Freshwater Communities Research Articles

Strong succession in prokaryotic association networks and community assembly mechanisms in an acid mine drainage-impacted riverine ecosystem.

Acid mine drainage (AMD) serves as an ideal model system for investigating microbial ecology, interaction, and assembly mechanism in natural environments. While previous studies have explored the structure and function of microbial communities in AMD, the succession patterns of microbial association networks and underlying assembly mechanisms during natural attenuation processes remain elusive. Here, we investigated prokaryotic microbial diversity and community assembly along an AMD-impacted river, from the extremely acidic, heavily polluted headwaters to the nearly neutral downstream sites. Microbial diversity was increased along the river, and microbial community composition shifted from acidophile-dominated to freshwater taxa-dominated communities. The complexity and relative modularity of the microbial networks were also increased, indicating greater network stability during succession. Deterministic processes, including abiotic selection of pH and high contents of sulfur and iron, governed community assembly in the headwaters. Although the stochasticity ratio was increased downstream, manganese content, microbial negative cohesion, and relative modularity played important roles in shaping microbial community structure. Overall, this study provides valuable insights into the ecological processes that govern microbial community succession in AMD-impacted riverine ecosystems. These findings have important implications for in-situ remediation of AMD contamination.

Flushing away the future: The effects of wastewater treatment plants on aquatic invertebrates.

Wastewater treatment plants (WWTP) are essential infrastructure in our developing world. However, with the development and release of novel entities and without modern upgrades, they are ineffective at fully removing micropollutants before treated effluents are released back into aquatic environments. Thus, WWTPs may represent additional point source impacts to freshwater environments, further pressuring aquatic fauna and already vulnerable insect communities. Previous studies - mostly focusing on single WWTPs - have shown general trends of freshwater invertebrate communities becoming dominated by pollution tolerant taxa. To expand on these findings, the current study is the first to comprehensively investigate data on the effects of 170 WWTPs on invertebrate taxonomic composition. We compared data for several diversity and pollution indices, as well as the taxonomic composition both upstream and downstream of the WWTPs (366 sampling sites). In terms of abundance, the three most frequent and negatively impacted orders were the Plecoptera, Trichoptera and Gastropoda, while the Turbellaria, Hirudinea and Crustacea increased in abundance. Although strong changes in community composition were observed between upstream and downstream sites (mean species turnover of 61%), commonly used diversity indices were not sensitive to these changes, highlighting their potential inadequacy in accurately assessing ecological health. Our results indicate that WWTPs change downstream conditions in favour of pollution tolerant taxa to the detriment of sensitive taxa. Order-level taxonomic responses can be informative but should be interpreted with caution, since they can be driven by a few taxa, or opposing responses of species in the same group can result in an overall low order-level response. Upgrading WWTPs via additional treatment steps or merging may be beneficial, provided upstream sections are unimpacted and/or are in a good chemical and structural condition.

Complex Dynamics Modes in a Simple Model of Prey-Predator Community: Bistability and Multistability

The paper proposes and studies a two-component discrete-time model of the prey-predator community considering zooplankton and fish interactions and their development features. Discrete-time systems of equations allow us to take into account naturally the rhythm of many processes occurring in marine and freshwater communities, which are subject to cyclical fluctuations due to the daily and seasonal cycle. We describe the dynamics of fish and zooplankton populations constituting the community by Ricker’s model, which is well-studied and widely used in population modeling. To consider the species interaction, we use the Holling-II type response function taking into account predator saturation. We carried out the study of the proposed model. The system is shown to have from one to three non-trivial equilibria, which gives the existence of the complete community. In addition to the saddle-node bifurcation, which generates bistability of stationary dynamics, a nontrivial equilibrium loses its stability according to the Neimark-Sacker scenario with an increase in the reproductive potential of both predator and prey species, as a result of which the community exhibits long-period oscillations similar to those observed in experiments. With the higher bifurcation parameter, the reverse Neimark-Sacker bifurcation is shown to occur followed by the closed invariant curve collapses, and dynamics of the population stabilizes, later losing stability through a cascade of period-doubling bifurcations. Multistability complicates the birth and disappearance of the invariant curve in the phase space scenario by the emergence of another irregular dynamics in the system with the single unstable nontrivial fixed point. At fixed values of the model parameters and different initial conditions, the system considered is shown to demonstrate various quasi-periodic oscillations. Despite extreme simplicity, the proposed discrete-time model of community dynamics demonstrates a wide variety and variability of dynamic modes. It shows that the influence of environmental conditions can change the type and nature of the observed dynamics.

From behaviour to complex communities: Resilience to anthropogenic noise in a fish-induced trophic cascade

Sound emissions from human activities represent a pervasive environmental stressor. Individual responses in terms of behaviour, physiology or anatomy are well documented but whether they propagate through nested ecological interactions to alter complex communities needs to be better understood. This is even more relevant for freshwater ecosystems that harbour a disproportionate fraction of biodiversity but receive less attention than marine and terrestrial systems. We conducted a mesocosm investigation to study the effect of chronic exposure to motorboat noise on the dynamics of a freshwater community including phytoplankton, zooplankton, and roach as a planktivorous fish. In addition, we performed a microcosm investigation to test whether roach's feeding behaviour was influenced by the noise condition they experienced in the mesocosms. Indeed, compared to other freshwater fish, the response of roach to motorboat noise apparently does not weaken with repeated exposure, suggesting the absence of habituation. As expected under the trophic cascade hypothesis, predation by roach induced structural changes in the planktonic communities with a decrease in the main grazing zooplankton that slightly benefited green algae. Surprisingly, although the microcosm investigation revealed persistent alterations in the feeding behaviour of the roach exposed to chronic noise, the dynamics of the roach-dominated planktonic communities did not differ between the noisy and noiseless mesocosms. It might be that roach's individual response to noise was not strong enough to cascade or that the biological cues coming from the conspecifics and the many planktonic organisms have diverted each fish's attention from noise. Our work suggests that the top-down structuring influence of roach on planktonic communities might be resilient to noise and highlights how extrapolating impacts from individual responses to complex communities can be tricky.

Biotic and Abiotic Interactions in Freshwater Mesocosms Determine Fate and Toxicity of CuO Nanoparticles.

Transformation, dissolution, and sorption of copper oxide nanoparticles (CuO-NP) play an important role in freshwater ecosystems. We present the first mesocosm experiment on the fate of CuO-NP and the dynamics of the zooplankton community over a period of 12 months. Increasingly low (0.08-0.28 mg Cu L-1) and high (0.99-2.99 mg Cu L-1) concentrations of CuO-NP and CuSO4 (0.10-0.34 mg Cu L-1) were tested in a multiple dosing scenario. At the high applied concentration (CuO-NP_H) CuO-NP aggregated and sank onto the sediment layer, where we recovered 63% of Cu applied. For the low concentration (CuO-NP_L) only 41% of applied copper could be recovered in the sediment. In the water column, the percentage of initially applied Cu recovered was on average 3-fold higher for CuO-NP_L than for CuO-NP_H. Zooplankton abundance was substantially compromised in the treatments CuSO4 (p < 0.001) and CuO-NP_L (p < 0.001). Community analysis indicated that Cladocera were most affected (bk = -0.49), followed by Nematocera (bk = -0.32). The abundance of Cladocera over time and of Dixidae in summer was significantly reduced in the treatment CuO-NP_L (p < 0.001; p < 0.05) compared to the Control. Our results indicate a higher potential for negative impacts on the freshwater community when lower concentrations of CuO-NP (<0.1 mg Cu L-1) enter the ecosystem.

The recovery of European freshwater biodiversity has come to a halt

Owing to a long history of anthropogenic pressures, freshwater ecosystems are among the most vulnerable to biodiversity loss1. Mitigation measures, including wastewater treatment and hydromorphological restoration, have aimed to improve environmental quality and foster the recovery of freshwater biodiversity2. Here, using 1,816 time series of freshwater invertebrate communities collected across 22 European countries between 1968 and 2020, we quantified temporal trends in taxonomic and functional diversity and their responses to environmental pressures and gradients. We observed overall increases in taxon richness (0.73% per year), functional richness (2.4% per year) and abundance (1.17% per year). However, these increases primarily occurred before the 2010s, and have since plateaued. Freshwater communities downstream of dams, urban areas and cropland were less likely to experience recovery. Communities at sites with faster rates of warming had fewer gains in taxon richness, functional richness and abundance. Although biodiversity gains in the 1990s and 2000s probably reflect the effectiveness of water-quality improvements and restoration projects, the decelerating trajectory in the 2010s suggests that the current measures offer diminishing returns. Given new and persistent pressures on freshwater ecosystems, including emerging pollutants, climate change and the spread of invasive species, we call for additional mitigation to revive the recovery of freshwater biodiversity.

Functional non‐equivalence in ecosystem engineers? Different freshwater mussels (Bivalvia: Unionidae) are associated with different macroinvertebrate communities in a subtropical lake

Abstract Freshwater mussels (Bivalvia: Unionidae) are often described as ecosystem engineers, with the capacity to alter abiotic habitats; this can facilitate certain macroinvertebrate taxa, increasing biodiversity and potentially altering community composition. Mussel species are often implicitly considered to be broad ecological equivalents, with similar impacts on abiotic environments and benthic communities. This assumption of redundancy is important in conservation approaches that aim to preserve ecosystem functioning, rather than species identity. We tested this assumption of equivalence in two species of subtropical freshwater mussels, Lamellidens marginalis and Parreysia caerulea, studying the associations between these mussel species and macroinvertebrate communities. We measured mussel density and abundance of macroinvertebrates, identified to family level, at 50 sites within Dhanmondi Lake, a heavily polluted sub‐tropical urban lake in Dhaka, Bangladesh. We investigated the associations of mussel density on different macroinvertebrate families, which we further classified into functional feeding groups (predators, scrapers, collectors and shredders). We found that densities of the two mussel species were negatively correlated in the 1–3‐m nearshore zone; average density of Lamellidens was 46.4 ± SE 3.6 individuals/0.25 m2, and average density of Parreysia was 6.4 ± SE 1.4 individuals/0.25 m2. In total, 2319 Lamellidens and 320 Parreysia specimens were recorded. Sites containing higher densities of Lamellidens had greater similarity in macroinvertebrate community composition, while there was no effect for Parreysia. Particular macroinvertebrate families and functional feeding groups were also associated with mussels; the majority of significant associations were with Lamellidens but not Parreysia. The clearest associations were with predators and scrapers, which increased in abundance with Lamellidens density. This provides support for a possible role for mussels in structuring freshwater invertebrate communities and emphasises the importance of species identity in these effects, with one possible keystone species (Lamellidens) largely accounting for mussel–invertebrate associations. While microhabitat variables (shading and sediment type) were poor predictors of abundance in any invertebrate taxa (including mussels), manipulative studies are needed to disentangle the direct effects of mussels from the role of overlapping habitat preference between mussels and other invertebrates. Our results highlight the importance of species identity in patterns of community composition, particularly for ecosystem engineer species. We also reinforce the need to test the assumption of ecological redundancy within communities.

Linking fine-scale behaviour to the hydraulic environment shows behavioural responses in riverine fish

BackgroundFish migration has severely been impacted by dam construction. Through the disruption of fish migration routes, freshwater fish communities have seen an incredible decline. Fishways, which have been constructed to mitigate the problem, have been shown to underperform. This is in part due to fish navigation still being largely misunderstood. Recent developments in tracking technology and modelling make it possible today to track (aquatic) animals at very fine spatial (down to one meter) and temporal (down to every second) scales. Hidden Markov models are appropriate models to analyse behavioural states at these fine scales. In this study we link fine-scale tracking data of barbel (Barbus barbus) and grayling (Thymallus thymallus) to a fine-scale hydrodynamic model. With a HMM we analyse the fish’s behavioural switches to understand their movement and navigation behaviour near a barrier and fishway outflow in the Iller river in Southern Germany.MethodsFish were tracked with acoustic telemetry as they approached a hydropower facility and were presented with a fishway. Tracking resulted in fish tracks with variable intervals between subsequent fish positions. This variability stems from both a variable interval between tag emissions and missing detections within a track. After track regularisation hidden Markov models were fitted using different parameters. The tested parameters are step length, straightness index calculated over a 3-min moving window, and straightness index calculated over a 10-min window. The best performing model (based on a selection by AIC) was then expanded by allowing flow velocity and spatial velocity gradient to affect the transition matrix between behavioural states.ResultsIn this study it was found that using step length to identify behavioural states with hidden Markov models underperformed when compared to models constructed using straightness index. Of the two different straightness indices assessed, the index calculated over a 10-min moving window performed better. Linking behavioural states to the ecohydraulic environment showed an effect of the spatial velocity gradient on behavioural switches. On the contrary, flow velocity did not show an effect on the behavioural transition matrix.ConclusionsWe found that behavioural switches were affected by the spatial velocity gradient caused by the attraction flow coming from the fishway. Insight into fish navigation and fish reactions to the ecohydraulic environment can aid in the construction of fishways and improve overall fishway efficiencies, thereby helping to mitigate the effects migration barriers have on the aquatic ecosystem.

Photoperiod effects in a freshwater community: Amphibian larvae develop faster and zooplankton abundance increases under an early-season photoperiod.

Organisms that shift their phenologies in response to global warming will experience novel photic environments, as photoperiod (daylength) continues to follow the same annual cycle. How different organisms respond to novel photoperiods could result in phenological mismatches and altered interspecific interactions. We conducted an outdoor mesocosm experiment exposing green frog (Rana clamitans) larvae, gray treefrog (Hyla versicolor) larvae, phytoplankton, periphyton, and zooplankton to a three-month shift in photoperiod: an early-season photoperiod (simulating April) and a late-season photoperiod (simulating July). We manipulated photoperiod by covering and uncovering tanks with clear or light-blocking lids to mimic realistic changes in daylength. We assessed amphibian life history traits and measured phytoplankton, periphyton, and zooplankton abundances. Green frog larvae and gray treefrog metamorphs were more developed under the early-season photoperiod. Gray treefrog total length was also reduced, but photoperiod did not affect green frog total length. Although phytoplankton and periphyton abundances were not affected by photoperiod, copepod nauplii were in greater abundance under the early-season photoperiod. Overall, this simplified aquatic community did not exhibit significant changes to structure when exposed to a three-month shift in photoperiod. Temperate amphibians that breed earlier in the year may develop faster, which may have long-term costs to post-metamorphic growth and performance. Asynchronous shifts in zooplankton abundances in response to altered photoperiods could subsequently affect freshwater community structure. While photoperiod has been shown to individually affect freshwater organisms, our study using replicated outdoor wetland communities shows that the comprehensive effects of photoperiod may be less important than other cues such as temperature and precipitation.

New species, new story: The impact of invasive non‐host predators on host‐trematode interactions

Abstract Predation is considered one of the key drivers shaping the relationships between animals. There is increasing evidence that non‐host aquatic taxa use free‐living parasite stages as prey, including motile cercariae of digenean trematodes. However, species‐specific predator differences in cercarial consumption are poorly understood, especially when invasive consumers are involved. We used experimental approaches to investigate the possible predator‐induced reduction in trematode‐host encounters in freshwater benthic communities. Four gammarid species, two native European (Gammarus jazdzewskii and G. pulex) and two invasive Ponto‐Caspian (Dikerogammarus villosus and Pontogammarus robustoides), were selected as predators of echinostomatid cercariae (Echinoparyphium aconiatum). We hypothesised that (i) all predators will prey on cercariae, but the amount of prey consumed will vary, the invaders will be more voracious than native species, and (ii) the effect of predator‐induced dilution of cercariae will depend on the environmental condition, i.e., the presence of the natural target host (the planorbid snail Planorbarius corneus) and alternative food resource; the exposure of predators to these two basic environmental factors will reduce their consumption rates of cercariae. The extent of the dilution effect caused by gammarids was highly predator species‐dependent and also varied depending on experimental conditions. Overall, all gammarids preyed on cercariae and reduced infection levels in the target snail host, but the invaders were more efficient than the native species. Furthermore, decreased consumption rates of cercarial prey were observed in the presence of both the target snail hosts and alternative food resources, except for invasive P. robustoides consuming more cercariae when a snail host was present. The observed differences among predators in the consumption of cercariae may be a result of complex interaction traits of parasites, predators and target hosts, such as different predatory behaviour of invasive and native amphipods, change in parasite behaviour in response to the presence/chemical signals of the target host and a predator‐induced “ecology of fear” interfering with the dilution effect of parasites. We provided evidence that gammarids can cause a significant reduction in parasite‐host encounters. The high consumption rates, especially in the presence of easily available alternative food resources, emphasise the importance of free‐living cercariae as an energy resource in aquatic food webs. The observed predator differences in the consumption of cercariae are further evidence of the higher competitive ability of invasive species. Simultaneously, our results indicate that the appearance of new predators in the environment may interfere with established host‐parasite interactions.

Response of the Coastal Phytoplankton Community to the Runoff from Small Rivers in the Northeastern Black Sea

River runoff is an important source of nutrients as well as suspended and dissolved organic matter that in coastal zones and on the shelf are transformed due to local production cycles. River runoff affects the hydrological regime, salinity, temperature, and irradiance in river–seawater mixing zone. Our study focuses on the response of phytoplankton to the impact of small Caucasian rivers in the Northeastern (NE) Black Sea, as one of the most sensitive components of marine ecosystems with respect to the changes in abiotic factors. The leading role of marine species of diatoms, dinoflagellates, and coccolithophores in the structure and functioning when impacted by runoff from small rivers is demonstrated in comparison to the freshwater community. Variability of the taxonomic composition and quantitative and productive characteristics of marine phytoplankton communities impacted by small rivers were comparable to or exceed the seasonal and interannual variability on the NE Black Sea shelf. This indicates the significant role of runoff from small Caucasian rivers in maintaining of a high production level of phytoplankton overall and of the coccolithophore Emiliania huxleyi in particular in the coastal zone.

Neonicotinoid mixture alters trophic interactions in a freshwater aquatic invertebrate community

Neonicotinoids are increasingly and widely used systemic insecticides in agriculture, residential applications, and elsewhere. These pesticides can sometimes occur in small water bodies in exceptionally high concentrations, leading to downstream non-target aquatic toxicity. Although insects appear to be the most sensitive group to neonicotinoids, other aquatic invertebrates may also be affected. Most existing studies focus on single-insecticide exposure and very little is known concerning the impact of neonicotinoid mixtures on aquatic invertebrates at the community level. To address this data gap and explore community-level effects, we performed an outdoor mesocosm experiment that tested the effect of a mixture of three common neonicotinoids (formulated imidacloprid, clothianidin and thiamethoxam) on an aquatic invertebrate community. Exposure to the neonicotinoid mixture induced a top-down cascading effect on insect predators and zooplankton, ultimately increasing phytoplankton. Our results highlight complexities of mixture toxicity occurring in the environment that may be underestimated with traditional mono-specific toxicological approaches.

Impacts of salinity stress caused by ballast water discharge on freshwater ecosystems

Global commodity trade and the transportation of goods rely heavily on shipping. But the discharge of large quantities of hypersaline ballast water carried by maritime vessels into freshwater ports, lakes, rivers and other water bodies cause salinity stress, which can be detrimental to native freshwater organisms. In this study, the effects of marine hypersaline ballast water discharge and the rate of discharge (abrupt vs. gradual) on inland freshwater lakes were assessed by observing changes in freshwater plankton communities under salinity stress. The findings indicate that increasing salinity has a negative impact on plankton community composition, density and biodiversity. Zooplankton were more sensitive to changes in salinity than phytoplankton through 10 consecutive days of experiments. Moreover, the plankton community showed some ability to recover itself at low salinities (S≤6.5 PSU). In terms of the rate of ballast water discharge, plankton were more likely to survive in aquatic environments with gradual changes in salinity than with abrupt changes. This research is of great significance for the scientific management of ballast water discharge in freshwater ports and the conservation of freshwater ecosystems.

Nutrient availability and grazing influence the strength of priority effects during freshwater bacterial community coalescence.

When bacterial communities mix, immigration history can fundamentally affect the community composition as a result of priority effects. Priority effects arise when an early immigrant exhausts resources and/or alters habitat conditions, thereby influencing the establishment success of the late arriver. The strength of priority effects is context-dependent and expected to be stronger if environmental conditions favour the growth of the first arriver. In this study, we conducted a two-factorial experiment testing the importance of nutrient availability and grazing on the strength of priority effects in complex aquatic bacterial communities. We did so by mixing two dissimilar communities, simultaneously, and with a 38 h time-delay. Priority effects were measured as the invasion resistance of the first community to the invading second community. We found stronger priority effects in treatments with high nutrient availability and absence of grazing, but in general, the arrival timing was less important than the selection by nutrients and grazing. At the population level, the results were complex, but priority effects may have been driven by bacteria belonging to for example, the genera Rhodoferax and Herbaspirillum. Our study highlights the importance of arrival timing in complex bacterial communities, especially if environmental conditions favour rapid community growth.

Trimethoprim resistance in surface and wastewater is mediated by contrasting variants of the dfrB gene.

Trimethoprim (TMP) is a low-cost, widely prescribed antibiotic. Its effectiveness is increasingly challenged by the spread of genes coding for TMP-resistant dihydrofolate reductases: dfrA, and the lesser-known, evolutionarily unrelated dfrB. Despite recent reports of novel variants conferring high level TMP resistance (dfrB10 to dfrB21), the prevalence of dfrB is still unknown due to underreporting, heterogeneity of the analyzed genetic material in terms of isolation sources, and limited bioinformatic processing. In this study, we explored a coherent set of shotgun metagenomic sequences to quantitatively estimate the abundance of dfrB gene variants in aquatic environments. Specifically, we scanned sequences originating from influents and effluents of municipal sewage treatment plants as well as river-borne microbiomes. Our analyses reveal an increased prevalence of dfrB1, dfrB2, dfrB3, dfrB4, dfrB5, and dfrB7 in wastewater microbiomes as compared to freshwater. These gene variants were frequently found in genomic neighborship with other resistance genes, transposable elements, and integrons, indicating their mobility. By contrast, the relative abundances of the more recently discovered variants dfrB9, dfrB10, and dfrB13 were significantly higher in freshwater than in wastewater microbiomes. Moreover, their direct neighborship with other resistance genes or markers of mobile genetic elements was significantly less likely. Our findings suggest that natural freshwater communities form a major reservoir of the recently discovered dfrB gene variants. Their proliferation and mobilization in response to the exposure of freshwater communities to selective TMP concentrations may promote the prevalence of high-level TMP resistance and thus limit the future effectiveness of antimicrobial therapies.

Different components of biodiversity mediate two ecosystem functions derived from naturally assembled filter‐feeder communities

Positive biodiversity–ecosystem functioning (BEF) relationships observed in experiments can be challenging to identify in natural communities. Freshwater animal communities are disproportionately harmed by global change that results in accelerated species loss. Understanding how animal‐mediated ecosystems functions may change as a result of global change can help determine whether biodiversity or species‐specific conservation will be effective at maintaining function. Unionid mussels represent half of imperiled species in freshwater ecosystems globally and perform important ecological functions such as water filtration and nutrient recycling. We explored BEF relationships for 22 naturally assembled mussel aggregations spanning three river basins. We used the Price equation to partition the contributions of species richness, composition, and context dependent interactions to two functions of interests: spatially‐explicit standing‐stock biomass (indirect proxy for function) and species‐specific nitrogen (N) excretion rates (direct measure of N recycling). Random and non‐random species loss each reduced biomass and N recycling. Many rare species with low contributions to biomass contributed to standing‐stock biomass in all basins. Widespread species had variable function across sites, such that context dependent effects (CDEs) outweighed richness effects on standing‐stock biomass in two basins, and were similar to richness effects in the third. Richness effects outweighed CDEs for N recycling. Thus, many species contributed a low proportion to overall N‐recycling; a product we attribute to the high evenness and functional effect trait diversity associated with these communities. The loss of low‐functioning species reduced the function of persisting species. This novel result using observational data adds evidence that positive species interactions, such as interspecific facilitation, may be a mechanism by which biodiversity enhances ecosystem functions. Our work stresses the importance of evaluating species‐specific contributions to functions in diverse systems, such as nutrient cycling when maintaining specific animal‐mediated functions is a management goal because indirect proxies may not completely characterize BEF relationships.

Habitat-specific regulation of bacterial community dynamics during phytoplankton bloom succession in a subtropical eutrophic lake

Phytoplankton blooms, an important indicator of severe eutrophication, are a globally significant consequence of anthropogenic activities and climate change on freshwater lakes. Shifts in microbial communities during phytoplankton blooms have been extensively investigated, yet we have a limited understanding of how distinct assembly processes underlying the temporal dynamics of freshwater bacterial communities within different habitats respond to the succession of phytoplankton blooms. To address this knowledge gap, we collected both water and sediment samples in a subtropical eutrophic lake over a complete period of phytoplankton blooms to assess the dynamics of bacterial communities and the temporal shifts in assembly processes. Our results showed that phytoplankton blooms strongly altered the diversity, composition, and coexistence patterns of both planktonic and sediment bacterial communities (PBC and SBC), but the successional patterns differed between PBC and SBC. PBC were less temporally stable under bloom-induce disturbances, with higher variations in temporal dynamics and greater sensitivity to environmental fluctuations. Furthermore, the temporal assembly patterns of bacterial communities in both habitats were mainly driven by homogeneous selection and ecological drift. In the PBC, the role of selection decreased over time, while ecological drift became increasingly important. Conversely, in the SBC, the relative impact of selection and ecological drift on community assemblages fluctuated less over time, with selection remaining the dominant process throughout the bloom.

Toward freshwater plant diversity surveys with eDNA barcoding and metabarcoding

AbstractProviding reliable, cost‐effective data on species distribution is critical to ensuring biodiversity conservation. However, many species may go unrecorded by conventional surveys, especially in aquatic environments. Environmental DNA (eDNA) barcoding and metabarcoding are alternative approaches that could complete biodiversity estimates based on species observations. While eDNA surveys are being standardized for some animal groups (e.g., fish and amphibians), research on eDNA approaches for freshwater plant communities is just starting to bear fruit. Here, we synthesized the 22 studies that used eDNA barcoding and metabarcoding to survey plant biodiversity in freshwater systems. We present evidence that contemporary aquatic plants (macrophytes) and terrestrial plants can be detected in water and surficial sediment eDNA from lakes and rivers. The phenology (e.g., senescence) of the target taxa strongly influences species detection. The main application of eDNA barcoding targets the monitoring of invasive macrophytes, and barcoding assays are available for 14 taxa. The metabarcoding approach shows a range of applications: the detection of rare macrophytes, catchment‐scale floristic inventories, and sediment fingerprinting. Barcodes on the plastid genome (cpDNA) are preferred for both approaches: matK and trnH‐psbA for barcoding, trnL, and rbcL for metabarcoding. The intergenic transcribed spacer 1 (ITS1) from the nuclear ribosomal DNA (nrDNA) was used for designing eDNA barcoding assays on five invasive macrophytes. In contrast, only three metabarcoding studies used the ITS1 or IST2 with newly designed primers. However, metabarcoding applications should routinely use a multi‐locus approach, combining cpDNA and nrDNA barcodes. Barcode combinations and existing primers need further testing on eDNA samples. We recommend using local barcode reference databases (BRDs), ideally self‐made, to circumvent taxonomic gaps and heterogeneous sequences in public BRDs. Finally, new technologies and developments like droplet digital PCR and hybridisation capture offer new perspectives for eDNA‐based biodiversity monitoring approaches.

Distribution and diversity of fishes and lampreys in Transylvania (Romania): a complete survey and suggestions for new protected areas.

Freshwater fishes are in a serious state of decline across the world, making them one of the most threatened groups of vertebrates. The Danube River catchment area in Europe holds the richest freshwater fish community, but our knowledge of the current distribution of these species is limited. Transylvania, the largest region of Romania, is one of the important tributaries of the Danube, from where 77 fish and two lamprey species were recorded until now. Despite this large diversity of freshwater fishes, there is a lack of systematic survey of the fish fauna in this region for the past 50 years. In this study, we present data on the occurrence and distribution of fishes and lampreys collected in Transylvania from 2007 to 2022. This data covers 43% of Romania's surface and includes all major rivers from Transylvania. 65 species of fish and three species of lampreys are recorded, and an additional nine fish species are also reported based on information from competent people. Of the 77 fish and lamprey species recorded 19 (24.7%) are non-native, although their relative abundance was low (5.1%) compared to other similar regions in Europe. The first records of Eudontomyzonmariae, Neogobiusmelanostomus, Piaractusbrachypomus, Pygocentrusnattereri, and Salvelinusalpinus in Transylvanian rivers are presented, as well as the first record of Cobitiselongata outside the Nera River basin (from the Caraș River) and the detection of three new populations of the vulnerable Umbrakrameri. Data on changes in distribution that have occurred since the last comprehensive survey 50 years ago are also provided and the importance of our results in conservation planning are discussed, including the designation of new protected areas for freshwater bodies and the compilation of the Romanian Red List of fishes.

Interspecific insights into direct and indirect effects of acetochlor on interactions between daphnids and microalgae: Toxic, trophic, and grazer-infochemical responses

As the herbicide acetochlor (ACT) affects the plankton food web, this study investigated the effects of ACT and exocrine infochemicals from daphnids (after ACT exposure and/or starvation) on Scenedesmus obliquus growth, as well as the effects of ACT and starvation on life-history traits of Daphnia magna. Filtered secretions from daphnids increased algal ACT tolerance, dependent on different ACT exposure histories and food intake. Endogenous and secretory metabolite profiles of daphnids following ACT and/or starvation appear to be regulated by the fatty acid synthesis pathway and sulfotransferases and were related to energy allocation trade-offs. Oleic acid (OA) and octyl sulfate (OS), screened by secreted and somatic metabolomics, affected algal growth and ACT behavior in the algal culture in opposite ways. Trophic and non-trophic interspecific effects caused by ACT were observed in microalgae-daphnid microcosms, including algal growth inhibition, daphnid starvation, OA down-regulation, and OS up-regulation. Based on these findings, risk assessment of ACT on the freshwater plankton communities should take species interactions into account.