Isotopic niche reflects stress-induced variability in physiological status

Resource partitioning of a Mexican clam in species-poor Baltic Sea sediments indicates the existence of a vacant trophic niche

Invasive species are often generalists that can take advantage of formerly unexploited resources. The existence of such vacant niches is more likely in species-poor systems like the Baltic Sea. The suspension feeding wedge clam, Rangia cuneata, native to estuarine environments in the Gulf of Mexico, was sighted for the first time in the southeastern Baltic in 2010 and a few years later in the northern Baltic along the Swedish coast. To explore possible competition for food resources between R. cuneata and the three native clams inhabiting Baltic shallow soft bottoms, stable isotope and fatty acid analyses were conducted. There was no overlap between R. cuneata and any of the native species in either stable isotope or fatty acid niches. This suggests efficient partitioning of resources; multivariate analyses indicate that separation was driven mainly by δ13C and by fatty acids reflecting diatoms and cyanobacteria, respectively (e.g. 16:1ω7 and 18:3ω3). R. cuneata reflected seasonal variation in phytoplankton more than other clams reflecting higher trophic plasticity. In conclusion, the addition of R. cuneata to the Baltic shallow soft bottoms suggests the existence of a vacant trophic niche in these sediment habitats, however the long-term effects on other species and nutrient cycling requires further studies focusing on the population dynamics of R. cuneata and its impact on the Baltic Sea ecosystem.

Why aquatic scientists should use sulfur stable isotope ratios (ẟ34S) more often

Over the last few decades, measurements of light stable isotope ratios have been increasingly used to answer questions across physiology, biology, ecology, and archaeology. The vast majority analyse carbon (δ13C) and nitrogen (δ15N) stable isotopes as the 'default' isotopes, omitting sulfur (δ34S) due to time, cost, or perceived lack of benefits and instrumentation capabilities. Using just carbon and nitrogen isotopic ratios can produce results that are inconclusive, uncertain, or in the worst cases, even misleading, especially for scientists that are new to the use and interpretation of stable isotope data. Using sulfur isotope values more regularly has the potential to mitigate these issues, especially given recent advancements that have lowered measurement barriers. Here we provide a review documenting case studies with real-world data, re-analysing different biological topics (i.e. niche, physiology, diet, movement and bioarchaeology) with and without sulfur isotopes to highlight the various strengths of this stable isotope for various applications. We also include a preliminary meta-analysis of the trophic discrimination factor (TDF) for sulfur isotopes, which suggest small (mean -0.4 ± 1.7 ‰ SD) but taxa-dependent mean trophic discrimination. Each case study demonstrates how the exclusion of sulfur comes at the detriment of the results, often leading to very different outputs, or missing valuable discoveries entirely. Given that studies relying on carbon and nitrogen stable isotopes currently underpin most of our understanding of various ecological processes, this has concerning implications. Collectively, these examples strongly suggest that researchers planning to use carbon and nitrogen stable isotopes for their research should incorporate sulfur where possible, and that the new 'default' isotope systems for aquatic science should now be carbon, nitrogen, and sulfur.

Seasonal Diet Changes and Trophic Links of Cold-Water Fish (Coregonus albula) within a Northern Lake Ecosystem

The seasonal feeding patterns of the cold-adapted fish, Coregonus albula, are poorly studied in high-latitude lakes but could provide insight for predicting the effects of global warming. We examined vendace's diet composition, traced the carbon and nitrogen isotope ratios from producers to consumers in the food web, and estimated vendace's trophic position in a subarctic lake (the White Sea basin). Results showed the vendace to be a typical euryphagous fish, but clear seasonal differences were found in the relative importance of plankton and benthos in the diet. The vendace consumed primarily benthic amphipods in the summer, planktonic cladocerans in the autumn, and copepods in the winter-spring (under ice); larvae of aquatic insects were the second-most important food items throughout the year. Because of the substantial proportion of fish embryos in its diet, the vendace had a trophic position similar to that of a predatory fish (perch). The Bayesian food source-mixing model revealed that the majority of vendace energy derives from planktonic copepods. The dominant Cyclops had the lowest carbon isotope values, suggesting a carbon-depleted diet typical for methanotrophic bacteria, as its probable food source was in a lake under ice. Understanding the feeding patterns of vendace provides information to better predict the potential biotic effects of environmental change on lake ecosystems.

Does pre-exposure to polluted sediment affect sub-cellular to population-level responses to contaminant exposure in a sentinel species?

Understanding how key-species respond to anthropogenic stress such as chemical pollution is critical for predicting ecosystem changes. Little is however known about the intra-specific variability in the physiological and biochemical traits involved in contaminant exposure responses. Here, we explored this idea by exposing the Baltic amphipod Monoporeia affinis from two sites, one moderately polluted and one more pristine, to a sediment spiked with PAHs and PCBs. We evaluated the amphipods responses related to feeding, growth, a stress biomarker (acetylcholinesterase [AChE] inhibition) and stable isotope (δ13C and δ15N) composition including isotope niche analyses. More adverse responses were expected in animals from the low-pollution site than those from the high-pollution site due to tolerance development in the latter. Amphipods from both populations showed a ∼30% AChE inhibition when exposed to the contaminant spiked sediment. However, both controls and exposed amphipods from the high-pollution site had higher survival, nutrient uptake and condition status than the amphipods from the low-pollution site, which did not feed on the added diatoms as indicated by their isotope values. We found no signs of population-specific responses in physiological adjustments to contaminants with regard to classic ecotoxicological biomarkers such as AChE inhibition and growth status. Instead, isotope niche analyses proved useful in assessing contaminant stress responses at the population level.

Stable isotopes and a changing world

The measurement of naturally occurring stable isotope ratios of the light elements (C, N, H, O, S) in animal tissues and associated organic and inorganic fractions of associated environments holds immense potential as a means of addressing effects of global change on animals. This paper provides a brief review of studies that have used the isotope approach to evaluate changes in diet, isotopic niche, contaminant burden, reproductive and nutritional investment, invasive species and shifts in migration origin or destination with clear links to evaluating effects of global change. This field has now reached a level of maturity that is impressive but generally underappreciated and involves technical as well as statistical advances and access to freely available R-based packages. There is a need for animal ecologists and conservationists to design tissue collection networks that will best answer current and anticipated questions related to the global change and the biodiversity crisis. These developments will move the field of stable isotope ecology toward a more hypothesis driven discipline related to rapidly changing global events.

Parasite effects on host's trophic and isotopic niches

Wild animals are usually infected with parasites that can alter their hosts' trophic niches in food webs as can be seen from stable isotope analyses of infected versus uninfected individuals. The mechanisms influencing these effects of parasites on host isotopic values are not fully understood. Here, we develop a conceptual model to describe how the alteration of the resource intake or the internal resource use of hosts by parasites can lead to differences of trophic and isotopic niches of infected versus uninfected individuals and ultimately alter resource flows through food webs. We therefore highlight that stable isotope studies inferring trophic positions of wild organisms in food webs would benefit from routine identification of their infection status.

High capacity for a dietary specialist consumer population to cope with increasing cyanobacterial blooms

We present a common-garden experiment to examine the amphipod Monoporeia affinis, a key deposit-feeder in the Baltic Sea, a low diversity system offering a good model for studying local adaptations. In the northern part of this system, the seasonal development of phytoplankton is characterized by a single diatom bloom (high nutritional quality), whereas in the south, the diatom bloom is followed by a cyanobacteria bloom (low nutritional quality) during summer. Therefore, the nutrient input to the benthic system differs between the sea basins. Accordingly, the amphipod populations were expected to be dietary specialists in the north and generalists in the south. We tested this hypothesis using a combination of stable isotope tracers, trophic niche analyses, and various endpoints of growth and health status. We found that when mixed with diatomes, the toxin-producing cyanobacteria, were efficiently incorporated and used for growth by both populations. However, contrary to expectations, the feeding plasticity was more pronounced in the northern population, indicating genetically-based divergence and suggesting that these animals can develop ecological adaptations to the climate-induced northward cyanobacteria expansion in this system. These findings improve our understanding regarding possible adaptations of the deposit-feeders to increasing cyanobacteria under global warming world in both limnic and marine ecosystems. It is possible that the observed effects apply to other consumers facing altered food quality due to environmental changes.

Patterns of dietary niche breadth and overlap are maintained for two closely related carnivores across broad geographic scales

Ecological studies investigating niche breadth and overlap often have limited spatial and temporal scale, preventing generalizations across varying environments and communities. For example, it is not clear whether species having restricted diets maintain such patterns relative to closely related species and across their geographic range of co-occurrence. We used stable isotope analysis of hair and fur samples collected from four regions of sympatry for Canada lynx (Lynx canadensis) and bobcat (Lynx rufus) spanning southern Canada and the northern United States, to test the prediction that the more generalist species (bobcat) exhibits a wider dietary niche than the more specialist species (Canada lynx) and that this pattern is consistent across different regions. We further predicted that Canada lynx diet would consistently exhibit greater overlap with that of bobcat compared to overlap of bobcat diet with Canada lynx. We found that Canada lynx had a narrower dietary niche than bobcat, with a high probability of overlap (85–95%) with bobcat, whereas the bobcat dietary niche had up to a 50% probability of overlap with Canada lynx. These patterns of dietary niche breadth and overlap were consistent across geographic regions despite some regional variation in diet breadth and position, for both species. Such consistent patterns could reflect a lack of plasticity in species dietary niches. Given the increasingly recognized importance of understanding dietary niche breadth and overlap across large spatial scales, further research is needed to investigate the mechanisms by which broad-scale patterns are maintained across species and systems.

Narrower isotopic niche size in fish infected by the intestinal parasite Pomphorhynchus sp. compared to uninfected ones

Examples of parasite-related effects on intermediate crustacean hosts are numerous but their ecological consequences on their vertebrate hosts are scarce. Here, we address the role of macroparasite infections on the trophic niche structure of definitive hosts and its potential physiological consequences using wild fish populations infected with an acantochephalan parasite Pomphorhynchus sp., a trophically transmitted intestinal worm. Infected and uninfected fish were sampled from six populations on the Marne River, France and the prevalence of intestinal parasites in the host populations ranged from 50% to 90%. Although the isotopic ratios (δ¹³ C and δ¹⁵ N) did not differ between infected and uninfected fish, we found a consistent pattern of isotopic niche size being considerably smaller in infected hosts when compared with noninfected ones. This was not explained by interindividual differences in intrinsic factors such as length/age or body condition between infected and uninfected fish. These results suggest a potential niche specialization of infected fish, which did not impair their energetic status.

Nested interactions between chemosynthetic lucinid bivalves and seagrass promote ecosystem functioning in contaminated sediments

In seagrass sediments, lucinid bivalves and their chemoautotrophic bacterial symbionts consume H₂S, relying indirectly on the plant productivity for the presence of the reduced chemical. Additionally, the role of lucinid bivalves in N provisioning to the plant (through N₂ fixation by the symbionts) was hypothesized. Thus, lucinids may contribute to sediment detoxification and plant fitness. Seagrasses are subject to ever-increasing human pressure in coastal environments. Here, disentangling nested interactions between chemosynthetic lucinid bivalves and seagrass exposed to pollution may help to understand seagrass ecosystem dynamics and to develop successful seagrass restoration programs that consider the roles of animal-microbe symbioses. We evaluated the capacity of lucinid bivalves (Loripes orbiculatus) to promote nutrient cycling and seagrass (Cymodocea nodosa) growth during a 6-week mesocosm experiment. A fully crossed design was used to test for the effect of sediment contamination (metals, nutrients, and hydrocarbons) on plant and bivalve (alone or interacting) fitness, assessed by mortality, growth, and photosynthetic efficiency, and for the effect of their nested interaction on sediment biogeochemistry. Plants performed better in the contaminated sediment, where a larger pool of dissolved nitrogen combined with the presence of other trace elements allowed for an improved photosynthetic efficiency. In fact, pore water nitrogen accumulated during the experiment in the controls, while it was consumed in the contaminated sediment. This trend was accentuated when lucinids were present. Concurrently, the interaction between clams and plants benefitted both organisms and promoted plant growth irrespective of the sediment type. In particular, the interaction with lucinid clams resulted in higher aboveground biomass of C. nodosa in terms of leaf growth, leaf surface, and leaf biomass. Our results consolidate the notion that nested interactions involving animal-microbe associations promote ecosystem functioning, and potentially help designing unconventional seagrass restoration strategies that exploit chemosynthetic symbioses.

Experimentally derived biochemical modelling parameters to improve understanding of aquaculture's effect on marine food webs

To construct robust biogeochemical models for application to marine-based aquaculture settings, careful selection of appropriate model parameters is necessary. This study used an experimental approach to establish biomarkers of farm and marine-derived organic matter, and to derive isotopic turnover rates, and trophic discrimination factors specific to aquaculture associated food webs. A shift towards a farm-derived resource base resulted in consumer tissues more depleted in the carbon-13 isotope (indicated by more negative δ¹³C values) and a higher proportion of oleic acid, linoleic acid, and alpha-linoleic acid in the fatty acid profile of consumers over time. Measured trophic discrimination factors between dietary sources and consumer tissues demonstrated high variability among species and tissue types, ranging from -0.25‰ to 0.82‰ for Δ¹³C and from -0.77‰ to 6.8‰ for Δ¹⁵N. Stable isotope half-lives were also diverse among species and tissue types, ranging from <7 days to 462 days. Results demonstrated that construction of robust models for tracing assimilation of farm-derived organic matter through marine food webs requires the use of taxa and tissue specific parameters. Turnover rates have applications for understanding assimilative capacity of communities and for managing populations within the ecological footprint of farms.

Isotopic niche size variability in an ecosystem engineer along a disturbance gradient in a South African lagoon

A key challenge for natural resource management is how to detect effects of environmental stress on individuals and populations before declines in abundance occur. Variability in carbon and nitrogen isotope composition (δ¹³C and δ¹⁵N) among consumers can provide information on the population trophic niche and how it may change in response to environmental stress. We measured δ¹³C and δ¹⁵N values in primary producers and in an ecosystem engineer, the bioturbating sandprawn Kraussillichirus kraussi, in Langebaan Lagoon, South Africa, along a human disturbance gradient. Diet partitioning mixing models were coupled with isotope niche analyses and individual body condition data to investigate shifts in resource utilisation and diet plasticity from minimally to highly disturbed sites. The δ¹⁵N values of seagrass, Zostera capensis indicated a nutrient gradient, with the highest δ¹⁵N values at highly disturbed sites indicating either anthropogenic or marine nitrogen inputs. A decreasing δ¹⁵N signal with distance from human disturbance/mouth of lagoon was however not evident for sandprawns nor their presumed dietary sources (phytoplankton, microphytobenthos or sediment organic matter), likely because of faster isotope turnover time compared to seagrass and/or differential fractionation for sandprawns among the sites. Sandprawn isotope niche sizes varied among sites, with no trend along the disturbance gradient. The smallest niche coincided with uniform feeding on microphytobenthos according to mixing models. On an individual level, deviating isotope values from population means were correlated to better body condition, suggesting that a divergent feeding strategy is beneficial. Our results support a generalist feeding behavior of the sandprawns with no evidence of reduced physiological status at the site with most human disturbance.

Are stable isotope ratios suitable for describing niche partitioning and individual specialization?

As concerns about anthropogenic and natural disturbance grow, understanding animal resource use patterns has been increasingly prioritized to predict how changes in environmental conditions, food web structure, and population dynamics will affect biological resilience. Among the tools used to assess resource use, stable isotope analysis has proliferated in ecological studies, particularly in relation to describing intra- and interspecific variation in trophic interactions. Despite a growing need to disseminate scientific information, the inherent limitations of stable isotope ratios and inappropriate synonymizing of distinct evolutionary and ecological processes may mislead ecological inferences in natural systems. This situation necessitates a re-evaluation of the utility of stable isotope ratios to address certain ecological questions. Here, we assess the efficacy of stable isotope ratios to describe two fundamental ecological processes, niche partitioning and individual specialization. Investigation of these processes has increased substantially in accordance with increased access to stable isotope data. This article discusses the circumstances and approaches that are necessary to evaluate niche partitioning and individual specialization, and outlines key considerations for the associated application of stable isotope ratios.

Microplastics intake and excretion: Resilience of the intestinal microbiota but residual growth inhibition in common carp

Aquatic animals can be influenced by exposure to microplastics (MPs), but little is known about their recovery capacity following MPs excretion. Here, common carp were exposed to environmentally relevant concentrations of MPs for 30 days and followed by MPs excretion for another 30 days. Growth, isotopic and elemental compositions and intestinal microbiota were investigated. We found that fish growth was not influenced by exposed to MPs but was significantly reduced following MPs excretion, indicating a delayed effect on growth. MPs intake and excretion, however, had no obvious effects on isotopic and elemental compositions. MPs altered the community structure and composition of intestinal microbiota and might reduce functional diversity. After MPs excretion, interestingly, bacterial community structures of MPs treatments were grouped together with the control, suggesting the general resilience of fish intestinal microbiota. Nevertheless, high abundance of pathogenic Shewanella, Plesiomonas and Flavobacterium was observed in MPs treatments but did not affect the functional potential of intestinal microbiota. The results of this study provide new information for the application of adverse outcome pathway (AOP) in MPs, suggesting the necessity of paying attention to recovery assay following MPs intake in the development of AOP frameworks.

Shark tooth collagen stable isotopes (δ15 N and δ13 C) as ecological proxies

The isotopic composition of tooth-bound collagen has long been used to reconstruct dietary patterns of animals in extant and palaeoecological systems. For sharks that replace teeth rapidly in a conveyor-like system, stable isotopes of tooth collagen (δ¹³ C_Teeth & δ¹⁵ N_Teeth ) are poorly understood and lacking in ecological context relative to other non-lethally sampled tissues. This tissue holds promise, because shark jaws may preserve isotopic chronologies from which to infer individual-level ecological patterns across a range of temporal resolutions. Carbon and nitrogen stable isotope values were measured and compared between extracted tooth collagen and four other non-lethally sampled tissues of varying isotopic turnover rates: blood plasma, red blood cells, fin and muscle, from eight species of sharks. Individual-level isotopic variability of shark tooth collagen was evaluated by profiling teeth of different ages across whole jaws for the shortfin mako shark Isurus oxyrinchus and sandbar shark Carcharhinus plumbeus. Measurements of δ¹³ C_Teeth and δ¹⁵ N_Teeth were positively correlated with isotopic values from the four other tissues. Collagen δ¹³ C was consistently ¹³ C-enriched relative to all other tissues. Patterns for δ¹⁵ N were slightly less uniform; tooth collagen was generally ¹⁵ N-enriched relative to muscle and red blood cells, but congruent with fin and blood plasma (values clustered around a 1:1 relationship). Significant within-individual variability was observed across whole shortfin mako shark (δ¹³ C range = 1.4‰, δ¹⁵ N range = 3.6‰) and sandbar shark (δ¹³ C range = 1.2‰-2.4‰, δ¹⁵ N range = 1.7‰-2.4‰) jaws, which trended with tooth age. We conclude that amino acid composition and associated patterns of isotopic fractionation result in predictable isotopic offsets between tissues. Within-individual variability of tooth collagen stable isotope values suggests teeth of different ages may serve as ecological chronologies, that could be applied to studies on migration and individual-level diet variation across diverse time-scales. Greater understanding of tooth replacement rates, isotopic turnover and associated fractionation of tooth collagen will help refine potential ecological inferences, outlining clear goals for future scientific inquiry.

A Review of the Effects of the Biopesticides Bacillus thuringiensis Serotypes israelensis (Bti) and kurstaki (Btk) in Amphibians

Insecticides are important in agriculture, to reduce human disease, and to decrease the nuisance of biting insects. Despite this, many have the potential for environmental impacts and toxicity in nontarget organisms. We reviewed data on the effects of insecticides based on toxins from Bacillus thuringiensis var. israelensis (Bti) and Bacillus thuringiensis var. kurstaki (Btk) on amphibians. The few peer-reviewed publications that are available for Bti provide variable conclusions, ranging from few observable effects to evidence of acute toxicity at high concentrations. We briefly highlight the current controversies and identify key areas for future investigation.

Electrophysiological Activity and Survival Rate of Rats Nervous Tissue Cells Depends on D/H Isotopic Composition of Medium

The deuterium content modification in an organism has a neuroprotective effect during the hypoxia model, affecting anxiety, memory and stress resistance. The aim of this work was to elucidate the possible mechanisms of the medium D/H composition modification on nerve cells. We studied the effect of an incubation medium with a 50 ppm deuterium content compared to a medium with 150 ppm on: (1) the activity of Wistar rats' hippocampus CA1 field neurons, (2) the level of cultured cerebellar neuron death during glucose deprivation and temperature stress, (3) mitochondrial membrane potential (MMP) and the generation of reactive oxygen species in cultures of cerebellar neurons. The results of the analysis showed that the incubation of hippocampal sections in a medium with a 50 ppm deuterium reduced the amplitude of the pop-spike. The restoration of neuron activity was observed when sections were returned to the incubation medium with a 150 ppm deuterium content. An environment with a 50 ppm deuterium did not significantly affect the level of reactive oxygen species in neuron cultures, while MMP decreased by 16-20%. In experiments with glucose deprivation and temperature stress, the medium with 50 ppm increased the death of neurons. Thus, a short exposure of nerve cells in the medium with 50 ppm deuterium acts as an additional stressful factor, which is possibly associated with the violation of the cell energy balance. The decrease in the mitochondrial membrane potential, which is known to be associated with ATP synthesis, indicates that this effect may be associated with the cell energy imbalance. The decrease in the activity of the CA1 field hippocampal neurons may reflect reversible adaptive changes in the operation of fast-reacting ion channels.

Ecological stoichiometric and stable isotopic responses to microplastics are modified by food conditions in koi carp

Microplastics (MPs) can be easily taken up by a wide range of aquatic animals and cause blockage of the digestive tract leading to starvation. Meanwhile, aquatic organisms are facing threats posed by food restriction in both wild and cultured environment. Little knowledge, however, exists on how MPs interact with food conditions to affect aquatic animals. Here, koi carp were exposed to polystyrene MPs (0, 100 or 1000 μg/L) under controlled feeding (satiated or starved) for 30 or 60 days. MPs reduced and interacted synergistically with food conditions on growth after 30 days but antagonistically after 60 days. MPs reduced crude lipid and carbohydrate but increased and antagonistically interacted with feeding conditions on crude protein. Food conditions interacted with MPs on C, N and P but stoichiometric responses were decoupled with macromolecules changes. Food conditions antagonistically interacted with MPs on δ¹³C after 60 days. Linear discriminant analysis revealed that C:P and N:P were the two most important measured parameters accounting for the response of koi towards MPs and food restriction, presenting an antagonistic interaction of MPs and food status with the prolonged exposure duration.

Nitrogen isotope composition of amino acids reveals trophic partitioning in two sympatric amphipods

According to ecological theory, two species cannot occupy the same niche. Using nitrogen isotope analyses (δ¹⁵N) of amino acids, we tested the extent to which two sympatric deposit‐feeding amphipods, Monoporeia affinis and Pontoporeia femorata, partition their trophic resources. We found that trophic position (TP) and resynthesis index (∑V; a proxy for degradation status of ingested material prior to assimilation by the consumer) differ between species. The surface‐feeding M. affinis had higher TP and intermediate ∑V, both pointing to a large contribution of metazoans in its diet. P. femorata, which feeds in the subsurface layers, had lower TP and a bimodal distribution of the ∑V values, supporting previous experimental evidence of a larger feeding niche. We also evaluated whether TP and ∑V values have consequences for amphipod fecundity and embryo viability and found that embryo viability in M. affinis was negatively linked to TP. Our results indicate that the amino acid‐δ¹⁵N data paired with information about reproductive status are useful for detecting differences in the trophic ecology of sympatric amphipods.

Trophic niche segregation allows range-extending coral reef fishes to co-exist with temperate species under climate change

Changing climate is forcing many terrestrial and marine species to extend their ranges poleward to stay within the bounds of their thermal tolerances. However, when such species enter higher latitude ecosystems, they engage in novel interactions with local species, such as altered predator-prey dynamics and competition for food. Here, we evaluate the trophic overlap between range-extending and local fish species along the east coast of temperate Australia, a hotspot for ocean warming and species range extensions. Stable isotope ratios (δ¹⁵ N and δ¹³ C) of muscle tissue and stomach content analysis were used to quantify overlap of trophic niche space between vagrant tropical and local temperate fish communities along a 730 km (6°) latitudinal gradient. Our study shows that in recipient temperate ecosystems, sympatric tropical and temperate species do not overlap significantly in their diet-even though they forage on broadly similar prey groups-and are therefore unlikely to compete for trophic niche space. The tropical and temperate species we studied, which are commonly found in shallow-water coastal environments, exhibited moderately broad niche breadths and local-scale dietary plasticity, indicating trophic generalism. We posit that because these species are generalists, they can co-exist under current climate change, facilitating the existence of novel community structures.

Variation in isotopic niche, digestive tract morphology, and mercury concentrations in two sympatric waterfowl species wintering in Atlantic Canada

Sympatric communities of organisms may exploit different ecological niches to avoid intra- and interspecific competition. We examined the isotopic niches of American black ducks ( Anas rubripes) and mallards ( A. platyrhynchos) wintering in coastal and urban areas of Atlantic Canada and compared isotopic niche with digestive tract morphologies and blood mercury (Hg) concentrations. Isotopic niche width (for δ13C and δ15N) varied between the three groups of ducks studied, with coastally foraging black ducks exhibiting the widest isotopic niche, followed by coastal mallards, while urban feeding black ducks had a narrow isotopic niche. These niche differences had physical and chemical consequences: coastal black ducks had longer digestive tracts, a larger range in gizzard sizes, and higher and more variable Hg concentrations than urban black ducks and coastal mallards. This plasticity in ecological niche may reduce competition among and within species, and subsequently explain why winter numbers of black ducks and mallards have increased in Atlantic Canada.

Possible Mechanisms of Biological Effects Observed in Living Systems during 2H/1H Isotope Fractionation and Deuterium Interactions with Other Biogenic Isotopes

This article presents the original descriptions of some recent physics mechanisms (based on the thermodynamic, kinetic, and quantum tunnel effects) providing stable 2H/1H isotope fractionation, leading to the accumulation of particular isotopic forms in intra- or intercellular space, including the molecular effects of deuterium interaction with 18O/17O/16O, 15N/14N, 13C/12C, and other stable biogenic isotopes. These effects were observed mainly at the organelle (mitochondria) and cell levels. A new hypothesis for heavy nonradioactive isotope fractionation in living systems via neutron effect realization is discussed. The comparative analysis of some experimental studies results revealed the following observation: "Isotopic shock" is highly probable and is observed mostly when chemical bonds form between atoms with a summary odd number of neutrons (i.e., bonds with a non-compensated neutron, which correspond to the following equation: Nn - Np = 2k + 1, where k ϵ Z, k is the integer, Z is the set of non-negative integers, Nn is number of neutrons, and Np is number of protons of each individual atom, or in pair of isotopes with a chemical bond). Data on the efficacy and metabolic pathways of the therapy also considered 2H-modified drinking and diet for some diseases, such as Alzheimer's disease, Friedreich's ataxia, mitochondrial disorders, diabetes, cerebral hypoxia, Parkinson's disease, and brain cancer.

Mosquito control based on Bacillus thuringiensis israelensis (Bti) interrupts artificial wetland food chains

The biocide Bacillus thuringiensis israelensis (Bti) has become the most commonly used larvicide to control mosquitoes in seasonal wetlands. Although Bti is considered non-toxic to most aquatic organisms, the non-biting chironomids show high susceptibilities towards Bti. As chironomids are a key element in wetland food webs, major declines in their abundance could lead to indirect effects that may be passed through aquatic and terrestrial food chains. We conducted two mesocosm experiments to address this hypothesis by assessing direct and indirect effects of Bti-modified availability of macroinvertebrate and zooplankton food resources on the predatory larvae of palmate and smooth newts (Urodelans: Lissotriton helveticus, Lissotriton vulgaris). We examined newt survival rates and dietary composition by means of stable isotope (δ¹⁵N and δ¹³C) analysis in the presence of Bti treatment and a predator (Odonata: Aeshna cyanea). We assessed palmate newts' body size at and time to metamorphosis while developing in Bti treated mesocosms. Chironomid larvae were the most severely affected aquatic invertebrates in all Bti treated food chains and experienced abundance reductions by 50 to 87%. Moreover, stable isotope analysis revealed that chironomids were preferred over other invertebrates and comprised the major part in newts' diet (56%) regardless of their availability. The dragonfly A. cyanea decreased survival of newt larvae by 27% in Bti treated mesocosms showing affected chironomid abundances. Increasing intraguild predation is most likely favored by the Bti-induced reduction of alternative prey such as chironomid larvae. The decreased food availability after Bti treatment led to slightly smaller L. helveticus metamorphs while their developmental time was not affected. Our findings highlight the crucial role of chironomids in the food webs of freshwater ecosystems. We are also emphasizing the importance of reconsidering human-induced indirect effects of mosquito control on valuable wetland ecosystems particularly in the context of worldwide amphibian and insect declines.

Trophodynamics of Southern Ocean pteropods on the southern Kerguelen Plateau

Pteropods are a group of small marine gastropods that are highly sensitive to multiple stressors associated with climate change. Their trophic ecology is not well studied, with most research having focused primarily on the effects of ocean acidification on their fragile, aragonite shells. Stable isotopes analysis coupled with isotope-based Bayesian niche metrics is useful for characterizing the trophic structure of biological assemblages. These approaches have not been implemented for pteropod assemblages. We used isotope-based Bayesian niche metrics to investigate the trophic relationships of three co-occurring pteropod species, with distinct feeding behaviors, sampled from the Southern Kerguelen Plateau area in the Indian Sector of the Southern Ocean-a biologically and economically important but poorly studied region. Two of these species were gymnosomes (shell-less pteropods), which are traditionally regarded as specialist predators on other pteropods, and the third species was a thecosome (shelled pteropod), which are typically generalist omnivores. For each species, we aimed to understand (a) variability and overlap among isotopic niches; and (b) whether there was a relationship between body size and trophic position. Observed isotopic niche areas were broadest for gymnosomes, especially Clione limacina antarctica, whose observed isotopic niche area was wider than expected on both δ13C and δ15N value axes. We also found that trophic position significantly increased with increasing body length for Spongiobranchaea australis. We found no indication of a dietary shift toward increased trophic position with increasing body size for Clio pyramidata f. sulcata. Trophic positions ranged from 2.8 to 3.5, revealing an assemblage composed of both primary and secondary consumer behaviors. This study provides a comprehensive comparative analysis on trophodynamics in Southern Ocean pteropod species, and supports previous studies using gut content, fatty acid and stable isotope analyses. Combined, our results illustrate differences in intraspecific trophic behavior that may be attributed to differential feeding strategies at species level.

The Importance of Isotopic Turnover for Understanding Key Aspects of Animal Ecology and Nutrition

Stable isotope-based methods have proved to be immensely valuable for ecological studies ranging in focus from animal movements to species interactions and community structure. Nevertheless, the use of these methods is dependent on assumptions about the incorporation and turnover of isotopes within animal tissues, which are oftentimes not explicitly acknowledged and vetted. Thus, the purpose of this review is to provide an overview of the estimation of stable isotope turnover rates in animals, and to highlight the importance of these estimates for ecological studies in terrestrial, freshwater, and marine systems that may use a wide range of stable isotopes. Specifically, we discuss 1) the factors that contribute to variation in turnover among individuals and across species, which influences the use of stable isotopes for diet reconstructions, 2) the differences in turnover among tissues that underlie so-called ‘isotopic clocks’, which are used to estimate the timing of dietary shifts, and 3) the use of turnover rates to estimate nutritional requirements and reconstruct histories of nutritional stress from tissue isotope signatures. As we discuss these topics, we highlight recent works that have effectively used estimates of turnover to design and execute informative ecological studies. Our concluding remarks suggest several steps that will improve our understanding of isotopic turnover and support its integration into a wider range of ecological studies.

Increase in stable isotope ratios driven by metabolic alterations in amphipods exposed to the beta-blocker propranolol

Anthropogenic pressures, such as contaminant exposure, may affect stable isotope ratios in biota. These changes are driven by alterations in the nutrient allocation and metabolic pathways induced by specific stressors. In a controlled microcosm study with the amphipod Gammarus spp., we studied effects of the β-blocker propranolol on stable isotope signatures (δ¹⁵N and δ¹³C), elemental composition (%C and %N), and growth (protein content and body size) as well as biomarkers of oxidative status (antioxidant capacity, ORAC; lipid peroxidation, TBARS) and neurological activity (acetylcholinesterase, AChE). Based on the known effects of propranolol exposure on cellular functions, i.e., its mode of action (MOA), we expected to observe a lower scope for growth, accompanied by a decrease in protein deposition, oxidative processes and AChE inhibition, with a resulting increase in the isotopic signatures. The observed responses in growth, biochemical and elemental variables supported most of these predictions. In particular, an increase in %N was observed in the propranolol exposures, whereas both protein allocation and body size declined. Moreover, both ORAC and TBARS levels decreased with increasing propranolol concentration, with the decrease being more pronounced for TBARS, which indicates the prevalence of the antioxidative processes. These changes resulted in a significant increase of the δ¹⁵N and δ¹³C values in the propranolol-exposed animals compared to the control. These findings suggest that MOA of β-blockers may be used to predict sublethal effects in non-target species, including inhibited AChE activity, improved oxidative balance, and elevated stable isotope ratios. The latter also indicates that metabolism-driven responses to environmental contaminants can alter stable isotope signatures, which should be taken into account when interpreting trophic interactions in the food webs.

Using Compound-Specific and Bulk Stable Isotope Analysis for Trophic Positioning of Bivalves in Contaminated Baltic Sea Sediments

Stable nitrogen isotopes (δ¹⁵N) are used as indicators of trophic position (TP) of consumers. Deriving TP from δ¹⁵N of individual amino acids (AAs) is becoming popular in ecological studies, because of lower uncertainty than TP based on bulk δ¹⁵N (TP_bulk). This method would also facilitate biomagnification studies provided that isotope fractionation is unaffected by toxic exposure. We compared TP_AA and TP_bulk estimates for a sediment-dwelling bivalve from two coastal sites, a pristine and a contaminated. Chemical analysis of PCB levels in mussels, sediments, and pore water confirmed the expected difference between sites. Both methods, but in particular the TP_AA underestimated the actual TP of bivalves. Using error propagation, the total uncertainty related to the analytical precision and assumptions in the TP calculations was found to be similar between the two methods. Interestingly, the significantly higher intercept for the regression between TP_AA and TP_bulk in the contaminated site compared to the pristine site indicates a higher deamination rate due to detoxification as a result of chronic exposure and a higher ¹⁵N fractionation. Hence, there is a need for controlled experiments on assumptions underlying amino acid-specific stable isotope methods in food web and bimagnification studies.