Nitrogen Stable Isotope Signatures Research Articles

Linking Transpiration to Reef Nitrogen Supply on a Tropical Coral Island

AbstractCoral reef islands are biodiversity hotspots with high conservation value, but we have a poor understanding of how island vegetation, through transpiration, influences groundwater nutrient supply to adjacent reef systems. Here we combine stable isotope tracing, geophysical surveys, and satellite analysis to unravel the links between transpiration and the discharge of groundwater nitrate to the waters surrounding a coral reef island (Lady Elliot Island in the Great Barrier Reef). Over a 2‐year study period (2020–2021) there was a net loss of freshwater from the island, that is, evapotranspiration (ET) exceeded rainfall, largely caused by high rates of transpiration (73% of ET). Transpiration was higher in forested areas, and groundwater salinity appeared to be higher there. By tracing the nitrogen stable isotope signature of groundwater nitrate into reef organism tissue we were able to map groundwater nitrogen discharge spatially and temporally. Groundwater nitrogen discharge was focussed on one side of the island and did not vary seasonally as expected, despite reduced rainfall and seabird guano inputs over the austral winter/spring. Based on our results, we propose that transpiration by island vegetation slows groundwater flow and concentrates nitrate in the groundwater being released to the surrounding reef system. High concentrations of nitrate in groundwater (up to 27 mmol L−) were observed on Lady Elliot Island and these seem to have increased since 2014, but further work is required to understand if this is a normal scenario for tropical coral islands or the result of the islands' revegetation program.

High urinary oxidative DNA damage in wild chimpanzees ranging in proximity of agricultural fields in Sebitoli area, Uganda.

Agriculture expansion is a major cause of habitat loss and exposure to phytochemical pollution for non-human primates. In addition to endocrine disruption, exposure to pesticides may have other sublethal physiological consequences for animals, such as generation of oxidative damage to macromolecules. In this study, we analyzed the pesticides contained in the river water across the home range of wild chimpanzees (Pan troglodytes) in Sebitoli area located on the Northern part of Kibale National Park (Uganda). We tested whether levels of three urinary markers of oxidative damage vary among individuals in relation to their ranging patterns, as a proxy for pesticide exposure intensity. To better characterize the foraging habitat use, the trophic level, and the energetic status of study individuals, we also quantified urinary levels of carbon and nitrogen stable isotope signatures and of C-peptide. Among the 511 pesticides screened, 18 compounds including herbicides, insecticides, and fungicides were found in the water sampled in the Western part of the home range of chimpanzees. In this area, chimpanzees used to feed on maize crops. By contrast, in the Eastern part where crop feeding was never observed, we found only seven pesticides. According to their ranging patterns and thus crop feeding frequency, the 139 urine samples collected from 43 Sebitoli chimpanzees were categorized as belonging to low, medium, and high exposure level. Chimpanzees from the high exposure zone had higher oxidative DNA damage (8-OHdG) than chimpanzees from both the low and medium exposure groups, who had similar levels of oxidative DNA damage. In addition, individuals with higher C-peptide tended to have significantly higher oxidative DNA damage and lipid peroxides. The three exposure groups had similar levels of urinary 8-isoprostanes and of urinary lipid peroxides. These results were robust for any potential confounding effect of other variables because neither age category nor sex or isotope levels were significantly associated with markers of oxidative damage. Our study points to genotoxic effects as one potential sublethal consequence of ranging in proximity of agricultural fields owing to exposure to pesticides or other unidentified sources of stress. Given our phylogenetic proximity, this information is relevant for the conservation of this species which is endangered and also sentinel for human health.

Small stream predators rely heavily on terrestrial matter energy input in the fall, regardless of riparian buffer size

Stream ecosystems are reliant on the reciprocal exchange of terrestrial and aquatic energy subsides to maintain a productive and stable food web. Land use around streams can have strong effects on the size and availability of resource subsidies for stream and riparian predators such as fish and spiders. A common forestry technique around streams is the establishment of forested buffers to protect aquatic and riparian ecosystems from upland disturbances. Buffer size may determine prey abundance, richness, and spatial extent of prey reach into both the aquatic and terrestrial systems. To test the effects of forested buffers subsidy direction, we explored the carbon and nitrogen stable isotope signatures of brown trout (Salmo trutta), Tetragnathidae and Lycosidae spiders, and their aquatic and terrestrial prey sources around twelve streams in southern Sweden. For both predator groups, buffer presence showed no effect on resource subsidy source. We found that both brown trout and spiders are significantly reliant on terrestrial sources of prey for their diets in the fall. To support the terrestrial subsidy into small streams it is vital to maintain ecologically functional riparian zones by conserving complex surrounding habitats that optimize habitat and both terrestrial and aquatic prey diversity.

Trophic ecology of the squid Doryteuthis gahi in the Southwest Atlantic inferred from stable isotope analysis

The Patagonian longfin squid Doryteuthis gahi has an annual life cycle with two seasonal cohorts (autumn and spring spawners). Previous studies estimated the trophic level of adult squid by stable isotope analysis of their muscle or gladius tissues, but few studies compared different sizes and spawning cohorts. In this study, mixed models were used to compare carbon (δ13C) and nitrogen (δ15N) stable isotope signatures of muscle and gladius tissues of the autumn and spring spawning cohorts and squid of different sizes. A published isoscape model provided δ15N baselines to calculate the trophic level of D. gahi. Sampled herbivorous prey such as Euphausiacea, planktonic juveniles Munida gregaria and salps Ihlea supported the trophic level estimates. The autumn spawning cohort had higher δ13C values than the spring spawning cohort. δ15N values increased with increasing dorsal mantle length, suggesting an ontogenetic change towards larger prey. Muscle tissue was enriched in 15N but slightly depleted in 13C compared to gladius tissue. Adult squid had an estimated trophic level of ∼3.2, with no significant differences between spawning cohorts. Stable isotope values followed a seasonal trend, with lower δ15N values and lower δ13C value between July and September. Large individuals of the spring spawning cohort were found to have the least niche overlap with other groups of D. gahi. Our study of the trophic levels and ecology of D. gahi can provide important information on its role in the food chain and can build the basis of studies on interaction with other species, and the impact of environmental changes on their populations. This information can inform the development of management strategies aimed at preserving the balance of the ecosystem and ensuring its sustainable use.

Carbon (δ13C) and nitrogen (δ15N) isotope ratios reveal geographic affinity and dietary status of Ajnala skeletal remains: A forensic anthropological study.

Stable isotope analysis from bones and teeth has been widely used to estimate the likely geographic locations and dietary status of individuals whose osseous remains have been retrieved from some forensic or bio-archaeological contexts. The carbon and nitrogen stable isotope signatures can provide insights into geographic affinity and dietary habits. Ajnala skeletal remains represent a serious crime against humanity committed in past by colonial rulers and by amateur archaeologists of modern times. In present study, isotopic concentrations of carbon (δ13C) and nitrogen (δ15N) estimated from 21 mandibular molars have been used to estimate the local or non-local status of badly damaged skeletal remains retrieved from an abandoned well at Ajnala (India). The collagen samples having C/N ratio within range of 2.8-3.6 were considered as well-preserved and non-contaminated ones. The isotope concentrations of carbon and nitrogen varied from -18.7‰ to -22.9‰ and +7.6‰ to +11.7‰, with an average of -20.49 ± 1.2‰ and +9.31 ± 1.1‰, respectively. The analysis of the obtained isotope values reflected the consumption of C3/C4 mixed diet by majority of the individuals, and such type of dietary habits are mainly restricted to the reported Indo-Gangatic plain of India to which slain soldiers reportedly belonged to. These observations corroborated the previous observations about the geographic affinity and dietary status of Ajnala individuals. Though C and N isotopes are by and large not the confirmed/direct indicators of geographic origin, they can provide corroboratory information to support other observations narrowing down the dietary habits of individuals of certain specific geographical regions.

Dietary reconstruction of the Bronze Age necropolis of Cova des Pas (Minorca Island): evidence from δ13C and δ15N analyses

Current paleodietary studies about the Naviform and Talayotic groups that took place in Minorca (Balearic Islands) during the Late Bronze Age–Early Iron Age (ca.1600–850/800 BC) have suggested a mixed and variable diet, largely depending on terrestrial sources of vegetables and meat. This study explores the nutritional pattern of the individuals buried in the Cova des Pas site (Minorca Island, Spain), a cave used as a collective sepulcher and the most exceptional and major human assemblage found in the Balearic Islands during this period. Carbon (δ13C) and nitrogen (δ15N) stable isotopic signatures were measured on extracted bone collagen from 49 individuals. Further, faunal remains from the Son Mercer de Baix site, the closest contemporaneous village to the collective sepulcher, were also analyzed to provide a baseline corpus of data to interpret human isotopic data. The results indicate a human diet based mainly on C3 plants with an important consumption of animal protein. The δ15N values of infants up to 4 years were high, indicating the enriched isotopic signature of breast milk, and weaning is assumed to occur around this age. Differences between sexes and the age subcategories were not statistically significant, assuming that the different groups of society had the same access to food. The data obtained in this isotopic study provides insight into the palaeodietary pattern of the human groups dated to the Late Bronze Age–Early Iron Age ages in the island of Minorca, contributing to the present debate on the emergence and development of complex societies on the Balearic archipelago.

Trophic ecology of northern gannets Morus bassanus highlights the extent of isotopic niche overlap with other apex predators within the Bay of Biscay

Understanding the linkages within complex and evolving marine food webs is essential to comprehend marine ecosystem structure and dynamics. Carbon (δ13C) and nitrogen (δ15N) stable isotope signatures are recognized to be powerful descriptors of the trophic ecology and trophic relationships within marine communities. Apex predators such as seabirds can influence the structure of communities by predating in lower trophic levels. They also convey information both over a range of spatial and temporal scales due to their high mobility and longevity, respectively. For this reason, here, we studied the trophic ecology of northern gannets and the extent of niche overlap with other apex predators within the Bay of Biscay (North-East Atlantic), a key feeding area for numerous predatory species. Mixing models indicated that northern gannets fed primarily on European anchovy Engraulis encrasicolus and on a mixed group composed of European sardine Sardina pilchardus, European sprat Sprattus sprattus, juvenile hake Merluccius merluccius, Atlantic mackerel Scomber scombrus and Atlantic horse mackerel Trachurus trachurus. Within the apex predator community, the northern gannets trophic niche overlapped the most with common guillemots Uria aalge (15.58%, based on Stable Isotope Bayesian Ellipses (SIBER)) and overlapped to a lesser extent with cetacean species occupying shelf habitats (i.e., the harbour porpoise Phocoena phocoena (9.99%) and the short-beaked common dolphin Delphinus delphis (10.37%)). Overall, the overlap found was moderate indicating trophic similarities and dissimilarities between these species. Information on trophic interactions between co-existing species is necessary for holistic environmental management, particularly in areas where several megafauna species share their foraging grounds with fisheries activity.

A preliminary study of macrofaunal communities and their carbon and nitrogen stable isotopes in the Haima cold seeps, South China Sea

The macrofaunal community in the Haima cold seeps of the South China Sea was investigated, and trophic relationships were evaluated using carbon and nitrogen stable isotope signatures (δ13C and δ15N). A total of 30 macrofaunal species were identified in this study, most of which were collected from the Gigantidas haimaensis mussel beds. Only four macrofaunal species were collected from the Calyptogena marissinica clam beds, where the environment may not be suitable for the survival of most macrofauna. The bush of slim tubeworms (Sclerolinum sp.) might be an important nursery ground for mussels. G. haimaensis was the predominant species, and its average δ13C and δ15N values were −52.7‰ and 0.6‰, respectively. Compared with the stable isotope signatures of particulate organic matter (POM), it suggested that the food source of macrofauna in the Haima cold seeps was mainly originated from chemosynthetic autotrophs. In the mussel bed, most of the macrofauna had a large δ13C isotopic distance compared with G. haimaensis, suggesting that G. haimaensis was not a direct food source for most cold-seep macrofauna in the mussel bed. The variation in stable isotope signatures of macrofauna might be attributed to different proportions of methanotrophic and thiotrophic endosymbionts. Neolithodes brodiei and Bathynomus jamesi showed the highest δ13C and δ15N values. However, the large isotopic distance indicated that N. brodiei and B. jamesi might just occasionally visit the cold-seep ecosystem and their food sources were mainly from the upper ecosystem relying on photosynthesis. Overall, our results suggested that there was no significant predator-prey relationship among these macrofaunal species in the Haima cold seeps. This study provided important baseline data of stable isotope signatures for further studies to elucidate the trophic interaction among seep organisms.

A trophic niche shift in a South American migrant: Stable nitrogen isotope signatures in feathers of Fork-tailed Flycatchers (Tyrannus savana)

Understanding how diet and life history strategies interact is important for exploring constraints of available nutrition on energetically expensive life history events in wild animals (i.e., reproduction, annual migration, molt). Previous research on migratory birds breeding in the Northern Hemisphere has demonstrated trophic niche shifts from invertebrates to fruit in order to fuel spring migration. We examined whether a trophic niche shift occurred in a Neotropical austral migrant, the Fork-tailed Flycatcher (Tyrannus savana savana), prior to spring migration by measuring stable nitrogen isotopes in feathers. We found that the austral migrant subspecies T. s. savana did appear to shift in diet from a higher to a lower trophic level (consistent in pattern with a shift from a higher to a lower ratio of invertebrates to fruit) but the shift occurred earlier than expected if it was preparation for migration. A sympatric sedentary subspecies occupying the same habitat (T. s. monachus) appeared to forage only at the lower trophic level during its annual molt and showed no evidence of a trophic niche shift; however, the molt of the 2 subspecies occurs at slightly different times of year. The timing of the trophic niche shift leads us to conclude that a higher trophic level diet early in molt is not related to preparation for spring migration but that it may be related to seasonal changes in food availability as the wet season concludes. A remaining challenge for understanding the ecological consequences of trophic niche shifts is to find ways to empirically measure trade-offs between different diets across energetically expensive life history events and compare these between taxa with differing life history strategies.

Elevated mercury and PCB concentrations in Dolly Varden (Salvelinus malma) collected near a formerly used defense site on Sivuqaq, Alaska

Environmental pollution causes adverse health effects in many organisms and contributes to health disparities for Arctic communities that depend on subsistence foods, including the Yupik residents of Sivuqaq (St. Lawrence Island), Alaska. Sivuqaq's proximity to Russia made it a strategic location for U.S. military defense sites during the Cold War. Two radar surveillance stations were installed on Sivuqaq, including at the Northeast Cape. High levels of persistent organic pollutants and toxic metals continue to leach from the Northeast Cape formerly used defense (FUD) site despite remediation efforts. We quantified total mercury (Hg) and polychlorinated biphenyl (PCB) concentrations, and carbon and nitrogen stable isotope signatures, in skin and muscle samples from Dolly Varden (Salvelinus malma), an important subsistence species. We found that Hg and PCB concentrations significantly differed across locations, with the highest concentrations found in fish collected near the FUD site. We found that 89% of fish collected from near the FUD site had Hg concentrations that exceeded the U.S. Environmental Protection Agency's (EPA) unlimited Hg-contaminated fish consumption screening level for subsistence fishers (0.049 μg/g). All fish sampled near the FUD site exceeded the EPA's PCB guidelines for cancer risk for unrestricted human consumption (0.0015 μg/g ww). Both Hg and PCB concentrations had a significant negative correlation with δ13C when sites receiving input from the FUD site were included in the analysis, but these relationships were insignificant when input sites were excluded. δ15N had a significant negative correlation with Hg concentration, but not with PCB concentration. These results suggest that the Northeast Cape FUD site remains a point source of Hg and PCB pollution and contributes to higher concentrations in resident fish, including subsistence species. Moreover, elevated Hg and PCB levels in fish near the FUD site may pose a health risk for Sivuqaq residents.

Simple extraction methods for pesticide compound-specific isotope analysis from environmental samples

Compound-specific isotope analysis (CSIA) is a powerful approach to evaluate the transformation of organic pollutants in the environment. However, the application of CSIA to micropollutants, such as pesticides, remains limited because appropriate extraction methods are currently lacking. Such methods should address a wide range of pesticides and environmental matrices, while recovering sufficient mass for reliable CSIA without inducing stable isotope fractionation. Here, we present simple extraction methods for carbon and nitrogen CSIA for different environmental matrices and six commonly used herbicides, i.e., atrazine, terbutryn, acetochlor, alachlor, butachlor, and S-metolachlor, and three fungicides, i.e., dimethomorph, tebuconazole, and metalaxyl. We examined the potential of several extraction methods for four types of soils or sediments, three types of environmental waters and aerial and root plant samples for multielement (ME)-CSIA.•Pesticide extraction recoveries varied depending on the physical characteristics of the pesticides and matrix properties for environmental water (77 to 87%), soil and sediment (35 to 82%), and plant (40 to 59%) extraction.•The tested extraction methods did not significantly affect the carbon and nitrogen stable isotope signatures of pesticides (Δ(13C) <0.9‰ for Δ(15N) <1.0‰).

Physiological traits of income breeding strategy in the sea cucumber Apostichopus japonicus

The sea cucumber, Apostichopus japonicus, is an important marine aquaculture species in Asian countries. The small body size but high fecundity of A. japonicus suggests that using income resources to support gonad development might be an appropriate life history strategy, but the physiological mechanisms involved in this approach remain unclear. In this study, we conducted three experiments to assess the physiological traits that underlie income breeding strategy of the A. japonicus. As inferred by the carbon and nitrogen stable isotope signatures, the results showed that income breeding allows all adults, even individuals with poor body condition and small maternal size, to reproduce annually by flexibly adjusting their reproductive output. In addition, functionally, we found that the sea cucumbers allocate income resources to the demands of reproduction, growth and storage simultaneously during reproduction. However, dynamic changes of activities of Na+/K+-ATPase, hexokinase and pyruvate kinase suggested that income breeding places a metabolic burden on the breeding adults. Our results provided new insights into the physiological mechanisms underlying income breeding strategy in sea cucumbers.

Temperature affects carbon and nitrogen stable isotopic signatures of aquatic plants

Aquatic plants are vital components of shallow aquatic ecosystems, and they can substantially contribute to food webs. However, the large spatial and temporal variations of δ13C and δ15N signatures of aquatic plants have hindered the interpretation of their trophic interactions with organisms at higher trophic levels, and the effects of temperature on plant isotopic signatures remain to be fully elucidated. Herein, we cultured three common submerged macrophytes [Elodea nuttallii (Planch.) St. John, Vallisneria spiralis L., and Potamogeton lucens L.] at four temperatures (10, 15, 20 and 25 °C) for 16 weeks and analyzed their δ13C and δ15N signatures. Results showed that temperature altered the isotopic signatures of all three plant species. δ13C and δ15N varied by 16.06‰ and 11.68‰ in P. lucens at different temperatures, respectively. Plant δ15N significantly decreased with rising temperature in all three plant species and was correlated with plant growth, N content, and pore water dissolved inorganic N (DIN) concentrations. Conversely, δ13C responded non-linearly with temperature: a hump-shaped response of δ13C with temperature was observed for P. lucens. Plant δ13C was not correlated with any of the measured parameters. Temperature can alter plant metabolism and photosynthesis and the compositions and concentrations of C and N sources, thereby influencing plant δ13C and δ15N signatures, respectively. Temperature plays a key role in altering plant C and N isotopic signatures. Therefore, we recommend future studies to carefully consider the effects of temperature on plant stable isotopic signatures when interpreting the food contribution of aquatic plants in food webs and long-term environmental changes via historical isotopic signatures of plants exposed to different temperatures, particularly in light of changing climate conditions.

Stable nitrogen isotopic signatures reveal the NH4+ evolution processes in pollution episodes in urban southwestern China

In most of the urban atmosphere, the formation of ammonium (NH4+) is mainly controlled by the initial isotope value of ammonia (δ15N-NH3 values) and the corresponding ammonia (NH3) conversion ratio (NH4+/(NH4+ + NH3)), as N isotope fractionation may occur under an NH3-sufficient environment. The significant influence of δ15N-NH3 values on the formation of NH4+ in the urban atmosphere has been demonstrated by many previous studies; however, exploration of the effect of the NH3 conversion ratios on NH4+ formation has been limited, especially during pollution episodes. To better understand the NH3 sources and the effect of the NH3 conversion ratios on the formation of NH4+ in the urban environment, NH4+ concentrations (0.2 to 13.4 μg·m−3, average 3.9 ± 2.8 μg·m−3) and their δ15N values (−4.0‰ to 29.2‰, average 20.6‰ ± 6.1‰) in PM2.5 were determined in the urban area of Nanning from September 1 to December 31, 2017. Based on the measured δ15N-NH4+ values and the isotope fractionation that occurred during gas-to-particle partitioning, we estimated that the initial δ15N-NH3 values were − 22.8‰ to 3.1‰ (average − 8.8‰ ± 5.5‰). These initial δ15N-NH3 values were influenced by agricultural emission sources (−31.0‰ to −4.4‰), fossil fuel-related NH3 emission sources (−14.6‰ to 10.1‰) and biomass combustion sources (12‰ and 23‰). Source apportionment indicated that these three potential NH3 emission sources contributed 43%, 33% and 24% to the ambient NH3, respectively. On clean days, similar trends in initial δ15N-NH3 and δ15N-NH4+ values were observed, and there was an apparent positive linear correlation between these values (R2 = 0.70, p < 0.01). This suggested that the δ15N-NH4+ signatures recorded changes in the NH3 sources and that NH4+ formation was controlled by the NH3 sources on clean days. However, during different pollution episodes the variations in δ15N-NH4+ values were consistent with the NH4+ concentrations and the NH4+/(NH4+ + NH3) values, and there were negative linear correlation between these values (R2 > 0.6, p < 0.01). This suggested that the δ15N-NH4+ signatures recorded NH4+ evolution processes and that the increased NH3 conversion ratios was the main reason for the development of NH4+ and PM2.5 in pollution episodes. We inferred that the decreased air temperature in cold months may stimulate the increased NH3 conversion ratios, based on the negative correlation of NH4+/(NH4+ + NH3) values with air temperature (r = −0.63, p < 0.01).

Ontogenetic diet shifts: an additional mechanism for successful invasion of a piranha species in a Neotropical floodplain.

Ontogenetic shifts in food preference reduces intraspecific competition as immature individuals eat different food types than adults. This diet plasticity could facilitate species' ability to successfully invade and establish itself in a new environment, even when co-occurring with phylogenetically close species. Here, ontogenetic diet shifts of a non-native piranha species (Serrasalmus marginatus) was tested by analyzing the relationship between its body length and the carbon and nitrogen stable isotope signatures. Carbon stable isotope was not correlated to fish length, but positive significant correlation between δ15N values and fish length was found for the non-native piranha. Also, immature and adult S. marginatus showed low isotopic niche overlap. The correlation between fish length and δ15N, and the low trophic overlap between immature and adult, indicate that the non-native S. marginatus had ontogenetic shifts in food preference, which may be viewed as an additional mechanism underlying its successful establishment in the upper Paraná River. Our findings indicate that ontogenetic shifts in food preference may be an invasive trait that facilitates the establishment of non-native fish species in tropical aquatic systems.

Effects of Tissue Preservation on Carbon and Nitrogen Stable Isotope Signatures in Syngnathid Fishes and Prey.

Simple SummaryStable isotope analysis (SIA) was used to assess the influence of various preservation methods (freezing, ethanol and formaldehyde) on syngnathid (seahorses and pipefishes) fins, seahorse newborns (seahorses), and prey (copepods and Artemia). The first available conversion models for syngnathids are provided, enabling their application to isotopic studies in the field and in the laboratory.Isotopic stable analysis (SIA) is a powerful tool in the assessment of different types of ecological and physiological studies. For that, different preservation methods for sampled materials are commonly used prior to isotopic analysis. The effects of various preservation methods (freezing, ethanol and formaldehyde) were analyzed for C:N, and δ13C and δ15N signals on a variety of tissues including dorsal fins (three seahorse and two pipefish species), seahorse newborns (three seahorses species), and prey (copepods and different stages of Artemia) commonly used to feed the fishes under rearing conditions. The aims of the study were: (i) to evaluate isotopic effects of chemical preservation methods across different types of organisms and tissues, using frozen samples as controls, and (ii) to construct the first conversion models available in syngnathid fishes. The chemical preservation in ethanol and, to a lesser extent, in formaldehyde significantly affected δ13C values, whereas the effects on δ15N signatures were negligible. Due to their low lipid content, the isotopic signals in fish fins was almost unaffected, supporting the suitability of dorsal fins as the most convenient material in isotopic studies on vulnerable species such as syngnathids. The regression equations provided resulted convenient for the successful conversion of δ13C between preservation treatments. Our results indicate that the normalization of δ15N signatures in preserved samples is unnecessary. The conversion models should be applicable in isotopic field studies, laboratory experiments, and specimens of historical collections.

Stable Isotopes Reveal Variation in Consumption of Pacific Salmon by Brown Bears, Despite Ready Access in Small Streams

AbstractBrown bears Ursus arctos consume a wide range of organisms, including ungulates and plants, but Pacific salmon Oncorhynchus spp. are especially important to their diet where their ranges overlap. Although some brown bears minimize antagonistic encounters with other brown bears or infanticide by avoiding streams where salmon spawn, studies generally assume that brown bears with ready access to salmon feed heavily on them. To test this assumption, and the hypothesis that male brown bears would feed more heavily on salmon than females (owing to their sexual size dimorphism), we collected hair samples from brown bears by using barbed wire placed on six small tributaries of Lake Aleknagik, Alaska, USA, where adult Sockeye Salmon Oncorhynchus nerka are readily accessible and frequently consumed by brown bears. Analysis of DNA distinguished among the different brown bears leaving the hair samples, some of which were sampled multiple times within and among years. We assessed the contribution of salmon to the diet of individual brown bears by using carbon and nitrogen stable isotope signatures. The 77 samples analyzed from 31 different bears over 4 y showed isotopic ratios consistent with reliance on salmon, but the wide range of isotopic signatures included values suggesting variable, and in one case considerable, use of terrestrial resources. Stable isotope signatures did not differ between male and female brown bears, nor did they differ between two sides of the lake, despite marked differences in Sockeye Salmon density. We collected the hair samples when salmon were present, so there was some uncertainty regarding whether they reflected feeding during the current or previous season. Notwithstanding this caveat, the results are consistent with the hypothesis that salmon were sufficiently available to provide food for the brown bears and that the considerable isotopic variation among brown bears with access to salmon reflected their age, status, and behavior.

Community size spectra provide indicators of ecosystem recovery on the Newfoundland and Labrador shelf

Species- and size-selective overexploitation often have ecosystem-wide impacts that are evident in community size spectra. To both derive potential ecosystem targets for community rebuilding and assess contemporary indicators relative to these targets, we constructed theoretical size spectra to predict pristine biomass densities using a combination of species- and size-specific nitrogen stable isotope signatures and a range of trophic efficiencies and primary productivity estimates within and among 3 sub-regions of the Newfoundland and Labrador shelf. Theoretical size spectra were compared to empirically derived size spectra using trawl survey data. The descending slopes of the empirically determined size spectra were between 1.25 and 2.42 times steeper than the theoretical slopes. The percentage of the theoretical distribution represented by the empirical size structure ranged between 1.1 and 29.4%, with the closest and furthest estimates associated with the smallest- and largest-sized fishes, respectively, and strongly influenced by estimates of trophic efficiency and primary productivity. Regional variation was also observed, with southern regions reaching 1.3-32.3% of the theoretical biomass density and the northernmost region reaching only 0-8.3%. Importantly, the descending slopes varied depending on fish size, with biomass density of larger sizes decreasing faster than that of smaller sizes. Variations among sub-regions and fish guilds were also observed. These analyses provide a means to derive potential ecosystem targets and indicators through which recovery of fish communities can be monitored and assessed.

Show your beaks and we tell you what you eat: Different ecology in sympatric Antarctic benthic octopods under a climate change context

Sympatry can lead to higher competition under climate change and other environmental pressures, including in South Georgia, Antarctica, where the two most common octopod species, Adelieledone polymorpha and Pareledone turqueti, occur side by side. Since cephalopods are typically elusive animals, the ecology of both species is poorly known. As beaks of cephalopods are recurrently found in top predator's stomachs, we studied the feeding ecology of both octopods through the evaluation of niche overlapping and specific beak adaptations that both species present. A multidisciplinary approach combining carbon (δ13C) and nitrogen (δ15N) stable isotope signatures, mercury (Hg) analysis and biomaterials' engineering techniques was applied to investigate the beaks. An isotopic niche overlap of 95.6% was recorded for the juvenile stages of both octopod species, dropping to 19.2% for the adult stages. Both A. polymorpha and P. turqueti inhabit benthic ecosystems around South Georgia throughout their lifecycles (δ13C: −19.21 ± 1.87‰, mean ± SD for both species) but explore trophic niches partially different during adult life stages (δ15N: 7.01 ± 0.40‰, in A. polymorpha, and 7.84 ± 0.65‰, in P. turqueti). The beaks of A. polymorpha are less dense and significantly less stiff than in P. turqueti. Beaks showed lower mercury concentration relative to muscle (A. polymorpha - beaks: 0.052 ± 0.009 μg g−1, muscle: 0.322 ± 0.088 μg g−1; P. turqueti - beaks: 0.038 ± 0.009 μg g−1; muscle: 0.434 ± 0.128 μg g−1). Overall, both octopods exhibit similar habitats but different trophic niches, related to morphology/function of beaks. The high Hg concentrations in both octopods can have negative consequences on their top predators and may increase under the present climate change context.

Carbon and nitrogen stable isotope signatures linked to anthropogenic toxic substances pollution in a highly industrialized area of South Korea

Sources of organic matter in a polluted coastal embayment (Lake Sihwa) indicated that δ13C and δ15N of suspended particulate matter (SPM) and sediments could be distinguished by land-use type and surrounding activities. Specifically, low δ15N occurred in inland creeks near industrial complex, where severe contamination by persistent toxic substances (PTSs) is evidenced. To identify the sources of 15N-depleted organic matter and PTSs, the SPM and/or sediments were collected along ~8 km of the Singil Creek and in stormwater drainage pipes that carried pollutants to the creeks from upland areas. Results indicated that stormwater originating in industrial areas was the main source of low δ15N and elevated PTSs, which appeared to flow into stormwater drains with rainwater and eventually into the creeks at stormwater outfalls. To improve the water quality of inland creeks, it will be necessary to reduce the amount of untreated stormwater entering stormwater drainage system from industrial complex.