45 1 Slovenian Environment Agency, Vojkova 1b, SI-1000 Ljubljana, Slovenia 2 Faculty of Civil and Geodetic Engineering, University of Ljubljana, Hajdrihova 28, SI-1000 Ljubljana, Slovenia * Corresponding author: E-mail address: aleksandrakrivograd@gmail.com Citation: Krivograd Klemenčič, A., Šter, T., (2025). Distribution of two invasive alien diatom species Achnanthidium delmontii and Achnanthidium druartii in Slovenia. Acta Biologica Slovenica 68 (4) Received: 26.05.2025 / Accepted: 01.08.2025 / Published: 08.08.2025 https://doi.org/10.14720/abs.68.4.22822 This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY SA) license Original Research Distribution of two invasive alien diatom species Achnanthidium delmontii and Achnanthidium druartii in Slovenia Aleksandra Krivograd Klemenčič 1,2, * , Tadeja Šter 1 Abstract The distribution of two invasive diatom species in Slovenia, Achnanthidium delmontii (ADMO) and Achnanthidium druartii (ADRU), was investigated in this study. Data from 87 rivers and 11 lakes collected between 2019 and 2024 in the frame of national monitoring of the ecological status of surface waters were included. ADMO was present in 40 rivers (46%) and was dominant (>5% relative abundance) in 27 rivers (31%), with the highest abundance reaching 77% in the Bolska River. It was rare in lakes, detected in only three lakes (27%). ADRU was detected in 12 rivers (14%) and was dominant in four rivers (5%), with the highest abundance reaching 27% in the Drava River. In contrast, ADRU was common in lakes, with a presence in eight lakes (73%), dominating Lake Slivnica and Lake Pernica. ADMO presence was associated with reduced diatom species diversity and evenness, although no direct ecological impact was observed. ADMO was present mainly in upland river sections, while ADRU was more frequent in lowland river sections and lakes. The results of this study confirmed the invasive character of ADMO, whereas ADRU did not affect species diversity and evenness of the diatom assemblages, and thus its invasive character could not be confirmed. Keywords Achnanthidium delmontii; Achnanthidium druartii; alien invasive species; diatoms; Slovenia 46 Acta Biologica Slovenica, 2025, 68 (4) Razširjenost dveh invazivnih tujerodnih vrst diatomej Achnanthidium delmontii in Achnanthidium druartii na območju Slovenije Izvleček V raziskavi smo proučevali razširjenost dveh invazivnih tujerodnih vrst diatomej v Sloveniji, Achnanthidium delmontii (ADMO) in Achnanthidium druartii (ADRU). V raziskavo smo vključili podatke iz 87 rek in 11 jezer zbranih med letoma 2019 in 2024 v okviru nacionalnega monitoringa ekološkega stanja površinskih voda. ADMO je bil prisoten v 40 rekah (46 %), dominantna vrsta (>5 % relativne abundance) je bil v 27 rekah (31 %), pri čemer je s 77 % v reki Bolski dosegel najvišjo zastopanost. ADMO je bil v jezerih redek, zaznan je bil le v treh jezerih (27 %). ADRU je bil prisoten v 12 rekah (14 %), dominantna vrsta je bil v štirih rekah (5 %), z največjo zastopanostjo 27 % v reki Dravi. V jezerih je bil ADRU pogost, prisoten je bil v osmih jezerih (73 %), prevladoval je v Slivniškem in Perniškem jezeru. Prisotnost ADMO je bila povezana z zmanjšano vrstno pestrostjo in enakomernostjo združb diatomej, čeprav neposrednega negativnega ekološkega vpliva nismo zaznali. ADMO se je večinoma pojavljal v povirnih delih rek, medtem ko je bil ADRU pogostejši v nižinskih delih rek in jezerih. Rezultati potrjujejo invazivni značaj ADMO, medtem ko invazivnost ADRU ni bila potrjena, saj njegova prisotnost ni pomembno vplivala na sestavo združb diatomej. Ključne besede Achnanthidium delmontii; Achnanthidium druartii; invazivne vodne vrste; diatomeje; Slovenija Introduction Primary producers in rivers and lakes form the basis of the food web, and as such, any change in their quantity or in the composition of their communities can result in a disturbed ecological balance in waterbodies, with a domino effect on higher trophic levels such as benthic macroinvertebrates and fish (Buczkó et al., 2022). Among invasive benthic diatoms, the most attention in the past has been paid to one of the largest freshwater diatom species, Didymosphenia geminata (Lyngbye) W.M. Schmidt, which is native to Europe and North America but highly invasive and aggressive in New Zealand. It causes a lot of problems in freshwater systems, such as the forma- tion of large and extensive mats that impact fish, aquatic plants, and insects, resulting in severe disturbances in food webs (Blanco and Ector, 2009). However, according to Taylor and Bothwell (2014), D. geminata blooms in New Zealand are not caused solely by the introduction of its cells in new areas, as similar, nearly synchronous blooms have occurred in areas where D. geminata is native, such as North America and Europe. Next to large-celled diatoms such as D. geminata, also small-celled diatoms (<25 µm), such as Achnanthidium delmontii Pérès, Le Cohu & Bar- thès (ADMO), and Achnanthidium druartii Rimet & Couté (ADRU), are considered invasive or potentially invasive species (e.g., Buczkó et al., 2022; Falasco et al., 2023; Ivanov, 2018). ADMO was discovered in 2007 and formally described in 2012, based on specimens from a French river (Pérès et al., 2012), while ADRU was first discovered in 2004 in the Rhone River in France and formally described in 2010 as a new species invading the rivers in France and Spain (Rimet et al., 2010). Since then, ADMO has been reported from several countries in Europe (France, Germany, Hungary, Italy, Neth- erlands, Switzerland) and Asia (China) (Guiry and Guiry, 2022), while ADRU has been reported from several coun- tries in Europe (Bulgaria, France, Germany, Netherlands, Serbia and Spain), North and South America, the Middle East, and Asia (Guiry and Guiry, 2022). ADMO has been reported in all Slovenian neighbouring countries, namely Hungary, Italy, Austria, and Croatia. Buczkó et al. (2022) reported the presence of ADMO in Hungary from 2015 onward and in Austria in several sections of the Danube River from 2013 onward. Falasco et al. (2023) reported on the first findings of ADMO in Italy in 2013 in the rivers of Liguria (NW-Italy). In Croatia, there are records of ADMO in the Drava River and the Danube River from 2019, which 47 Acta Biologica Slovenica, 2025, 68 (4) were collected in the framework of Joint Danube Survey 4 (JDS4) (ICPDR, 2019). Information on the presence of ADRU in Slovenia’s neighbouring countries is scarce. There is a record of ADRU from the Danube River in Austria from 2019, collected in the framework of JDS4 (ICPDR, 2019). However, according to our information, there is no available information on the presence of ADRU in Croatia, Hungary, or Italy. In this research, the distribution of two potentially invasive, alien benthic diatom species, namely ADMO and ADRU, in the territory of Slovenia is presented for the first time. Moreover, for the sampling sites where ADMO and ADRU were identified, nutrient and organic matter load- ing, as well as species diversity and evenness, are also reported. Materials and Methods Phytobenthos sampling Sampling of the phytobenthos was carried out in 2019–2024 as part of the national monitoring of surface water quality according to the Water Framework Directive (WFD) (Directive 2000/60/EC). Altogether, 247 phytobenthos samples were collected at 175 sampling sites in rivers, and 35 samples were collected at 32 sampling sites in lakes and reservoirs (hereinafter referred to as lakes) across Slovenia according to the national programmes for monitoring chemical and ecological status of surface waters (ARSO, 2017, 2022). In total, 87 rivers and 11 lakes were included in this research. According to the Slovenian national methodologies for ecological status assessment of rivers and lakes using phytobenthos and macrophytes (MOP, 2016a, 2016b), river samples were collected once per year from June to September, up to a depth of 60 cm and at a distance of at least 1 m from the riverbanks, or in cases of smaller rivers, at least 10% of the river’s width from the banks. A multi-habitat approach was used, meaning that the sam- ples were collected from a range of habitats (e.g., gravel, mud, riffle, and pool) differing in substrate type, depth, current velocity, and shadiness. For lakes, the national methodology applies only to natural lakes (Lake Bled and Lake Bohinj). Multi-habitat sampling was also performed in these and other lakes. Phytobenthos was collected along a 50 m shoreline stretch that included various habitats characterised by varying depth, substrate, and shading conditions. In each lake, phytobenthos was sampled once per year, typically between June and September, and usually at three different sampling sites representing the dominant substrate type. An exception was Lake Vogršček, where only two sites were sampled. Sampling in Lake Pernica deviated from the recommended timeframe, as it was conducted in October, outside the June-September window defined by national methodology for natural lakes. In both rivers and lakes, phytobenthos was removed from the substrate (stones, pebbles, wood, macrophytes, etc.) using a toothbrush in a tray containing a small amount of river or lake water. The material was homogenised and poured into a wide-necked plastic bottle. Each sample was fixed with ethanol to a final concentration of 70%. Laboratory analyses The phytobenthos samples were transferred to the labo- ratory and treated with concentrated nitric acid (HNO 3 ) to remove cell contents and other organic matter, following the standard procedure (SIST EN 14407, 2014) and the instructions of the Slovenian national methodologies for ecological status assessment of rivers and lakes using phy- tobenthos and macrophytes (MOP, 2016a, 2016b). Cleaned samples were mounted in Naphrax® (Brunel Microscopes, Chippenham, Wiltshire, UK), a medium with a high refrac- tive index, for permanent slide preparation. Permanent slides were examined using a light microscope (Leica DM RB, Germany) at 1000× magnification. Diatom identification and enumeration were performed according to the stan- dard procedure (SIST EN 14407, 2014) and the Slovenian national methodologies (MOP, 2016a, 2016b). For each permanent slide, at least 500 valves were counted and identified to the species or lower taxonomic level. Diatom identification and nomenclature followed the identification monograph by Lange-Bertalot et al. (2017). Scanning elec- tron microscope (SEM) images were obtained using a JEOL JSM-7500F microscope and sample preparation according to Hasle and Fryxell (1970). Statistical analyses The abundance of diatom species was expressed as relative counts (in %). The levels of nutrient and organic loading in the investigated rivers were evaluated using the Trophic Index (TI) (Rott et al., 1999) and Saprobic Index (SI) (Rott et al., 1997), respectively. For the investigated lakes, 48 Acta Biologica Slovenica, 2025, 68 (4) nutrient loading levels were evaluated using the TI (Rott et al., 1999). Both indices were calculated solely based on diatom data, followed by an assessment of the ecological trophic status (for both rivers and lakes) and the ecological saprobic status (for rivers only), in accordance with the Slovenian national methodologies for ecological status assessment of rivers and lakes using phytobenthos and macrophytes (MOP, 2016a, 2016b). Species diversity was assessed using the Shannon-Wie- ner diversity index (SW), and species evenness was assessed using the Evenness index (E). Both indices were calculated using OMNIDIA 6.0.9, a software tool for the calculation of 18 diatom water quality indices. Correlations between the abundance of target species, species diversity, and species evenness were calculated using the Pearson correlation coefficient and Microsoft Excel software. Results and Discussion Altogether, 403 diatom taxa were identified in river samples and 238 diatom taxa in lake samples. In Slovenia, benthic diatoms are used, together with macrophytes, phytoplank- ton, fish, benthic macroinvertebrates, and macroalgae, as a biological quality element for the assessment of the ecological status of surface waters, namely the quality of the structure and functioning of aquatic ecosystems, as defined by the WFD (Directive 2000/60/EC). Two diatom indices, the Trophic index (Rott et al., 1999) and the Sapro- bic index (Rott et al., 1997), are used in Slovenia to assess nutrient and organic matter loading in rivers, respectively. For natural lakes in Slovenia (Lake Bled and Lake Bohinj), the trophic state is assessed using benthic diatoms and the Trophic index (Rott et al., 1999), in combination with phytoplankton, in accordance with national methodology (MOP, 2016b). For other lakes, where there is no national methodology, benthic diatoms and the Trophic Index (Rott et al., 1999) may still be used as an additional or indicative assessment tool, but are not formally part of the ecological status assessment under national monitoring. Morphology of Achnanthidium delmontii and Achnanthidium druartii The morphology of ADMO is described in detail in the work of Pérès et al. (2012) and that of ADRU in the work of Rimet et al. (2010), in which both species were first formally described. ADMO and ADRU both belong to the group of taxa related to Achnanthidium pyrenaicum (Hustedt) Kobayasi based on the stria density, which is around 20 μm, and the linear-lanceolate valve shape. According to Pérès et al. (2012), the main morpho- logical characteristics of ADMO (Figure 1) are as follows: Valves are linear with rounded apices, becoming elliptical in smaller individuals; valve length ranges from 7.3 to 21.4 μm, and valve width from 3.3 to 5.1 μm. On the raphe valve, the axial area is narrow, and the central area is irregular, typically forming a rectangular fascia, but a shortened stria may be present on one of the margins. The raphe is filiform and straight, with distinct central pores. Striae are slightly radial, numbering 20–26 in 10 μm in the central part of the valve, and up to 35 in 10 μm near the apices. On the rapheless valve, the axial area is acicular. Striae are parallel to slightly radiate near the apices; in most cases, two striae in the central part are slightly more widely spaced. Striae are slightly radial, numbering 18–22 in 10 μm in the central part and up to 25 in 10 μm at the apices. Under light microscopy, ADMO can be distinguished from similar taxa by a rectangular fascia on the raphe valve and a typical irregular cell shape. The main morphological characteristics of ADRU (Figure 2) according to Rimet et al. (2010) are: Valves are lanceolate with slightly subrostrate ends, never capitate; valve length ranges from 12 to 29 μm, and valve width from 3.9 to 5.8 μm. The raphe sternum is larger in the middle of the valve than in the extremities. The rapheless valve is convex, with a narrow, straight sternum that is only slightly enlarged in the centre. Striae are very weakly radiate throughout on both valves. Occasionally, short striae are inserted near the middle of the valve. For both valves, stria density is 15-22 in 10 μm in the central part of the valve and approximately 40-50 in 10 μm near the apices. Under light microscopy, ADRU can be distinguished from similar taxa, namely A. convergens (Kobayasi) Kobayasi, A. deflexum (Reimer) Kingston, A. japonicum (Kobayasi) Kobayasi, A. latecephalum Kobayasi, A. pyrena- icum (Hustedt) Kobayasi and A. rivulare Potapova & Pon- ader, by its generally wider and longer valve dimensions. However, in many cases, the size may overlap (Rimet et al., 2010). According to current knowledge, only A. pyrenaicum and A. rivulare have been recorded in Slovenia among the above-listed taxa. Another interesting characteristic that differentiates ADRU is that it shows an important stria density difference between the centre and the apices. 49 Acta Biologica Slovenica, 2025, 68 (4) Figure 1. Scanning electron microscope (SEM) images of Achnanthidium delmontii (ADMO). SEM: external view of the raphe valve with a rectangular fascia (marked with an arrow) (A), external view of the rapheless valve (B), external girdle view of the raphe valve (C), internal view of the rapheless valve (D). One of the main characteristics of ADMO is irregular cell shape (B, C, D). Slika 1. Fotografije z vrstičnim elektronskim mikroskopom (SEM) Achnanthidium delmontii (ADMO). SEM: pogled na zunanji del valve z rafo in dobro vidnim pravokotnim praznim prostorom (puščica) (A), pogled na zunanji del valve brez rafe (B), pogled na zunanji del valve z rafo z bočne strani (C), pogled na notranji del valve brez rafe (D). Ena glavnih značilnosti ADMO je nepravilna oblika celic (B, C, D). Figure 2. Scanning electron microscope (SEM) images of Achnanthidium druartii (ADRU). SEM: external views of the raphe valve (A, D), external view of the rapheless valve (B), internal view of the raphe valve (C). Slika 2. Fotografije z vrstičnim elektronskim mikroskopom (SEM) Achnanthidium druartii (ADRU). SEM: pogled na zunanji del valve z rafo (A, D), pogled na zunanji del valve brez rafe (B), pogled na notranji del valve z rafo (C). 50 Acta Biologica Slovenica, 2025, 68 (4) Occurrence of Achnanthidium delmontii and Achnanthidium druartii in Slovenia Table 1 provides an overview of the sampling sites where ADMO and ADRU were detected, including national sampling site codes, date of sampling, and the relative abundance of both species expressed as a percentage of counted valves. The table also includes information on nutrient and organic matter loading at each site, as well as measures of species diversity and evenness, expressed as the Shannon-Wiener diversity index and the Evenness index, respectively. Regarding rivers, a total of 247 phytobenthos samples collected at 175 sampling sites located on 87 rivers were included in the study. Among all investigated rivers, ADMO was present in 100 samples (41%) collected at 74 sampling sites (42%) from 40 rivers (46%) (Table 1). ADMO was a dominant species (more than 5% in relative abundance) in 56 phytobenthos samples (23%) collected at 45 sampling sites (26%) from 27 rivers (31%). The highest abundance of ADMO was recorded in the Bolska River (Figure 3), specif- ically at sampling sites Dolenja vas and Čeplje in August 2022, with relative abundances of 77% and 64%, respec- tively. Regarding ADMO representation, the Bolska River was followed by the Mirna River, where at the sampling site Dolenji Boštanj, the ADMO relative abundance reached 69%, also in August 2022. ADRU was present in 32 river samples (13%) collected at 22 sampling sites (13%) from 12 rivers (14%) (Table 1). ADRU was a dominant species (more than 5% in relative abun- dance) in 11 phytobenthos samples (5%) collected at eight sampling sites (5%) from 4 rivers (5%). The highest abun- dance of ADRU was detected in the Drava River (Figure 4), specifically at sampling site Ruše in September 2021, with a relative abundance of 27%, followed by sampling site Ranca in August 2024 with 20%, and sampling site Tribej in September 2021 with a relative abundance of 19%. In Slovenian lakes, 35 phytobenthos samples were collected at 32 sampling sites across 11 lakes. Among the two investigated species, ADMO was relatively rare, being detected in only five samples (14%) across three lakes. It was never dominant in any of the samples, with the highest recorded abundance reaching just 0.38% in the Gajševsko Lake (GaFB03, June 2023). In contrast, ADRU was more widespread and frequently encountered, being present in 15 samples (43%) in 8 of the 11 studied lakes. It showed dominance (relative abundance >5%) in 3 samples from two lakes. The highest relative abundance of ADRU was observed in Lake Slivnica (10% at SlFB04, May 2023), followed by Lake Pernica (6.6% at P2FB05, October 2024). River/Lake Sampling site Date of sampling ADMO (%) ADRU (%) TI SI EQR TI EQR SI SW E Mura Bad Radkersburg 21.01.2021 33.84 2.63 2.03 0.71 0.60 3.45 0.77 Mura Bad Radkersburg 30.08.2024 17.15 1.92 1.69 1.00 1.00 2.68 0.62 Mura Ceršak 21.01.2021 11.04 2.90 2.09 0.61 0.58 3.96 0.82 Mura Ceršak 30.08.2024 4.16 + 2.28 1.87 0.85 0.81 3.35 0.73 Mura Mele 21.01.2021 6.13 2.93 2.07 0.61 0.58 3.84 0.78 Mura Mele 30.08.2024 11.30 1.93 1.71 1.00 1.00 3.02 0.66 Mura Mota 14.02.2023 0.78 2.71 1.87 0.69 0.80 3.85 0.78 Mura Gibina 14.02.2023 5.86 3.15 2.16 0.47 0.55 3.75 0.77 Ledava Domajinci 7.03.2019 2.39 2.72 1.89 0.75 0.77 4.07 0.75 Table 1. List of sampling sites where invasive diatoms Achnanthidium delmontii (ADMO) and Achnanthidium druartii (ADRU) were detected, including national sampling site codes, date of sampling, relative abundance of both species expressed in percentage of counted valves, Trophic index (TI), Sap- robic index (SI), ecological trophic status (EQR TI), ecological saprobic status (EQR SI), Shannon-Wiener diversity index (SW), and Evenness index (E). + indicates that the species was observed during the qualitative examination of the sample but not during valve counting. / indicates that the ecological status assessment methodology has not been developed. Blue colour indicates high ecological status, green indicates good ecological status, and yellow indicates moderate ecological status. Tabela 1. Seznam vzorčnih mest z ugotovljeno prisotnostjo invazivnih diatomej Achnanthidium delmontii (ADMO) in Achnanthidium druartii (ADRU) z navedenimi nacionalnimi šiframi vzorčnih mest, datumom vzorčenja, relativno pogostostjo obeh vrst izraženo v odstotkih od preštetih polovic lupinic, trofičnim indeksom (TI), saprobnim indeksom (SI), ekološkim trofičnim (EQR TI) in ekološkim saprobnim (EQR SI) stanjem, Shannon-Wiennerjevim diver- zitetnim indeksom (SW) in indeksom Evenness (E). Z znakom + je označeno, da je bila vrsta zaznana le med kvalitativnim pregledom vzorca in ne med štetjem polovic lupinic. Z znakom / je označeno, da metodologija vrednotenja ekološkega stanja ni razvita. Modra barva označuje zelo dobro ekološko stanje, zelena označuje dobro ekološko stanje in rumena zmerno ekološko stanje. 51 Acta Biologica Slovenica, 2025, 68 (4) Drava Tribej 22.09.2021 18.70 2.52 1.94 0.75 0.71 4.4 0.82 Drava Tribej 25.09.2024 17.81 2.34 1.89 0.83 0.77 3.85 0.74 Drava Ruše 22.09.2021 12.08 26.53 2.28 1.79 0.85 0.93 3.86 0.75 Drava Ruše 21.08.2024 3.28 13.68 2.19 1.76 0.89 0.96 3.96 0.77 Drava Krčevina pri Ptuju 22.09.2021 21.97 1.98 2.04 1.66 0.96 1.00 3.12 0.67 Ptujsko jezero Ranca 16.09.2021 0.39 13.80 1.83 1.51 1.00 1.00 3.87 0.76 Drava Ranca 21.08.2024 19.68 2.55 1.69 0.74 1.00 4 0.75 Drava Prepolje 6.08.2019 0.19 5.78 2.88 2.03 0.62 0.60 4.53 0.79 Drava Prepolje 24.08.2022 1.93 2.80 1.90 0.65 0.76 4.53 0.85 Drava Gorišnica 6.08.2019 0.77 4.44 2.67 1.90 0.70 0.76 4.37 0.82 Drava Gorišnica 24.08.2022 5.08 2.83 1.98 0.64 0.66 4.54 0.83 Drava Borl 16.09.2021 55.00 3.14 2.57 2.06 0.73 0.59 2.49 0.57 Drava Borl 23.08.2024 12.28 1.56 2.52 1.71 0.75 1.00 3.84 0.75 Drava Ormož 6.08.2019 4.79 1.80 2.63 1.98 0.71 0.66 5.01 0.83 Drava Ormož 23.12.2020 4.23 0.38 2.54 1.99 0.75 0.64 4.41 0.79 Drava Ormož 4.08.2022 2.47 0.38 2.95 1.94 0.59 0.70 4.61 0.83 Drava Ormož 23.08.2024 5.81 6.97 2.54 1.77 0.75 0.95 4.39 0.79 Drava Grabe 15.09.2021 26.21 + 1.50 1.48 1.00 1.00 2.03 0.55 Meža Topla 17.07.2024 + 1.93 1.58 0.94 1.00 2.87 0.63 Mislinja Mala vas 17.07.2024 0.39 1.85 1.53 1.00 1.00 2.33 0.57 Mislinja Otiški vrh 17.07.2024 0.78 2.15 1.67 0.98 0.92 2.72 0.59 Dravinja Videm pri Ptuju 6.08.2024 15.5 1.84 1.50 1.00 1.00 2.54 0.59 Pesnica Zamušani 21.09.2023 1.89 2.28 1.98 0.96 0.74 3.98 0.85 Sava Dolinka Zelenci 11.07.2023 1.75 1.43 1.41 1.00 1.00 3.07 0.61 Sava Bohinjka nad izlivom Jezernice 29.08.2022 1.37 2.14 1.65 1.00 1.00 3.91 0.82 Sava Bohinjka Bodešče 29.08.2022 4.89 2.54 2.02 1.00 0.75 2.66 0.58 Sava Struževo 28.07.2021 6.9 1.39 1.51 1.00 1.00 2.72 0.72 Sava Dragočajna 21.06.2022 1.36 0.39 1.78 1.65 1.00 1.00 3.71 0.71 Sava Dragočajna 14.08.2024 9.23 2.45 1.79 0.70 0.77 4.68 0.87 Sava Medno 14.08.2024 12.08 1.70 1.53 1.00 1.00 2.57 0.58 Sava Šentjakob 26.07.2019 0.78 1.84 1.78 1.00 0.78 3.06 0.6 Sava Kresnice 12.07.2023 4.9 1.82 1.56 1.00 1.00 2.53 0.58 Sava Podkraj 16.08.2024 9.76 2.00 1.55 0.95 1.00 2.86 0.59 Sava Vrhovo 20.08.2021 8.57 2.66 1.98 0.70 0.66 4.31 0.83 Sava Vrhovo 23.06.2023 0.39 2.34 1.94 0.82 0.71 4.08 0.74 Sava Brestanica 20.08.2021 1.86 2.81 2.03 0.65 0.59 4.04 0.74 Sava Podgračeno 14.09.2021 0.77 2.53 1.99 0.75 0.64 3.95 0.76 Sava Jesenice na Dolenjskem 21.08.2019 50.65 2.68 1.92 0.69 0.74 2.95 0.56 Sava Jesenice na Dolenjskem 11.08.2020 1.36 2.23 1.59 0.88 1.00 3.1 0.6 Sava Jesenice na Dolenjskem 14.09.2021 6.81 2.62 2.07 0.72 0.58 3.93 0.78 Sava Jesenice na Dolenjskem 17.08.2022 4.64 0.77 2.85 2.01 0.63 0.61 3.98 0.79 Sava Jesenice na Dolenjskem 9.10.2023 9.3 2.03 1.75 0.97 0.98 3.61 0.74 Sava Jesenice na Dolenjskem 14.08.2024 16.01 1.61 1.48 1.00 1.00 1.93 0.49 Kokra Kranj 26.07.2021 28.85 1.89 1.61 1.00 1.00 3.29 0.71 Sora Medvode 22.06.2021 31.52 1.44 1.45 1.00 1.00 1.67 0.42 River/Lake Sampling site Date of sampling ADMO (%) ADRU (%) TI SI EQR TI EQR SI SW E 52 Acta Biologica Slovenica, 2025, 68 (4) Poljanska Sora Na Dobravi 1.07.2019 1.32 1.90 1.72 1.00 1.00 3.21 0.67 Poljanska Sora Na Dobravi 22.06.2021 26.21 1.48 1.48 1.00 1.00 2.27 0.58 Selška Sora Vešter 22.06.2021 6.83 1.76 1.59 1.00 1.00 3.31 0.78 Kamniška Bistrica Ihan 26.08.2024 28.37 2.34 1.70 1.00 1.00 3.19 0.64 Kamniška Bistrica Beričevo 26.08.2024 3.1 2.46 1.88 1.00 0.87 2.38 0.49 Pšata Bišče 1.07.2021 13.23 2.74 1.99 0.52 0.73 4.09 0.79 Pšata Bišče 11.07.2024 21.79 2.74 1.97 0.52 0.74 4.18 0.77 Mirna Dolenji Boštanj 17.06.2019 0.39 2.60 2.06 0.58 0.71 4.05 0.76 Mirna Dolenji Boštanj 1.08.2022 68.95 0.30 2.47 1.87 0.64 0.78 1.99 0.42 Sotla Rigonce 14.09.2021 0.39 2.64 1.92 1.00 1.00 3.44 0.69 Bistrica Zagaj 17.06.2019 25.58 2.06 1.58 1.00 1.00 3.48 0.73 Kolpa Osilnica 4.07.2024 6.99 1.42 1.55 1.00 1.00 3.14 0.71 Kolpa Radenci 14.06.2021 44.53 1.51 1.53 1.00 1.00 2.75 0.62 Kolpa Radoviči (Metlika) - Bubnjarci 4.08.2021 24.43 0.38 2.11 1.60 0.68 1.00 4.03 0.76 Kolpa Radoviči (Metlika) - Bubnjarci 4.07.2024 14.45 0.40 2.29 1.68 0.61 1.00 4.37 0.86 Lahinja Geršiči 12.09.2023 + 5.56 2.32 1.67 0.76 1.00 4.23 0.81 Gruberjev prekop Ljubljana 25.07.2022 0.39 2.73 2.02 0.56 0.75 4.26 0.81 Gruberjev prekop Ljubljana 13.09.2023 5.09 2.76 1.99 0.55 0.76 3.83 0.79 Iščica Ižanska cesta 17.08.2022 + 2.54 1.95 0.61 0.72 3.7 0.78 Iščica Ižanska cesta 11.07.2024 0.78 2.56 1.86 0.60 0.75 3.21 0.69 Mali Graben Dolgi most 21.06.2022 1.15 2.40 1.73 0.67 1.00 3.16 0.64 Gradaščica Dvor 21.06.2022 15.59 2.15 1.73 1.00 1.00 3.29 0.71 Savinja Grušovlje 16.07.2024 1.94 2.05 1.52 1.00 1.00 2.34 0.54 Savinja Medlog 4.09.2024 5.03 1.98 1.57 1.00 1.00 2.46 0.57 Savinja Veliko Širje 4.09.2024 17.58 2.05 1.63 1.00 1.00 3.03 0.65 Dreta Spodnje Kraše 10.09.2021 15.67 2.16 1.70 1.00 1.00 3.48 0.7 Paka Ločan 10.09.2021 8.43 2.60 1.87 0.77 1.00 3.82 0.75 Paka Šoštanj 16.07.2024 2.73 2.47 1.93 1.00 1.00 3.23 0.67 Paka Slatina 16.07.2024 10.4 3.05 2.54 0.69 0.60 3.06 0.73 Bolska Čeplje 2.08.2022 63.99 2.51 1.95 1.00 0.92 2.12 0.45 Bolska Dolenja vas 2.08.2022 76.67 2.03 1.81 1.00 1.00 1.59 0.33 Gračnica Gračnica 25.08.2023 8.54 1.87 1.64 1.00 1.00 2.59 0.61 Krka Soteska 20.08.2020 0.19 2.38 1.94 0.68 0.78 3.31 0.64 Krka Krška vas 20.08.2020 1.74 2.50 1.86 0.92 1.00 4.32 0.8 Radulja Grič pri Klevevžu 25.08.2023 3.50 1.85 1.65 1.00 1.00 2.97 0.64 Soča Solkanski jez 30.08.2019 4.14 1.73 1.37 1.00 1.00 3.43 0.7 Soča Solkanski jez 26.08.2020 4.82 1.77 1.47 0.99 1.00 3.12 0.69 Idrijca nad Divjim jezerom 15.06.2021 3.88 1.41 1.49 0.91 1.00 3.09 0.7 Idrijca Hotešk 18.09.2020 59.72 2.22 1.90 0.64 0.77 2.36 0.53 Trebuščica Most pri Sovi 5.10.2023 8.15 1.71 1.59 0.74 1.00 3.02 0.72 Bača Grapa 5.10.2023 8.81 1.55 1.44 1.00 1.00 1.39 0.37 Vipava Velike Žablje 27.08.2020 1.51 2.18 1.88 0.83 0.87 2.91 0.63 Vipava Velike Žablje 18.07.2023 0.77 2.39 1.82 0.73 1.00 2.2 0.45 Vipava Miren 27.08.2020 5.71 2.66 1.90 0.43 0.73 3.92 0.76 Vipava Miren 18.07.2023 5.87 2.63 1.94 0.44 0.71 4.05 0.8 River/Lake Sampling site Date of sampling ADMO (%) ADRU (%) TI SI EQR TI EQR SI SW E 53 Acta Biologica Slovenica, 2025, 68 (4) Hubelj Ajdovščina 5.03.2021 + 1.69 1.67 1.00 1.00 3.17 0.67 Nadiža Robič 3.07.2024 3.14 0.39 1.65 1.65 0.98 1.00 3.18 0.76 Reka Podgraje 10.03.2021 2.09 1.75 1.78 1.00 0.78 3.56 0.78 Reka Topolc 21.06.2021 27.72 1.94 1.63 1.00 1.00 2.86 0.6 Reka Topolc 7.09.2023 25.48 2.48 1.77 0.75 1.00 3.33 0.72 Reka Cerkvenikov mlin 8.03.2021 6.73 2.39 1.98 1.00 0.82 4.21 0.83 Klivnik Brid 25.02.2020 1.17 1.54 1.48 1.00 1.00 2.28 0.48 Molja Zarečica 13.09.2023 6.65 2.36 1.81 0.81 0.77 4.33 0.8 Rižana Dekani nad pregrado 17.06.2022 12.4 2.34 1.81 0.75 1.00 3.43 0.72 Rižana Dekani nad pregrado 7.09.2023 14.85 1.60 1.53 1.00 1.00 3.25 0.72 Dragonja Planjave 8.03.2021 4.31 1.37 1.46 1.00 0.80 3.29 0.71 Dragonja Podkaštel 8.03.2021 5.34 1.65 1.71 0.98 0.72 3.74 0.74 Blejsko jezero BlFB08 12.08.2019 0.20 2.41 0.43 3.52 0.68 Bohinjsko jezero BOFB05 21.06.2022 + 1.32 0.75 3.87 0.74 Bohinjsko jezero BOFB08 21.06.2022 + 1.25 0.77 3.77 0.74 Šmartinsko jezero SmFB04 26.05.2023 1.23 2.02 4.64 0.87 Šmartinsko jezero SmFB05 26.05.2023 0.97 1.59 3.04 0.61 Šmartinsko jezero SmFB06 26.05.2023 1.95 1.57 3.77 0.77 Slivniško jezero SlFB02 29.05.2023 5.34 2.12 3.8 0.76 Slivniško jezero SlFB04 29.05.2023 10.02 2.30 3.77 0.75 Slivniško jezero SlFB05 29.05.2023 4.30 2.43 3.91 0.76 Gajševsko jezero GaFB02 14.06.2023 + 2.67 3.85 0.75 Gajševsko jezero GaFB03 14.06.2023 0.38 + 2.64 4.05 0.77 Mola MoFB02 1.08.2024 0.38 1.80 2.23 0.54 Vogršček V2FB04 7.08.2024 0.60 1.62 2.65 0.65 Vogršček V2FB06 7.08.2024 0.38 1.61 3.45 0.68 Klivnik KLFB03 8.08.2024 + + 1.68 2.62 0.63 Perniško jezero 2 P2FB03 23.10.2024 2.94 3.17 4.36 0.8 Perniško jezero 2 P2FB05 23.10.2024 6.61 2.98 2.3 0.54 Perniško jezero 2 P2FB06 23.10.2024 + 3.06 2.85 0.64 River/Lake Sampling site Date of sampling ADMO (%) ADRU (%) TI SI EQR TI EQR SI SW E For the purpose of this study, SW and E of the samples with ADMO and/or ADRU were recorded (Table 1). More- over, the correlations between ADMO and ADRU abun- dance and SW and E were calculated using the Pearson correlation coefficient. The results showed moderate neg- ative correlations of -0.46 and -0.50 between ADMO abun- dance and SW, and ADMO abundance and E, respectively. These results indicate that the presence of ADMO in the samples is associated with lower diatom species diversity and lower diatom species evenness, which is most evident in the samples with the highest abundance of ADMO (Table 1). High abundance of ADMO in the samples, reaching up to 77%, is most probably due to its invasive character. According to Buczkó et al. (2022), AD, MO has a wide eco- logical range, which serves to confirm its potential invasive behaviour. High biodiversity and high species evenness are often associated with balanced environmental condi- tions and low anthropogenic pressure, which reflect good or high ecological status of water bodies. However, in the dataset used in this research, the river sections in which ADMO abundance was the highest (more than 60%) and diatom species diversity and evenness were the lowest are associated with good or high ecological saprobic and eco- logical trophic status (Table 1). Moreover, ADMO was pres- ent mainly in the upland river sections (Figure 5), which are, in most cases, hydromorphologically undisturbed, which 54 Acta Biologica Slovenica, 2025, 68 (4) corresponds with the findings of Falasco et al. (2023). We can conclude that even if ADMO is very abundant, the consequent disturbance of the aquatic environment is not noticed, at least as far as the physical appearance of river sections (Figure 3) and ecological saprobic and trophic status assessment are concerned. These findings are in accordance with the findings from the literature (Buczkó et. al., 2022). In the lakes, ADMO was detected mainly during the qualitative inspection of the samples (Lake Bohinj, Gajševsko Lake and Lake Klivnik). With a slightly higher abundance of 0.4% ADMO was detected only in one sample in Gajševsko Lake in June 2023 (Table 1). The results of the correlations between ADRU abun- dance and SW and E were the opposite compared with ADMO, showing a weak positive correlation of 0.25 and 0.23 between ADRU abundance and SW, and between ADRU abundance and E, respectively. Rimet et al. (2010) considered ADRU as an invasive species; however, in the dataset used in this research, ADRU was present in much lower proportions (up to 27%) compared to ADMO (up to 77%) and did not affect the species diversity and evenness of the diatom assemblages in the samples. These findings are in accordance with other studies in which the invasive- ness of ADRU could not be confirmed (Ivanov, 2018). In the dataset used in this research, ADRU was present mainly in the lowland river sections, lakes and reservoirs (Figure 6), which corresponds with the findings of Ivanov (2018). ADRU reached the highest abundance in the Drava River, a river with a high number of reservoirs above the dams of Hydro-Power Plants, namely at the sampling sites Ranca, Ruše, and Tribej. In the dataset used in this research, the river sections in which ADRU abundance was the highest (>5%) are associated with good or high ecological saprobic and ecological trophic status (Table 1). Among the 11 lakes included in this study, ADRU was confirmed in eight, with the highest abundance of 10% recorded in Lake Slivnica in May 2023. In Slovenia, there are two natural lakes, namely the subalpine Lake Bled and the alpine Lake Bohinj. ADRU was found in Lake Bled in August 2019 with a low abundance of 0.2%; however, in Lake Bohinj, which is considered one of the most ecologically pristine lakes in Slovenia (good ecological status; Table 1), it was not found. Moreover, Lake Bled was included in the dataset used in this research in the years 2019 and 2022; however, ADRU was found only in one sample in 2019 and was not detected again in 2022. The distribution of ADMO and ADRU in Slovenian rivers and lakes in the years 2019–2024, accompanied by the rel- ative abundance of both species expressed in percentage, is presented in Figures 5 and 6. Buczkó et al. (2022) found that ADMO started to spread in the Danube River from the source downstream. In 2013, it was found in Germany and Austria, and three years later, it was found in the Hungarian section of the Danube River. However, this pattern was not confirmed in Slovenian rivers. Figure 3. River Bolska has the highest occurrence of invasive benthic diatom Achnanthidium delmontii (ADMO) in Slovenia. A-Bolska-Čeplje and B-Bolska-Dolenja vas in August 2022. Photo: archive ARSO. Slika 3. Reka Bolska z najvišjo številčnostjo invazivne bentoške diatomeje Achnanthidium delmontii (ADMO) v Sloveniji. A-Bolska-Čeplje in B-Bolska-Dolenja vas avgusta 2022. Foto: arhiv ARSO. 55 Acta Biologica Slovenica, 2025, 68 (4) Figure 4. River Drava has the highest abundance of invasive alien diatom species, Achnanthidium druartii (ADRU), in Slovenia. A-Drava-Ruše and B-Drava-Ranca in August 2024. Photo: archive ARSO. Slika 4. Reka Drava z najvišjo številčnostjo invazivne tujerodne vrste diatomeje Achnanthidium druartii (ADRU) v Sloveniji. A-Drava-Ruše in B-Drava-Ranca avgusta 2022. Foto: arhiv ARSO. Figure 5. Distribution and relative abundance (in percentage) of Achnanthidium delmontii (ADMO) in Slovenia between 2019 and 2024. Slika 5. Porazdelitev in relativna abundanca (v odstotkih) vrste Achnanthidium delmontii (ADMO) v Sloveniji med leti 2019 in 2024. 56 Acta Biologica Slovenica, 2025, 68 (4) The first record of ADMO and ADRU in Slovenia is from the national monitoring of surface waters in 2019. The most probable reason is that in 2019, the Slovenian Environment Agency switched from the diatom taxonomy of Süßwasser- flora von Mitteleuropa (Krammer and Lange-Bertalot, 1986, 1988, 1991a, 1991b) to the taxonomy of Freshwater Benthic Diatoms of Central Europe (Lange-Bertalot et al., 2017). In the monograph of Lange-Bertalot et al. (2017), both ADMO and ADRU were already included, while in older mono- graphs, they are missing since both species were only for- mally described as new diatom species in 2012 (ADMO) and 2010 (ADRU). Although ADMO was formally first observed in Slovenia in 2019, it was most likely present in Slovenian freshwaters several years earlier and had been counted under Achnanthes biasolettiana Grunow, which is currently regarded as a synonym of Achnanthidium pyrenaicum (Hustedt) H. Kobayasi. One reason to assume the earlier occurrence of ADMO in Slovenia is that it had already been reported in the freshwaters of neighbouring countries: as early as 2013 in Austria (Buczkó et al., 2022) and Italy (Falasco et al., 2023), and in 2015 in Hungary (Buczkó et al., 2022). Buczkó et al. (2022) reported that in 2013, when ADMO was first recorded in German and Austrian sections of the Danube River, it was present in low abundances (<3%); however, by 2019, ADMO had become one of the most abundant and frequent diatom species found in the Danube River. Given that ADMO, when first recorded in Slovenian samples in 2019, was already present with very high abundances, reaching up to 51% in Sava - Jesenice na Dolenjskem (Table 1, Figure 5), we can conclude that it had already been present in Slovenia well before 2019. In con- trast, according to our information, there are no records of ADRU occurring in neighbouring countries before 2019, and thus it may not have been present in Slovenian waters earlier. Furthermore, ADRU, when first recorded in Slovenian samples in 2019, showed a relative abundance Figure 6 Distribution and relative abundance (in percentage) of Achnanthidium druartii (ADRU) in Slovenia between 2019 and 2024. Slika 6. Porazdelitev in relativna abundanca (v odstotkih) vrste Achnanthidium druartii (ADRU) v Sloveniji med leti 2019 in 2024. 57 Acta Biologica Slovenica, 2025, 68 (4) of up to 6% in Drava - Prepolje, which is 8.5 times lower than that of ADMO. There is a possibility that ADRU will be more widely distributed in Slovenian waters with a higher relative abundance after a certain amount of time, as was the case with ADMO. The data presented in this study show only the present state, which might be temporally biased. Although there are no records of ADRU in Croatia, Hungary, and Italy, we can conclude from its presence in Slovenian rivers near the border of the above-mentioned countries, such as the Kolpa, Sotla, Dragonja, Ledava, and Nadiža (Table 1, Figure 6), that ADRU is most likely present in the territory of those countries. Although diatoms are useful indicators of various pres- sures such as nutrient and organic loading, acidification, or salinity in running and standing surface waters, ADMO and ADRU are not yet considered as indicator organisms. However, ongoing research is exploring their potential as indicator organisms, which would be reasonable given their increasing prevalence across Europe. Conclusions Altogether 247 phytobenthos samples collected at 87 rivers and 35 phytobenthos samples collected at 11 lakes in the frame of the national monitoring of surface water quality from 2019 to 2024 were included in this study. Achnanthidium delmontii (ADMO) was present in 100 river samples (41%) from 40 rivers (46%) and was a dominant species in 56 phytobenthos samples (23%) from 27 rivers (31%). The highest abundance of ADMO was recorded in the Bolska River at sampling sites Dolenja vas and Čeplje with relative abundances of 77% and 64%, respectively. Achnanthidium druartii (ADRU) was present in 32 river sam- ples (13%) from 12 rivers (14%) and was a dominant species in 11 phytobenthos samples (5%) from 4 rivers (5%). The highest abundance of ADRU was detected in the Drava River at sampling site Ruše with a relative abundance of 27%. In lakes, ADMO was relatively rare, being detected in only five samples (14%) across three lakes and was never dominant in any of the lake samples. ADRU was present in 15 samples (43%) from 8 lakes and was a dominant species in 3 samples from two lakes. The highest relative abundance of ADRU was observed in Lake Slivnica with a relative abundance of 10%. The results of this study show that the presence of ADMO in the samples is associated with lower diatom species diversity and evenness, indicating, together with very high relative abundances, its invasive character. How- ever, the river sections in which ADMO abundance was the highest and diatom species diversity and evenness were the lowest are associated with good or high ecological sap- robic and ecological trophic status. Moreover, ADMO was present mainly in the upland river sections, which are, in most cases, hydromorphologically undisturbed. Although ADMO was present in the investigated rivers in very high numbers, the consequent disturbance of the aquatic environment was not noticed, at least as far as the physical appearance of river sections and ecological saprobic and trophic status assessment are concerned. ADRU, which was present mainly in the lowland river sections and lakes, did not affect the species diversity and evenness of the diatom assemblages in the samples, and thus its invasive character could not be confirmed. Author Contributions Conceptualization, A.K.K.; methodology, A.K.K. and T.Š.; investigation, A.K.K. and T.Š.; resources, A.K.K.; data cura- tion, A.K.K. and T.Š.; writing—original draft preparation, A.K.K.; writing—review and editing, A.K.K. and T.Š.; funding acquisition, A.K.K. All authors have read and agreed to the published version of the manuscript. Acknowledgement Special thanks are extended to Mr. Matej Cunder from the Slovenian Environment Agency for creating the distribution maps used in this study. Funding The authors acknowledge the financial support from the Slovenian Environment Agency and the Slovenian Research Agency (research core funding No. P2-0180). Conflicts of Interest The authors declare no conflict of interest. 58 Acta Biologica Slovenica, 2025, 68 (4) References ARSO, 2017. Program monitoringa kemijskega in ekološkega stanja voda, program za obdobje 2016 do 2021. Agencija Republike Slovenije za okolje, Ministrstvo za okolje in prostor, Ljubljana. https://www.gov.si/assets/organi-v-sestavi/ARSO/Vode/Stanje-voda/Program-monitoringa-kemijskega-in-ekoloskega-stanja- voda-za-obdobje-2016-do-2021.pdf (accessed 20.4.2025) ARSO, 2022. Program monitoringa kemijskega in ekološkega stanja voda, program za obdobje 2022 do 2027. 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