Fominykh et al. 2016). Origi- disappeared, and what was a delta swamp became a nally a subtropical species, in colder regions of Europe predominantly agricultural area within the last fifty E. densa often forms self-sustaining populations in years. Only fragments of the old Mediterranean wet- water bodies with artificially increased water tempe- lands have survived (Margeta and Fistanić 2000; rature (Hussner and Lösch 2005; Wasowicz et al. Kralj et al. 2016). To gain more arable land, people 2014; Fominykh et al. 2016). It is found in both used a method of digging and depositing soil and lentic and lotic environments (Yarrow et al. 2009) and created a unique landscape of dense networks of appears to be truly naturalized in warm-temperate channels and arable fields which can be cultivated and cool subtropical conditions. Within subtropical from boats (Curić 1994; Kralj et al. 2016). Due to and tropical areas, E. densa is limited to high altitudes the above mentioned interventions and changes, the or cold-water springs (Cook and Urmi-König 1984). majority of the water bodies in the Delta are now This paper presents new localities for E. densa, very well interconnected. and is the first to document its spread to the Balkan Along the left and right edge of the Delta numerous Peninsula and Southeast Europe. Along with distri- big karst springs, such as Bađula, Bili Vir, Mislina, bution data, we present data on characteristics Mlinište, Modro Oko, Prud appear, as well as smaller and the observed trend of its spread. Furthermore, ones. These are ascending springs in which water we aim to emphasize the importance and benefits of upwells through cavernous karst formations and forms national monitoring of surface waters, which has small lakes (limnocrene springs) on the surface. been carried out according to the Water Framework Directive (European Community 2000), not only as Description of the sites a tool for water quality assessment but also as a way in which alien aquatic taxa may be rapidly detected. The Norin River is a tributary of the Neretva River, and is a medium sized, lowland river on limestone Material and methods bedrock (European Community 2000; Mihaljević 2011) (Figure 1). The Mislina River is also a medium sized Study area watercourse on limestone bedrock (European Com- The field survey was performed in over 600 locations munity 2000; Mihaljević 2011), strongly channelized covering most of the Croatian territory, including and connected both to the Neretva River and to the 277 rivers and 46 lakes (including reservoirs). Adriatic Sea. The Prud Spring and the Modro Oko All sites at which Egeria densa occurred are Spring supply the Norin River and the Neretva situated within the alluvial delta that extends along River, respectively. The Prud Spring is protected as the final 30 km of the course of the Neretva River, an ornithological Special Reserve and is situated before its mouth into the Adriatic Sea. The Neretva close to the ornithological reserve Pod Gredom. The River and its tributaries comprise the largest complex Modro Oko Spring is a limnocrene karst spring, of wetland habitats in the Croatian coastal zone. The situated at the right bank of the Neretva River, in the Delta is rich in underground water, which supplies immediate vicinity of the Significant Landscape numerous springs, streams and lakes, and is charac- Modro Oko and Lake Desne (Figure 1). terized by well-developed aquatic, wetland and coastal vegetation. It is one of a few preserved wetland areas Macrophyte survey in the Mediterranean region of Europe and conse- quently a Ramsar site of great importance for the Data on the distribution of E. densa, as well as breeding, migration and wintering of numerous physicochemical and phytosociological characteristics wetland bird species. Furthermore, it is protected on of its habitats, were collected within the national a national level and includes four Special Reserves system for monitoring surface waters and the scientific and two Significant Landscapes. The area is also project MULTISEC (Multimetric System of Evaluating included in the Natura 2000 network as an SPA Crenobiocoenosis). Over 600 locations throughout (Special Protection Area) and a pSCI (proposed Site Croatia have been periodically visited within the of Community Importance). monitoring scheme, while the particular occurrence The whole Delta area has been strongly anthro- sites for E. densa were visited 1–4 times within the pologically influenced since the 19th century, when study period.

382 Egeria densa invades SE Europe

Figure 1. Study area and occurrence sites for Egeria densa in Croatia: 1 – the Prud Spring, 2 – middle reach of the Norin River, 3 – lower reach of the Norin River, 4 – the Modro Oko Spring, 5 – the Mislina River.

Monitoring of macrophytes was carried out during Conductivity, salinity and pH were measured in situ the vegetation seasons (June–September) from 2010 with a SevenMulti Modular Meter System (Mettler to 2017. Watercourses were surveyed for macrophytes Toledo) under standard conditions. Dissolved oxygen along 100 m-long transects, while 6 × 100 m transects was measured in situ using a Hach HQ40D Portable were used when surveying macrophytes in lakes. In Multi Meter using an optical sensor. Water samples less-accessible areas, the river/lake bottom was for alkalinity measurement were stored at 4–5 °C and raked to reach the macrophytes, with the rake either alkalinity was determined by titration within 24 h of on a long pole or at the end of a rope. Cover and sampling. Water samples for total phosphorus analysis abundance of each plant species were estimated were preserved in sulphuric acid and analysed with a according to the five degree Kohler scale (1 – very rare, UV-VIS spectrometer (Perkin Elmer Lambda 25). 2 – rare, 3 – common, 4 – frequent, 5 – abundant, Water samples for total nitrogen and total organic predominant), which estimates the frequency of each carbon (TOC) were collected in dark bottles, filled to species and takes into account the three-dimensional the top, stored at 4–8 °C and analysed within 24 h development of the plant stands (Kohler 1978). using a Shimadzu TOC-VCPH equipped with an All macrophyte species were recorded and analyser for total nitrogen and TOC. Water samples identified to species level. Egeria densa was for ammonium, nitrate and orthophosphate analyses identified based on morphological characteristics were immediately filtered through glass filters with using several identification keys (Pignatti 1982; 0.45 µm pores, stored at 4–8 °C and analysed within Cook and Urmi-König 1984; Koehler and Bove 2001; 24 hours using an ionic chromatographer (Dionex Dandy 2010; van de Weyer et al. 2011). Collected 3000). The values of nutrients were used to assess specimens of E. densa were conserved in 50% water quality status according to the Regulation on ethanol with 10% glycerol added, or desiccated in a the Water Quality Standard (Official Gazette 73/2013, herbarium press and deposited in herbarium ZA 151/2014, 78/2015), with predefined thresholds (Thiers 2018) under the ID numbers: ZA5686, delimiting between the categories: very good, good, ZA5687, ZA5688, ZA5689. moderate, poor, very poor. All localities within the national surface water Water flow velocity was estimated according to monitoring system were also sampled for basic water Janauer (2003), who distinguishes 4 categories (1 – no physicochemical parameters and nutrients, while flow, stagnant, 2 – low, slow flow velocity, from just localities outside national monitoring were sampled visible to ca. 30 cm/s, 3 – medium flow velocity, ca. only for basic water physicochemical parameters. 35–65 cm/s, 4 – high flow velocity, 470 cm/s).

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Table 1. A summary of the establishment and spread of Egeria densa in the Neretva Delta. Presence and abundance of E. densa at surveyed localities according to Kohler’s scale: 0 – absent, 1 – very rare, 2 – rare, 3 – common, 4 – frequent, 5 – abundant, predominant (Kohler 1978), X – longitude and Y – latitude coordinates in WGS84 coordinate system. *Location was not surveyed in that particular year. The Norin River Modro Oko Spring The Mislina River The Prud Spring Middle reach Lower reach Lower reach Prud Village Vid Village Romići Village X=17,510198 Mlinište Village X=17,620067 X=17,629113 X=17,595837 Y=43,057283 X=17,615770 Y=43,094843 Y=43,080423 Y=43,053128 Y=42,992470 2010  0    2013 0  4   2014 3 2 2 3  2015 5   5  2016 5   5 5 2017  4 2  

Results relatively abundant at the beginning of the vegetation season (Table 1). In shallow water next to the bank, Egeria densa was first recorded in the lower reach of E. densa was associated with B. erecta and Alisma the Norin River in 2013, near the settlement Romići. lanceolatum With., while in deeper water it was After the initial discovery, a detailed search of the associated with nodosus Poir. (Table S1). location was conducted and E. densa was subsequently The species was absent from fast-flowing water. recorded in nearby sites (Table 1, Table S4). Periodical During the vegetation season, the abundance of searches of the same sites have indicated its invasive E. densa increased and it finally formed dense, sub- potential, whereby the species tended to increase its merged monospecific stands in up to 1 m deep slow- abundance in particular localities (Table 1). flowing water (Figure 2). In deeper water, up to 2.2 m, Naturalized populations of E. densa were observed it was present in lower abundance. The predominance in five locations (Figure 1), located in the Mediterra- of E. densa remained constant in subsequent years. nean subecoregion of the Dinaric ecoregion of The levels of nitrates, total nitrogen, orthophos- Croatia, in rivers and springs of the Neretva River phates and total phosphorous were slightly elevated Basin and include two limnocrene karst springs (Table S3), and water status was assessed as good (Prud and Modro Oko), as well as two rivers (Norin regarding nutrients. and Mislina). Middle reach Habitat characterization of the sites In the middle reach, E. densa was first recorded in 2014 when it was frequent but codominant with the The Norin River indigenous B. erecta, Hydrocharis morsus-ranae L., Water at this site is clear, slightly basic, oligohaline N. lutea, Potamogeton lucens L., S. emersum, Spar- and characterized by high conductivity and alkalinity ganium erectum L. and Ceratophyllum demersum L. (Table S2). Vegetation is typical of slow-flowing (Table S1). In 2017, the abundance of E. densa water, characterized by a high abundance of was higher than in 2014, although no major change Sparganium emersum Rehmann, Berula erecta in indigenous vegetation was observed (Table 1). (Huds.) Coville and Nuphar lutea (L.) Sm. and a well- Although E. densa was clearly self-sustaining here, developed belt of Phragmites australis (Cav.) Trin. in 2017 it was still associated, and codominant, with ex. Steud. along the watercourse (Table S1). Three native vegetation (Table S1). stable populations of E. densa were found and Physicochemical analysis of water showed only monitored in the Norin River over the course of our slightly elevated levels of ammonium, nitrates and research, distributed in the Prud Spring, and in the total nitrogen (Table S3), and water status was assessed middle and lower reach of the river (Figure 1). Based as good. on the data collected from 2010 to 2017, we can assume Lower reach that the spread in the Norin River began in the lower section and moved upstream (Table 1). In the lower reach of the river, the population of E. densa gradually declined from 2013, when it was The Prud Spring recorded for the first time and estimated as frequent Although the site was visited in 2013, dense stands (Table 1). In surveys carried out in 2014 and 2017, of E. densa were first recorded in 2014, already only individual plants were present. Native aquatic

384 Egeria densa invades SE Europe

Figure 2. Egeria densa habitat in the Neretva Delta. A – monospecific stand at the Prud Spring, B – submerged stand at the Prud Spring, C and D – invasion at Modro Oko Spring, E – codominance with native species at the Norin River, F – monospecific stand at Mlinište Spring. Photographs by Sanja Gottstein and Igor Stanković.

vegetation was abundantly developed in the lower reach, The Modro Oko Spring dominated by N. lutea, Nymphaea alba L., Oenanthe aquatica (L.) Poir. and S. emersum, all characteristic The population of E. densa was well established and for slow and medium flowing waters (Table S1). abundant in the Modro Oko Spring. Male plants Water status was assessed as good due to only were recorded for the first time in 2014, when slightly elevated levels of nitrates and total nitrogen abundance scored 3 (medium) on Kohler’s scale (Table S3). (Table 1). In the following years the species was

385 A. Rimac et al. more abundant, surpassing indigenous vegetation water is oligohaline because of the intrusion of the (Table 1). It was rooting in 0.3–2.2 m deep, slow- sea water. Nutrient content suggests moderate flowing water forming monospecific stands, or was eutrophication, at least during some months, which associated with the species N. lutea, which can access can be explained by the land use of the surrounding light above the dense canopy of E. densa. In deeper area which is composed mainly of arable land. water, E. densa was associated with P. lucens, while Water turbidity is often a limiting factor for in shallower water it was more abundant than Hyppuris aquatic plants, reducing light availability which, in vulgaris L. and B. erecta. Notably, at the peak of the more plastic and adaptable plants like E. densa, can vegetation season, E. densa was completely dominant cause a shift in plant growth. In waters with high in the area inhabited by aquatic vegetation (Figure 2), turbidity, E. densa shows an increase in shoot elon- while flowering occurred at the end of the September. gation, which improves light harvesting capability, Water on this locality is clear, oligohaline, with while in clear water and suitable light conditions it high alkalinity and conductivity (Table S2). has greater productivity and branching, thus having a greater invasive potential (Tanner et al. 1993; Riis et The Mislina River al. 2012). Consequently, E. densa is frequently Flowering male plants of E. densa were recorded at dominant and appears to perform best in clear water the beginning of the vegetation season, in March conditions (Bini et al. 1999; Carrillo et al. 2006). 2016, forming monospecific stands in the lower Such conditions are present not only in our study reach of the river (Table 1). In these stands, the sites but also in the majority of watercourses and predominance of E. densa over the indigenous flora lakes in the surrounding area. Occurrence sites in the was evident as no other species were observed. Neretva Delta also provide favourable temperature Notably, this population extended to the nearby conditions for the species, since E. densa shows Mlinište Spring, which is situated on the left bank of relatively constant growth between 16 and 28 °C the river and is connected to the main course (Figure 2). (Barko and Smart 1981). Although E. densa is more The water in the Mislina River is clear, slightly basic, competitive in warm waters (Riis et al. 2012), and and characterized by high conductivity and alkalinity the summer temperatures in most of our study sites (Table S2). Values of ammonium, nitrates and total do not exceed 20 °C, E. densa was still dominant nitrogen were only slightly elevated (Table S3), and over native vegetation in some localities. water status was assessed as good regarding nutrients. Salinity measured at E. densa occurrence sites ranges between 0.3 and 0.4‰. These oligohaline Discussion conditions support the growth of the species, while salinity around 1.2‰ is the most favourable for its Potential invasion of E. densa poses a serious threat growth and productivity. Development and distribu- to aquatic ecosystems by affecting water-flow, tion of E. densa are limited at salinities higher than sedimentation, water quality and hydrochemistry, 5‰ (Hauenstein and Ramirez 1986). Therefore, we light penetration and native species (Yarrow et al. can expect the further spread of E. densa in water 2009). Aquatic alien species can outcompete and bodies in the Neretva Delta, but we presume it will displace native aquatic species, as well as negatively be somewhat restricted since the salinity in parts of affect populations and assemblages of aquatic fauna the area exceeds the level for optimum growth (Boylen et al. 1999; Stiers et al. 2011; Hussner (Romić et al. 2008). Regarding alkalinity, the mea- 2012). Furthermore, for invasive clonal aquatic sured values are high in all occurrence sites and are plants, such as E. densa, high phenotypic plasticity expected to be similar in the whole area due to the enables relatively fast adaptation to a wide range of limestone bedrock. This may have a positive impact habitats in introduced areas (Riis et al. 2010) and on the spread and potential invasion of E. densa − augments their invasive potential. In the Neretva since the species can efficiently use HCO3 as an Delta, high biomass production of E. densa could alternative source of inorganic carbon (Browse et al. have a negative impact on irrigation systems and 1979; Pierini and Thomaz 2004). recreational activities such as fishing, boating and Although concentrations of nutrients in the occur- swimming. Moreover, it could have a serious negative rence sites are moderately elevated and thus provide impact on the biodiversity of protected areas and suitable conditions for macrophytes, we did not observe Natura 2000 sites located in the Delta. any specific pattern that would provide a link between In the Neretva Delta, E. densa grows in clear E. densa abundance or dominance and the concen- water with low flow velocity. In all localities the tration of nutrients. water is characterized by high alkalinity and is During our study, we recorded only male E. densa slightly basic, due to the limestone bedrock. The plants, which is in accordance with previous records

386 Egeria densa invades SE Europe of flowering populations in Europe (Medina and which E. densa was introduced into the studied area, Cirujano 1995; Gros et al. 2009; Aymerich 2012; we believe that introduction via watercourses is Fominykh et al. 2016). Male have generally unlikely, as the species has not yet been recorded been observed across the whole introduced range from neighbouring countries, or from Southeast (Mccullough 1997; DiTomaso et al. 2013; but see Europe (Hussner 2012). Coetzee et al. 2011), while the plant reproduces and The total number of alien species in spreads very efficiently via vegetative shoot fragments Croatian freshwater ecosystems is relatively low, that contain “double nodes”—a special nodal region especially in comparison with Central European that consists of two single nodes separated by a countries (Hussner 2012). There are only seven alien greatly shortened internode which can produce aquatic plant species recorded from Croatian water- lateral buds, branches and adventitious (Cook courses so far, including E. densa. Of these, only and Urmi-König 1984). Dispersal via fragments is Michx. is considered invasive an efficient strategy in aquatic plants and is parti- (Boršić et al. 2008). In the Dinaric ecoregion, cularly effective in areas such as the Neretva Delta. including the Mediterranean part of the country, the This Mediterranean wetland area is characterized by situation appears to be even more benign, since there a vast net of very well interconnected springs, rivers, are few records of alien aquatic plant species so far small lakes, melioration channels and ditches, repre- (Nikolić 2018). This is quite surprising considering senting a suitable environment for the establishment the warm Mediterranean climate, which should be and growth of E. densa. Additionally, this area is very suitable for the establishment of most alien subject to human activities such as boating and aquatic plant species. The majority of these plants fishing, which could greatly contribute to the dispersal are thermophilous subtropical and tropical species, of E. densa fragments (Brundu 2015). Furthermore, negatively affected primarily by low temperatures the whole area hosts a large number of wetland bird (Chytrý et al. 2009). Nonetheless, prior to our record species, which can also play a significant role in the of E. densa, only one alien aquatic species was dispersal of fragments (Reynolds et al. 2015; Green recorded from the Mediterranean part of Croatia - M. 2016; Coughlan et al. 2017). heterophyllum Michx (Starmühler 2009). Similarly, Since E. densa is a widely used aquarium plant, it in other SE European countries the climate is is presumed that the most common route of intro- suitable but the available distributional data shows a duction is the ornamental trade and release from low number of alien aquatic plants (Hussner 2012). aquaria (Mccullough 1997; Les and Mehrhoff 1999; The main reason may be insufficient botanical Hussner and Lösch 2005; Wasowicz et al. 2014; research within these countries (Lambdon et al. Fominykh et al. 2016). This is the most probable 2008). We believe, however, that this is not the case scenario for the introduction of the plant into the with the Neretva Delta as a considerable amount of Neretva Delta as well. On the other hand, migratory floristic data is available for the area (Topić 1995; water birds also play an important role in the Glasnović et al. 2015; Nikolić 2018); yet E. densa dispersal of aquatic alien species, through both epi- was not recorded until 2013 as the first alien aquatic zoochory and endozoochory, thus enabling transport plant species in the area, followed by M. across considerable distances (Reynolds et al. 2015; heterophyllum in 2016 (Jasprica et al. 2017). Green 2016; Coughlan et al. 2017). Although endo- Implementation of the Water Framework Directive zoochory is considered a dominant form of dispersal (European Community 2000), which includes monito- of aquatic organisms and enables transport over ring of the ecological status of inland waters, provides larger distances, ectozoochory by birds has also been the most comprehensive data on aquatic flora in Croatia documented as an efficient dispersal mechanism of but also confirms the low number and limited distri- aquatic plants. In the case of E. densa, transport of bution of alien aquatic species, especially within the vegetative fragments could be the only possible form Dinaric ecoregion. Since the ornamental trade is the of transport by birds, since the species does not most common path for the introduction of aquatic produce fruits and seeds outside the native range. species worldwide (Kay and Hoyle 2001; Brundu 2015), The Neretva Delta is a Ramsar site with over 300 the present situation might be a consequence of the registered bird species, representing an important almost non-existent water garden industry in Croatia, stop-over site during migration of birds from middle and the low intensity of both intentional and and northeast Europe to Africa. Since the area is unintentional introduction by humans. Another reason situated on the route of the Central European (Black might be the features of the Adriatic Sea Basin in Sea/Mediterranean) Flyway, it is possible that water Croatia, which consists of several larger, unconnected birds have been transporting E. densa fragments. karst river basins, with no possibility for the exchange Although we cannot ascertain the exact vector by of taxa.

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In invasion management, the benefits of preven- Chytrý M, Pyšek P, Wild J, Pino J, Maskell LC, Vilà M (2009) tative measures far exceed the potential economic European map of alien plant invasions based on the quantitative assessment across habitats. Diversity and Distributions 15: 98– and ecological costs (Leung et al. 2002) and are 107, https://doi.org/10.1111/j.1472-4642.2008.00515.x much more effective than eradication. Preventative Coetzee JA, Bownes A, Martin GD (2011) Prospects for the biological management requires international coordination of control of submerged macrophytes in South Africa. African early-warning systems, immediate access to critical Entomology 19: 469–487, https://doi.org/10.4001/003.019.0203 Cook CDK, Urmi-König K (1984) A revision of the Egeria information, specialized training of personnel, and (Hydrocharitaceae). Aquatic Botany 19: 73–96, https://doi.org/10. rapid-response strategies (Ricciardi et al. 2011). 1016/0304-3770(84)90009-3 Although the current situation in Croatian water- Coughlan NE, Kelly TC, Davenport J, Jansen MAK (2017) Up, up courses seems favourable in comparison to Central and away: bird-mediated ectozoochorous dispersal between aquatic environments. Freshwater Biology 62: 631–648, European countries, a careful and systematic approach https://doi.org/10.1111/fwb.12894 to the research and management of invasive alien Curić Z (1994) Donjoneretvanski kraj - Potencijalni i valorizirani species is still necessary. In addition to the regular turistički činitelji. Hrvatsko geografsko društvo, Zagreb, Croatia, national monitoring conducted in Croatia, we have 224 pp Dandy JE (2010) Egeria. In: Tutin TG, Heywood VH, Burges NA, started a project of detailed mapping of E. densa in Moore DM, Valentine DH, Walters SM, Webb DA (eds), Flora the Neretva Delta, with a plan to establish periodical Europea. Cambridge University Press, New York, United States monitoring of the present populations in order to of America, pp 3–4 estimate their trend and promptly detect the possible DiTomaso JM, Kyser GB, Oneto SR, Wilson RG, Orloff SB, Anderson LW, Wright SD, Roncoroni JA, Miller TL, Prather threat to resident species and habitats. TS, Ransom C, Beck KG, Duncan C, Wilson KA, Mann JJ (2013) Weed control in natural areas in the Western United Acknowledgements States. Weed Research and Information Center, University of , Davis, California, 544 pp The research was undertaken within the Project of Water Bodies European Community (2000) Directive 2000/60/EC of the European Surveillance, financed by the State Institution for Water Manage- Parliament and of the Council of 23 October 2000 establishing a ment “Hrvatske vode” and MULTISEC Project (Multimetric System framework for Community action in the field of water policy. of Evaluating Crenobiocoenosis) funded by the Environmental Official Journal of the European Communities L327: 1–72 Protection and Energy Efficiency Fund (EPEEF) of Croatia. We Fominykh AS, Mukhutdinov VF, Kipriyanova LM (2016) Findings would like to thank to Hrvatske vode for the provision of the data of Brazilian elodea in cooling ponds of the Verkhnii Tagil Power on physicochemical and chemical properties of water. Plant (Middle Urals). 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Supplementary material The following supplementary material is available for this article: Table S1. List of species at occurrence sites of Egeria densa, with the estimated cover and abundance according to Kohlers’ scale. Table S2. Basic physicochemical parameters of water based on monthly measurements in 2016 at Egeria densa occurrence sites. Table S3. Chemical parameters of water based on monthly measurements in 2016 at Egeria densa occurrence sites. Table S4. Finding sites of Egeria densa in the Neretva Delta (Croatia). This material is available as part of online article from: http://www.reabic.net/journals/bir/2018/Supplements/BIR_2018_Rimac_etal_SupplementaryTables.xlsx

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