This study presents the first occurrences of the diatom species Discostella asterocostata in Europe’s second largest river, the Danube River and the earliest occurrence in European freshwater ecosystems. Samples collected between 2016 and 2022 in Croatia and Hungary were analysed using light and scanning electron microscopy (SEM) along with molecular techniques to confirm the presence of the species. Morphometric comparisons with known populations from Asia and the USA revealed no significant morphological differences. Molecular analyses facilitated the detection of D. asterocostata even in samples with low abundance and confirmed the genetic similarity with already sequenced populations. Environmental correlations showed that D. asterocostata had a significant positive correlation with temperature and a negative correlation with total organic carbon and nitrogen forms, suggesting that it favours warmer waters with lower nutrient forms. Given the ongoing increase in water temperatures due to climate change, the occurrence and potential spread of this species in European rivers may be related to changing environmental conditions. Our results emphasise the importance of integrating traditional and molecular approaches for the early detection of non-indigenous diatom species. The establishment of D. asterocostata in Europe raises questions regarding its ecological role and long-term impact on native phytoplankton communities and emphasises the need for continuous monitoring and research on the responses of large river ecosystems to global environmental change.

Metabarcoding is a rapidly developing field for studying aquatic life and provides a promising alternative to microscopy. However, accurate assessments require that database errors and species boundaries be addressed, yet the use of only a short gene sequences in metabarcoding, may be insufficient for accurate species identification. This study examines the potential of metabarcoding in replacing traditional microscopic methods in planktic diatom identification. Phytoplankton samples were collected monthly from May 2021 to April 2022 at 13 sites on the Hungarian section of the River Danube. Environmental variables were measured, and electron microscopy and metabarcoding analyses were conducted. Both morphological and DNA analysis methods were used to study the Thalassiosirales order. Although there was some overlap in the taxa identified by both methods, there were also discrepancies, with certain taxa detected exclusively by one method. P-distance analysis and BLAST search were used to correct misidentifications, revealing mismatches between database sequences and observed species. Phytoplankton community exhibited varying temperature and nutrients optima and tolerance ranges, which influenced their distribution patterns. During spring, diatoms-particularly Thalassiosirales-dominated the phytoplankton, with proportions decreasing in summer. Algal biomass in the Danube was highest in March, decreasing sharply by the end of summer and remaining low until the end of the growing season, the decrease relating to changes in the TN:TP ratio, which was very low in the warm water period (mostly below 10), leading to nitrogen limitation. Discrepancies between bioinformatic analysis and SEM observations revealed errors in the reference database. Our results clarify the functional group (FG) classification, which is important for both ecological status assessment and understanding of ecosystem functioning. Temperature is one of the most fundamental drivers of microbial nitrogen dynamics in rivers. Global warming is driving up the average temperature of the Danube, creating more favourable conditions for denitrification since the speed of microbial processes is higher in warm water. The increasingly common nitrogen limitation could potentially limit algal growth. Environmental factors, especially temperature and nutrient concentrations, significantly influenced the Danube's phytoplankton communities, with implications for ecosystem health and water quality assessment. Integrated approaches combining molecular techniques with traditional morphological analysis are needed for comprehensive ecological assessments, but accurate species identification and ecological status assessment require completeness in reference databases and the correction of database errors.

We explored seasonal changes in community assembly processes shaping periphytic diatom distribution in hilly streams of the Danube River basin. Particularly, we were interested (1) whether compositional stochasticity of diatom community changes with increasing hydrological disturbance in spring compared to favorable conditions in autumn; (2) how the relative strength of environmental filtering and dispersal-related processes varies across seasons; and (3) whether the relative importance of these processes depends on dispersal abilities of diatom guilds (low-profile, high-profile, motile, and planktic). The compositional stochasticity increased in autumn, indicating that diatom assemblages were more prone to ecological drift in benign hydrological conditions. Variation partitioning showed that environmental filtering remained consistently more important than dispersal-related processes across seasons. The stronger taxonomic homogenization observed for low-profile guild in spring suggests that this guild was influenced by mass effects during high-flow period. Thus, low-profile diatoms were strongly driven by dispersal, while high-profile and motile diatoms were more controlled by the environment. However, the increased role of mass effects was mediated by high abundance and low disturbance sensitivity of low-profile diatoms, rather than their dispersal ability. Overall, our results highlight the important role of hydrology in the balance between deterministic and stochastic processes structuring lotic diatom metacommunities.

During our study on small and shallow waters in Hungary, a new invasive species of copepod, Sinodiaptomus sarsi (Rylov, 1923), was found in our zooplankton samples collected in 2017. In Hungary, we found it in 17 lakes, with significant biomass in the 4th fishing lake on the outskirts of Püspökladány, Szénaréti-tó in Nyírbátor, the Fegyverneki-Holt-Tisza, and Lake Déli-tó in Gyopárosfürdő. As a result, we confirm that Sinodiaptomus sarsi has successfully established populations in Hungarian waters, yet we found no clear evidence that its presence has significantly altered the overall zooplankton community structure. As a selective filter feeder, it can potentially fulfil the functional role of native calanoid copepods in lentic ecosystems. However, its distribution may be restricted by its thermal requirements, which could confine its presence to specific habitats or seasons. S. sarsi appears to be an invasive and opportunistic member of Hungarian freshwater zooplankton, with its role and distribution likely determined by the interplay of temperature regimes, trophic state, and predator pressure. Monitoring its dispersion will be essential to detect potential shifts in population dynamics, particularly under changing climatic and eutrophication scenarios.

Global climate changes have led to dramatic increases in drought durations in previously permanent streams, impacting the biodiversity and functioning of river ecosystems. However, the response of benthic diatom communities to hydrological intermittency remains poorly understood. In this study, we compared the taxonomic and functional compositions of the diatom communities between permanent and intermittent sections in two hilly stream systems in southwestern Hungary. Our results showed that both the taxonomic and functional compositions of diatom communities were significantly affected by changes in the hydrological regime, leading to a decline in species richness and diversity and functional richness in intermittent sections. Functional richness and dispersion decreased significantly with declining taxonomic richness, likely as a consequence of species loss driven by flow intermittency. Aquatic–subaerial diatoms with moderate oxygen requirements were indicative of intermittent sections, while large, occasionally aerophilic and oxybiontic diatoms characterized permanent sections. The relative abundance of low-profile diatoms increased in intermittent sections, indicating that the natural successional process of communities was disrupted due to streambed drying. Furthermore, intermittent sections were marked by elevated abundances of α-mesosaprobous and α-meso-polysaprobous diatoms, indicating a reduced self-purification capacity under intermittent-flow conditions. These findings provide detailed insight into the responses of diatom communities to drought and water scarcity in intermittent streams, which are becoming increasingly common in warm temperate regions.

The relative importance of environmental filtering and dispersal in structuring metacommunities of littoral periphytic diatoms was assessed within a large lake (Lake Ladoga, north-western Russia). We hypothesized that different diatom functional guilds (high-profile, low-profile, motile and planktic) would respond differently to environmental and spatial variables, depending on their mode of attachment and dispersal ability. Guilds showed distinct distributional patterns along a gradient of wave-induced disturbance: low-profile diatoms dominated in exposed shorelines (high disturbance), whereas high-profile and motile diatoms were more abundant in coastal wetlands (low disturbance). Redundancy analysis with variation partitioning of the entire diatom community revealed that environmental variables (littoral geomorphology and water chemistry) outperformed spatial variables, indicating a predominant role of species sorting. When the diatom community was deconstructed into functional groups, benthic guilds (i.e., low-profile, high-profile and motile) showed higher dependence on environmental factors, whereas the planktic guild was primarily predicted by spatial factors. Thus, benthic taxa with lower dispersal abilities were less impacted by mass effects than planktic taxa. Among benthic guilds, motile diatoms were less sensitive to water chemistry compared to low-profile and high-profile diatoms, suggesting that a group-specific habitat preference may interfere with diatom responses to water quality changes. Hence, the response of diatom guilds to water quality parameters was related to their resistance to wave action. Overall, we concluded that the higher importance of environmental filtering than dispersal-related processes in shaping the diatom metacommunity resulted from a combination of a strong environmental gradient in geomorphology/water chemistry and the intermediate spatial scale of our study.

Egy kísérleti kavitációs berendezés tervezésével, létrehozásával és kisüzemi alkalmazásával a szennyezett vizek egyik lehetséges tisztítási módját vizsgáltuk. A kísérleti berendezés mobil kivitelű, és alkalmas különböző szennyezettségű vizek tisztítására. Bemutatjuk a kavitáció hatását különböző mikroszkopikus méretű élőlényekre. Először egy hígított zöldalgatenyészetet vizsgáltunk, megállapítottuk, hogy 16–32 perces kavitáció több mint 10–20%-kalcsökkenti a klorofillkoncentrációt és az ép sejtek arányát. Biológiailag bontható szennyvízzel kevert algás halastóvízben, kavitáció hatására a cianobaktériumok, ostoros algák mennyisége 40–80%-kalcsökkent, a zöldalgáknál minimális volt a csökkenés. Ezeknél a méréseknél síkszelepes, nagynyomású kavitációgenerátort alkalmaztunk. Bizonyítottuk,hogy a hajók ballasztvize és a szennyvizek mikrobiótája mennyiségének csökkentésére a kavitációs vízkezelés ígéretes megoldásnak tekinthető, de figyelemmel kell lenni arra, hogy csak elpusztul a mikrobióta, de nem tűnik el a vízből.

The research presented investigates whether DNA-based metabarcoding can replace the morphology-based identification of diatom taxa in the ecological status assessments of aquatic habitats. When comparing data obtained with microscopy and metabarcoding, significant deviations have been noticed. One of the main reasons includes the incompleteness of the reference database used for taxonomic annotation of sequences. The database library should be complemented with species inhabiting unique habitats and having specific environmental requirements representing environmental endpoints for genetic diversification. Soda pans and soda lakes are examples of an extreme habitat with the loss of sodic character as the main threat; thus, accurate identification of species and exact information on their salinity tolerance is essential for adequate ecological status assessment. In the present study, by using microscopy and metabarcoding, we investigated taxa of the genus Halamphora that are common in soda pans and soda lakes. We detected six species of which Halamphora dominici and H. veneta occurred frequently and often in high abundance (it was often dominant having relative abundance higher than 5%). Analyses of DNA data confirmed the separation of the two species; as a result, the reference database library has been supplemented with sequences of H. dominici. Furthermore, we have confirmed that this species, which is a significant indicator of sodic character, shows a positive correlation with salinity.

In this work the rapid expansion of a small-celled (<15 µm) monoraphid aquatic invasive species (AIS) along the Central European lotic systems is reported, using the integrated dataset of two large-scale monitoring programs, as supplemented by additional records. Achnanthidium delmontii Pérès, Le Cohu & Barthès 2012 (ADMO) was discovered in 2007 and formally described in 2012, on the basis of specimens from a French river. ADMO was first detected in the upper sections of River Danube in 2013, and has been detectable since 2013, and from 2015 onwards in Hungary. The abundance and the number of occupied habitat types by the species have gradually increased. In 2019, ADMO was found to be among the most abundant and the most frequent species in the River Danube, with a mean relative abundance of at least 5%, and a frequency of at least 10% in samples. To extend the Danubian dataset, the relative abundances of ADMO from 79 freshwater lotic samples were studied to assess their potential of the species as an indicator organism. Weighted average regression was employed to determine the species’ optima and tolerances for 18 environmental variables. ADMO has a wide ecological range, which serves to confirm its potential invasive behaviour. In the case of the following variables, the values were found to be consistent with previously published data on the requirements of the taxon. Despite ADMO prefers high temperatures (estimated optimum = 21.8 °C), its spread shows a downstream pattern of the main watercourses of Europe. The species was first found abundantly from the upper section of River Danube, then with increasing abundance in the middle section, while remaining rare in the lower section. This distribution demonstrates that the River Danube serves as an important linear route for the species' invasion.The results of the study will help improve the ecological classification of water bodies and inform water protection actions undertaken under the auspices of the Water Framework Directive (WFD).

Diatoms are valuable bioindicators and their traditional classification and identification are mainly based on the morphological characteristics of their frustules. However, in recent years, DNA-based methods have been proposed and are rapidly growing in the scientific literature as a complementary tool to assess the ecological status of freshwaters. Diatom-based ecological status assessment uses indices calculated from sensitivity and tolerance values as well as relative abundance of species. Correct assessment requires an accurate identification of species. In the present study, diatom assemblages of an oxbow lake were investigated using light and scanning electron microscopy as well as metabarcoding using rbcL marker, and the identification results were compared, intending to match barcode sequences of species that are currently missing in the diatom reference database. The investigated oxbow is an important wetland for bird conservation, although it is impacted by land use. Taxon lists based on morphology and metabarcoding considerably differed when bioinformatics analysis involved DADA2 pipeline with Diat.barcode database. Previously unknown sequence variants of four pennate species were found with additional BLAST search. Using phylogeny and p-distance calculations sequences could be matched to three small-celled naviculoid species that were found under a microscope. One of them was found to be a new species of the genus Mayamaea and was described as a new species, Mayamaea ectorii. Additionally, spatial distribution maps for several small-celled naviculoid species are provided for the Hungarian territory.

Diatoms are widely applied in the ecological status assessment of aquatic ecosystems using indices calculated from pollution sensitivity and indicator values of species. Traditional, morphology-based identification of species requires in-depth taxonomic knowledge and expertise. Identifying taxa according to their barcode sequences obtained with high-throughput sequencing (metabarcoding) would be a promising alternative. In this pilot study, we tested the applicability of metabarcoding of benthic diatom assemblages for the ecological status assessment of Hungarian water bodies, comparing its performance to that of morphology-based identification of species. The barcode region of the rbcL gene was investigated in samples from running waters with various trophic states and unique lentic habitats, namely soda pans. For running waters, the Specific Pollution Sensitivity Index (IPS) and for soda pans the H index and the Indice Biologique Diatomées (IBD) were calculated from the morphology- and sequence-based abundances. The ecological quality ratio was determined for running waters. Overall, more infrageneric taxa were found with microscopy than with metabarcoding in the lotic and lentic samples. The correspondence between taxon lists detected by the two methods was relatively low but increased considerably in the case of morphologically dominant taxa. Community composition based on microscopy and DNA sequence analysis showed a significant correlation and was determined by the same main environmental drivers. Morphology-based indices strongly correlated with sequence-based indices. Both aspects indicated the same ecological status class for more than half of the lotic samples. In other cases, a status shift from good to moderate was frequent, a phenomenon that could prove problematic because the Water Framework Directive prescribes intervention for waters with a moderate or worse status. Considering discrepancies between the results obtained with microscopy and metabarcoding, using both methods in parallel could be proposed until the reference database has been suitably updated.

On-site wastewater treatment systems are gaining popularity in areas where centralized wastewater treatment is not available. In the current case study a domestic activated sludge system was investigated, where treated effluent was stored in a short-term (1 week turn-over time) and a long-term (over 2-3 months) storage tank and was then used for irrigation. This design provided a unique opportunity to assess the chemical and microbial changes of the effluent upon storage. Long-term storage greatly improved both the chemical quality and the degradation efficiency of most organic micropollutants examined, including petroleum hydrocarbons and the pesticide diethyltoluamide. Taxonomic profile of the core microbiome of the effluent was also influenced upon storage. Relative abundance values of the members of Azoarcus and Thauera genera, which are important in degrading polycyclic aromatic hydrocarbons compounds, clearly indicated the biodegrading activity of these microbes across samples. The abundance of xenobiotics degradation functions correlated with the observed organic micropollutant degradation values indicating efficient microbial decomposition of these contaminants. Functions related to infectious diseases also had the highest abundance in the short-term storage tank corresponding well with the relative abundance of indicator organisms and implying to the significance of storage time in the elimination of pathogens. Based on these results, small, on-site wastewater treatment systems could benefit from long-term storage of wastewater effluent.

The body size of aquatic invertebrates is, to a great extent, dependent on ambient temperature, but size distributions are also determined by other factors like food supply and predation. The effect of temperature on organisms is formulated in the temperature–size hypothesis, which predicts a smaller body size with increasing temperature. In this study, the effect of temperature on the subfossil remains of three littoral Cladocera (Alona affnis, A. quadrangularis, and Chydorus cf. sphaericus) was investigated. Exoskeletal remains of these species can be found in large numbers in lacustrine sediments and over a wide north–south range in Europe. The total length of both headshield and postabdomen for A. affinis and A. quadrangularis and carapace length for C. cf. sphaericus were measured to observe their response to changes in latitude and temperature. A different response to ambient temperature in the growth of body parts was observed. The size of the headshields of both Alona species and of the carapace of Chydorus was significantly larger in colder regions as opposed to warm ones. It turned out that the postabdomen was not a good predictor of ambient temperature. While the sizes of all remains increased with latitude, the sizes of the Alona remains was smaller in the mountain lakes of the Southern Carpathians than in other cold lakes, in this case in Finland, a fact indicative of the importance of other factors on size distribution. This study demonstrates that a morphological response to climate is present in littoral cladocerans, and, therefore, changes in the length of headshield and carapace may be used as a proxy for climate changes in paleolimnological records.