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Estuaries, acting as transitional habitats receiving species introductions from both freshwater and marine sources, undergo significant impacts from global climate changes. Planktonic microorganisms contribute significantly to estuarine biodiversity and ecological stability. These microorganisms primarily fall into three groups: eukaryotic plankton, particle-associated bacteria, and free-living bacteria. Understanding the structural characteristics and interactions within these subcommunities is crucial for comprehending estuarine dynamics. We collected samples from three distinct locations (< 0.1 PSU, 6.6 PSU, and 19 PSU) within the Yangtze River estuary. Samples underwent analysis for physicochemical indicators, while microbial communities were subjected to 16S/18S rRNA amplicon sequencing. Additionally, simulated mixing experiments were conducted using samples of varying salinities. Estuary samples, combined with simulated experiments, were employed to collectively examine the structural characteristics and assembly processes of estuarine microbes. Our research highlights the considerable impact of phylogenetic classification on prokaryotic behavior in these communities. We observed a transition in assembly processes from primarily stochastic for particle-associated bacteria to a predominant influence of homogeneous selection as salinity increased. Particle-associated bacterial communities exhibited a greater influence of stochastic processes compared to free-living bacteria, showcasing higher stability in diversity. The variations in composition and structure of estuarine microbial subcommunities were influenced by diverse environmental factors. Particle-associated bacteria displayed elevated network characterization values and established closer interactions with eukaryotic plankton. Structural equation modeling (SEM) analysis revealed that free-living bacteria displayed a heightened sensitivity to environmental factors and exerted a more significant influence on assembly processes and network characteristics. Simulated mixing in these environments resulted in the loss of species with similar microbial taxonomic relationships. The functioning of bacterioplankton is influenced by salinity and the processes governing their assembly, particularly in relation to different living states. These findings significantly contribute to our understanding of the intricate interplay between prokaryotic and eukaryotic plankton microorganisms in highly dynamic environments, laying a robust foundation for further exploration into the ecological mechanisms governing microbial dynamics in estuaries. • Differences in assembly processes among estuarine sub-communities. • Sourcetracking analysis reveals community response to water mixing of estuary. • Bacterial lifestyles (Free-living & particle-associated) alter community interactions and functions. [ABSTRACT FROM AUTHOR]

The widespread salinisation of freshwater ecosystems poses a major threat to the biodiversity, functioning, and services that they provide. Human activities promote freshwater salinisation through multiple drivers (e.g., agriculture, resource extraction, urbanisation) that are amplified by climate change. Due to its complexity, we are still far from fully understanding the ecological and evolutionary consequences of freshwater salinisation. Here, we assess current research gaps and present a research agenda to guide future studies. We identified different gaps in taxonomic groups, levels of biological organisation, and geographic regions. We suggest focusing on global- and landscape-scale processes, functional approaches, genetic and molecular levels, and eco-evolutionary dynamics as key future avenues to predict the consequences of freshwater salinisation for ecosystems and human societies. The global acceleration of freshwater salinisation due to human activities such as agriculture, resource extraction, and urbanisation and its amplification by climate change is unequivocal. Although research in this field is growing, there are key aspects at the ecological and evolutionary levels that remain unaddressed. Increasing salinisation is a problem as it can increase the stress or mortality of freshwater organisms, leading to a loss of diversity and/or functionality of freshwater ecosystems but also the services and benefits to human societies that they provide. We identify the main gaps of recent research and suggest a research agenda to facilitate future research efforts in order to achieve a more comprehensive understanding on freshwater salinisation. [ABSTRACT FROM AUTHOR]

Climate change increasingly threatens alpine natural areas and notably aquatic systems. Alpine waterbodies, such as ponds, are particularly vulnerable to changes in temperature. Their biodiversity faces rising threats, especially for cold stenothermal species. Located at high elevation (>2600 m a.s.l.), the aquatic network of the Macun catchment (35 ponds and several streams) was put under a strict protection framework in 2000 ("Swiss National Park") to mitigate threats to alpine biodiversity. The pond network has been regularly monitored as a sentinel of change, through aquatic macroinvertebrate sampling and physico-chemical measurements. Monitoring shows an alarmingly sharp increase in summer water temperature, rising by 4 °C between 2005 and 2020 (2.7 °C per decade). In contrast, nutrient concentrations have remained low and stable over these 15 years. The aquatic macroinvertebrate species richness has also been mostly stable between 2002 and 2021, with no negative impacts on the monitored cold stenotherms. Indeed, as the ponds are located at high elevation, these cold stenotherms are still living within their thermal range. Some cold stenothermal species have colonised the ponds from lower elevations, so the Macun pondscape acts as a refuge for these migrating taxa. In synthesis, our investigation highlights the alarming abiotic condition of the high-elevation freshwater biodiversity habitats, including a sharp temperature increase. It also provides evidence that their high elevation allows theses waterbodies to remain refuges for cold stenotherms, for now. In the longer term, however, a tipping point is likely to be reached, with a potential collapse of cold stenothermal biodiversity. This demonstrates the critical need to continue longer-term monitoring of high-elevation ecosystems to assess the magnitude of the impairment and to implement suitable conservation strategies, such as enhanced protection or the creation of new suitable habitats. [Display omitted] • Water temperature increased sharply in high-elevation ponds in the last 15 years. • Warming has not yet impaired macroinvertebrate communities in high-elevation ponds. • High-elevation pondscapes (>2600 m) act today as refuges for cold-specialist species. • Protection and monitoring are required to conserve these unique alpine habitats. [ABSTRACT FROM AUTHOR]

Diadromous fish species have a complex life cycle during which they migrate between marine and freshwater habitats. They experience multiple human-induced pressures in both environments, likely exacerbated by climate change, leading to dramatic population declines across their distribution ranges. Currently Species Distribution Models (SDMs) have been applied separately in both their continental and marine habitats to improve our understanding of their lifecycles and help with species management. Integrating the freshwater-sea continuum into the decisions would now be a step further in improving their management. With this objective, we developed a decision tree that links marine and freshwater SDM outputs with current observations of population functionality and suggested management guidance options for the viability of these species. Potential effects of climate change were included through future SDM projections to guide integrative and long-term management. Several criteria were proposed to assess the SDM validity considering the main sources of SDM uncertainties and local expert knowledge on habitat and population status. The framework was applied to approximately one hundred catchments from southern Portugal to southern Scandinavia for four diadromous species. At the European level, management guidance options differed between the two anadromous and two catadromous species. Platichthys flesus and Chelon ramada European populations seemed in better state than those of Alosa alosa and A. fallax. Finally, with the help of national diadromous species experts, we focused on four catchments distributed along the European latitudinal gradient to test the proposed methodology and demonstrate local management challenges in terms of freshwater-sea continuity. [ABSTRACT FROM AUTHOR]

• Parasite infections often peak in certain seasons in aquatic ecosystems. • Meta-analysis of 693 seasonal infection contrasts reveals no universal pattern. • Taxon-specific patterns rule among winter-summer and wet-dry season infection dynamics. • The same parasite taxa show contrasting patterns between freshwater and marine habitats. • Sensitivity to abiotic conditions therefore varies widely among host-parasite systems. Seasonal infection dynamics characterise many host-parasite interactions, as abiotic conditions drive fundamental biological processes in both hosts and parasites. The extent to which seasonal cycles in parasite abundance are universal, and whether the same underlying mechanisms are responsible for seasonality across different host-parasite associations, have received little attention to date. Here, global patterns of seasonal infection dynamics are tested using a meta-analysis of 693 seasonal contrasts in the local abundance of metazoan parasites in aquatic vertebrates, representing 343 parasite species and 148 host species. These include both contrasts between winter and summer in temperate systems, and between wet and dry seasons in tropical systems. Overall, the analysis found no evidence that seasonal infection dynamics are universal, nor that they are controlled by the same factors. More specifically, the results indicate no overall effect of the magnitude of local seasonal differences in abiotic conditions (temperature or precipitation), type of host (amphibian or fish), habitat (freshwater, coastal marine or deep-sea), host body length, or mode of transmission (skin contact or trophic) on the extent of seasonal differences in parasite infections. Instead, the results revealed many clear taxon-specific or habitat-specific patterns of seasonal infection dynamics, with some parasite or host taxa generally showing distinct peaks in infections in summer or dry seasons, and others showing no seasonal changes. The present findings serve to identify empirically which host-parasite associations are most likely to respond to changing climatic conditions, and thus provide useful information for fisheries and aquaculture. [ABSTRACT FROM AUTHOR]

Freshwater ecosystems are threatened worldwide, with 1/3 of freshwater species being under extinction risk. Yet, systematic conservation planning has received little attention as a means to prioritize freshwater conservation areas. Here, we develop a comprehensive approach to spatially delineate key areas for freshwater conservation across Europe, considering directed-connectivity, climate change and human-related disturbances. We assess the adequacy of the current reserve network in protecting European freshwater ecosystems and provide guidelines for expansion. We conducted 13 prioritization scenarios for 18,300 sub-basins across Europe, accounting for the distribution of 1,668 freshwater species. Connectivity properties, metrics of climatic stability and human induced pressures were incorporated into the spatial prioritization framework. We highlight the critical importance of specific regions in Central and Southern Europe, the Mediterranean Basin, and parts of the Balkan Peninsula for future freshwater conservation efforts. However, our analysis also reveals the complete inadequacy of the current reserve network, as protected sub-basins only cover 4 % of the study area and encompass on average only 3 % of freshwater species distributions. In contrast, expanding current reserves to cover the top 17 % of high priority areas across Europe could substantially improve the overall coverage of species distributions, reaching an average of 61.5 % for all freshwater species and even exceeding 93.1 % for species of conservation interest (critically endangered, endangered and vulnerable species). The priority freshwater conservation network derived from the analysis provides specific spatial recommendations to help achieve conservation goals if the current reserve network is appropriately expanded. • Freshwater species are at risk worldwide and conservation planning is overlooked. • A new comprehensive approach prioritizes key areas for European freshwaters. • Critical regions for conservation exist in Central Europe and the Mediterranean. • Current reserves are inadequate covering only 4 % of Europe's freshwater ecosystems. • Expanding reserves can result in a 20-fold increase in species coverage within PAs. [ABSTRACT FROM AUTHOR]

Climate change is a growing global issue with many countries and institutions declaring a climate state of emergency. Excess CO 2 from anthropogenic sources and changes in land use practices are contributing to many detrimental changes, including increased global temperatures, ocean acidification and hypoxic zones along coastal habitats. All senses are important for aquatic animals, as it is how they can perceive and respond to their environment. Some of these environmental challenges have been shown to impair their sensory systems, including the olfactory, visual, and auditory systems. While most of the research is focused on how ocean acidification affects olfaction, there is also evidence that it negatively affects vision and hearing. The effects that temperature and hypoxia have on the senses have also been investigated, but to a much lesser extent in comparison to ocean acidification. This review assembles the known information on how these anthropogenic challenges affect the sensory systems of fishes, but also highlights what gaps in knowledge remain with suggestions for immediate action. Olfaction, vision, otolith, pH, freshwater, seawater, marine, central nervous system, electrophysiology, mechanism. [Display omitted] • Conditions in abiotic factors of oceans and freshwater habitats are changing quickly due to anthropogenic activity. • Low pH impairs olfaction and vision, alters otolith growth, and affects CNS functioning. • High temperatures increase the signalling speed of nerves, alters sensory processing, and increases ROS in the CNS. • Low oxygen impairs energy production, nerve conduction speed, negatively affects vision and causes apoptosis in the brain. [ABSTRACT FROM AUTHOR]

The well documented fossil record of European Quaternary freshwater gastropods offers a unique resource for continental-scale biogeographical analyses. Here, we assembled a dataset including 338 freshwater gastropod taxa from 1058 localities across Europe, which we used to explore how freshwater gastropod communities varied in space and time across six distinct time intervals of the Quaternary, i.e. Gelasian, Calabrian, Middle Pleistocene, Last Interglacial, Last Glacial and Holocene. We took into consideration both species richness and qualitative structural patterns, comparing turnover rates between time intervals and examining variations in community nestedness-segregation patterns. Species richness differed significantly between time intervals. The Early Pleistocene showed the highest diversity, likely because of the contribution of long-lived aquatic systems like the lakes Bresse and Tiberino that promoted speciation and endemism. The rich Middle to Late Pleistocene and Holocene assemblages were mostly linked to fluvial and/or lacustrine systems with short temporal durations. We identified a major turnover event at the Plio-Pleistocene boundary, related to the demise of long-lived lakes and of their rich, endemic faunas at the end of the Pliocene. In the subsequent intervals, little or no turnover was observed. We also observed a pattern of high segregation in Early Pleistocene communities, associated with the abundance of endemic species with small distribution ranges, and reflecting the provincial character of the aquatic freshwater systems at that time. This structured pattern disintegrated gradually towards the Middle Pleistocene and remained unstructured up to present. In particular, spatial patterns of community nestedness-segregation in the Last Interglacial and Holocene suggest a random recolonization of freshwater habitats mostly by generalist species following deglaciation. [ABSTRACT FROM AUTHOR]

• A new hybrid species distribution framework was proposed for diadromous species. • It combined dispersal, multi-habitat suitability, and population dynamics. • HyDiaD was used to predict range-shift responses for shads under climate change. • The relative contribution of habitat suitability vs biological forces was assessed. • Population dynamics parameters were calibrated using expert knowledge elicitation. Diadromous species are particularly vulnerable to climate change because they utilize both marine and freshwater habitat to complete their life cycles. Dispersal plays an important role in restraining the distribution of plant and animal species, and is a key mechanism to allow diadromous species to adapt to changes in habitat suitability, but it is often not included in species distribution models that explore population trends under climate scenarios. The objective of this study was to develop a model to estimate potential shifts in diadromous populations in the Atlantic area of Europe under two climate change scenarios and multiple global climate models. To address the question of range-shift responses, a hybrid approach for diadromous species distribution (HyDiaD) was developed that incorporated two components: i) statistical static models of habitat suitability describing the influence of environmental factors on species occurrence, and ii) biological processes relevant for the distribution of the species, such as population demography and dispersal dynamics. Hybrid models were developed using a novel approach that incorporated both population and between-catchment dispersal dynamics specific to each species. Occupancy data for diadromous species in a subset of Atlantic Area catchments were first validated by regional experts, and boosted regression trees were applied to estimate habitat suitability within each catchment based on historical physical and climatic environmental predictors from the continental and marine domains. Habitat suitability was then used in a population dynamics model that incorporated between-catchment dispersal and local population growth. Results for different-sized catchments were compared using time series of spawner density and saturation rate, which estimated how much of the available habitat was being utilized. Many of the species-specific values used in HyDiaD were estimated through a survey of diadromous species experts, and group consensus was reached by calculating weighted averages. The HyDiaD model was applied to two shad species (Alosa alosa and A. fallax) to explore population trends projected annually from 1951 to 2100. Projected trends indicated that under XXIst century climate scenarios, habitat suitability is expected to increase for A. fallax , but decrease for A. alosa. Projected trends also indicated an increase in the rate of annual variability for A. alosa , particularly in the southern part of its range. Future studies can utilize the HyDiaD model to explore distribution trends for other diadromous species under climate change scenarios. [ABSTRACT FROM AUTHOR]

• Cell concentration increased more with temperature than pCO2 in continuous cultures. • Species and strains differed in growth response to increased temperature and pCO2. • Straight R. raciborskii strains have five more carbon acquisition genes than coiled. • Straight strain cell concentration increased more with higher pCO2 than coiled strain. Global increases in atmospheric CO 2 and temperatures will impact aquatic systems, with freshwater habitats being affected. Some studies suggest that these conditions will promote cyanobacterial dominance. There is a need for a clearer picture of how algal species and strains within species will respond to higher temperatures and CO 2 , especially in combination. This study examined two chlorophytes (Monoraphidium and Staurastrum), and two strains of the cyanobacterium Raphidiopsis raciborskii (straight S07 and coiled C03), to determine how the combination of higher temperature and CO 2 levels will affect their growth and maximum cell concentrations. Continuous cultures were used to compare the steady state cell concentrations at 28 °C and 30 °C, and CO 2 partial pressures (pCO 2), 400 and 750 ppm for all cultures, and in addition 1000 ppm at 28 °C for R. raciborskii strains. This study showed that, for all species, water temperature had a greater effect than higher pCO 2 on cell concentrations. There were clear differences in response between the chlorophyte species, with Monoraphidium preferring 28 °C and Staurastrum preferring 30 °C. There were also differences in response of the R. raciborskii strains to increasing temperature and pCO 2 , with S07 having a greater increase in cell concentration. Genome analysis of R. raciborskii showed that the straight strain has five additional carbon acquisition genes (β-CA , chpY , cmpB , cmpD and NdhD4), indicative of increased carbon metabolism. These differences in the strains' response to elevated pCO 2 will lead to changes in the species population structure and distribution in the water column. This study shows that it is important to examine the effects of both pCO 2 and temperature, and to consider strain variation, to understand how species composition of natural systems may change under future climate conditions. [ABSTRACT FROM AUTHOR]