Differences In Isotope Values Research Articles

Variations in inter-specific and sex-related niche partitioning in pelagic boobies during their annual cycle.

Animals that co-occur in a region (sympatry) may share the same environment (syntopy), and niche differentiation is expected among closely related species competing for resources. The masked booby (Sula dactylatra) and smaller congeneric red-footed booby (Sula sula) share breeding grounds. In addition to the inter-specific size difference, females of both species are also larger than the respective males (reversed sexual size dimorphism). Although both boobies consume similar prey, sometimes in mixed-species flocks, each species and sex may specialize in terms of their diet or foraging habitats. We examined inter- and intra-specific differences in isotopic values (δ13C and δ15N) in these pelagically feeding booby species during the incubation period at Clarion Island, Mexico, to quantify the degrees of inter- and intra-specific niche partitioning throughout the annual cycle. During incubation, both species preyed mainly on flyingfish and squid, but masked boobies had heavier food loads than red-footed boobies. There was no overlap in isotopic niches between masked and red-footed boobies during breeding (determined from whole blood), but there was slight overlap during the non-breeding period (determined from body feathers). Female masked boobies had a higher trophic position than conspecific males during breeding; however, no such pattern was detected in red-footed boobies. These results provide evidence of inter- and intra-specific niche partitioning in these tropical seabird species, particularly during the breeding period and in the more-dimorphic species. Our results suggest that these closely related species use different strategies to cope with the same tropical marine environment.

Stable isotopes in the shell organic matrix for (paleo)environmental reconstructions

Stable isotope ratios of mollusc shell carbonates have long been used to reconstruct past environmental conditions. Although shells also contain organics, they are seldom used in (paleo)climatic studies. Here, we extract the acid-soluble and insoluble fractions of the organic matrix of modern Mytilus galloprovincialis shells from three sites along a coast–to-upper-estuary environmental gradient to measure their hydrogen (δ2H) and oxygen (δ18O) isotope compositions. Both organic fractions showed isotopic signatures significantly different from those of carbonate and water at each site, indicating the involvement of different fractionation mechanisms. The soluble fraction showed gradual differences in isotope values along the gradient, while the insoluble fraction showed δ2H-δ18O correlation regressions subparallel to the Global and Local Meteoric Water Lines. These results showed the great potential of the shell organic matrix stable isotopes as possible (paleo)environmental proxies, stimulating further research to better define the fractionation mechanisms involved.

Isotopic trajectories and interspecific niche partitioning in tropical pelagic sharks

Pelagic sharks are top predators with key role in the structure and functioning of ocean ecosystems. However, little is known on their assemblage-wide trophic ecology. Specifically, fundamental data gaps exist over how trophic niches of pelagic sharks diverge both intra- and inter-specifically. To address this question, multi-tissue stable isotope analysis is a powerful tool that enables the quantification of trophic dynamics of mobile predators throughout ontogeny and across species. In this study, we utilized stable isotope analysis of carbon (δ13C) and nitrogen (δ15N) of tissues with varying turnover rates (muscle, red blood cells, liver, and plasma) to assess individual niche trajectories (i.e., temporal isotopic variation), population isotopic niche width, and isotopic niche overlap across five shark species (n = 155) in the eastern tropical Pacific Ocean: silky (Carcharhinus falciformis), blue (Prionace glauca), smooth hammerhead (Sphyrna zygaena), bigeye thresher (Alopias superciliosus), and the pelagic thresher shark (A. pelagicus). Overall, the relationship between isotopic niche trajectory length and body size was not significant, indicating that the magnitude of ontogenetic dietary variation does not increase in larger individuals. Pelagic sharks had variable feeding strategies over short-long time scales, likely reflecting species-specific responses to seasonality of prey availability and foraging location. Observed interspecific differences in isotope values indicate a degree of trophic niche partitioning across the five pelagic shark species. These data suggest ontogenetic niche shifts in pelagic sharks potentially differ from more coastal tied species and reveal species-specific ecological roles, but further studies on the diet and fine-scale habitat use are required to verify these results.

Trophic Assessment of an Artificial Kelp Eisenia bicyclis Bed Off the Eastern Coast of Korea Based on Stable Isotope Analyses

Effective trophic assessment of restoration success after the creation of an artificial habitat is essential for ecological management. In this study, to evaluate the ecological trophic function of restored kelp beds of artificial macroalgal reefs (AMRs) attached with the kelp species Eisenia bicyclis, we compared carbon and nitrogen stable isotope ratios of organic matter sources and macrobenthic consumers and isotopic indices of functional feeding groups between artificial and reference (natural bed and barren ground, respectively) habitats. There were no significant differences in isotopic values of consumer feeding groups (except for carnivores) and their potential food sources between the artificial and natural sites, suggesting that resource diversity and resource use by consumers was similar across all sites. The isotopic data also exhibited similar or relatively higher trophic niches of feeding groups at the artificial site compared to those at the natural site, indicating that the artificial kelp bed can play important roles in similar ecological functions and services as a natural counterpart. In addition, the present study revealed that the isotopic ranges and niche indices of feeding groups were significantly wider at the artificial and natural kelp beds compared to those at the barren ground, suggesting the trophic importance of macroalgal-derived organic matter to consumer nutrition in coastal ecosystems. Overall, our results support the conclusion that the creation of an Eisenia bicyclis kelp bed by the AMRs may lead to the restoration success of the macroalgae habitat through the recovery of the ecological function of a natural food web structure.

Priming effects induced by degradable microplastics in agricultural soils

Microplastics (MPs) in farmland soils have the potential to cause environmental and human health risks due to their ubiquitous and persistent nature. Degradable MPs may significantly alter the decomposition of soil organic carbon by the so-called priming effect (PE). However, the amplitude and direction of the PE induced by degradable MPs remain uncertain in farmland soils. In this study, a 56-d laboratory incubation experiment was conducted by adding three types of degradable MPs, i.e., poly-hydroxyalkanoates (PHA), polybutylene succinate (PBS), and polylactic acid (PLA) to three farmland soils planted with strawberry, blueberry, and paddy-rape rotation, respectively. The differences in isotopic values (δ 13C) between soils (−25.30−-27.46‰) and MPs (−11.94−-9.99‰) were used to assess the PE and net carbon balance by the natural 13C isotopic tracing method. We found that the stronger the degradability of MPs, the greater the PE induced. Subsequently, PHA-induced PE was the highest, followed by PBS- and PLA-induced PE corresponding to their biodegradability. Moreover, PHA-induced PE reached an impressive 552−1744%, while PBS- and PLA-induced PE achieved 44−179%, and −29−43% across the above three soils, respectively. Overall, PHA resulted in the highest cumulative PE and the lowest net carbon balance relative to PBS and PLA in all three soils. This study, for the first time, demonstrates that the amplitude of PE is dependent on the degradability of MPs, emphasizing that microbial nitrogen mining may play an important role in driving the PE in the long term due to the nitrogen deficiency of MPs.

Deciphering the Trophic Ecology of Three Marlin Species Using Stable Isotope Analysis in Temperate Waters Off Southeastern Australia

Understanding the unique feeding behaviours of oceanic fish, such as marlin, is key to their effective management. Marlin are notoriously difficult to study, however, and the limited research on marlin feeding shows that diet can vary greatly between species and geographic regions. One region where marlin feeding behaviours are particularly poorly understood are temperate eastern Australian waters. This study collected marlin tissue from game fishing tournaments between latitudes 32°43′06.5″S/152°08′50.1″E to 34°40′12.9″S/150°51′34.3″E between 2010 and 2021, and used stable isotope analysis (SIA) to assess the trophic ecology of the three species of marlin occurring in the region: black (Istiompax indica), blue (Makaira nigricans), and striped (Kajikia audax) marlin. All species had similar δ13C values, but δ15N differed between species, with higher variability observed in blue marlin than in the other two species. Sulphur isotopes were key in identifying the relative contribution of coastal or benthic influences on marlin diet, with δ34S suggesting that blue marlin had less coastal/benthic dietary influence than black or striped marlin. Incorporation of δ34S into SIA for marlin is thus recommended for future studies. Some differences in isotope values across locations and dates were found, however, the uneven sample sizes due to the opportunistic sampling limited the ability to understand spatial or seasonal differences. These findings show that marlin followed similar dietary trends to conspecifics in other regions despite temperate eastern Australian waters being one of the few with three marlin species commonly co-occuring. This suggests that interspecies resource competition is not a major force driving the demography of these species in eastern Australian waters. This research highlights a need for specific management strategies at a species level, particularly for blue marlin. Future research incorporating prey isoscapes and baselines assessed over a wider range of marlin sizes is suggested to further improve our knowledge and capacity to manage the marlin of eastern Australian waters.

Trophic niche of Australian cownose rays (Rhinoptera neglecta) and whitespotted eagle rays (Aetobatus ocellatus) along the east coast of Australia.

Australian cownose rays (Rhinoptera neglecta) and whitespotted eagle rays (Aetobatus ocellatus) are large myliobatiform rays that co‐occur off temperate eastern Australia. Here, we performed stable‐isotope analyses (δ13C, δ15N and δ34S) on fin clips of both species to gain insights into their trophic interactions and isotopic niches, and assess the effect of preservation (ethanol‐stored versus frozen) on isotopic values of fin‐clip tissue of R. neglecta. Linear mixed models identified species as the main factor contributing to variation among δ15N and δ34S values, and disc width for δ13C. Bayesian ecological niche modelling indicated a 57.4% to 74.5% overlap of trophic niches, with the niche of R. neglecta being smaller and more constrained. Because values of δ13C were similar between species, variation in isotopic niches were due to differences in δ15N and δ34S values. Linear mixed models failed to detect differences in isotopic values of ethanol‐stored and frozen fin tissue of R. neglecta. This study provides the first examination of the trophic ecology of R. neglecta and the comparison of isotopic niche with A. ocellatus, which will facilitate future research into the trophic interactions of these species and aid better resource management.

Temporal and spatial isotopic variability of marine prey species in south-eastern Australia: Potential implications for predator diet studies.

Stable isotope analyses, particularly of carbon (δ13C) and nitrogen (δ15N), are used to investigate ecological relationships among species. For marine predators, research has shown the main factors influencing their intra-specific and intra-individual isotopic variation are geographical movements and changes in the composition of diet over time. However, as the differences seen may be the result of changes in the prey items consumed, a change in feeding location or the combination of both, knowledge of the temporal and spatial consistency in the isotopic values of prey becomes crucial for making accurate inferences about predator diets. This study used an abundant marine predator, the Australasian gannet (Morus serrator), as prey sampler to investigate the annual variation in fish and squid prey isotope values over a four-year period (2012–2015) and the geographic variation between two sites with contrasting oceanographic conditions. Significant inter-annual variation was observed in δ13C and/or δ15N values of five of the eight prey species analysed. The strongest inter-annual variation in both δ13C and δ15N values occurred in 2015, which coincided with a strong El Niño-Southern Oscillation (ENSO). This may suggest a temporal fluctuation in the geographic source of prey or the origin of their nutrients. These results suggest that it is important to consider the potential significant differences in isotopic values within the prey assemblages that predators consume. This is important to improve the interpretation of marine predator isotope results when determining the influence of environmental variability on their diets.

Gaining insight into the assimilated diet of small bear populations by stable isotope analysis

Apennine brown bears (Ursus arctos marsicanus) survive in an isolated and critically endangered population, and their food habits have been studied using traditional scat analysis. To complement current dietary knowledge, we applied Stable Isotope Analysis (SIA) to non-invasively collected bear hairs that had been individually recognized through multilocus genotyping. We analysed carbon (δ13C) and nitrogen (δ15N) stable isotopes of hair sections and bear key foods in a Bayesian mixing models framework to reconstruct the assimilated diet on a seasonal basis and to assess gender and management status effects. In total, we analysed 34 different seasonal bear key foods and 35 hair samples belonging to 27 different bears (16 females and 11 males) collected during a population survey in 2014. Most bears showed wide δ15N and δ13C ranges and individual differences in seasonal isotopic patterns. Vegetable matter (herbs, fleshy fruits and hard mast) represented the major component of the assimilated diet across the dietary seasons, whereas vegetable crops were rarely and C4 plants (i.e., corn) never consumed. We confirmed an overall low consumption of large mammals by Apennine bears consistently between sexes, with highest values in spring followed by early summer but null in the other seasons. We also confirmed that consumption of fleshy fruits peaked in late summer, when wild predominated over cultivated fleshy fruits, even though the latter tended to be consumed in higher proportion in autumn. Male bears had higher δ 15N values than females in spring and autumn. Our findings also hint at additional differences in the assimilated diet between sexes, with females likely consuming more herbs during spring, ants during early summer, and hard mast during fall compared to males. In addition, although effect sizes were small and credibility intervals overlapped considerably, management bears on average were 0.9‰ lower in δ 13C and 2.9‰ higher in δ 15N compared to non-management bears, with differences in isotopic values between the two bear categories peaking in autumn. While non-management bears consumed more herbs, wild fleshy fruits, and hard mast, management bears tended to consume higher proportions of cultivated fruits, ants, and large mammals, possibly including livestock. Although multi-year sampling and larger sample sizes are needed to support our findings, our application confirms that SIA can effectively integrate previous knowledge and be efficiently conducted using samples non-invasively collected during population surveys.

Different trophic positions among social vespid species revealed by stable isotopes.

The social vespid wasps are common insect predators and several species behave in unison in the same biotopes. It is commonly accepted that social wasps are mainly opportunistic generalist predators without differences in prey selection and hence they compete for the same food resources. Trophic positions of six vespid wasp species and their potential prey from four sites in Finland and one in the UK were evaluated using carbon and nitrogen stable isotopes (δ13C and δ15N). The difference in isotope values indicated different trophic positions among species. In general, Dolichovespula spp. showed higher δ15N values than Vespula spp., which suggests that Dolichovespula forage on higher trophic levels. Dolichovespula media (Retzius, 1783) showed the highest δ15N values, whereas Vespula vulgaris showed the lowest. Dolichovespula media partly expresses apex predator-like δ15N values, whereas Vespula species tend to forage on primary consumers. The largest species Vespa crabro (Linnaeus, 1758) showed also similar δ15N values as Vespula spp. However, δ13C and δ15N values of V. vulgaris workers varied slightly during the season. This study offers novel insights about the trophic segregation in the social wasp community, suggesting specialization in diet resource utilization, especially between Dolichovespula and Vespula.

Assessment of restoration success in a transplanted seagrass bed based on isotopic niche metrics

A major highlight of restoration efforts is to improve the ecological structure and function of the natural ecosystem in the restored habitat. Assessment of restoration success is a crucial component of an optimal ecological management strategy. In studies to determine the restoration success of a transplanted seagrass habitat by assessing trophic recovery, we examined carbon and nitrogen stable isotope ratios of organic matter sources and macrobenthic assemblages in a transplanted eelgrass Zostera marina bed. The eelgrass bed was restored about 2 years after transplantation in a southern coastal bay of Korea, and consequently, the food web structure in the bed was compared with that in a natural reference site. Our results revealed no significant differences in isotopic values of both macrobenthic consumers and their putative food sources between the transplanted and natural seagrass beds. These isotopic similarities in florae and faunae in the two beds suggest a uniformity in food web structure formed by the diversity and availability of resources, and thereby suggest similarities in the resource–consumer relationship. Isotopic niche indices and high dietary overlaps of feeding guilds in the transplanted and natural beds further suggest the transplanted habitat provides similar ecological functions and ecosystem services to its natural counterpart. Collectively, our results suggest the eelgrass transplantation led to successful restoration of a common seagrass bed, with recovery of the functional properties of the food web structure. Finally, our findings support the idea that stable isotope measures can provide a better understanding of the functioning of restored ecosystems, and improve post-transplantation monitoring efforts for the future planning and managing of successful habitat restoration.

Groundwater-fed oasis in arid Northwest China: Insights into hydrological and hydrochemical processes

As an inland groundwater-fed oasis in northwest China, the Dunhuang West Lake Wetland (DWLW) has been threatened by decreasing groundwater recharge over the past several decades. Understanding recharging processes for DWLW is a key step for better protection of the oasis, but poorly studied. To this end, we carried out a comprehensive water chemistry and isotope sampling and analysis, by taking the oasis and its water sources as a complete system to reveal the oasis water recharging mechanism and flow paths. Water samples, including mountain snowmelt water, river water, groundwater, spring and lake water, were collected within and around the DWLW, and their environmental isotopes (δ18O and δD) and water chemistry were analysed. Results showed that the stable isotope values exhibited strong spatial variations, ranging from −90.69‰ to −42.45‰ for δD and from −13.81‰ to −2.90‰ for δ18O. The isotope values were extremely depleted for snowmelt water, followed by river water, groundwater, spring water, and were most enriched in lake water. The difference in isotopic values from the different water bodies indicates that seasonal rivers primarily originate from the snowmelt water in the adjacent mountains, discharge into the aquifer through infiltration, then groundwater rises up by forms of springs to the lake oases, the river and groundwater interacted frequently, causing significant changes in solute in different positions. The ion concentrations accumulated continuously along the flow path, the total dissolved solid content varied from 416 mg/L in river water to 20360 mg/L in lake water, and the dominant anion facies changed systematically from HCO3- to SO42- then Cl-. The ionic ration plot, chlor-alkali index and saturation index demonstrated that dissolution of minerals plays a major role in the change in chemical composition, and other processes such as carbonate precipitation associated with cation exchange reaction also influence the chemical composition. Our results are helpful for understanding the hydrological and hydrochemical processes in arid groundwater-fed oases, and can be also useful for rational management and assessment of regional-scale groundwater resources in oases.

Trophic relationships and use of area of two sympatric small cetaceans in the Southwestern Atlantic Ocean determined by carbon and nitrogen stable isotopes.

As cetaceans are sentinels of the marine environment, studying their life history is of utmost importance in understanding their habits and their interaction with the environment. To achieve this goal, it is important to study their ecological niches through the investigation of their habitat use patterns and trophic relationships. This study aimed to evaluate if there are differences in the habitat use patterns and the trophic ecology of franciscana (Pontoporia blainvillei) and Guiana dolphins (Sotalia guianensis) accidentally caught off the south coast of São Paulo State, and Guiana dolphins biopsied in the Cananeia estuary, southeastern Brazil, based on the carbon and the nitrogen stable isotopes analysis. Isotopic ratios were estimated from skin samples. The isotopic niche size and overlap were calculated using the SIBER package (R software). The gathered results showed evidence of spatial segregation between Guiana dolphins and franciscanas. The Guiana dolphins sampled inside and outside the estuary showed evidence of trophic and spatial segregation. No difference in isotopic values between sexes was found for both species. Such differences between franciscanas and Guiana dolphins were expected as both species have distinct life histories as a result of different evolutionary pathways.

Expanding on incremental dentin methodology to investigate childhood and infant feeding practices on Taumako (southeast Solomon Islands)

Though many ethnohistoric sources in the tropical Pacific recount chiefly feasting events, few describe childhood feeding practices despite the impact childhood under-nutrition may have had on morbidity and early mortality. Bioarchaeological investigation of the Namu burial ground (circa 750–300 BP) on the island of Taumako (southeast Solomon Islands) provides a direct means of understanding prehistoric life on a Polynesian Outlier in the south western Pacific. We investigate infant and childhood (0–10 years) feeding behavior in prehistoric Taumako by creating δ13Ccollagen, δ15Ncollagen, and δ13Ccarbonate profiles from 20 individuals using horizontal dentin sections of permanent first molars. The high-resolution data created using novel sample preparation offers insight into childhood diet in the absence of documentary evidence, incrementally sampling δ13Ccollagen and δ15Ncollagen but also expanding on the method to provide carbonate data from the same sequential dentin samples.The individuals who died in adolescence have significantly lower δ15Ncollagen values in early life than those who died in adulthood, which may suggest a link between diet, nutritional health, and morbidity. There were no significant differences in isotope values between social status groups, suggesting shared childhood experiences regarding types of foods consumed. Longitudinal assessment of δ13Ccollagen and δ15Ncollagen shows a strong relationship between the two values, likely a result of the typical tropical Pacific diet consisting largely of high protein marine foods that overshadows low protein terrestrial foods. This highlights the utility of δ13Ccarbonate in order to more effectively investigate consumption of low protein foodtypes in this region.

Stable isotope patterns in lake food webs reflect productivity gradients

AbstractStable isotopes 13C and 15N are often used in lake ecosystems to assess energy sources and trophic positions, respectively. However, δ13C and δ15N are also influenced by internal biogeochemical processes in epilimnetic and hypolimnetic habitats in lakes, but the extent to which biogeochemical processing mediates isotope values between these two habitats, and whether these patterns are influenced by lake productivity is not known. We sampled δ13C and δ15N in epilimnetic mussels, Chaoborus, cisco (Coregonus artedi), and seston and zooplankton in the epilimnia and hypolimnia of 22 Minnesota (USA) lakes ranging from oligotrophic to eutrophic. We also measured lake temperature–oxygen profiles and light levels to assess factors influencing isotope patterns. Isotope samples were baseline‐corrected using epilimnetic mussels in each lake (sample—mussel) to control for watershed‐level differences in isotope values. Results showed δ13C in epilimnetic and hypolimnetic zooplankton, hypolimnetic seston, Chaoborus, and cisco became more depleted in δ13C relative to epilimnetic mussels in low‐productivity lakes where light penetrated into the hypolimnion, while epilimnetic seston δ13C stayed similar to mussel δ13C in all lakes. This pattern was likely due to hypolimnetic phytoplankton in clearwater lakes incorporating more respired CO2, which is depleted in δ13C, and subsequently passing depleted δ13C values up the food chain. Results also showed habitat differences in δ15N with epilimnetic and hypolimnetic zooplankton, hypolimnetic seston, Chaoborus, and cisco becoming more enriched relative to epilimnetic mussels in low‐productivity lakes with higher O2 levels in the hypolimnion, while epilimnetic seston δ15N remained similar to mussel values. The δ15N pattern is consistent with the idea that denitrification and microbial degradation enriched hypolimnetic seston relative to epilimnetic seston in low nutrient lakes, while enhanced epilimnetic primary production enriched epilimnetic δ15N seston relative to hypolimnetic seston in high nutrient lakes. Our results indicate isotopic differences between epilimnetic and hypolimnetic organisms that change along productivity gradients and suggest that microbial processes and the light regime are important drivers.

Characteristics of Water Isotopes and Water Source Identification During the Wet Season in Naqu River Basin, Qinghai-Tibet Plateau

Climate change is affecting the discharge of headstreams from mountainous areas on the Qinghai–Tibet Plateau. To constrain future changes in discharge, it is important to understand the present-day formation mechanism and components of runoff in the basin. Here we explore the sources of runoff and spatial variations in discharge through measurements of δ2H and δ18O in the Naqu River, at the source of the Nu River, on the Qinghai–Tibet plateau, during the month of August from 2016 to 2018. We established thirteen sampling sites on the main stream and tributaries, and collected 39 samples from the river. We examined all the water samples and analyzed them for isotopes. We find a significant spatial variation trend based on one-way analysis of variance (ANOVA) (p < 0.05) between Main stream-2 and tributaries. The local meteoric water-line (LMWL) can be described as: δ2H = 7.9δ18O + 6.29. Isotopic evaporative fractionation in water and mixing of different water sources are responsible for the spatial difference in isotopic values between Main stream-2 and tributaries. Based on isotopic hydrograph separation, the proportion of snowmelt in runoff components ranges from 15% to 47%, and the proportion of rainwater ranges from 3% to 35%. Thus, the main components of runoff in the Naqu River are snowmelt and groundwater.

δ13C and δ15N in Humboldt squid beaks: understanding potential geographic population connectivity and movement

We quantified the δ13C and δ15N values in the lower beaks of Humboldt squid, Dosidicus gigas, collected from international waters off Costa Rica, Ecuador, Peru and Chile by Chinese squid jigging vessels during 2009, 2010 and 2013. There was a significant difference in the isotopic values among regions with the lowest value off Ecuador and the highest off Chile, which were interpreted as a function of trophic effects as well as baseline values. However, constant trophic level of D. gigas across its geographic range showed that spatial variation in the baseline of primary production is the main driver responsible for the observed geographic isotope variability. Inter-regional difference and intra-regional convergence of isotope values indicated squid off Costa Rica, Ecuador and Chile belong to different geographically segregated populations, which were previously proved by integrated population identifying method. In contrast, the higher variations in δ13C and δ15N values in a given size group suggest the squid off Peru move and forage in different places. Moreover, potential population exchange could be responsible for the overlap of the isotope values between the squid off Peru and off Chile. On the whole, the spatial difference in isotopic values of Humboldt squid beaks improves our understanding of potential geographic population connectivity and movement.

Advanced approach to analyzing calcareous protists for present and past pelagic ecology: Comprehensive analysis of 3D-morphology, stable isotopes, and genes of planktic foraminifers.

Marine protists play an important role in oceanic ecosystems and biogeochemical cycles. However, the difficulties in culturing pelagic protists indicate that their ecology and behavior remain poorly understood; phylogeographic studies based on single-cell genetic analyses have often shown that they are highly divergent at the biological species level, with variable geographic distributions. This indicates that their ecology could be complex. On the other hand, the biomineral (calcareous) shells of planktic foraminifers are widely used in geochemical analyses to estimate marine paleoenvironmental characteristics (i.e., temperature), because the shell chemical composition reflects ambient seawater conditions. Among the pelagic protists, planktic foraminifers are ideal study candidates to develop a combined approach of genetic, morphological, and geochemical methods, thus reflecting environmental and ecological characteristics. The present study precisely tested whether the DNA extraction process physically and chemically affects the shells of the planktic foraminifer Globigerinoides ruber. We used a nondestructive method for analyzing physical changes (micro-focus X-ray computed tomography (MXCT) scanning) to compare specimens at the pre- and post-DNA extraction stages. Our results demonstrate that DNA extraction has no significant effect on shell density and thickness. We measured stable carbon and oxygen isotopes on the shell of each individual in a negative control or one of two DNA-extracted groups and detected no significant differences in isotopic values among the three groups. Moreover, we evaluated isotopic variations at the biological species level with regard to their ecological characteristics such as depth habitat, life stages, and symbionts. Thus, our examination of the physiochemical effects on biomineral shells through DNA extraction shows that morphological and isotopic analyses of foraminifers can be combined with genetic analysis. These analytical methods are applicable to other shell-forming protists and microorganisms. In this study, we developed a powerful analytical tool for use in ecological and environmental studies of modern and past oceans.

Stable isotope value and trophic position of fishes in Three Gorges Reservoir and Changshou Reservoir

为探究人为活动对水库鱼类食物来源和营养级关系的影响，选择人为影响程度较低的三峡水库和影响程度较高的长寿湖水库为研究对象，应用碳、氮稳定同位素技术对两个水库鱼类δ¹³C值、δ¹⁵N值和营养级进行比较分析.结果显示：长寿湖水库鱼类δ¹³C平均值（-22.66‰±1.94‰）与三峡水库（-22.59‰±1.87‰）较为接近，但长寿湖鱼类的δ¹⁵N值（13.95‰ ±3.02‰）显著高于三峡水库鱼类的δ¹⁵N值（11.98‰ ±2.25‰）.根据δ¹⁵N值计算不同食性鱼类间的营养级，三峡水库营养级为肉食性鱼类 > 杂食性鱼类 > 浮游生物食性鱼类 > 草食性鱼类；而长寿湖中则表现为浮游生物食性鱼类 > 肉食性鱼类 > 杂食性鱼类 > 草食性鱼类，营养级出现了异常现象.这一结果表明，对于人为影响（如人工喂食鱼饲料等）较大的水体，采用δ¹⁵N值计算不同食性鱼类间的营养级存在不确定性.;To investigate the effect of the food sources and trophic position of fish in different anthropogenic interference of Three Gorges Reservoir (TGR) and Changshou Reservoir, we used stable isotopes analysis to investigate the δ¹³C and δ¹⁵N signatures of fish in these two reservoirs. Meanwhile, differences in isotopic values and trophic position of fish were also compared. The result showed that little variation in δ¹³C values of fish was observed between TGR and Changshou Reservoir. The average δ¹³C values of fish observed in Changshou Reservoir (-22.66‰±1.94‰) was similar to that in TGR (-22.59‰±1.87‰). While, the δ¹⁵N values of fish in Changshou Reservoir (13.95‰ ±3.02‰) were significantly higher than that in TGR (11.98‰ ±2.25‰). We calculated the trophic level among different fishes based on the value of δ¹⁵N. The trophic level decreased in order of carnivorous fish > omnivorous fish > planktivore fish > herbivorous fish in TGR. While, a relative difference was occurred at Lake Changshou in terms of the trophic level which was conducted as planktivore fish > carnivorous fish > omnivorous fish > herbivorous fish when compared with those in TGR. The result indicated the uncertainty of trophic level derived from value of δ¹⁵N for aqueous systems with intensive influences of human activities.

Stable isotope evidence from formalin\u2013ethanol-preserved specimens indicates dietary shifts and increasing diet overlap in Lake Victoria cichlids

Stable isotopes are increasingly being used to infer past and present trophic interactions in light of environmental changes. The Lake Victoria haplochromine cichlids have experienced severe environmental changes in the past decades that, amongst others, resulted in a dietary shift towards larger prey. We investigated how the changed environment and diet of the haplochromines influenced stable isotope values of formalin-then-ethanol-preserved cichlid specimens, and then investigated how these values differed among species before (1977–1982) and after substantial environmental changes (2005–2007). We found a small preservation effect on both δ13C and δ15N values, and significant differences in isotope values among haplochromine species collected before the environmental changes. In contrast, there was a remarkable similarity in δ13C and δ15N values among species collected from the contemporary ecosystem and two out three species showed significantly different stable isotope values compared to species of the historic ecosystem. In addition, we found a putative isotopic gradient effect along our 5-km-long research transect indicating that the studied demersal species are more stenotopic than previously thought. The environmental changes have resulted in dietary change and overlap of the haplochromines which provides insight into the trophic plasticity of these species, which are often considered trophic specialists.