Coupling of green and brown food webs and ecosystem stability

Ecological impacts of treated effluent on multitrophic biodiversity and their interactions

The reuse of water, particularly treated effluent from wastewater treatment plants (WWTPs), is a crucial and sustainable strategy for mitigating water scarcity, especially in megacities with high water demand and limited resources. However, the ecological risks associated with effluent discharge into receiving waterbodies have gained significant global attention. Understanding the dynamic effects of WWTP effluent on multi-trophic groups and their interactions is essential for assessing ecological impacts in aquatic ecosystems and informing management strategies. In this study, we examined five taxonomic groups representing different trophic levels of the freshwater food web - bacteria (decomposers), algae (primary producers), zooplankton (primary consumers), and benthic macroinvertebrates and fish (predators) - across two rivers to elucidate ecological responses to WWTP effluent from a multi-trophic perspective. Our results revealed significant but variable biological responses among these groups, depending on river conditions and trophic level. In the nutrient-rich river, primary consumers (zooplankton) were most affected, whereas in the nutrient-poor river, primary producers (algae) exhibited the strongest responses primarily derived from environmental disturbances. Notably, interactions between environmental variables and taxa were highly diverse, with trophic dynamics influenced by both bottom-up and top-down processes in the nutrient-rich river, whereas bottom-up effects dominated in the nutrient-poor river. Furthermore, niche overlap in algae-zooplankton networks was higher in the nutrient-rich river than in the nutrient-poor river. This study underscores the importance of integrating multi-trophic biodiversity profiling and trophic interaction analyses to comprehensively assess the ecological effects of WWTP effluent in receiving aquatic ecosystems with contrasting environmental contexts. Our findings highlight the importance of conservation and sustainable management practices, especially in urban aquatic ecosystems located in (semi-)arid regions that experience prolonged periods of low precipitation.

Plant neighbors differentially alter a focal species' biotic interactions through changes to resource allocation

Plant resource allocation strategies are thought to be largely a consequence of changing abiotic conditions and evolutionary history. However, biotic interactions also influence how a plant allocates resources. As a result, plants mediate indirect interactions between organisms above- and belowground through resource allocation. Neighboring plants can influence plant fitness directly through competition for resources, and indirectly by altering associated community interactions (associational effects), such as pollination, herbivory, and a suite of belowground interactions. Given the importance of community interactions for plant success, and the known ability for plant neighbors to change these interactions, the goal of this "pandemic project" was to understand how heterospecific plant neighbors alter plant resource allocation, whether this occurred through above- or belowground mechanisms, and whether this in turn alters biotic interactions and the relationship between a focal plant and its herbivore and soil community interactions. To do so, we established a common garden experiment, manipulating plant neighbor identity and the extent of interaction among neighbors (aboveground only, vs. above- and belowground interactions, using customized pot types), and measured changes to a focal plant and its biotic interactions over two growing seasons. We found evidence of both neighbor effects and pot type, showing that neighbor interactions affect a focal plant through both above- and belowground processes, and how the focal plant is affected depends on neighbor identity. Though neighbors did not directly alter herbivory or most soil microbial interactions, they did alter the relationship between belowground microbial communities and a plant response trait (specific leaf area). Plant resource allocation responses were reduced with time, showing the importance of extending experiments beyond a single growing season, and are an important consideration when making predictions about plant responses to changing conditions. This study contributes to a growing body of work showing how community contexts affect the above- and belowground interactions of a plant through plant resource allocation strategies.

Putting the Asymmetric Response Concept to the test: Modeling multiple stressor exposure and release in a stream food web

Communities in stream ecosystems often respond asymmetrically to increase and release of stressors, as indicated by slow and incomplete recovery. The Asymmetric Response Concept (ARC) posits that this is due to a shift in the relative importance of three mechanisms: tolerance, dispersal, and biotic interactions. In complex natural communities, these mechanisms may produce alternative outcomes through poorly understood indirect effects. To understand how the three mechanisms respond to different temporal stressor scenarios, we studied multiple scenarios using a stream food web model. We asked the following questions: Do groups of species decline as expected on the basis of individual tolerance rankings derived from laboratory experiments when they are embedded in a complex dynamic food web? Does the response of ecosystem function match that of communities? To address these questions, we aggregated data on individual tolerances at the level of functional groups and studied how single and multiple stressors affect food web dynamics and nutrient cycling. Multiple stressor scenarios involved different intensities of salt and temperature increase. Functional groups exhibited a different relative tolerance ranking between the laboratory and dynamic food web contexts. Salt as a single stressor had only minor and transient effects at low level but led to the loss of one or more functional groups at high level. In contrast, high temperature, alone or in combination with salt, caused the loss of functional groups at all tested levels. Patterns often differed between the response of communities and ecosystem function. We discuss our findings with respect to the ARC.

Predator mass mortality events restructure food webs through trophic decoupling

Predators have a key role in structuring ecosystems1-4. However, predator loss is accelerating globally4-6, and predator mass-mortality events7 (MMEs)-rapid large-scale die-offs-are now emblematic of the Anthropocene epoch6. Owing to their rare and unpredictable nature7, we lack an understanding of how MMEs immediately impact ecosystems. Past predator-removal studies2,3 may be insufficient to understand the ecological consequences of MMEs because, in nature, dead predators decompose in situ and generate a resource pulse8, which could alter ensuing ecosystem dynamics by temporarily enhancing productivity. Here we experimentally induce MMEs in tritrophic, freshwater lake food webs and report ecological dynamics that are distinct from predator losses2,3 or resource pulses9 alone, but that can be predicted from theory8. MMEs led to the proliferation of diverse consumer and producer communities resulting from weakened top-down predator control1-3 and stronger bottom-up effects through predator decomposition8. In contrast to predator removals alone, enhanced primary production after MMEs dampened the consumer community response. As a consequence, MMEs generated biomass dynamics that were most similar to those of undisturbed systems, indicating that they may be cryptic disturbances in nature. These biomass dynamics led to trophic decoupling, whereby the indirect beneficial effects of predators on primary producers are lost and later materialize as direct bottom-up effects that stimulate primary production amid intensified herbivory. These results reveal ecological signatures of MMEs and demonstrate the feasibility of forecasting novel ecological dynamics arising with intensifying global change.

Seasonality in land-ocean connectivity and local processes control sediment bacterial community structure and function in a High Arctic tidal flat

Climate change is altering patterns of precipitation, cryosphere thaw, and land-ocean influxes, affecting understudied Arctic estuarine tidal flats. These transitional zones between terrestrial and marine systems are hotspots for biogeochemical cycling, often driven by microbial processes. We investigated surface sediment bacterial community composition and function from May to September along a river-intertidal-subtidal-fjord gradient. We paired metabarcoding of in situ communities with in vitro carbon-source utilization assays. Bacterial communities differed in space and time, alongside varying environmental conditions driven by local seasonal processes and riverine inputs, with salinity emerging as the dominant structuring factor. Terrestrial and riverine taxa were found throughout the system, likely transported with runoff. In vitro assays revealed sediment bacteria utilized a broader range of organic matter substrates when incubated in fresh and brackish water compared to marine water. These results highlight the importance of salinity for ecosystem processes in these dynamic tidal flats, with the highest potential for utilization of terrestrially derived organic matter likely limited to tidal flat areas (and times) where sediments are permeated by freshwater. Our results demonstrate that intertidal flats must be included in future studies on impacts of increased riverine discharge and transport of terrestrial organic matter on coastal carbon cycling in a warming Arctic.

metanetwork: A R package dedicated to handling and representing trophic metanetworks

Trophic networks describe interactions between species at a given location and time. Due to environmental changes, anthropogenic perturbations or sampling effects, trophic networks may vary in space and time. The collection of network time series or networks in different sites thus constitutes a metanetwork. We present here the R package metanetwork, which will ease the representation, the exploration and analysis of trophic metanetwork data sets that are increasingly available. Our main methodological advance consists in suitable layout algorithm for trophic networks, which is based on trophic levels and dimension reduction in a graph diffusion kernel. In particular, it highlights relevant features of trophic networks (trophic levels, energetic channels). In addition, we developed tools to handle, compare visually and quantitatively and aggregate those networks. Static and dynamic visualisation functions have been developed to represent large networks. We apply our package workflow to several trophic network data sets.

Extreme drought exacerbates plant nitrogen‑phosphorus imbalance in nitrogen enriched grassland

The nitrogen‑phosphorus (N-P) imbalance induced by N enrichment has received increasing concerns, because N:P ratios play a critical role in driving many fundamental ecological processes. Given the simultaneous occurrence of different global change drivers, it is important to understand whether and how would such N-induced N-P imbalance would be mediated by other global change factors. We examined the interactive effects of N addition (10 g N m^-2 yr^-1) and extreme drought (-66 % rainfall during the growing season) on species- and community-level N:P ratios in both green and senesced leaves in a temperate grassland of northern China. Extreme drought did not alter soil available N:P ratio under ambient N conditions, but increased that under N enriched conditions. Further, extreme drought did not alter the community-level N:P in both green and senesced leaves under ambient N conditions but significantly enhanced that under N enriched conditions. The drought-induced species turnover made a significant positive contribution to the changes in the community-level N:P ratio under N enriched conditions, but not under ambient N conditions. Our results suggest that the N-induced ecosystem N-P imbalance would be exacerbated by extreme drought event, the frequency of which is predicted to increase across global drylands. Such N-P imbalance would have consequences on litter decomposition, nutrient cycling, and the structures of above- and below-ground food webs. Our findings highlighted the complexity in predicting ecosystem N-P imbalance given the interactions between different global change drivers.

The coordination of green-brown food webs and their disruption by anthropogenic nutrient inputs

Aim

Our goal was to quantify nitrogen flows and stocks in green-brown food webs in different ecosystems, how they differ across ecosystems and how they respond to nutrient enrichment.

Location

Global.

Time period

Contemporary.

Major taxa studied

Plants, phytoplankton, macroalgae, invertebrates, vertebrates and zooplankton.

Methods

Data from >500 studies were combined to estimate nitrogen stocks and fluxes in green-brown food webs in forests, grasslands, brackish environments, seagrass meadows, lakes and oceans. We compared the stocks, fluxes and metabolic rates of different functional groups within each food web. We also used these estimates to build a dynamical model to test the response of the ecosystems to nutrient enrichment.

Results

We found surprising symmetries between the green and brown channels across ecosystems, in their stocks, fluxes and consumption coefficients and mortality rates. We also found that nitrogen enrichment, either organic or inorganic, can disrupt this balance between the green and brown channels.

Main conclusions

Linking green and brown food webs reveals a previously hidden symmetry between herbivory and detritivory, which appears to be a widespread property of natural ecosystems but can be disrupted by anthropogenic nitrogen additions.

Dam-induced flow velocity decrease leads to the transition from heterotrophic to autotrophic system through modifying microbial food web dynamics

The impoundment of reservoirs changes the river from a riverine heterotrophic system to a lacustrine autotrophic system, which could be attributed to the shift of pelagic microbial food webs in response to the dam-induced disturbances. However, little is known about what is the key factor controlling this variation and how different underlying interactions affect the food web dynamics. This study investigated the effects of flow velocity and nutrient supply on microbial plankton using a microcosm experiment. The results showed that flow velocity decrease was the main factor inducing the detritus-based food web transformed to the autotroph-based food web, with heterotrophic bacteria and protozoan dominated at high velocity, whereas phytoplankton and metazoan were prevalent in the lentic environment. The lentic-acclimated genera, such as Chlorella sp., Mallomonas sp. and Microcystis sp., showed hysteresis after the velocity recovery, suggesting the potential of algae bloom in reservoirs and even downstream of dams. We further conducted a flow-velocity manipulating experiment and constructed a multi-trophic nitrogen cycling model to provide a mechanistic explanation for the microbial food web dynamics and the nitrogen transformation performances. As indicated in model prediction and sensitivity analysis, the abiotic and biotic variations were directly or indirectly controlled by nutrient utilization and predator-prey interactions. Quantification of these bottom-up and top-down forces revealed the buffer role of predators in mitigating the positive effects of nutrient availability on autotrophs at low velocity and on heterotrophs at high velocity, respectively. This study highlights the importance of mastering the whole information of different trophic levels, in order to better capture the complex microbial food web interactions and the consequent biogeochemical processes in river-reservoir systems.

Protist Predation Influences the Temperature Response of Bacterial Communities

Temperature strongly influences microbial community structure and function, in turn contributing to global carbon cycling that can fuel further warming. Recent studies suggest that biotic interactions among microbes may play an important role in determining the temperature responses of these communities. However, how predation regulates these microbiomes under future climates is still poorly understood. Here, we assess whether predation by a key global bacterial consumer-protists-influences the temperature response of the community structure and function of a freshwater microbiome. To do so, we exposed microbial communities to two cosmopolitan protist species-Tetrahymena thermophila and Colpidium sp.-at two different temperatures, in a month-long microcosm experiment. While microbial biomass and respiration increased with temperature due to community shifts, these responses changed over time and in the presence of protists. Protists influenced microbial biomass and respiration rate through direct and indirect effects on bacterial community structure, and predator presence actually reduced microbial respiration at elevated temperature. Indicator species analyses showed that these predator effects were mostly determined by phylum-specific bacterial responses to protist density and cell size. Our study supports previous findings that temperature is an important driver of microbial communities but also demonstrates that the presence of a large predator can mediate these responses to warming.

The role of cladocerans in green and brown food web coupling

ABSTRACT Cladocerans, an important zooplankton community, are consumers from the base of the food web of aquatic environments. We investigated the contribution of producers (phytoplankton and periphytic biofilm) and particulate organic carbon (POC). Collections were carried out in lakes of the Upper Paraná River Floodplain, the last stretch free of dams in the second-largest South American basin. Isotope ratios (δ13C and δ15N) were measured, besides the contributions of probable food sources to the cladocerans biomass. The phytoplankton constituted the source of carbon for cladocerans, followed by POC. Thus this work, in addition to emphasizing the importance of cladocerans in nutrient cycling, highlighted the need for the conservation of environments surrounding the lakes as they are the sources of organic matter for aquatic communities. Besides, the analyzed zooplanktonic organisms demonstrated their role in the interconnection between the green and brown food webs, which have been studied separately for a long time.

Metatranscriptomic outlook on green and brown food webs in acid mine drainage

Acid mine drainages (AMDs), metal-rich acidic effluents generated by mining activities, are colonized by prokaryotic and eukaryotic microorganisms widely distributed among different phyla. We compared metatranscriptomic data from two sampling stations in the Carnoulès AMD and from a third station in the nearby Amous River, focussing on processes involved in primary production and litter decomposition. A synergistic relationship between the green and brown food webs was favoured in the AMD sediments by the low carbon content and the availability of mineral nutrients: primary production of organic matter would benefit C-limited decomposers whose activity of organic matter mineralization would in turn profit primary producers. This balance could be locally disturbed by heterogeneous factors such as an input of plant debris from the riparian vegetation, strongly boosting the growth of Tremellales which would then outcompete primary producers. In the unpolluted Amous River on the contrary, the competition for limited mineral nutrients was dominated by the green food web, fish and bacterivorous protists having a positive effect on phytoplankton. These results suggest that in addition to direct effects of low pH and metal contamination, trophic conditions like carbon or mineral nutrient limitations also have a strong impact on assembly and activities of AMDs' microbial communities.

Energy pathways modulate the resilience of stream invertebrate communities to drought

While climate change is altering ecosystems on a global scale, not all ecosystems are responding in the same way. The resilience of ecological communities may depend on whether food webs are producer- or detritus-based (i.e. 'green' or 'brown' food webs, respectively), or both (i.e. 'multi-channel' food web). Food web theory suggests that the presence of multiple energy pathways can enhance community stability and resilience and may modulate the responses of ecological communities to disturbances such as climate change. Despite important advances in food web theory, few studies have empirically investigated the resilience of ecological communities to climate change stressors in ecosystems with different primary energy channels. We conducted a factorial experiment using outdoor stream mesocosms to investigate the independent and interactive effects of warming and drought on invertebrate communities in food webs with different energy channel configurations. Warming had little effect on invertebrates, but stream drying negatively impacted total invertebrate abundance, biomass, richness and diversity. Although resistance to drying did not differ among energy channel treatments, recovery and overall resilience were higher in green mesocosms than in mixed and brown mesocosms. Resilience to drying also varied widely among taxa, with larger predatory taxa exhibiting lower resilience. Our results suggest that the effects of drought on stream communities may vary regionally and depend on whether food webs are fuelled by autochthonous or allochthonous basal resources. Communities inhabiting streams with large amounts of organic matter and more complex substrates that provide refugia may be more resilient to the loss of surface water than communities inhabiting streams with simpler, more homogeneous substrates.