Warming impairs trophic transfer efficiency in a long-term field experiment

Long-neglected contribution of nitrification to N2O emissions in the Yellow River

Rivers play a significant role in the global nitrous oxide (N2O) budget. However, the microbial sources and sinks of N2O in river systems are not well understood or quantified, resulting in the prolonged neglect of nitrification. This study investigated the isotopic signatures of N2O, thereby quantifying the microbial source of N2O production and the degree of N2O reduction in the Yellow River. Although denitrification has long been considered to be the dominant pathway of N2O production in rivers, our findings indicated that denitrification only accounted for 18.3% (8.2%-43.0%) of the total contribution to N2O production in the Yellow River, with 50.2%-80.2% being concurrently reduced. The denitrification contribution to N2O production (R2 = 0.44, p < 0.01) and N2O reduction degree (R2 = 0.70, p < 0.01) were positively related to the dissolved organic carbon (DOC) content. Similar to urban rivers and eutrophic lakes, denitrification was the primary process responsible for N2O production (43.0%) in certain reaches with high organic content (DOC = 5.29 mg/L). Nevertheless, the denitrification activity was generally constrained by the availability of electron donors (average DOC = 2.51 mg/L) throughout the Yellow River basin. Consequently, nitrification emerged as the primary contributor in the well-oxygenated Yellow River. Additionally, our findings further distinguished the respective contribution of ammonia-oxidizing bacteria (AOB) and archaea (AOA) to N2O emissions. Although AOB dominated the N2O production in the Yellow River, the AOA specie abundance (AOA/(AOA + AOB)) contributed up to 32.6%, which resulted in 25.6% of the total nitrifier-produced N2O, suggesting a significant occurrence of AOA in the oligotrophic Yellow River. Overall, this study provided a non-invasive approach for quantifying the microbial sources and sinks to N2O emissions, and demonstrated the substantial role of nitrification in the large oligotrophic rivers.

Experimental warming promotes phytoplankton species sorting towards cyanobacterial blooms and leads to potential changes in ecosystem functioning

A positive feedback loop where climate warming enhances eutrophication and its manifestations (e.g., cyanobacterial blooms) has been recently highlighted, but its consequences for biodiversity and ecosystem functioning are not fully understood. We conducted a highly replicated indoor experiment with a species-rich subtropical freshwater phytoplankton community. The experiment tested the effects of three constant temperature scenarios (17, 20, and 23 °C) under high-nutrient supply conditions on community composition and proxies of ecosystem functioning, namely resource use efficiency (RUE) and CO2 fluxes. After 32 days, warming reduced species richness and promoted different community trajectories leading to a dominance by green algae in the intermediate temperature and by cyanobacteria in the highest temperature treatments. Warming promoted primary production, with a 10-fold increase in the mean biomass of green algae and cyanobacteria. The maximum RUE occurred under the warmest treatment. All treatments showed net CO2 influx, but the magnitude of influx decreased with warming. We experimentally demonstrated direct effects of warming on phytoplankton species sorting, with negative effects on diversity and direct positive effects on cyanobacteria, which could lead to potential changes in ecosystem functioning. Our results suggest potential positive feedback between the phytoplankton blooms and warming, via lower net CO2 sequestration in cyanobacteria-dominated, warmer systems, and add empirical evidence to the need for decreasing the likelihood of cyanobacterial dominance.

Experimental warming promotes CO2 uptake but hinders carbon incorporation toward higher trophic levels in cyanobacteria-dominated freshwater communities

Shallow freshwaters can exchange large amounts of carbon dioxide (CO2) with the atmosphere and also store significant quantities of carbon (C) in their sediments. Current warming and eutrophication pressures might alter the role of shallow freshwater ecosystems in the C cycle. Although eutrophication has been widely associated to an increase in total phytoplankton biomass and particularly of cyanobacteria, it is still poorly understood how warming may affect ecosystem metabolism under contrasting phytoplankton community composition. We studied the effects of experimental warming on CO2 fluxes and C allocation on two contrasting natural phytoplankton communities: chlorophytes-dominated versus cyanobacteria-dominated, both with a similar zooplankton community with a potentially high grazing capacity (i.e., standardized density of large-bodied cladocerans). The microcosms were subject to two different constant temperatures (control and +4 °C, i.e., 19.5 vs 23.5 °C) and we ensured no nutrient nor light limitation. CO2 uptake increased with warming in both communities, being the strongest in the cyanobacteria-dominated communities. However, only a comparatively minor share of the fixed C translated into increased phytoplankton (Chl-a), and particularly a negligible share translated into zooplankton biomass. Most C was either dissolved in the water (DIC) or sedimented, the latter being potentially available for mineralization into DIC and CO2, or methane (CH4) when anoxic conditions prevail. Our results suggest that C uptake increases with warming particularly when cyanobacteria dominate, however, due to the low efficiency in transfer through the trophic web the final fate of the fixed C may be substantially different in the long run.

Warming and pollution interact to alter energy transfer efficiency, performance and fitness across generations in zebrafish (Danio rerio)

Energy transfer efficiency across different trophic levels, from food to new biomass, can determine population dynamics and food-web function. Here we show that the energy needed to produce a unit of new biomass increases with warming and exposure to bisphenol A (BPA), an endocrine disrupting compound. These environmental effects are at least partially transmitted across generations via DNA methylation. We raised parental (F0) and their offspring (F1) zebrafish (Danio rerio) of two genotypes (DNA methyltransferase 3a knock-out [DNMT3a-/-] and wild type [DNMT3a+/+]) at different temperatures (24 and 30 °C), with and without BPA (0 and 10 μg l-1) to test whether the effects of BPA are i) temperature specific, ii) mediated by DNA methylation, and iii) transmitted across generations even if offspring are not exposed. All experimental factors interacted to influence growth in length and mass, and metabolic rates with the result that wild-type F0 and F1 fish experienced the greatest energetic cost of growth under warm conditions in the presence of BPA. However, this response was not observed in DNMT3a-/- fish, indicating that DNA methylation is at least partly responsible for mediating these effects. Under the same conditions (warm + BPA) wild-type parents had reduced swimming performance, and reduced fecundity, and offspring embryonic survival was reduced significantly; genotype affected these responses significantly. Our results indicate that the conditions that are becoming increasingly common globally - warming and endocrine disrupting compounds from plastic pollution and production - can have detrimental effects on energy transfer efficiency and thereby potentially on food-web structure. These effects can be transmitted across generations even if offspring are not exposed to the pollutant, and are likely to have ramifications for conservation and fisheries.

Steeper size spectra with decreasing phytoplankton biomass indicate strong trophic amplification and future fish declines

Under climate change, model ensembles suggest that declines in phytoplankton biomass amplify into greater reductions at higher trophic levels, with serious implications for fisheries and carbon storage. However, the extent and mechanisms of this trophic amplification vary greatly among models, and validation is problematic. In situ size spectra offer a novel alternative, comparing biomass of small and larger organisms to quantify the net efficiency of energy transfer through natural food webs that are already challenged with multiple climate change stressors. Our global compilation of pelagic size spectrum slopes supports trophic amplification empirically, independently from model simulations. Thus, even a modest (16%) decline in phytoplankton this century would magnify into a 38% decline in supportable biomass of fish within the intensively-fished mid-latitude ocean. We also show that this amplification stems not from thermal controls on consumers, but mainly from temperature or nutrient controls that structure the phytoplankton baseline of the food web. The lack of evidence for direct thermal effects on size structure contrasts with most current thinking, based often on more acute stress experiments or shorter-timescale responses. Our synthesis of size spectra integrates these short-term dynamics, revealing the net efficiency of food webs acclimating and adapting to climatic stressors.

Assessment of the impact of dams on aquatic food webs using stable isotopes: Current progress and future challenges

Dams have disrupted natural river systems worldwide and although population and community level effects on aquatic biota have been well documented, food web responses remain poorly understood and difficult to characterize. The application of stable isotope analysis (SIA) provides a means to assess the effect of dams on food webs. Here we review the effect of dams on aquatic food webs using SIA, aiming to detect knowledge gaps in the field of dam impacts on aquatic food webs and propose a conceptual framework to help formulate hypotheses about dam impacts on food webs guided by food web theory. Dams can affect aquatic food webs via two pathways: a bottom-up pathway with altered basal food sources and their transfer to consumers through changes in flow, nutrients, temperature and sediment, and a top-down pathway with consumer species composition altered mainly through habitat fragmentation and related physiochemical changes. Taking these mechanisms into consideration, the impact of dams on food web attributes derived from SIA was evaluated. These studies generally apply mixing models to determine how dams alter the dominant carbon sources supporting food webs, use δ15N to examine how dams alter food-chain length, or use Layman metrics of isotope variability to assess niche changes for invertebrate and fish assemblages. Most studies compare the patterns of SIA metrics spatially (e.g. upstream vs reservoir vs downstream of dams; regulated vs unregulated rivers) and temporally (before vs after dam construction), without explicit hypotheses and/or links to theoretical concepts of food webs. We propose several steps to make SIA studies of dam impacts more rigorous and enhance their potential for producing novel insights. Future studies should quantify the shape and strength of the effect of dams on SIA-measured food web response, be conducted at larger temporal and spatial scales (particularly along the river longitudinal continuum and the lateral connected ecosystems (e.g., floodplains)), and consider effects of dams on food web resilience and tipping points.

Changes in sea floor productivity are crucial to understanding the impact of climate change in temperate coastal ecosystems according to a new size-based model

The multifaceted effects of climate change on physical and biogeochemical processes are rapidly altering marine ecosystems but often are considered in isolation, leaving our understanding of interactions between these drivers of ecosystem change relatively poor. This is particularly true for shallow coastal ecosystems, which are fuelled by a combination of distinct pelagic and benthic energy pathways that may respond to climate change in fundamentally distinct ways. The fish production supported by these systems is likely to be impacted by climate change differently to those of offshore and shelf ecosystems, which have relatively simpler food webs and mostly lack benthic primary production sources. We developed a novel, multispecies size spectrum model for shallow coastal reefs, specifically designed to simulate potential interactive outcomes of changing benthic and pelagic energy inputs and temperatures and calculate the relative importance of these variables for the fish community. Our model, calibrated using field data from an extensive temperate reef monitoring program, predicts that changes in resource levels will have much stronger impacts on fish biomass and yields than changes driven by physiological responses to temperature. Under increased plankton abundance, species in all fish trophic groups were predicted to increase in biomass, average size, and yields. By contrast, changes in benthic resources produced variable responses across fish trophic groups. Increased benthic resources led to increasing benthivorous and piscivorous fish biomasses, yields, and mean body sizes, but biomass decreases among herbivore and planktivore species. When resource changes were combined with warming seas, physiological responses generally decreased species' biomass and yields. Our results suggest that understanding changes in benthic production and its implications for coastal fisheries should be a priority research area. Our modified size spectrum model provides a framework for further study of benthic and pelagic energy pathways that can be easily adapted to other ecosystems.

Protection promotes energetically efficient structures in marine communities

The sustainability of marine communities is critical for supporting many biophysical processes that provide ecosystem services that promote human well-being. It is expected that anthropogenic disturbances such as climate change and human activities will tend to create less energetically-efficient ecosystems that support less biomass per unit energy flow. It is debated, however, whether this expected development should translate into bottom-heavy (with small basal species being the most abundant) or top-heavy communities (where more biomass is supported at higher trophic levels with species having larger body sizes). Here, we combine ecological theory and empirical data to demonstrate that full marine protection promotes shifts towards top-heavy energetically-efficient structures in marine communities. First, we use metabolic scaling theory to show that protected communities are expected to display stronger top-heavy structures than disturbed communities. Similarly, we show theoretically that communities with high energy transfer efficiency display stronger top-heavy structures than communities with low transfer efficiency. Next, we use empirical structures observed within fully protected marine areas compared to disturbed areas that vary in stress from thermal events and adjacent human activity. Using a nonparametric causal-inference analysis, we find a strong, positive, causal effect between full marine protection and stronger top-heavy structures. Our work corroborates ecological theory on community development and provides a quantitative framework to study the potential restorative effects of different candidate strategies on protected areas.

Direct biological fixation provides a freshwater sink for N2O

Nitrous oxide (N2O) is a potent climate gas, with its strong warming potential and ozone-depleting properties both focusing research on N2O sources. Although a sink for N2O through biological fixation has been observed in the Pacific, the regulation of N2O-fixation compared to canonical N2-fixation is unknown. Here we show that both N2O and N2 can be fixed by freshwater communities but with distinct seasonalities and temperature dependencies. N2O fixation appears less sensitive to temperature than N2 fixation, driving a strong sink for N2O in colder months. Moreover, by quantifying both N2O and N2 fixation we show that, rather than N2O being first reduced to N2 through denitrification, N2O fixation is direct and could explain the widely reported N2O sinks in natural waters. Analysis of the nitrogenase (nifH) community suggests that while only a subset is potentially capable of fixing N2O they maintain a strong, freshwater sink for N2O that could be eroded by warming.

Tipping points of nitrogen use efficiency in freshwater phytoplankton along trophic state gradient

Eutrophication and harmful algal blooms have severe effects on water quality and biodiversity in lakes and reservoirs. Ecological regime shifts of phytoplankton blooms are generally thought to be driven by the rapidly rising nutrient use efficiency of bloom-forming species over short periods, and often exhibit nonlinear dynamics. Regime shifts of trophic state, eutrophication, stratification, and clear or turbid waters are well-studied topics in aquatic ecology. However, information on the prevalence of regime shifts in relationships between trophic states and phytoplankton resource transfer efficiencies in ecosystems is still lacking. Here, we provided a first insight into regime shifts in nitrogen use efficiency of phytoplankton along the trophic state gradient. We explored the regime shifts of phytoplankton resource use efficiency and detected the tipping points by combining four temporal or spatial datasets from tropical to temperate zones in Asia and Europe. We first observed significant abrupt transitions (abruptness > 1) in phytoplankton nitrogen use efficiency along the trophic state gradient. The tipping point values were lower in subtropical/tropical waterbodies (mesotrophic states; TSIc: around 50) than those in temperate zones (eutrophic states; TSIc: 60-70). The regime shifts significantly reduced the primary production transfer efficiency via zooplankton (from 0.15 ± 0.03 to 0.03 ± 0.01; mean ± standard error) in the aquatic food web. Nitrogen-fixing filamentous cyanobacteria can drive eutrophication under mesotrophic state. Our findings imply that the time-window of opportunity for harmful algae prevention and control in lakes and reservoirs is earlier in subtropical/tropical regions.

High Concentration of Sulphate Coupled with Climate Warming Generates Ecosystem Feedback Under Sub-Oxic Conditions at Sediment-Water Interface in the Ganga River

Here, we quantified sediment phosphorus (P) release in relation to concentrations of dissolved oxygen (DO) and sulphate, and increase in temperature in a major river of India subjected to long-term human perturbations. We found a substantial increase in sediment P release, an ecosystem feedback, at higher concentrations of sulphate, more towards the lower end of DO concentrations. A 2°C warming increased sediment P release upto 25.21% and caused a drop in DO level by 16%. Our findings reconcile the observed sulphate-driven changes in sediment P release across systems, and provide first experimental evidence of warming-induced increases. Our results imply that aquatic ecosystems will undergo self-fertilizing effect as the planet warming interacts with other human perturbations. This has implications for eutrophication linkages and ecosystem functioning.

How can physiology best contribute to wildlife conservation in a warming world?

Global warming is now predicted to exceed 1.5°C by 2033 and 2°C by the end of the 21st century. This level of warming and the associated environmental variability are already increasing pressure on natural and human systems. Here we emphasize the role of physiology in the light of the latest assessment of climate warming by the Intergovernmental Panel on Climate Change. We describe how physiology can contribute to contemporary conservation programmes. We focus on thermal responses of animals, but we acknowledge that the impacts of climate change are much broader phylogenetically and environmentally. A physiological contribution would encompass environmental monitoring, coupled with measuring individual sensitivities to temperature change and upscaling these to ecosystem level. The latest version of the widely accepted Conservation Standards designed by the Conservation Measures Partnership includes several explicit climate change considerations. We argue that physiology has a unique role to play in addressing these considerations. Moreover, physiology can be incorporated by institutions and organizations that range from international bodies to national governments and to local communities, and in doing so, it brings a mechanistic approach to conservation and the management of biological resources.

What the geological past can tell us about the future of the ocean's twilight zone

Paleontological reconstructions of plankton community structure during warm periods of the Cenozoic (last 66 million years) reveal that deep-dwelling 'twilight zone' (200-1000 m) plankton were less abundant and diverse, and lived much closer to the surface, than in colder, more recent climates. We suggest that this is a consequence of temperature's role in controlling the rate that sinking organic matter is broken down and metabolized by bacteria, a process that occurs faster at warmer temperatures. In a warmer ocean, a smaller fraction of organic matter reaches the ocean interior, affecting food supply and dissolved oxygen availability at depth. Using an Earth system model that has been evaluated against paleo observations, we illustrate how anthropogenic warming may impact future carbon cycling and twilight zone ecology. Our findings suggest that significant changes are already underway, and without strong emissions mitigation, widespread ecological disruption in the twilight zone is likely by 2100, with effects spanning millennia thereafter.

Disruption of ecological networks in lakes by climate change and nutrient fluctuations

Climate change interacts with local processes to threaten biodiversity by disrupting the complex network of ecological interactions. While changes in network interactions drastically affect ecosystems, how ecological networks respond to climate change, in particular warming and nutrient supply fluctuations, is largely unknown. Here, using an equation-free modelling approach on monthly plankton community data in ten Swiss lakes, we show that the number and strength of plankton community interactions fluctuate and respond nonlinearly to water temperature and phosphorus. While lakes show system-specific responses, warming generally reduces network interactions, particularly under high phosphate levels. This network reorganization shifts trophic control of food webs, leading to consumers being controlled by resources. Small grazers and cyanobacteria emerge as sensitive indicators of changes in plankton networks. By exposing the outcomes of a complex interplay between environmental drivers, our results provide tools for studying and advancing our understanding of how climate change impacts entire ecological communities.

Warming Affects Bioconcentration and Bioaccumulation of Per- and Polyfluoroalkyl Substances by Pelagic and Benthic Organisms in a Water-Sediment System

Warming and exposure to emerging global pollutants, such as per- and polyfluoroalkyl substances (PFAS), are significant stressors in the aquatic ecosystem. However, little is known about the warming effect on the bioaccumulation of PFAS in aquatic organisms. In this study, the pelagic organisms Daphnia magna and zebrafish, and the benthic organism Chironomus plumosus were exposed to 13 PFAS in a sediment-water system with a known amount of each PFAS at different temperatures (16, 20, and 24 °C). The results showed that the steady-state body burden (C_b-ss) of PFAS in pelagic organisms increased with increasing temperatures, mainly attributed to increased water concentrations. The uptake rate constant (k_u) and elimination rate constant (k_e) in pelagic organisms increased with increasing temperature. In contrast, warming did not significantly change or even mitigate C_b-ss of PFAS in the benthic organism Chironomus plumosus, except for PFPeA and PFHpA, which was consistent with declined sediment concentrations. The mitigation could be explained by the decreased bioaccumulation factor due to a more significant percent increase in k_e than k_u, especially for long-chain PFAS. This study suggests that the warming effect on the PFAS concentration varies among different media, which should be considered for their ecological risk assessment under climate change.

Temperature and nutrients drive eco-phenotypic dynamics in a microbial food web

Anthropogenic increases in temperature and nutrient loads will likely impact food web structure and stability. Although their independent effects have been reasonably well studied, their joint effects-particularly on coupled ecological and phenotypic dynamics-remain poorly understood. Here we experimentally manipulated temperature and nutrient levels in microbial food webs and used time-series analysis to quantify the strength of reciprocal effects between ecological and phenotypic dynamics across trophic levels. We found that (1) joint-often interactive-effects of temperature and nutrients on ecological dynamics are more common at higher trophic levels, (2) temperature and nutrients interact to shift the relative strength of top-down versus bottom-up control, and (3) rapid phenotypic change mediates observed ecological responses to changes in temperature and nutrients. Our results uncover how feedback between ecological and phenotypic dynamics mediate food web responses to environmental change. This suggests important but previously unknown ways that temperature and nutrients might jointly control the rapid eco-phenotypic feedback that determine food web dynamics in a changing world.

Warming of surface water in the large and shallow lakes across the Yangtze River Basin, China, and its driver analysis

A variety of physical, chemical, and biological processes within the lakes relies on the surface water temperature while the spatial pattern of large lakes of different warming trends and their connections with climate change remain unclear. Using correlation analysis, regression tree analysis (RTA), and general linear models (GLMs), we have estimated the warming trends of 192 lakes since 2000 in the populated Yangtze River Basin, China, to identify dominant climate drivers and quantify their contributions. The results show that surface water temperature has increased substantially in the majority of the investigated lakes (179 from a total of 192 lakes) at a rate of 0.29 (- 0.12 to 0.62) °C/decade (median and 95% confidence interval). The shallower lakes (< 13.1 m in depth) usually have the faster median warming rates than the deeper lakes (i.e., 0.37 °C/decade versus 0.16 °C/decade). We find that in the shallow lakes, rising air temperatures and declining wind speeds can explain the majority of variation in surface water temperature (i.e., 31.4‒80.3% and 13.0‒21.0%, respectively). In contrast, in deeper lakes, change of air temperatures plays a dominant role in water warming (75.4‒91.2%). This study has emphasized the importance of declining wind speed in water warming in large and shallow lakes and illustrated a difference of dominant climatic drivers in water warming between the shallow and deep lakes.

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.

Highly conserved thermal performance strategies may limit adaptive potential in corals

Increasing seawater temperatures are expected to have profound consequences for reef-building corals' physiology. Understanding how demography changes in response to chronic exposure to warming will help forecast how coral communities will respond to climate change. Here, we measure growth rates of coral fragments of four common species, while exposing them to temperatures ranging from 19°C to 31°C for one month to calibrate their thermal-performance curves (TPCs). Our results show that, while there are contrasting differences between species, the shape of the TPCs was remarkably consistent among individuals of the same species. The low variation in thermal sensitivity within species may imply a reduced capacity for rapid adaptive responses to future changes in thermal regimes. Additionally, interspecific differences in thermal responses show a negative relationship between maximum growth and thermal optima, contradicting expectations derived from the classic 'warmer-is-better' hypothesis. Among species, there was a trade-off between current and future growth, whereby most species perform well under current thermal regimes but are susceptible to future increases in temperature. Increases in water temperature with climate change are likely to reduce growth rates, further hampering future coral reef recovery rates and potentially altering community composition.

Rapid eco-phenotypic feedback and the temperature response of biomass dynamics

Biomass dynamics capture information on population dynamics and ecosystem-level processes (e.g., changes in production over time). Understanding how rising temperatures associated with global climate change influence biomass dynamics is thus a pressing issue in ecology. The total biomass of a species depends on its density and its average mass. Consequently, disentangling how biomass dynamics responds to increasingly warm and variable temperatures ultimately depends on understanding how temperature influences both density and mass dynamics. Here, we address this issue by keeping track of experimental microbial populations growing to carrying capacity for 15 days at two different temperatures, and in the presence and absence of temperature variability. We develop a simple mathematical expression to partition the contribution of changes in density and mass to changes in biomass and assess how temperature responses in either one influence biomass shifts. Moreover, we use time-series analysis (Convergent Cross Mapping) to address how temperature and temperature variability influence reciprocal effects of density on mass and vice versa. We show that temperature influences biomass through its effects on density and mass dynamics, which have opposite effects on biomass and can offset each other. We also show that temperature variability influences biomass, but that effect is independent of any effects on density or mass dynamics. Last, we show that reciprocal effects of density and mass shift significantly across temperature regimes, suggesting that rapid and environment-dependent eco-phenotypic dynamics underlie biomass responses. Overall, our results connect temperature effects on population and phenotypic dynamics to explain how biomass responds to temperature regimes, thus shedding light on processes at play in cosmopolitan and abundant microbes as the world experiences increasingly warm and variable temperatures.

Resource limitation determines realized thermal performance of consumers in trophodynamic models

Recent work has demonstrated that changes in resource availability can alter a consumer's thermal performance curve (TPC). When resources decline, the optimal temperature and breadth of thermal performance also decline, leading to a greater risk of warming than predicted by static TPCs. We investigate the effect of temperature on coupled consumer-resource dynamics, focusing on the potential for changes in the consumer TPC to alter extinction risk. Coupling consumer and resource dynamics generally reduces the potential for resource decline to exacerbate the effects of warming via changes to the TPC due to a reduction in top-down control when consumers near the limits of their thermal performance curve. However, if resources are more sensitive to warming, consumer TPCs can be reshaped by declining resources, leading to increased extinction risk. Our work elucidates the role of top-down and bottom-up regulation in determining the extent to which changes in resource density alter consumer TPCs.

Interactive effects of anthropogenic environmental drivers on endocrine responses in wildlife

Anthropogenic activity has created unique environmental drivers, which may interact to produce unexpected effects. My aim was to conduct a systematic review of the interactive effects of anthropogenic drivers on endocrine responses in non-human animals. The interaction between temperature and light can disrupt reproduction and growth by impacting gonadotropins, thyroid hormones, melatonin, and growth hormone. Temperature and endocrine disrupting compounds (EDCs) interact to modify reproduction with differential effects across generations. The combined effects of light and EDCs can be anxiogenic, so that light-at-night could increase anxiety in wildlife. Light and noise increase glucocorticoid release by themselves, and together can modify interactions between individuals and their environment. The literature detailing interactions between drivers is relatively sparse and there is a need to extend research to a broader range of taxa and interactions. I suggest that incorporating endocrine responses into Adverse Outcome Pathways would be beneficial to improve predictions of environmental effects.

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.

Water depth and land-use intensity indirectly determine phytoplankton functional diversity and further regulate resource use efficiency at a multi-lake scale

Biodiversity-ecosystem functioning relationships under multiple pressures have recently been the subject of broad studies. For the key primary producer in aquatic ecosystems, phytoplankton, several studies have focused on trait-based functional diversity (FD) and the related functioning (e.g., resource use efficiency, RUE), and their linkages. However, investigations of the effects of environmental factors at different levels (e.g., land use, lake morphometry, climate and nutrients) on FD and RUE are sparse. We developed a data-driven-model framework to simultaneously elucidate the effects of multiple drivers on FD (functional diversity based on dendrograms, FDc and functional richness, FRic) and RUE (of nitrogen and phosphorus) of phytoplankton based on data from 68 Yunnan-Guizhou Plateau lakes, Southwest China. We found that the concentration of total phosphorus, which is mainly affected by land-use intensity and influenced by water depth, was the primary (positive) driver of changes in both FDc and FRic, while RUE was mainly explained by phytoplankton FD (i.e., FRic). These results indicate that water depth and land-use intensity influence indirectly phytoplankton FD and further regulate RUE. Moreover, nonlinear correlations of RUE with FRic were found, which may be caused by interspecific competition and niche differentiation of the phytoplankton community related to nutrient levels. Our finding may help managers to set trade-off targets between FD and RUE in lake ecosystems except for extremely polluted ones, in which the thresholds derived from the Bayesian network, of total phosphorus, total nitrogen and land-use intensity were approximately 0.04 mg/L, 0.50 mg/L and 244 (unitless), respectively. The probability of meeting the RUE objectives was lower in shallow lakes than in deep lakes, but for FRic the opposite was observed.

Variations in cost of transport and their ecological consequences: a review

Movement is essential in the ecology of most animals, and it typically consumes a large proportion of individual energy budgets. Environmental conditions modulate the energetic cost of movement (cost of transport, COT), and there are pronounced differences in COT between individuals within species and across species. Differences in morphology affect COT, but the physiological mechanisms underlying variation in COT remain unresolved. Candidates include mitochondrial efficiency and the efficiency of muscle contraction-relaxation dynamics. Animals can offset increased COT behaviourally by adjusting movement rate and habitat selection. Here, we review the theory underlying COT and the impact of environmental changes on COT. Increasing temperatures, in particular, increase COT and its variability between individuals. Thermal acclimation and exercise can affect COT, but this is not consistent across taxa. Anthropogenic pollutants can increase COT, although few chemical pollutants have been investigated. Ecologically, COT may modify the allocation of energy to different fitness-related functions, and thereby influence fitness of individuals, and the dynamics of animal groups and communities. Future research should consider the effects of multiple stressors on COT, including a broader range of pollutants, the underlying mechanisms of COT and experimental quantifications of potential COT-induced allocation trade-offs.

Trophic Transfer Efficiency in Lakes

Trophic transfer efficiency (TTE) is usually calculated as the ratio of production rates between two consecutive trophic levels. Although seemingly simple, TTE estimates from lakes are rare. In our review, we explore the processes and structures that must be understood for a proper lake TTE estimate. We briefly discuss measurements of production rates and trophic positions and mention how ecological efficiencies, nutrients (N, P) and other compounds (fatty acids) affect energy transfer between trophic levels and hence TTE. Furthermore, we elucidate how TTE estimates are linked with size-based approaches according to the Metabolic Theory of Ecology, and how food-web models can be applied to study TTE in lakes. Subsequently, we explore temporal and spatial heterogeneity of production and TTE in lakes, with a particular focus on the links between benthic and pelagic habitats and between the lake and the terrestrial environment. We provide an overview of TTE estimates from lakes found in the published literature. Finally, we present two alternative approaches to estimating TTE. First, TTE can be seen as a mechanistic quantity informing about the energy and matter flow between producer and consumer groups. This approach is informative with respect to food-web structure, but requires enormous amounts of data. The greatest uncertainty comes from the proper consideration of basal production to estimate TTE of omnivorous organisms. An alternative approach is estimating food-chain and food-web efficiencies, by comparing the heterotrophic production of single consumer levels or the total sum of all heterotrophic production including that of heterotrophic bacteria to the total sum of primary production. We close the review by pointing to a few research questions that would benefit from more frequent and standardized estimates of TTE in lakes.

System analysis of the fast global coronavirus disease 2019 (COVID-19) spread. Can we avoid future pandemics under global climate change?

The recent fast global spread of COVID-19 caused by a severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) questions why and how the disease managed to be so effective against existing health protection measures. These measures, developed by many countries over centuries and strengthened over the last decades, proved to be ineffective against COVID-19. The sharp increase in human longevity and current transport systems in economically developing countries with the background of persisting cultural frameworks and stable local pools of high bacterial and viral mutations generated the wide gap between the established health protection systems and the new emerging diseases. SARS-CoV-2 targets human populations over the world with long incubation periods, often without symptoms, and serious outcomes. Hence, novel strategies are necessary to meet the demands of developing economic and social environments. Moreover, the ongoing climate change adds extra challenges while altering the existing system of interactions in biological populations and in human society. Climate change may lead to new sources of viral and microbial mutations, new ways of zoonotic disease transmission and to huge social and economic transformations in many countries. The present short Opinion applies a system approach linking biomedical, climate change, social and economic aspects and, accordingly, discusses the measures and more efficient means to avoid future pandemics.

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.

Metabolic plasticity can amplify ecosystem responses to global warming

Organisms have the capacity to alter their physiological response to warming through acclimation or adaptation, but the consequence of this metabolic plasticity for energy flow through food webs is currently unknown, and a generalisable framework does not exist for modelling its ecosystem-level effects. Here, using temperature-controlled experiments on stream invertebrates from a natural thermal gradient, we show that the ability of organisms to raise their metabolic rate following chronic exposure to warming decreases with increasing body size. Chronic exposure to higher temperatures also increases the acute thermal sensitivity of whole-organismal metabolic rate, independent of body size. A mathematical model parameterised with these findings shows that metabolic plasticity could account for 60% higher ecosystem energy flux with just +2 °C of warming than a traditional model based on ecological metabolic theory. This could explain why long-term warming amplifies ecosystem respiration rates through time in recent mesocosm experiments, and highlights the need to embed metabolic plasticity in predictive models of global warming impacts on ecosystems.

Organisms can alter their physiological response to warming. Here, the authors show that the ability to raise metabolic rate following exposure to warming is inverse to body size and provide a mathematical model which estimates that metabolic plasticity could amplify energy flux through ecosystems in response to warming.

Endocrine disruption from plastic pollution and warming interact to increase the energetic cost of growth in a fish

Energetic cost of growth determines how much food-derived energy is needed to produce a given amount of new biomass and thereby influences energy transduction between trophic levels. Growth and development are regulated by hormones and are therefore sensitive to changes in temperature and environmental endocrine disruption. Here, we show that the endocrine disruptor bisphenol A (BPA) at an environmentally relevant concentration (10 µgl^-1) decreased fish (Danio rerio) size at 30°C water temperature. Under the same conditions, it significantly increased metabolic rates and the energetic cost of growth across development. By contrast, BPA decreased the cost of growth at cooler temperatures (24°C). BPA-mediated changes in cost of growth were not associated with mitochondrial efficiency (P/O ratios (i.e. adenosine diphosphate (ADP) used/oxygen consumed) and respiratory control ratios) although BPA did increase mitochondrial proton leak. In females, BPA decreased age at maturity at 24°C but increased it at 30°C, and it decreased the gonadosomatic index suggesting reduced investment into reproduction. Our data reveal a potentially serious emerging problem: increasing water temperatures resulting from climate warming together with endocrine disruption from plastic pollution can impact animal growth efficiency, and hence the dynamics and resilience of animal populations and the services these provide.

A climate-resilient marine conservation network for Canada

Climate change and biodiversity loss are twin crises that are driving global marine conservation efforts. However, if unaccounted for, climate change can undermine the efficacy of such efforts. Despite this, integration of climate change adaptation and resilience into spatial marine conservation and management has been limited in Canada and elsewhere. With climate change impacts becoming increasingly severe, now is the time to anticipate and reduce impacts wherever possible. We provide five recommendations for an inclusive, proactive, climate-ready approach for Canada’s growing marine conservation network: (1) integrating climate-resilience as a universal objective of the Canadian Marine Conservation Network, creating and implementing (2) national transdisciplinary working groups with representation from all knowledge holders and (3) necessary tools that integrate climate change into conservation design, (4) defining operational and climate-relevant monitoring and management objectives, and (5) strengthening communication and increasing knowledge exchange around the roles and benefits of protected areas within government and towards the public. Canada’s extensive marine and coastal areas reflect national and international responsibility to engage on this issue. Canada is well positioned to assume a leading role in climate change adaptation for marine conservation and help accelerate progress towards international commitments around mitigating ongoing biodiversity loss and climate change.

Food web interactions in a human dominated Mediterranean coastal ecosystem

Mediterranean coastal ecosystems provide various valuable ecosystem goods and services; however, they are vulnerable to ecological degradation due to a dramatic increase in resource use and environmental stress. Disentangling the effects of multiple human interventions on coastal ecosystems requires whole description of food web interactions using quantitative tools. A mass balance Ecopath model has been developed here for Saronikos Gulf, a naturally oligotrophic Mediterranean coastal ecosystem with a long history of human interventions. Our main focus was to describe the structure and functioning of the ecosystem, investigate the trophic interplay among the various compartments of the food web under the impact of mixed multi-gear fisheries, and to quantify resilience related emergent ecosystem properties. To this end, we reviewed a large amount of local and regional biological information which was integrated in 40 functional groups covering all trophic levels, while fishing activities were described with 7 fleets. The model shared characteristics of both productive (e.g., high amount of flows) and oligotrophic systems (e.g., low biomass accumulation) and presented typical features of Mediterranean ecosystem functioning, such as the importance of detritus as an energy source, strong benthic-pelagic coupling and the dominance of the pelagic compartment in terms of total production and consumption. Trophic forcing in the ecosystem of Saronikos Gulf was complex with both top-down and bottom-up drivers being important. Zooplankton was the central nexus between basal resources and higher trophic levels, while top predators such as hake, squids and anglerfish were identified as keystone species presenting a significant overall effect on the food web via direct and indirect trophic interactions. Ecological indicators depicted a moderately complex food-web of a large and immature ecosystem with its strengths in reserve being affected by environmental degradation. Additionally, exploitation indices classified fishing activities in Saronikos Gulf as unsustainable, affecting several target groups, including high trophic level species. However, the morphological and bathymetric complexity of Saronikos Gulf seems to function as a natural ecological reserve for the ecosystem by providing nursery grounds to various species (e.g., hake, small pelagic fishes) and supporting important fish stocks for local fisheries.

Bottom-Heavy Trophic Pyramids Impair Methylmercury Biomagnification in the Marine Plankton Ecosystems

Methylmercury (CH₃Hg⁺, MMHg) in the phytoplankton and zooplankton, which form the bottom of marine food webs, is a good predictor of MMHg in top predators, including humans. Therefore, evaluating the potential exposure of MMHg to higher trophic levels (TLs) requires a better understanding of relationships between MMHg biomagnification and plankton dynamics. In this study, a coupled ecological/physical model with 366 plankton types of different sizes, biogeochemical functions, and temperature tolerance is used to simulate the relationships between MMHg biomagnification and the ecosystem structure. The study shows that the MMHg biomagnification becomes more significant with increasing TLs. Trophic magnification factors (TMFs) in the lowest two TLs show the opposite spatial pattern to TMFs in higher TLs. The low TMFs are usually associated with a short food-chain length. The less bottom-heavy trophic pyramids in the oligotrophic oceans enhance the MMHg trophic transfer. The global average TMF is increased from 2.3 to 2.8 in the warmer future with a medium climate sensitivity of 2.5 °C. Our study suggests that if there are no mitigation measures for Hg emission, MMHg in the high-trophic-level plankton is increased more dramatically in the warming future, indicating greater MMHg exposure for top predators such as humans.