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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 ecoevolutionary dynamics as key future avenues to predict the consequences of freshwater salinisation for ecosystems and human societies., This work is part of the AQUACOSM and AQUACOSM-plus projects that have received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement Nos 731065 and 871081. D.C.M., M.B., E.J., R.P., S.A.B., M.G., Z.H., S.L., J.C.N., M.R., and C.F.V. were supported by the H2020 EU-funded project AQUACOSM-plus (no. 871081). D.C.M., M.B., S.A.B., and J.C.N. were supported by the H2020 EU-funded project AQUACOSM (no. 731065). M.B. and E.J. were supported by the TÜBITAK program BIDEB2232 (project 118C250). M.B. and S.B. were supported by the H2020 EU-funded project PONDERFUL (no. 869296). Z.H. and C.F.V. were supported by the NKFIH-471-3/2021 project. Z.H. was supported by the János Bolyai Research Scholarship of the Hungarian Academy of Sciences. P.U-C. was supported by the Swedish Research Council Formas (grant: 2020-01825). M.M. was supported by the Grant RyC2016-19348 funded by MCIN/AEI/10.13039/501100011033 and by ESF ‘Investing in your future’

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 ecoevolutionary dynamics as key future avenues to predict the consequences of freshwater salinisation for ecosystems and human societies. This work is part of the AQUACOSM and AQUACOSM-plus projects that have received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement Nos 731065 and 871081. D.C.M., M.B., E.J., R.P., S.A.B., M.G., Z.H., S.L., J.C.N., M.R., and C.F.V. were supported by the H2020 EU-funded project AQUACOSM-plus (no. 871081). D.C.M., M.B., S.A.B., and J.C.N. were supported by the H2020 EU-funded project AQUACOSM (no. 731065). M.B. and E.J. were supported by the TÜBITAK program BIDEB2232 (project 118C250). M.B. and S.B. were supported by the H2020 EU-funded project PONDERFUL (no. 869296). Z.H. and C.F.V. were supported by the NKFIH-471-3/2021 project. Z.H. was supported by the János Bolyai Research Scholarship of the Hungarian Academy of Sciences. P.U-C. was supported by the Swedish Research Council Formas (grant: 2020-01825). M.M. was supported by the Grant RyC2016-19348 funded by MCIN/AEI/10.13039/501100011033 and by ESF ‘Investing in your future’

Planktivorous fish predation directly affects zooplankton biomass, community and size structure, and may indirectly induce a trophic cascade to phytoplankton. However, it is not clear how quickly the zooplankton community structure and the cascading effects on phytoplankton recover to the unaffected state (i.e. resilience) once short-term predation by fish stops. The resilience has implications for the ecological quality and restoration measures in aquatic ecosystems. To assess the short-term zooplankton resilience against fish predation, we conducted a mesocosm experiment consisting of 10 enclosures, 6 with fish and 4 without fish. Plankton communities from a natural lake were used to establish phytoplankton and zooplankton in the mesocosms. High biomasses (about 20 g wet mass m-3) of juvenile planktivorous fish (perch, Perca fluviatilis) were allowed to feed on zooplankton in fish enclosures for four days. Thereafter, we removed fish and observed the recovery of the zooplankton community and its cascading effect on trophic interactions in comparison with no fish enclosures for four weeks. Short-term fish predation impaired resilience in zooplankton community by modifying community composition, as large zooplankton, such as calanoids, decreased just after fish predation and did not re-appear afterwards, whereas small cladocerans and rotifers proliferated. Total zooplankton biomass increased quickly within two weeks after fish removal, and at the end even exceeded the biomass measured before fish addition. Despite high biomass, the dominance of small zooplankton released phytoplankton from grazer control in fish enclosures. Accordingly, the zooplankton community did not recover from the effect of fish predation, indicating low short-term resilience. In contrast, in no fish enclosures without predation disturbance, a high zooplankton:phytoplankton biomass ratio accompanied by low phytoplankton yield (Chlorophyll-a:Total phosphorus ratio) reflected phytoplankton control by zooplankton over the experimental period. Comprehensive views on short and long-term resilience of zooplankton communities are essential for restoration and management strategies of aquatic ecosystems to better predict responses to global warming, such as higher densities of planktivorous fish.

Many zooplankton species typically exhibit diel vertical migration (DVM), where zooplankton migrate from the hypolimnion to the epilimnion of lakes at night. Zooplankton exhibit this behavior to avoid visual predators and UV radiation by remaining in the bottom waters during the day and ascending to the surface waters to feed on phytoplankton at night. However, hypoxic conditions in the hypolimnion of lakes mayinterfere with DVM and force zooplankton to increase diel horizontal migration (DHM) to find predation refuge in littoral zones. Climate change and eutrophication are expected to increase the prevalence and severity of hypoxic conditions worldwide and thereby possibly alter zooplankton migration patterns. We hypothesize that hypoxia will force zooplankton to shift their migration patterns from predominantly DVM to DHM to avoid oxygen-depleted bottom waters. To test our hypothesis, we are conducting a standardized global sampling program to test whether pelagic, full water column estimates of zooplankton are greater at night versus the day under hypolimnetic hypoxic versus oxic conditions. Participants are aiming to sample at least one lake with an oxic hypolimnion and one lake with a hypoxic hypolimnion during the thermally-stratified period at midday and midnight. With our global dataset (currently expecting about 60 lakes in 22 countries), our goal is to improve our understanding of how global change may alter zooplankton migration behavior and patterns in lakes. info:eu-repo/semantics/publishedVersion

International audience; no abstract

1. The size structure of communities is shaped by biotic and abiotic interactions. Therefore, comparative analyses of size spectra may reveal the major drivers governing patterns and processes in size-structured communities. 2. We tested the suitability of non-taxonomic, size-related variables as tools for elucidating systematic shifts in lake fish assemblages along the gradients of environmental factors and lake-use intensity. Catch data of multimesh gillnets from 78 lowland lakes in northern Germany were analysed. 3. We first identified the correlations, and hence inherent redundancy, among 18 sizerelated variables. The correspondence between eight weakly correlated size variables and descriptors of lake morphometry, lake productivity, lake-use intensity and taxonomic and functional fish-assemblage composition was tested using ordination by non-metric multidimensional scaling (NMDS). The three axes of the NMDS analysis were strongly correlated with five size variables, which in turn corresponded to lake area, mean and maximum depth, total phosphorus and chlorophyll a concentration, predator abundance and predator ⁄prey length ratios (PPLR). 4. The number of size classes increased with increasing lake area. The slopes of normalised length spectra were flatter (less negative) and size diversity was higher in deep, less nutrient-rich lakes and in lakes with a higher numerical predator abundance, indicating a higher relative abundance of large fish. The exponent of the Pareto type II mass spectra was larger and maximum fish length was smaller in shallow, nutrient-rich lakes and in lakes with lower predator biomass and smaller PPLR, indicating a higher relative proportion of medium-sized fish. 5. Analyses of size spectra at regional scales can contribute important information to the evaluation of the ecological quality of lakes. We suggest further studies at a broader range of environmental and geographical scales to understand the subtle response of size-related variables to biotic interactions, abiotic stressors and geographical patterns., JRC.H.5-Rural, water and ecosystem resources

Is Microtox® toxicity related to potentially harmful algae proliferation in Mediterranean salt marshes? Mediterranean salt marshes are ecosystems that are highly influenced by sea changes and freshwater inputs from runoff. In these ecosystems, toxic and nontoxic algae blooms often produce Jarge and unpredictabJe biomasses of phytoplankton. The Microtox@) test has been described as a successful, quick method for detecting toxicity in various phytoplankton taxa. Our study sought to test the efficiency of Microtox® in detecting toxic HAB in Mediterranean salt marshes. The results showed that the Microtox® test was able to detect toxic substances in the particulate matter of several lagoons in the Emporda salt marshes. This Microtox® toxicity coincided with periods when potentially harrnful cyanobacteria, dinoflagellates and haptophytes had a high biomass. The results suggest that potentially harmful phytoplankton cannot be ruled out as a source of Microtox® toxicity or as a source of other organism kills and subacute effects in Mediterranean salt marshes. Las marismas mediterráneas son ecosistemas altamente injluenciados por la entrada de agua marina y de agua dulce, procedente de la escorrentía. En estos ecosistemas, las proliferaciones de algas tóxicas y no tóxicas sin patrón temporal o espacial, alcanzan altas e impredecibles biomasas de fitoplancton. El ensayo Microtox® ha sido descrito como un rápido y eficiente método para detectar la toxicidad de diferentes taxones de fitoplaneton. Nosotros hemos hecho una primera aproximación para determinar la eficiencia del test Microtox® en la detección de proliferaciones algales tóxicas en las marismas mediterráneas. Se ha podido concluir que hay substancias tóxicas, en la materia particulada de diversas lagunas de las marismas del Emporda, que el método Microtox® es capaz de detectar. Esta toxicidad detectada por Microtox® se da en periodos en que cianobacterias, dinojlagelados y haptófitos potencialmente tóxicos presentan alta biomasa. Los resultados sugieren que el fitoplancton potencialmente tóxico no puede ser descartado como causante de la mortalidad o de efectos subagudos a otros organismos de las marismas mediterráneas.

Mediterranean salt marshes are ecosystems that are highly influenced by sea exchanges and freshwater inputs from runoffs. In these ecosystems, toxic and no toxic algae blooms, without a temporal or spatial pattern, reach often high and unpredictable biomasses of phytoplankton. The Microtox® test have been described as a successful, quick method for detecting toxicity of different phytoplankton taxa. We made a first approach to determine the efficiency of Microtox® for detecting toxic HAB in Mediterranean salt marshes. It could be concluded that there are toxic substances in the particulate matter of several lagoons in the Empordà salt marshes that the Microtox® test was able to detect. This Microtox® toxicity coincided with periods when potentially harmful cyanobacteria, dinoflagellates and haptophytes have a high biomass. The results suggest that potentially harmful phytoplankton cannot be ruled out as a source Microtox® toxicity and also as a source of other organism kills or sub-acute effects in Mediterranean salt marshes., JRC.H.1-Water Resources

Mediterranean salt marshes are ecosystems that are highly influenced by sea changes and freshwater inputs from runoff. In these ecosystems, toxic and non-toxic algae blooms often produce large and unpredictable biomasses of phytoplankton. The Microtox R test has been described as a successful, quick method for detecting toxicity in various phytoplankton taxa. Ourstudy sought to test the efficiency of Microtox R in detecting toxic HAB in Mediterranean salt marshes. The results showed that the Microtox R test was able to detect toxic substances in the particulate matter of several lagoons in the Empordà salt marshes. This Microtox R toxicity coincided with periods when potentially harmful cyanobacteria, dinoflagellates and haptophytes had a high biomass. The results suggest that potentially harmful phytoplankton cannot be ruled out as a source of Microtox R

Coastal wetlands are characterized by high biodiversity, which is one of the main criteria considered when establishing protection policies or when proposing adequate management actions. In this study, the crustacean and aquatic insect composition of the Empord`a wetlands is described. These two faunal groups contribute highly to the total biodiversity in these wetlands but are seldom considered when managing natural areas. A selection (84 sampling points) of all water body types present in the Empord`a wetlands were sampled monthly (surber and dip net with a 250 μm mesh). Sampling was carried out during 3 surveys (1991-92, 1996-97 and 1999-2000). A rich fauna of 125 crustacean and 295 aquatic insect taxa was identified. We characterized each water body type using the most abundant species and the relative species richness of the taxonomic groups. A classification of the water body types, according to similarity between inventories, groups the brackish and hyperhaline systems in one cluster and the various freshwater systems in another one. Among freshwater systems, lotic waters and freshwater wetlands have a high similarity, whereas rice fields and freshwater springs have a low similarity Los humedales costeros se caracterizan por una alta biodiversidad, que es uno de los principales criterios considerados alestablecer políticas de protección o al proponer acciones de gestión adecuada. En este estudio se describe la composición de crustáceos e insectos acuáticos de los aiguamolls del Empordà. Estos dos grupos faunísticos contribuyen de forma importante al total de la biodiversidad en los humedales, pero raramente se tienen en consideración al gestionar las áreas naturales. Se muestreó mensualmente una selección (84 puntos de muestreo) de todas las tipologías de masas de agua presentes en los aiguamolls de l’Empordà mediante redes surber y salabres de poro de malla de 250 μm. El muestreo se llevó a cabo durante tres campañas (1991-92, 1996-97 y 1999-2000). Se identificó una riqueza faunística de 125 taxones de crustáceos y 295 taxones de insectos acuáticos. Cada tipología de masa de agua se caracterizó utilizando las especies más abundantes y la riqueza específica de los grupos faunísticos. La clasificación de las tipologías de masas de agua, según la similaridad entre inventarios faunísticos, agrupó los sistemas salobres y hiperhalinos en un grupo. Entre los sistemas de agua dulce, los sistemas lóticos y los humedales de agua dulce presentaron una alta similaridad, mientras que los arrozales y las surgencias de agua dulce mostraron una baja similaridad