Quagga (QM) and zebra (ZM) mussels (Dreissena bugensis and Dreissena polymorpha) are invasive bivalves forming freshwater fouling communities of high economic and environmental importance in Europe and North America. They may experience immersion in soft sediments by sedimenting particles (mostly at deeper locations or areas with high hydrodynamic activity) or while attached to burrowing unionid mussels. On the other hand, mussels located near the water surface may encounter water level decreases and air exposure during droughts. We experimentally tested mussel survival and behavioral responses to immersion in sediments and water level reduction. Interspecific differences in these responses can help understand the ongoing replacement of ZM by QM in invaded communities and less common cases of their co-existence. ZM were more resistant to both stressors, surviving longer when immersed in sediments or exposed to air. Moreover, both species survived better in coarse versus fine sediments. QM re-emerged from sediments more efficiently than ZM. In turn, ZM relocated over longer downward distances facing water level reduction compared to the behavior of individuals tested at the constant water level. Thus, ZM seem better adapted to remain at their attachment sites, survive temporary adverse conditions, and cope with dangers typical for shallow nearshore areas. On the other hand, QM perform better on soft bottoms, abandoning their attachment sites to cope with immersion in sediments. These adaptations allow QM to dominate deep-water soft sediments, whereas ZM can find refuges in areas periodically exposed to air and on unionid mussels.

Filter feeders, like mussels, can significantly lower the concentration of harmful substances in the water body. In the present study, we examined the distribution of organic pollutants (polycyclic aromatic hydrocarbons [PAHs], non-steroidal anti-inflammatory drugs [NSAIDs]) in Lake Balaton, the largest shallow lake of Central Europe. We also investigated the sensitivity of the invasive quagga mussel to these substances and its potential to reduce their concentration in the water column. Our findings show that organic pollutant levels in Lake Balaton were generally below concentrations known to harm mussels, as indicated by the stress gene activity patterns observed along the lake's longitudinal axis. However, in the most urbanized eastern part of the lake, especially in spring, we detected signs of environmental contamination from certain pollutants (e.g. diclofenac), highlighting potential risks to local ecosystems and communities. Removal capacity of the mussels for PAHs reached the maximum after four days of exposure to 5-10 % diluted water accommodated fraction of fuel-oil fraction #4 when the mussels (20 ind. L-1) reduced the PAH level by 100-85 %. Mussels (50 ind. L-1) removed 28 % and 21 % of ibuprofen and ketoprofen, respectively, from 1 µg L-1 concentrated solutions within 24 h. Many of the stress response genes were activated in the quagga mussel after their exposure to PAHs. These results suggest a significant role of gregarious invasive bivalves in the removal of organic pollutants from lake water.

Unionid mussels are globally threatened by several human disturbances, including the introduction of non-native species. Among these, biofouling zebra and quagga mussels of Ponto-Caspian origin are considered to be especially detrimental to unionid locomotion, filtration and physical condition. The aim of our study was to determine and compare the impact of dreissenid fouling and/or presence on locomotion and burrowing of the native Unio tumidus and invasive Sinanodonta woodiana, a novel invader expanding its range in Europe in recent decades. We tested unionids collected from Lake Balaton (central Europe) that were fouled by dreissenids (zebra and quagga mussels mixed), cleaned of fouling or non-fouled (collected without any signs of dreissenid fouling). Moreover, unionids were tested in the presence or absence of other fouled individuals and dreissenids isolated in mesh bags to determine the influence of direct fouling and presence of dreissenids in the environment on unionid behaviour. Movement initiation time, locomotion distance and burrowing level were retrieved from videos recorded for 24 hours. Direct fouling affected only the behaviour of U. tumidus, limiting their burrowing and delaying movements. After removal of fouling, movement timing returned back to normal, but mussels still burrowed less than the control non-fouled individuals, indicating persisting effects of fouling on physical condition. Moreover, U. tumidus reduced their locomotion in the presence of fouled unionids. Sinanodonta woodiana responded to the presence of dreissenids (especially quagga mussels) with increased burrowing. These different responses of the two unionid species to Dreissena spp. indicate that biofoulers may influence biotic interactions between the unionids by promoting the invasive species (less susceptible to negative effects of fouling). Moreover, S. woodiana may indirectly affect U. tumidus through apparent competition, constituting an environmental reservoir of biofoulers exerting a stronger impact on the native species.

Byssate bivalves are ecosystem engineers with world-wide impact on aquatic communities through habitat forming and biofouling of hard-shelled organisms. In fresh waters, they are represented by invasive Ponto-Caspian dreissenid mussels spreading throughout Europe and North America. They negatively affect globally threatened unionid mussels by fouling, which deteriorates their condition and survival. The appearance of quagga mussels (D. rostriformis bugensis, QM) in areas occupied by zebra mussels (Dreissena polymorpha, ZM) usually has led to the replacement of ZM by QM. We combined long-term field survey (Lake Balaton, Hungary) and experimental data to check differences in fouling of unionid mussels (Unio tumidus and Sinanodonta woodiana) by the two dreissenids, determine their mechanisms and predict environmental consequences of the species replacement. ZM fouled unionids evenly throughout the year, whereas QM exhibited high fluctuations, being common on unionid shells during their recruitment peak (summer), decreasing towards autumn and almost completely absent in spring. Such fluctuations did not occur on stony substrata. This pattern suggests that interspecific differences in fouling did not result from recruitment preferences, but from greater detachment of QM from unionid substratum, whereas ZM more often remained attached to their initial recruitment sites. This was supported by the results of the laboratory experiments, in which dreissenid mussels did not show any consistent preference or avoidance of unionid mussels. Whereas, QM attached less often than ZM to hard objects and showed a higher detachment rate. Furthermore, dreissenids increased detachment after substratum immersion into soft sediments, indicating their capability of coping with suffocation after the burrowing of the living substratum or its siltation. The observed pattern indicates that the replacement of ZM by QM in the dreissenid assemblage may reduce fouling pressure on unionids. On the other hand, unionids may become a refuge for ZM in habitats invaded by competitively superior QM.

The flow direction forms a west-east nutrient gradient in Lake Balaton and separates two basins with different food conditions indicated by the annual mean of water chlorophyll-a concentration. Trends of protein and carbohydrate contents of the invasive quagga mussel decline along the longitudinal coordinates, whereas lipids increase in mussels living between the two basins under moderate food conditions. Lipid accumulation might rescue the mussels when carbohydrate stores deplete.

Dreissena rostriformis bugensis (quagga mussel, QM) has spread into areas occupied by an earlier invader, Dreissena polymorpha (zebra mussel, ZM) in Europe and North America. Usually QM displaces ZM within a few years or both species coexist, although the mechanisms driving these outcomes have not been uncovered clearly.In Lake Balaton (central-eastern Europe), QM displaced ZM in the oligotrophic (food-limited) basin, whereas they coexist in the eutrophic (food-rich) basin. Searching for the drivers of interactions in dreissenid assemblages, we compared survival, growth, allometry, shell hardness, biomacromolecule content and superoxide dismutase (SOD) expression (indicating nutrition stress) of dreissenids collected in both basins in a field survey, and in individuals collected from the food-rich basin and experimentally transplanted (10 weeks) to the food-limited or food-rich (i.e. the same) basin.In the field survey, QM from the food-rich basin showed the greater height increment per unit length than coexisting ZM and food-limited conspecifics. ZM had the hardest shells of all the mussel populations. In the food-rich basin, ZM did not differ from QM in weight, protein, and carbohydrate contents, but had higher lipid content and SOD expression. Food-limited QM, compared to conspecifics from the food-rich basin, had weaker shells, but their protein, carbohydrate, and lipid contents showed faster increments per unit size, thus adults made up for the initial advantage of the food-rich population.QM survived better than ZM after transplantation irrespective of the basin. Shells were harder in ZM versus QM and in the food-rich versus food-limited conditions. QM grew at both locations, whereas ZM only in the food-rich basin. The protein and carbohydrate contents were greater in the food-rich versus food-limited basin, with no interspecific differences. Lipid content in QM was higher in the food-limited versus food-rich basin, whereas the opposite held for ZM.We demonstrated that the dreissenid species could coexist in food-rich conditions, despite the higher level of stress in ZM (as shown by weaker survival, higher SOD expression), whereas QM displaced ZM under food-limiting conditions, probably due to the ability to replace missing storage carbohydrates with accumulated lipids. Nevertheless, QM from the food-limited basin also showed symptoms of nutritional stress (changes in biomacromolecule content, lower shell hardness). Results suggest that the ability to show a rapid change in metabolism could be an important advantage of QM over ZM in their competition.

The impacts of species invasions can subside over time as ecosystems ‘adapt’ and invaders decline or increase over time as additional species invade. The character and timescales of invasion impacts provide important insights into ecosystem dynamics and management. Yet long-term studies of invasion impacts remain rare and often confound invasive species impacts with coincident environmental change. One way to address this challenge is to ask: what ecological changes over time since invasion are recapitulated in ecosystems that span a range of conditions, are located in different regions, and were invaded in different decades? We synthesize many-decade time series across seven ecosystems to resolve shared changes in seven key ecosystem features following invasion by zebra mussels and subsequent invasion by quagga mussels. These two congeners are among the most widespread invasive species that re-engineer and increasingly co-invade freshwater ecosystems. Seven polymictic shallow lakes with long-term data sets reveal remarkably similar trends, with the strongest ecosystem impacts occurring within 5–10 years of zebra mussel invasion. Surprisingly, plankton communities then exhibited a partial, significant recovery. This recovery was absent, and impacts of initial invasion amplified, in four lakes where quagga mussels outcompeted zebra mussels and more completely depleted phytoplankton. Thus, we show that the ecosystem impacts of invasive species can subside over time but amplify with serial introductions of competing, even closely similar, taxa.

Selective predation may affect interspecific competition between coexisting prey species. Ponto-Caspian zebra (Dreissena polymorpha) and quagga mussels (Dreissena rostriformis bugensis) have become common components of benthic communities in invaded ecosystems in Europe and North America, where they are exposed to predation by molluscivorous fish. In a pairwise food selection experiment, we examined whether an efficient predator of dreissenids, the common carp (Cyprinus carpio), selects between the two mussel species and, if so, which mussel traits (attachment, shell strength, nutritional value) are responsible for the discrimination. The fish were offered simultaneously the two mussel species in three separate treatments: (1) attached individuals; (2) unattached individuals; and (3) freshly removed soft tissue, as well as in three mussel size classes. The fish consistently selected quagga over zebra mussels, irrespective of mussel size and attachment status, including even soft tissues of both species. The success rate of fish attacks did not differ between the prey species. Smaller mussels were easier to crush and swallow, but more difficult to detach from the substratum. Thus, although interspecific differences in attachment and shell strength existed (small and medium zebra mussels had stronger shells and were more strongly attached than corresponding quagga mussels), these traits did not affect fish preferences. However, the differences in chemical composition of the body-the glycogen content (higher in large quagga vs. zebra mussels) and the caloric content (higher in small and medium quagga vs. zebra mussels)-were responsible for fish selection. The higher predatory pressure on the quagga versus zebra mussel suggests that differential predation is not a likely reason for the displacement of zebra by quagga mussels, currently occurring in many European and North American ecosystems. Thus, this phenomenon must be driven by other traits making the quagga mussel more competitive.

After introduction, the invasive bivalve dreissenids became key species in the biota of Lake Balaton, the largest shallow lake in Central Europe. The contribution of dreissenid soft tissue and shell, as biotic phases, in element distribution and its interaction with the water and upper sediment phases were examined in two basins with different trophic conditions in spring and autumn. Six metals (Ba, Cu, Fe, Mn, Pb, Zn) were detected in all investigated phases. In general, metals were abundant in the water and soft tissue in the eastern basin in spring, and in the sediment and shells in the western basin in autumn. This might be associated with the more urbanized surroundings in the eastern, and the enhanced organic matter production in the western basin. High relative shares of Ba, Cu, Mn, and Pb were associated with the water and shell samples, whereas high shares of Fe and Zn were noted in the soft mussel tissue and sediments. Results suggest that dynamics of metal uptake by dreissenids depend on the seasonal change in metabolic activity. Shell metal content is less changeable; shells might absorb metals from both the soft tissue and water phases. Metallothionein peptides, the scavengers of intracellular metals, were determined to be biomarkers of the bulk contaminants rather than only metals. The present study shows that invasive bivalves, with high abundance, filtering activity, and storing capacity can significantly contribute to element distribution in the shoreline of a shallow lake ecosystem.

The invasive zebra mussel, Dreissena polymorpha (ZM), established in Europe for a long time, has recently been joined and commonly outcompeted by a new invader, the quagga mussel, Dreissena rostriformis bugensis (QM). To identify factors contributing to this displacement, we studied behavioural differences between the species: aggregation, movement, and responses to conspecifics, congeners and their alarm cues. Compared to ZM, QM were more aggregated and less motile, crawling shorter distances for a shorter time at a slower speed. Conversely, QM exhibited more nonlocomotor movements. Both species aggregated and burrowed less and showed more nonlocomotor movements in response to conspecific and heterospecific alarm cues. They also moved shorter distances in the presence of conspecific alarm cues. ZM delayed their locomotion and nonlocomotor movements, whereas QM started locomotion earlier in the presence of both alarm cues. Mussel responses to living heterospecifics resembled those to alarm cues. In mixed-species aggregations, ZM attached to conspecifics more often than to QM shells, whereas QM were nonselective. To summarize, QM are less mobile, less selective regarding attachment site and more aggregated than ZM. This allows QM to perform better in mixed-species assemblages by spending less energy on relocation and overgrowing ZM to a higher extent than vice versa. Both species responded to heterospecific signals, which is helpful in mixed-species assemblages, particularly in novel areas occupied by these invasive species. Nevertheless, similar responses to alarm cues and living heterospecifics suggest a negative interaction between the congeners. (C) 2020 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

In invasive dreissenid communities, the zebra mussel usually appears earlier and then is displaced by the quagga mussel. We analysed length-weight allometric relationships, attachment strength (2 days, 1 week and 1 month of exposure), shell crushing resistance and glycogen content across the entire size range of both species in large shallow European lakes where this displacement has recently occurred. In Lake Balaton (Hungary) and Ijsselmeer (The Netherlands), the soft tissue dry weight increment of zebra mussels per unit length decreased after the quagga mussel invasion and became lower than that of quagga mussels. In Lake Markermeer (the Netherlands), having relatively worse environmental conditions, dry weight increment per unit length was always higher in quagga mussels than in zebra mussels, but no negative change in dry weight increment occurred in zebra mussels during the quagga mussel invasion. Small zebra mussels had more resistant shells and stronger attachment than quagga mussels. These differences were reduced (shell hardness) or reversed (long-term attachment) in larger individuals. Zebra mussels had lower glycogen content than quagga mussels across the entire size range. Thus, the quagga mussel advantage over zebra rnussel likely consists in the faster dry weight increment per unit length and higher storage product contents of the former, due to its lower investrnents in attachment strength and shell crushing resistance. (C) 2019 International Association for Great Lakes Research. Published by Elsevier B.V. All rights reserved.

Dreissenid mussels (including the zebra mussel Dreissena polymorpha and the quagga mussel D. rostriformis) are among the world's most notorious invasive species, with large and widespread ecological and economic effects. However, their long-term population dynamics are poorly known, even though these dynamics are critical to determining impacts and effective management. We gathered and analyzed 67 long-term (>10 yr) data sets on dreissenid populations from lakes and rivers across Europe and North America. We addressed five questions: (1) How do Dreissena populations change through time? (2) Specifi- cally, do Dreissena populations decline substantially after an initial outbreak phase? (3) Do different measures of population performance (biomass or density of settled animals, veliger density, recruitment of young) follow the same patterns through time? (4) How do the numbers or biomass of zebra mussels or of both species combined change after the quagga mussel arrives? (5) How does body size change over time? We also considered whether current data on long-term dynamics of Dreissena populations are adequate for science and management. Individual Dreissena populations showed a wide range of temporal dynamics, but we could detect only two general patterns that applied across many populations: (1) Populations of both species increased rapidly in the first 1-2 yr after appearance, and (2) quagga mussels appeared later than zebra mussels and usually quickly caused large dedines in zebra mussel populations. We found little evidence that combined Dreissena populations declined over the long term. Different measures of population performance were not congruent; the temporal dynamics of one life stage or population attribute cannot generally be accurately inferred from the dynamics of another. We found no consistent patterns in the long-term dynamics of body size. The long-term dynamics of Dreissena populations probably are driven by the ecological characteristics (e.g., predation, nutrient inputs, water temperature) and their temporal changes at individual sites rather than following a generalized time course that applies across many sites. Existing long-term data sets on dreissenid populations, although dearly valuable, are inadequate to meet research and management needs. Data sets could be improved by standardizing sampling designs and methods, routinely collecting more variables, and increasing support.

Although the ecosystem transforming impact of the invasive dreissenid mussels has been widely reported in short-to-mid time scale studies, little is known about the contribution of the spent shells to sediments accumulating on the lake bottom. The question whether the shell production significantly reduces the lifespan of the lake by increasing sedimentation rate is particularly interesting in those shallow lakes where the calcium supply is sufficient to maintain the high mussel biomass production permanently, and where the alkaline water does not favor shell dissolution. Lake Balaton, a large calcareous, shallow lake in Central Europe invaded by dreissenids ('Dreissena polymorpha, Dreissena rostriformis bugensis'), provides an ideal testing ground for this scenario. Therefore, we made calculations based on recent population abundance datasets (2000–2018), estimated the whole habitable, hard surface coastline and the muddy bottom of the pelagic area which is also gradually becoming inhabited by 'D. r. bugensis', using high resolution aerial photographs and analyzing seismic sections. We created four scenarios: (1) if no dreissenids are present (applying basic sedimentation rate); (2) if 'D. r. bugensis' had not been introduced to the lake (only 'D. polymorpha'); (3) if 'D. r. bugensis' occupies the hard surfaces of the coastline (the current dominant situation); (4) if 'D. r. bugensis' colonizes the entire lake bottom (a probable future model). Different sedimentation rates obtained from the literature were used to model the filling of Lake Balaton. The shell production of the new invader, D. r. bugensis can shorten the lake’s lifespan by one to two-thirds, depending on the model, and whether the mussel density currently observed at the shoreline is extended to the whole lake bottom. Attention is called to shallow calcareous lakes with low pre-invasion sedimentation rates in which the shell contribution of invasive mollusks has the potential to shorten lifespan.

The colonization progress of the invasive bivalve dreissenids, the formerly dominant Dreissena polymorpha and the recently (2008) introduced Dreissena rostriformis bugensis was studied between 2009 and 2013 in the largest Central European shallow lake, Lake Balaton, Hungary. The density of dreissenid planktonic veligers, new settlers (post-veligers and early juveniles), and the population structure (density, length frequency, relative abundance) of the two species were monitored on experimentally introduced natural stone substrata, on different time scales. Dreissenids started dynamic settling following a sudden veliger bloom. As substratum saturation progressed, competition between species for places was suggested, which, after two years, led to an increased number of large individuals (>20mm) and also recruits of D. r. bugensis. By contrast, the population of D. polymorpha was confined to middle size (11–18mm) individuals of the first settler generation. On local substrata, where the benthic community was already established, the replacement of D. polymorpha by D. r. bugensis took longer, but it happened in a similar way. The invasion speed of D. r. bugensis in Lake Balaton resembled the speed obtained in other European water bodies where D. r. bugensis, similar to Lake Balaton, was introduced much later than D. polymorpha. However, a longer replacement process was found in North America, where both species invaded new habitats at the same time. This suggests that the speed, and probably the success, of D. r. bugensis invasion depends on new surface availability, and whether the two dreissenid species are introduced together or at different times.

Arctodiaptomus spinosus (Daday, 1891) is a characteristic species of the soda pan zooplankton in the Great Hungarian Plain. The biogeographical distribution of the species is interesting, since its range expands from the Pannonian Biogeographic region to the other side of the Carpathians, occurring in saline lakes in Eastern Anatolia, Armenia, Iran and in temporary waters in Ukraine. Our investigations focused on the morphometric characteristics and the COI haplotype diversity of four Hungarian populations in the Kiskunság area. We detected substantial morphological differences between the Böddi-szék population and the rest of the sampling sites, however considerable differences were not observable in the COI haplotypes in the populations. The 20 animals investigated for COI haplotypes belonged to the same haplotype network. Tajima's D indicated departures from the neutral Wright – Fisher population model and suggested population expansion. The genetic composition of Arctodiaptomus spinosus populations in the Kiskunság area is rather uniform.

Olfaction, a chemosensory modality, plays a pivotal role in the orientation and behavior of invertebrates. The central olfactory processing unit in terrestrial stylomatophoran snails is the procerebrum, which contains NO synthesizing interneurons, whose oscillatory currents are believed to be the base of odor evoked memory formation. Nevertheless, in this model the up- and downstream events of molecular cascades that trigger and follow NO release, respectively, have not been studied. Immunocytochemistry and flow cytometry studies performed on procerebral neural perikarya isolated from the snail Helix pomatia revealed cell populations with discrete DAF-2 fluorescence, indicating the release of different amounts of NO. Glutamate increased the intensity of DAF-2 fluorescence, and the number of DAF-2 positive non-bursting interneurons, through a mechanism likely to involve an NMDA-like receptor. Similarly to glutamate, NO activation induced an increase in intracellular cGMP levels through activation of soluble guanylyl cyclase. Immunohistochemical localization of proteins possessing the phosphorylated target sequence of AGC family kinases (RXXS/T-P), among them protein kinase A (RRXS/T-P), showed striking similarities to the distribution of NOS/cGMP. Activators of cyclic nucleotide synthesis increased the AGC-kinase-dependent phosphorylation of discrete proteins with 28, 45, and 55kDamw. Importantly, exposure of snails to an attractive odorant induced hyperphosphorylation of the 28kDa protein, and increased levels of cGMP synthesis. Protein S-nitrosylation and intercellular activation of protein kinase G were also suggested as alternative components of NO signaling in the snail procerebrum. The present results from Helix pomatia indicate an important role for procerebrum NO/cGMP/PKA signaling pathways in the regulation of olfactory (food-finding) behavior.

As a novel approach to characterize the phenolic pollutants of Lake Balaton (Central Europe, western Hungary), 26 endocrine disrupting phenols (chlorophenols, nitrophenols, alkylphenols, triclosan, bisphenol-A) were quantified in dissolved and suspended particulate matter (SPM) phases, alike. Sample collection was performed in the western and eastern basins, at 20 sites in April and October 2014. Solid-phase and ultrasound-assisted extractions to withdraw target phenols from dissolved and suspended phases were employed. Compounds were derivatized with hexamethyldisilazane and trifluoroacetic acid for their quantification as trimethylsilyl derivatives by gas chromatography-tandem mass spectrometry. In Lake Balaton’s dissolved phase, 2-chlorophenol (103–164 ng/L), 4-chlorophenol (407–888 ng/L), 2,4-dichlorophenol (20.2–72.0 ng/L), 2,4,6-trichlorophenol (10.4–38.1 ng/L), 2-nitrophenol (31.0–66.5 ng/L), 4-nitrophenol (31.5–94.1 ng/L), and bisphenol-A (20.6–112 ng/L), while in its SPM, 4-chlorophenol (<LOQ-1274 μg/kg, dry matter), 4-nitrophenol (423–714 μg/kg), 4-nonylphenol isomers (1500–2910 μg/kg), and bisphenol-A (250–587 μg/kg) were determined. Since phenolics appear partially or exclusively in the SPM, the analysis of both phases proved to be of primary importance.

In this study, we tested whether the spatial distribution of waterbirds is influenced by shoreline urbanization or other habitat characteristics. We conducted monthly censuses along shoreline sections of a continental lake (Lake Balaton, Hungary) to assess the abundance of 11 common species that use this lake as a feeding and staging area during migration and winter. We estimated the degree of urbanization of the same shoreline sections and also measured other habitat characteristics (water depth, extent of reed cover, biomass of zebra mussels, distances to waste dumps and to other wetlands). We applied linear models and model averaging to identify habitat variables with high relative importance for predicting bird distributions. Bird abundance and urbanization were strongly related only in one species. Other habitat variables exhibited stronger relationships with bird distribution: (1) diving ducks and coots preferred shoreline sections with high zebra mussel biomass, (2) gulls preferred sites close to waste dumps, and (3) the abundances of several species were higher on shoreline sections close to other wetlands. Our findings suggest that the distribution of waterbirds on Lake Balaton is largely independent of shoreline urbanization and influenced by food availability and connectivity between wetlands.

During the unusually long European drought between 2000 and 2003, the water level of the large and shallow Lake Balaton, Hungary (area = 596 km2, mean depth = 3.25 m), decreased by 28%. Although food availability for zooplankton remained unchanged, and the fish stock declined more than the water mass, the density of populations of several planktonic rotifers, cladocerans, calanoid copepodes, and veligers decreased by 60‐90% simultaneously with the water‐level decrease and regenerated only after the drought. The generally strong turbulence of the lake was intensified during the four consecutive years of low water, as verified by instrumental monitoring of the turbulence intensity and by the estimation of the turbulent kinetic‐energy dissipation rate. In our tank experiments, turbulence conditions similar to those that existed in the lake during low water were simulated, but mineral suspended material was minimized and food was regularly resupplied. Under these experimental conditions, zooplankton taxa showing the highest mortality were the same as those that were most susceptible in situ. Increased turbulence coupled with the water‐level decrease is especially unfavorable for rotifer Keratella, the cladoceran Daphnia, Bosmina, and copepodit and adult stages of the calanoid copepod Eudiaptomus gracilis in this lake.