Effect of multiple agricultural stressors on freshwater ecosystems: The role of community structure, trophic status, and biodiversity-functioning relationships on ecosystem responses

Azolla Pinnata: Sustainable Floating Oil Cleaner of Water Bodies

Various plant-based materials effectively absorb oil contaminants at the water/air interface. These materials showcase unparalleled efficiency in purging oil contaminants, encompassing rivers, lakes, and boundless oceans, positioning them as integral components of environmental restoration endeavors. In addition, they are biodegradable, readily available, and eco-friendly, thus making them a preferable choice over traditional oil cleaning materials. This study explores the phenomenal properties of the floating Azolla fern (Azolla pinnata), focusing on its unique hierarchical leaf surface design at both the microscale and nanoscale levels. These intricate structures endow the fern with exceptional characteristics, including superhydrophobicity, high water adhesion, and remarkable oil or organic solvent absorption capabilities. Azolla's leaf surface exhibits a rare combination of dual wettability, where hydrophilic spots on a superhydrophobic base enable the pinning of water droplets, even when positioned upside-down. This extraordinary property, known as the parahydrophobic state, is rare in floating plants, akin to the renowned Salvinia molesta, setting Azolla apart as a natural wonder. Submerged in water, Azolla leaves excel at absorbing light oils at the air-water interface, demonstrating a notable ability to extract high-density organic solvents. Moreover, Azolla's rapid growth, doubling in the area every 4-5 days, especially in flowing waters, positions it as a sustainable alternative to traditional synthetic oil-cleaning materials with long-term environmental repercussions. This scientific lead could pave the way for more environmentally friendly approaches to mitigate the negative impacts of oil spills and promote a cleaner water ecosystem.

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.

Increased invasion of submerged macrophytes makes native species more susceptible to eutrophication in freshwater ecosystems

Invasion and eutrophication are considered to pose serious threats to freshwater biodiversity and ecosystem function. However, little is known about the synergistic effects of invasion density and nutrient concentration on native submerged macrophytes. Here, we selected a common invasive species (Elodea nuttallii) and two native plants (Hydrilla verticillata and Potamogeton maackianus) to elucidate the effects of invasion density and eutrophication on native submerged plants. We found that (1) high nutrient concentrations inhibited the growth of both invasive and native species, but E. nuttallii, with a wide ecological niche, was more tolerant to eutrophication than the two native species. (2) High invasion density had a remarkable negative effect on the growth of the two native species under the medium and high nutrient concentrations. (3) Medium and high invasion densities of E. nuttallii made native macrophytes more susceptible to eutrophication. (4) The two native macrophytes had species-specific responses to medium and high invasion densities under medium and high nutrient concentrations. Specifically, a high invasion density of E. nuttallii significantly delayed the growth of H. verticillata rather than P. maackianus. Thus, it is necessary to consider the synergistic effects of invasion with eutrophication when assessing invasion in freshwater ecosystems. And our results implied that invasion with eutrophication was a powerful factor determining the results of interspecific competition among submerged macrophytes, which could change the biodiversity, community structure and functions of freshwater ecosystems.

The combination of multiple environmental stressors strongly alters microbial community assembly in aquatic ecosystems

Microorganisms play a critical role in maintaining the delicate balance of ecosystem services. However, the assembly processes that shape microbial communities are vulnerable to a range of environmental stressors, such as climate change, eutrophication, and the use of herbicides. Despite the importance of these stressors, little is known about their cumulative impacts on microbial community assembly in aquatic ecosystems. To address this knowledge gap, we established 48 mesocosm experiments that simulated shallow lake ecosystems and subjected them to warming (including continuous warming (W) and heat waves (H)), glyphosate-based herbicides (G), and nutrient loading (E). Our study revealed that in the control group, both deterministic and stochastic processes codominated the assembly of microbial communities in water, whereas in sediment, the processes were primarily stochastic. Interestingly, the effects of multiple stress factors on assembly in these two habitats were completely opposite. Specifically, stressors promoted the dominance of stochastic processes in water but increased the importance of deterministic processes in sediment. Furthermore, warming amplified the effects of herbicides but exerted an opposite and stronger influence on assembly compared to nutrients, emphasizing the complexity of these mechanisms and the significance of considering multiple stressors. The interaction of some factors significantly affected assembly (p < 0.05), with the effects of WEG being most pronounced in water. Both water and sediment exhibited homogeneous assembly of microbial communities (mean NTI >0), but the phylogenetic clustering of microbial communities in water was more closely related (NTI >2). Our research revealed the response model of microbial community assembly in aquatic ecosystems to multiple environmental stresses, such as agricultural pollution, climate change, and eutrophication, and indicated that microbial community changes in sediment may be an important predictor of lake ecosystem development. This provides scientific evidence that better environmental management can reduce impacts on aquatic ecosystems under the threat of future warming.

Use of CMIP6 scenarios as a reference to understand the responses of macrophyte germination and seedling growth to future warming and allelopathy co-stressors

The application of appropriate references such as CMIP6 climate scenarios for benchmarking studies of climate change on ecosystems can promote consistency among different climate change research. However, the use of CMIP6 climate scenarios is not common among experiments on the effects of climate change on freshwater ecosystems. Also, little is known about the impact of ecological factor such as allelopathy of alien species on macrophyte germination and seedling growth under different climate scenarios. In our study, by simulating three annual mean temperature changes at global warming levels of 1.5 °C (low warming scenario), 2 °C (medium warming scenario) and 4 °C (high warming scenario) corresponding to CMIP6 multi-model mean change at the corresponding global warming level, we conducted a mesocosm experiment to investigate their possible effects of different climate scenarios and allelopathy co-stressors on macrophyte germination and seedling growth. Our study showed that three warming scenarios all can facilitate macrophyte propagule germination and seedling growth, but the effect paths vary with CMIP6 warming scenarios and there are more influence pathways under high warming scenarios than under low and medium warming scenarios. Higher aqueous extract concentrations of Eichhornia crassipes can significantly stimulate macrophyte propagule germination and seedling growth. And the medium and high warming scenarios may exacerbate the impacts of allelopathic substances on macrophyte germination and seedling growth, and their effects depend on the combination of the two stressors. These results indicated that medium- and high-temperature scenarios may have greater ecological effects on macrophytes than low-temperature scenarios. Thus, our results highlighted that future climate studies need proper benchmarks such as CMIP6 warming scenarios, because it can provide relatively more accurate and realistic simulations, valid comparative results, comprehensive understanding and supportive coordination among researchers.

Diversity loss from multiple interacting disturbances is regime-dependent

Anthropogenic activities expose many ecosystems to multiple novel disturbances simultaneously. Despite this, how biodiversity responds to simultaneous disturbances remains unclear, with conflicting empirical results on their interactive effects. Here, we experimentally test how one disturbance (an invasive species) affects the diversity of a community over multiple levels of another disturbance regime (pulse mortality). Specifically, we invade stably coexisting bacterial communities under four different pulse frequencies, and compare their final resident diversity to uninvaded communities under the same pulse mortality regimes. Our experiment shows that the disturbances synergistically interact, such that the invader significantly reduces resident diversity at high pulse frequency, but not at low. This work therefore highlights the need to study simultaneous disturbance effects over multiple disturbance regimes as well as to carefully document unmanipulated disturbances, and may help explain the conflicting results seen in previous multiple-disturbance work.

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.

Chemodiversity of Cyanobacterial Toxins Driven by Future Scenarios of Climate Warming and Eutrophication

Climate change and eutrophication are two environmental threats that can alter the structure of freshwater ecosystems and their service functions, but we know little about how ecosystem structure and function will evolve in future scenarios of climate warming. Therefore, we created different experimental climate scenarios, including present-day conditions, a 3.0 °C increase in mean temperature, and a "heatwaves" scenario (i.e., an increase in temperature variability) to assess the effects of climate change on phytoplankton communities under simultaneous stress from eutrophication and herbicides. We show that the effects of climate warming, particularly heatwaves, are associated with elevated cyanobacterial abundances and toxin production, driven by a change from mainly nontoxic to toxic Microcystis spp. The reason for higher cyanobacterial toxin concentrations is likely an increase in abundances because under the dual pressures of climate warming and eutrophication individual Microcystis toxin-producing ability decreased. Eutrophication and higher temperatures significantly increased the biomass of Microcystis, leading to an increase in the cyanobacterial toxin concentrations. In contrast, warming alone did not produce higher cyanobacterial abundances or cyanobacterial toxin concentrations likely due to the depletion of the available nutrient pool. Similarly, the herbicide glyphosate alone did not affect abundances of any phytoplankton taxa. In the case of nutrient enrichment, cyanobacterial toxin concentrations were much higher than under warming alone due to a strong boost in biomass of potential cyanobacterial toxin producers. From a broader perspective our study shows that in a future warmer climate, nutrient loading has to be reduced if toxic cyanobacterial dominance is to be controlled.

Marsh frog response to urea fertilizer during the embryonic, larval, and metamorphosis stages: a new perspective into urea toxicity on amphibians

Amphibian populations are declining worldwide. These declines are caused by a variety of factors, one of which is the use of fertilizers in agriculture. This is especially true for tadpoles, which may develop in fertilizer-polluted agricultural water bodies. Nevertheless, there is little data on the toxicological consequences of fertilizers on amphibians. The goal of this study was to determine the acute and chronic toxicity of urea fertilizer on marsh frogs' (Pelophylax sp.) embryonic, larval, and metamorphic stages. For this purpose, in a static-renewal test, individuals were exposed to twelve nominal concentrations (0 to 15000 mg/L) of urea for 122 days to determine hatching success, survival, growth, development, and metamorphic traits, as well as histological consequences. Based on the results, at concentrations greater than 500 mg/L, no hatching occurred. Survivorship was unaffected for the first 72 hours, but it reached 0% on day 26 at concentrations greater than 150 mg/L. Survival and development rates decreased significantly in 100 and 150 mg/L treatments after a longer duration (day 86). Growth was reduced as well, but it was only significant at 150 mg/L. Metamorphosis time and percentage were significantly impacted, but not metamorphosis size. Increased urea fertilizer concentrations had significant histopathological consequences for the skin, gills, liver, kidneys, and striated muscles. Our results suggest that urea fertilizer, at concentrations commonly found in agroecosystems, may pose a serious threat to temperate anuran species inhabiting these conditions.

Impacts of extreme climatic events on trophic network complexity and multidimensional stability

Untangling the relationship between network complexity and ecological stability under climate change is an arduous challenge for theoretical and empirical ecology. Even more so, when considering extreme climatic events. Here, we studied the effects of extreme climatic events (heatwaves) on the complexity of realistic freshwater ecosystems using topological and quantitative trophic network metrics. Next, we linked changes in network complexity with the investigation of four stability components (temporal stability, resistance, resilience, and recovery) of community's functional, compositional, and energy flux stability. We found reduction in topological network complexity to be correlated with reduction of functional and compositional resistance. However, temperature-driven increase in link-weighted network complexity increased functional and energy flux recovery and resilience, but at the cost of increased compositional instability. Overall, we propose an overarching approach to elucidate the effects of climate change on multidimensional stability through the lens of network complexity, providing helpful insights for preserving ecosystems stability under climate change.

Nature-based tourism influences ecosystem functioning along waterways: Implications for conservation and management

Nature-based tourism has an influence on ecosystem functioning around watercourses, but this influence lacks scientific evidence. Additionally, strategic and operational management of streams necessitates trade-offs between the recreational activities and values of tourists and riparian zone hospitality services. This paper aims to assist environmentalists and planners by exploring the effects of tourism-based recreational activities on ecosystem functioning along the drawdown zone. The study uses multivariate statistical techniques to delineate the relevant global tourism issues for planners. Kruskal-Wallis tests (p < 0.01) were conducted using quantitative data from 284 transects within the Three Gorges Dam Reservoir in China. The results revealed higher ecosystem function indices than tourism indices. Indicators of tourism contributed both positively and negatively to ecological indicators, with the Pearson correlation coefficients ranging from minor to moderate (r = ̶ 0.24 to 0.38, p < 0.05). Principal component analysis revealed that the critical variables of ecosystem functioning and tourism activities explained 72.26 % of the overall variance. Nevertheless, hierarchical cluster analysis revealed that these indicators responded differently in the upstream, midstream, and downstream sections. Our findings suggest that policymakers should consider the different characteristics of riparian zones in future planning, as doing so will improve both national and global strategic and operational management.

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.

Spatiotemporal Variations of Human Pressure on Key Biodiversity Areas in the Qinghai–Tibet Plateau: A Comparative Analysis

A key biodiversity area (KBA) is one of the important emerging area-based conservation measures that is being implemented recently in China; however, the human pressure faced by a KBA is still unclear. This study analyzed the spatiotemporal variation of human pressure on KBAs from 1990 to 2017 and compared it with the human pressure on national natural reserves (NNRs) through a case study of the Qinghai–Tibet Plateau. In addition, changes in the trend of human pressure before and after 2010 were analyzed to examine the influence of conservation policies on human pressure. Results showed that human pressure on KBAs and NNRs gradually increased from 1990 to 2017. Furthermore, the growth rates and mean values of human pressure in KBAs were higher than those in NNRs. After the implementation of conservation policies in 2010, the growth rates of human pressure on both KBAs and NNRs have significantly slowed, and the areas with negative growth in both KBAs and NNRs have gradually expanded. In addition to providing an understanding of the changing spatiotemporal trends of human pressure on KBAs, this study can serve as a reference to formulate policies for the improvement of the effectiveness of conservation.

Food web rewiring drives long-term compositional differences and late-disturbance interactions at the community level

Multiple anthropogenic disturbances affect the structure and functioning of communities. Recent evidence highlighted that, after pulse disturbance, the functioning a community performs may be recovered fast due to functional redundancy, whereas community multivariate composition needs a longer time. Yet, the mechanisms that drive the different community recovery times have not been quantified empirically. We use quantitative food-web analysis to assess the influence of species interactions on community recovery. We found species-interactions strength to be the main mechanism driving differences between structural and functional recovery. Additionally, we show that interactions between multiple disturbances appear in the long term only when both species-interaction strength and food-web architecture change significantly.

Ecological communities are constantly exposed to multiple natural and anthropogenic disturbances. Multivariate composition (if recovered) has been found to need significantly more time to be regained after pulsed disturbance compared to univariate diversity metrics and functional endpoints. However, the mechanisms driving the different recovery times of communities to single and multiple disturbances remain unexplored. Here, we apply quantitative ecological network analyses to try to elucidate the mechanisms driving long-term community-composition dissimilarity and late-stage disturbance interactions at the community level. For this, we evaluate the effects of two pesticides, nutrient enrichment, and their interactions in outdoor mesocosms containing a complex freshwater community. We found changes in interactions strength to be strongly related to compositional changes and identified postdisturbance interaction-strength rewiring to be responsible for most of the observed compositional changes. Additionally, we found pesticide interactions to be significant in the long term only when both interaction strength and food-web architecture are reshaped by the disturbances. We suggest that quantitative network analysis has the potential to unveil ecological processes that prevent long-term community recovery.