Éva Ács

The increasing focus on ecosystem restoration in European environmental policy highlights the need for reliable methods to quantify the nature conservation value (NCV) of aquatic habitats. Current Water Framework Directive (WFD) metrics assess ecological quality mainly through the phytoplankton community composition and abundance. However, these approaches do not reflect properly the real conservation value of the habitats, because they overlook the contribution of rare and unique algal species. Currently, the method proposed by Coesel is the only microalgae-based framework explicitly incorporating rarity, using desmid assemblages to assess the conservation value of aquatic habitats. In this study, we investigated its applicability to the Hungarian aquatic ecosystems using a database containing 379 desmid species from 101 localities, representing 14 habitat types. We calculated the NCV value of all localities. The highest NCV values were found in old bog lakes, marshlands, oxbows and large saline lakes, while reservoirs, large rivers and heavily modified habitats showed lower scores. No significant relationship was detected between the NCV value and the WFD-compliant Ecological Quality Ratio (EQR) metrics, indicating that ecological status metrics are weak predictors of conservation value. Although the desmid-based NCV method provides a promising foundation, its components require refinement and habitat-specific calibration to support more robust conservation assessments.

The Bogdałów post-mining reservoir (Poland) represents a slightly alkaline, moderately mineralised ecosystem formed by flooding a former lignite pit. Its anthropogenic origin and stable physicochemical conditions have enabled the development of species-rich diatom assemblages, particularly numerous Nitzschia (Bacillariaceae) species. To explore this diversity, an integrative approach combining microscopy and DNA-based analyses was employed. Morphological examinations were performed using light and scanning electron microscopy, as well as confocal laser scanning microscopy. Molecular phylogenetic analyses were based on the sequencing of the nuclear SSU rDNA and the chloroplast rbcL and psbC gene markers. This comprehensive study led to the discovery and formal description of Nitzschia excavata sp. nov., distinguishable by unique morphological features and a phylogenetically distinct lineage. Furthermore, environmental DNA metabarcoding and metagenomic database searches revealed sequences identical or closely related to the N. excavata sp. nov. lineage in freshwater habitats across Europe and China, indicating that this taxon has an unexpectedly broad distribution. These findings underscore the value of integrating classical morphological analysis with multi-marker molecular data in diatom taxonomy and demonstrate that anthropogenic habitats may support taxa with broader distributions than previously recognized. The study highlights the important role of metabarcoding and metagenomics in revealing cryptic diversity and clarifying the biogeographic patterns of newly described species.

A comparative analysis of beta-diversity (i.e., spatial variation in community structure) of periphytic diatoms under the influence of seasonal variability of the hydrological regime in the streams of the Danube River basin was carried out. In spring, high-flow events following heavy rainfall resulted in decreased species diversity and evenness with a concomitant increase in the abundance of disturbance-resistant low-profile diatoms. In autumn, benign hydrological conditions resulted in a recovery in species diversity and evenness with increased abundance of disturbance-sensitive motile diatoms. Variance partitioning analysis showed that although environmental factors dominated in both seasons, spatial variables played a more important role in spring than in autumn. The distance-decay of similarity analysis revealed that spring floods are accompanied by a significant increase in the homogeneity of community structure between nearby stations due to the effect of excessive dispersal. At the same time, high heterogeneity of the environment under hydrological disturbance conditions leads to a rapid change in community structure along the spatial gradient. In autumn, with a weak environmental filter, the role of ecological drift increased. Thus, seasonal changes in the hydrological regime affect the balance of deterministic and stochastic processes shaping beta-diversity patterns of periphytic diatoms in riverine conditions.

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.

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.

DNA metabarcoding of benthic diatoms has been successfully applied for biomonitoring at the national scale and can now be considered technically ready for routine application. However, protocols and methods still vary between and within countries, limiting their transferability and the comparability of results. In order to overcome this, routine use of DNA metabarcoding for diatom biomonitoring requires knowledge of the sources of variability introduced by the different steps of the procedure. Here, we examine how elements of routine procedures contribute to variability between European laboratories. A set of four experiments were performed focusing on DNA extraction and PCR amplification steps to evaluate their reproducibility between different laboratories and the variability introduced by different protocols currently applied by the scientific community. Under the guidance of a reference laboratory, 17 participants from 14 countries performed DNA extraction and PCR amplification in parallel, using the same fixed protocol and their own choice of protocol. Experiments were performed by each participant on a set of standardised DNA and biofilm samples (river, lake and mock community) to investigate potential systematic and random errors. Our results revealed the successful transferability of a protocol amongst labs and a highly similar and consistent ecological assessment outcome obtained regardless of the protocols used by each participant. We propose an “all for one but prove them all” strategy, suggesting that distinct protocols can be used within the scientific community, as long as their consistency is be proven by following minimum standard requirements.

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.

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.

In this study, we aim to investigate how the functional properties of microalgae help to delineate the major groups of aquatic habitats. Using functional trait-based and Reynolds’ functional group-based approaches similarities of the microalgal flora of all aquatic habitats occurring in Hungary were compared. The habitats covered the whole size spectrum of standing waters (10^-2–10⁸ m²) and water currents (watershed: 10²–10¹¹ m²), , limnological and chemical properties. Both functional trait-based and functional group-based habitat classifications overrode the hydromorphology-based typology, however, functional group-based clusters showed closer resemblance to limnological-hydromorphological types than clusters created by trait-based approaches both for qualitative and quantitative data. Most of the aquatic habitats that have similar limnological characteristics showed resemblance in the functional properties of their microflora. Rivers and river-related habitats were the most diverse functionally. These were followed by standing waters with extended macrophyte coverage. The small, unique habitats displayed the lowest functional richness. The occurrence of several functional groups in some extreme habitats implies two alternative explanations: first, the habitat template of the groups is wider than defined in the original description; second, detailed information on the autecology of species assigned to a functional group necessitates the creation of new groups specific for the unique habitats.