Multiple-stressor exposure of aquatic food webs: Nitrate and warming modulate the effect of pesticides

Non-pathogenic microbiome associated to aquatic plants and anthropogenic impacts on this interaction

The microbiota associated with aquatic plants plays a crucial role in promoting plant growth and development. The structure of the plant microbiome is shaped by intricate interactions among hosts, microbes, and environmental factors. Consequently, anthropogenic pressures that disrupt these interactions can indirectly impact the ecosystem services provided by aquatic plants, such as CO2 fixation, provision of food resources, shelter to animals, nutrient cycling, and water purification. Presently, studies on plant-microbiota interactions primarily focus on terrestrial hosts and overlook aquatic environments with their unique microbiomes. Therefore, there is a pressing need for a comprehensive understanding of plant microbiomes in aquatic ecosystems. This review delves into the overall composition of the microbiota associated with aquatic plant, with a particular emphasis on bacterial communities, which have been more extensively studied. Subsequently, the functions provided by the microbiota to their aquatic plants hosts are explored, including the acquisition and mobilization of nutrients, production of auxin and related compounds, enhancement of photosynthesis, and protection against biotic and abiotic stresses. Additionally, the influence of anthropogenic stressors, such as climate change and aquatic contamination, on the interaction between microbiota and aquatic plants is discussed. Finally, knowledge gaps are highlighted and future directions in this field are suggested.

Land use and river-lake connectivity: Biodiversity determinants of lake ecosystems

Lake ecosystems confront escalating challenges to their stability and resilience, most intuitively leading to biodiversity loss, necessitating effective preservation strategies to safeguard aquatic environments. However, the complexity of ecological processes governing lake biodiversity under multi-stressor interactions remains an ongoing concern, primarily due to insufficient long-term bioindicator data, particularly concerning macroinvertebrate biodiversity. Here we utilize a unique, continuous, and in situ biomonitoring dataset spanning from 2011 to 2019 to investigate the spatio-temporal variation of macroinvertebrate communities. We assess the impact of four crucial environmental parameters on Lake Dongting and Lake Taihu, i.e., water quality, hydrology, climate change, and land use. These two systems are representative of lakes with Yangtze-connected and disconnected subtropical floodplains in China. We find an alarming trend of declining taxonomic and functional diversities among macroinvertebrate communities despite improvements in water quality. Primary contributing factors to this decline include persistent anthropogenic pressures, particularly alterations in human land use around the lakes, including intensified nutrient loads and reduced habitat heterogeneity. Notably, river-lake connectivity is pivotal in shaping differential responses to multiple stressors. Our results highlight a strong correlation between biodiversity alterations and land use within a 2-5 km radius and 0.05-2.5 km from the shorelines of Lakes Dongting and Taihu, respectively. These findings highlight the importance of implementing land buffer zones with specific spatial scales to enhance taxonomic and functional diversity, securing essential ecosystem services and enhancing the resilience of crucial lake ecosystems.

Combined stress of an insecticide and heatwaves or elevated temperature induce community and food web effects in a Mediterranean freshwater ecosystem

Ongoing global climate change will shift nature towards Anthropocene's unprecedented conditions by increasing average temperatures and the frequency and severity of extreme events, such as heatwaves. While such climatic changes pose an increased threat for freshwater ecosystems, other stressors like pesticides may interact with warming and lead to unpredictable effects. Studies that examine the underpinned mechanisms of multiple stressor effects are scarce and often lack environmental realism. Here, we conducted a multiple stressors experiment using outdoor freshwater mesocosms with natural assemblages of macroinvertebrates, zooplankton, phytoplankton, macrophytes, and microbes. The effects of the neonicotinoid insecticide imidacloprid (1 µg/L) were investigated in combination with three temperature scenarios representing ambient, elevated temperatures (+4 °C), and heatwaves (+0 to 8 °C), the latter two having similar energy input. We found similar imidacloprid dissipation patterns for all temperature treatments with lowest average dissipation half-lives under both warming scenarios (DT50: 3 days) and highest under ambient temperatures (DT50: 4 days) throughout the experiment. Amongst all communities, only the zooplankton community was significantly affected by the combined treatments. This community demonstrated low chemical sensitivity with lagged and significant negative imidacloprid effects only for cyclopoids. Heatwaves caused early and long-lasting significant effects on the zooplankton community as compared to elevated temperatures, with Polyarthra, Daphnia longispina, Lecanidae, and cyclopoids being the most negatively affected taxa, whereas Ceriodaphnia and nauplii showed positive responses to temperature. Community recovery from imidacloprid stress was slower under heatwaves, suggesting temperature-enhanced toxicity. Finally, microbial and macrofauna litter degradation were significantly enhanced by temperature, whereas the latter was also negatively affected by imidacloprid. A structural equation model depicted cascading food web effects of both stressors with stronger relationships and significant negative stressor effects at higher than at lower trophic levels. Our study highlights the threat of a series of heatwaves compared to elevated temperatures for imidacloprid-stressed freshwaters.

Prediction of Potential Risk for Flupyradifurone and Its Transformation Products to Hydrobionts

Flupyradifurone (FPF) is considered the latest generation of neonicotinoid insecticides. Here, we investigated the toxicity and ecological risk of FPF and its aerobic transformation products (TPs) to aquatic species using the method of prediction. We found that FPF exhibited moderate or high toxicity to some aquatic species. The 5% hazardous concentration of FPF was 3.84 μg/L for aquatic organisms. We obtained 91 aerobic TPs for FPF, and almost half of FPF TPs exhibited toxicity to fish or Daphnia. Eleven of the TPs of FPF exhibited a high or moderate risk to aquatic ecosystems. All FPF TPs with high and moderate risks contained a 6-chloropyridine ring structure, indicating that the derivant of a pyridine ring exhibits potential risks to aquatic ecosystems. Our results provide insight into the potential risk of FPF to aquatic ecosystems and could be used to help set criteria to control pollution caused by FPF.

Assessing the ecological impact of pesticides/herbicides on algal communities: A comprehensive review

The escalating use of pesticides in agriculture for enhanced crop productivity threatens aquatic ecosystems, jeopardizing environmental integrity and human well-being. Pesticides infiltrate water bodies through runoff, chemical spills, and leachate, adversely affecting algae, vital primary producers in marine ecosystems. The repercussions cascade through higher trophic levels, underscoring the need for a comprehensive understanding of the interplay between pesticides, algae, and the broader ecosystem. Algae, susceptible to pesticides via spillage, runoff, and drift, experience disruptions in community structure and function, with certain species metabolizing and bioaccumulating these contaminants. The toxicological mechanisms vary based on the specific pesticide and algal species involved, particularly evident in herbicides' interference with photosynthetic activity in algae. Despite advancements, gaps persist in comprehending the precise toxic effects and mechanisms affecting algae and non-target species. This review consolidates information on the exposure and toxicity of diverse pesticides and herbicides to aquatic algae, elucidating underlying mechanisms. An emphasis is placed on the complex interactions between pesticides/herbicides, nutrient content, and their toxic effects on algae and microbial species. The variability in the harmful impact of a single pesticide across different algae species underscores the necessity for further research. A holistic approach considering these interactions is imperative to enhance predictions of pesticide effects in marine ecosystems. Continued research in this realm is crucial for a nuanced understanding of the repercussions of pesticides and herbicides on aquatic ecosystems, mainly algae.

Eutrophication and loss of riparian shading influence food quality and trophic relation in stream food webs

Eutrophication induced by excessive inputs of nutrient is one of the main stressors in aquatic ecosystems. Deforestation in riparian zones alter riparian shading, which together with eutrophication is expected to exert a complex control over stream food webs. We manipulated two levels of riparian shading (open canopy vs. shading canopy) and nutrient supply (ambient vs. nutrient addition) in three headwater streams to investigate the individual and combined effects of eutrophication and loss of riparian shading on carbon sources and nutritional quality of biofilms, and the subsequent trophic effects on macroinvertebrate grazers. Nutrient enrichment increased the autochthonous carbon (i.e., algae especially diatoms) indicated by fatty acid (FA) biomarkers within biofilms and grazers. The nutritional quality indicated by eicosapentaenoic acid (EPA) content of biofilms was increased with nutrient enrichment and more so with the combined effect of an increase in riparian shading, consequently leading to an increase in the nutritional quality, density, and biomass of grazers. In particular, the trophic linkages between biofilms and grazers were mainly influenced by EPA concentration in the biofilms, and strengthened with the combined effects of riparian shading and additional nutrients. Our study emphasizes the nutritional significance of EPA for consumers at higher trophic levels and proposes its potential as an indicator for monitoring the health of aquatic ecosystems.

Effects of nutrient enrichment on freshwater macrophyte and invertebrate abundance: A meta-analysis

External nutrient loading can cause large changes in freshwater ecosystems. Many local field and laboratory experiments have investigated ecological responses to nutrient addition. However, these findings are difficult to generalize, as the responses observed may depend on the local context and the resulting nutrient concentrations in the receiving water bodies. In this research, we combined and analysed data from 131 experimental studies containing 3054 treatment-control abundance ratios to assess the responses of freshwater taxa along a gradient of elevated nutrient concentrations. We carried out a systematic literature search in order to identify studies that report the abundance of invertebrate, macrophyte, and fish taxa in relation to the addition of nitrogen, phosphorus, or both. Next, we established mixed-effect meta-regression models to relate the biotic responses to the concentration gradients of both nutrients. We quantified the responses based on various abundance-based metrics. We found no responses to the mere addition of nutrients, apart from an overall increase of total invertebrate abundance. However, when we considered the gradients of N and P enrichment, we found responses to both nutrients for all abundance metrics. Abundance tended to increase at low levels of N enrichment, yet decreased at the high end of the concentration gradient (1-10 mg/L, depending on the P concentration). Responses to increasing P concentrations were mostly positive. For fish, we found too few data to perform a meaningful analysis. The results of our research highlight the need to consider the level of nutrient enrichment rather than the mere addition of nutrients in order to better understand broad-scale responses of freshwater biota to eutrophication, as a key step to identify effective conservation strategies for freshwater ecosystems.

Interactions between climate warming, herbicides, and eutrophication in the aquatic food web

Understanding the interactive effects of multiple environmental stressors on biological communities is crucial for effective environmental management and biodiversity conservation. Here, we present the results of an outdoor mesocosm experiment examining how an aquatic food web responds to the individual and combined effects of climate warming, heat waves, nutrient enrichment, and herbicide exposure. To assess ecosystem functioning, we examined energy flow, using stable isotope analysis integrated with the bioenergetics food web approach to quantify energy fluxes among trophic levels. Our results revealed that the combined effects of these stressors altered the pattern of energy fluxes within the food web. Under warming conditions, there was an increase in energy flux from producers and primary consumers to secondary consumers. However, we did not observe a significant increase in energy flux in primary consumers, potentially due to enhanced top-down control. Nutrient enrichment increased energy flux from producers to higher trophic levels while simultaneously decreasing detrital energy flux. Herbicide exposure did not significantly affect herbivory energy flux but did reduce detritivory energy flux, particularly from detritus to primary consumers. The interactive effects we observed were primarily antagonistic or additive, although we also detected reversed and synergistic effects. The responses to multiple stressors varied across different energy flow pathways, leading to an asymmetric response. Furthermore, our results also revealed significant differences in the effects of constant warming and heat waves, either alone or in combination with water pollution. The asymmetric response of energy flow pathways and the prevalence of antagonistic effects present significant challenges for ecosystem restoration. Together, our findings provide novel and clear evidence of the complex mechanisms by which the coexistence of stressors can differently affect the pathways of energy flux across trophic levels in aquatic ecosystems. Regulatory strategies for ecosystems should comprehensively consider responses at multi-trophic levels using a network perspective, especially in the face of combinations of global and local stressors.

Unlocking secrets of microbial ecotoxicology: recent achievements and future challenges

Environmental pollution is one of the main challenges faced by humanity. By their ubiquity and vast range of metabolic capabilities, microorganisms are affected by pollution with consequences on their host organisms and on the functioning of their environment. They also play key roles in the fate of pollutants through the degradation, transformation, and transfer of organic or inorganic compounds. Thus, they are crucial for the development of nature-based solutions to reduce pollution and of bio-based solutions for environmental risk assessment of chemicals. At the intersection between microbial ecology, toxicology, and biogeochemistry, microbial ecotoxicology is a fast-expanding research area aiming to decipher the interactions between pollutants and microorganisms. This perspective paper gives an overview of the main research challenges identified by the Ecotoxicomic network within the emerging One Health framework and in the light of ongoing interest in biological approaches to environmental remediation and of the current state of the art in microbial ecology. We highlight prevailing knowledge gaps and pitfalls in exploring complex interactions among microorganisms and their environment in the context of chemical pollution and pinpoint areas of research where future efforts are needed.

Microcosm experiment combined with process-based modeling reveals differential response and adaptation of aquatic primary producers to warming and agricultural run-off

Fertilizers, pesticides and global warming are threatening freshwater aquatic ecosystems. Most of these are shallow ponds or slow-flowing streams or ditches dominated by submerged macrophytes, periphyton or phytoplankton. Regime shifts between the dominance of these primary producers can occur along a gradient of nutrient loading, possibly triggered by specific disturbances influencing their competitive interactions. However, phytoplankton dominance is less desirable due to lower biodiversity and poorer ecosystem function and services. In this study, we combined a microcosm experiment with a process-based model to test three hypotheses: 1) agricultural run-off (ARO), consisting of nitrate and a mixture of organic pesticides and copper, differentially affects primary producers and enhances the risk of regime shifts, 2) warming increases the risk of an ARO-induced regime shift to phytoplankton dominance and 3) custom-tailored process-based models support mechanistic understanding of experimental results through scenario comparison. Experimentally exposing primary producers to a gradient of nitrate and pesticides at 22°C and 26°C supported the first two hypotheses. ARO had direct negative effects on macrophytes, while phytoplankton gained from warming and indirect effects of ARO like a reduction in the competitive pressure exerted by other groups. We used the process-based model to test eight different scenarios. The best qualitative fit between modeled and observed responses was reached only when taking community adaptation and organism acclimation into account. Our results highlight the importance of considering such processes when attempting to predict the effects of multiple stressors on natural ecosystems.

Heatwaves, elevated temperatures, and a pesticide cause interactive effects on multi-trophic levels of a freshwater ecosystem

Climate impacts of elevated temperatures and more severe and frequent weather extremes like heatwaves are globally becoming discernible on nature. While a mechanistic understanding is pivotal for ecosystem management, stressors like pesticides may interact with warming, leading to unpredictable effects on freshwater ecosystems. These multiple stressor studies are scarce and experimental designs often lack environmental realism. To investigate the multiple stressor effects, we conducted a microcosm experiment for 48 days comprising benthic macroinvertebrates, zooplankton, phytoplankton, macrophytes, and microbes. The fungicide carbendazim (100 μg/L) was investigated combined with temperature scenarios representing elevated temperatures (+4 °C) or heatwaves (+0 to +8 °C), both applied with similar energy input on a daily fluctuating ambient temperature (18 °C ± 1.5 °C), which served as control. Measurements showed the highest carbendazim dissipation in water under heatwaves followed by elevated and ambient temperatures. Average carbendazim concentrations were about 50% in water and 16% in sediment of the nominal concentration. In both heated cosms, zooplankton community dynamics revealed an unexpected shift from Rotifera to Cladocera and Copepoda nauplii, indicating variations in their thermal sensitivity, tolerance and resilience. Notably, warming and heatwaves shaped community responses similarly, suggesting heat intensity rather than distribution patterns determined the community structure. Heatwaves led to significant early and longer-lasting adverse effects that were exacerbated over time with Cladocera and Copepoda being most sensitive likely due to significant carbendazim interactions. Finally, a structural equation model demonstrated significant relationships between zooplankton and macrophytes and significantly negative carbendazim effects on zooplankton, whereas positive on macroinvertebrate abundances. The relationship between macroinvertebrate feeding and abundance was masked by significantly temperature-affected microbial leaf litter decomposition. Despite the thermal tolerance of zooplankton communities, our study highlights an increased pesticide threat under temperature extremes. More intense heatwaves are thus likely to cause significant alterations in community assemblages which will adversely affect ecosystem's processes and functions.

Nitrate with enriched heavy oxygen isotope linked to changes in nitrogen source and transformation as groundwater table rises

Nitrate is a significant constituent of the total nitrogen pool in shallow aquifers and poses an escalating threat to groundwater resources, making it crucial to comprehend the source, conversion, and elimination of nitrogen using appropriate techniques. Although dual-isotope dynamics in nitrate have been widely used, uncertainties remain regarding the asynchronously temporal changes in δ¹⁸O-NO₃^- and δ¹⁵N-NO₃^- observed in hypoxic aquifers. This study aimed to investigate changes in nitrogen sources and transformations using temporal changes in field-based NO₃^- isotopic composition, hydro-chemical variables, and environmental DNA profiling, as the groundwater table varied. The results showed that the larger enrichment in δ¹⁸O-NO₃^- (+13‰) compared with δ¹⁵N-NO₃^- (-2‰) on average during groundwater table rise was due to a combination of factors, including high ¹⁸O-based atmospheric N deposition, canopies nitrification, and soil nitrification transported vertically by rainfalls, and ¹⁸O-enriched O₂ produced through microbial and root respiration within denitrification. The strong association between functional gene abundance and nitrogen-related indicators suggests that anammox was actively processed with nitrification but in small bacterial population during groundwater table rise. Furthermore, bacterial species associated with nitrogen-associated gradients provided insight into subsurface nitrogen transformation, with Burkholderiaceae species and Pseudorhodobacter potentially serving as bioindicators of denitrification, while Candidatus Nitrotogn represents soil nitrification. Fluctuating groundwater tables can cause shifts in hydro-chemical and isotopic composition, which in turn can indicate changes in nitrogen sources and transformations. These changes can be used to improve input sources for mixture models and aid in microbial remediation of nitrate.

Global historical trends and drivers of submerged aquatic vegetation quantities in lakes

Submerged aquatic vegetation (SAV) in lake littoral zones is an inland water wetland type that provides numerous essential ecosystem services, such as supplying food and habitat for fauna, regulating nutrient fluxes, stabilizing sediments, and maintaining a clear water state. However, little is known on how inland SAV quantities are changing globally in response to human activities, where loss threatens the provisioning of these ecosystem services. In this study, we generate a comprehensive global synthesis of trends in SAV quantities using time series (>10 years) in lakes and identify their main drivers. We compiled trends across methods and metrics, integrating both observational and paleolimnological approaches as well as diverse measures of SAV quantities, including areal extent, density, or abundance classes. The compilation revealed that knowledge on SAV is mostly derived from temperate regions, with major gaps in tropical, boreal, and mountainous lake-rich regions. Similar to other wetland types, we found that 41% of SAV times series are largely decreasing mostly due to land use change and resulting eutrophication. SAV is, however, increasing in 28% of cases, primarily since the 1980s. We show that trends and drivers of SAV quantities vary regionally, with increases in Europe explained mainly by management, decreases in Asia due to eutrophication and land use change, and variable trends in North America consistent with invasive species arrival. By providing a quantitative portrait of trends in SAV quantities worldwide, we identify knowledge gaps and future SAV research priorities. By considering the drivers of different trends, we also offer insight to future lake management related to climate, positive restoration actions, and change in community structure on SAV quantities.

Heat waves rather than continuous warming exacerbate impacts of nutrient loading and herbicides on aquatic ecosystems

Submerged macrophytes are vital components in shallow aquatic ecosystems, but their abundances have declined globally. Shading by periphyton and phytoplankton/turbidity plays a major role in this decline, and the competing aquatic primary producers are subject to the complex influence of multiple stressors such as increasing temperatures, nutrient loading and herbicides. Their joint impact has rarely been tested and is difficult to predict due to potentially opposing effects on the different primary producers, their interactions and their grazers. Here, we used 48 mesocosms (2500 L) to simulate shallow lakes dominated by two typical submerged macrophytes, bottom-dwelling Vallisneria denseserrulata and canopy-forming Hydrilla verticillata, and associated food web components. We applied a combination of nutrient loading, continuous warming, heat waves and glyphosate-based herbicides to test how these stressors interactively impact the growth of submerged macrophytes, phytoplankton and periphyton as competing primary producers. Warming or heat waves alone did not affect phytoplankton and periphyton abundance, but negatively influenced the biomass of V. denseserrulata. Nutrient loading alone increased phytoplankton biomass and water turbidity and thus negatively affected submerged macrophyte biomass, particularly for V. denseserrulata, by shading. Glyphosate alone did not affect biomass of each primary producer under ambient temperatures. However, heat waves facilitated phytoplankton growth under combined nutrient loading and glyphosate treatments more than continuous warming. As a consequence, H. verticillata biomass was lowest under these conditions indicating the potential of multiple stressors for macrophyte decline. Our study demonstrated that multiple stressors interactively alter the biomass of primary producers and their interactions and can eventually lead to a loss of macrophyte communities and shift to phytoplankton dominance. These results show the risks in shallow lakes and ponds in agricultural landscapes and underline the need for multiple stressor studies as a base for their future management.

Warming lowers critical thresholds for multiple stressor-induced shifts between aquatic primary producers

In aquatic ecosystems, excessive nutrient loading is a global problem that can induce regime shifts from macrophyte- to phytoplankton-dominated states with severe consequences for ecosystem functions. Most agricultural landscapes are sites of nutrient and pesticide loading, which can interact with other stressors (e.g., warming) in additive, antagonistic, synergistic or reversed forms. The effects of multiple stressors on the resilience of macrophyte-dominated states and on critical thresholds for regime shifts are, however, unknown. We test the effects of individual and combined stressors of warming, nitrate, and various pesticides typically found in agricultural run-off (ARO) on the growth of macrophytes, periphyton, and phytoplankton in microcosms. We applied a one-level replicated design to test whether ARO induces a regime shift and a multifactorial dose-response design to model stressor thresholds and disentangle stressor interactions along a gradient. The individual stressors did not induce a regime shift, but the full ARO did. Nitrate and pesticides acted synergistically, inducing a shift with increasing phytoplankton biomass and decreasing macrophyte biomass. Warming amplified this effect and lowered critical thresholds for regime shifts. Shallow aquatic ecosystems in agricultural landscapes affected by global warming thus increasingly risk shifting to a turbid, phytoplankton-dominated state, and negatively impacting ecosystem service provisioning. Multiple stressor interactions must be considered when defining safe operating spaces for aquatic systems.