Stable Nitrogen Isotopic Composition Research Articles

Sources of NH4+ in PM2.5 and Their Seasonal Variations in Urban Tianjin China: New Insights From the Seasonal δ15N Values of NH3 Source

AbstractThe stable nitrogen isotopic composition (δ15N) has been widely used to quantify sources of ammonium (NH4+) in PM2.5. However, the overlap and uncertainty in δ15N values from different NH3 sources, coupled with their seasonal variability, hinder accurate identification of NH4+ source. Here, the δ15N values of various NH3 source samples collected by the active sampler were determined. Subsequently, we measured the δ15N values of NH4+ in PM2.5, which were collected seasonally in Tianjin. We found that the combustion‐related NH3 (c‐NH3) exhibiting higher δ15N values compared to volatile NH3 (v‐NH3), but all δ15N values was fell within the range reported by previous studies. Furthermore, inconsistent seasonal variations were observed in the δ15N‐NH3 values originating from emissions of agricultural soil and human excreta. The application of the Bayesian isotope mixing model (MixSIAR model) revealed a significant increase in the contribution of v‐NH3 to NH4+ when incorporating current source data, as opposed to previous data, for δ15N of NH3 source. Notably, the contribution of v‐NH3 (53.1%) to NH4+ was almost equivalent to that of c‐NH3 (46.9%) when considering the seasonal δ15N signatures of NH3 source. Additionally, the estimated contribution of v‐NH3 to NH4+ exhibited significant seasonal variability, which is more reasonable than in the non‐seasonal scenario. This study demonstrated that v‐NH3 and c‐NH3 contributed to NH4+ in PM2.5 in Tianjin almost equally, and it is highlighted that the seasonal δ15N values of NH3 sources should be considered when estimating the contributions of different NH3 sources to NH4+ in PM2.5 by the MixSIAR model.

Stable isotopic composition of Bothrops atrox (common lancehead) in four distinct land covers in eastern Amazonia

AbstractSince consumers reflect the isotopic composition of an assimilated diet, stable isotopes can be a useful tool to address the feeding ecology of tropical snakes. This is the first study reporting carbon and nitrogen stable isotopic composition of Bothrops atrox (Linnaeus, 1758) living in different landscapes located in the lower Amazon river, encompassing four main natural landscapes of the Amazon: old‐growth forests, várzeas (flooded forests), savannas, and pastures. Our null hypothesis is that the δ13C of forest specimens of B.atrox is more negative because forests are dominated by C3 plants, while C4 plants are common in the other landscapes. On the other hand, δ15N of forest specimens should be more positive, since the δ15N of old‐growth forests are higher than plants of savanna, várzea, and pastures. Confirming our hypothesis, the δ13C of B. atrox scales of the Tapajós National Forest was approximate −25‰ to −24‰, increased to approximately −23.5‰ to −23.0‰ in the savanna and pasture, and to −21‰ in the várzea, showing an increased contribution of C4‐derived carbon. Some specimens of B. atrox had δ15N as high as 18‰, which is much higher than the average δ15N of the snake's prey (7‰), confirming the apex position of B. atrox in the Amazon region. The δ15N values of the forest specimens were 5‰ higher than the savanna specimens, and this difference decreased to 3‰ between the forest and the pasture, and the várzea specimens. Finally, there were not large differences between δ15N values of livers and scales in any of the landscapes, suggesting a constant diet through time, and reinforcing the possibility of the use of snake's scale as a less invasive and non‐lethal tissue to analyze.Abstract in Portuguese is available with online material.

Solid Phase Extraction Methodology for Robust Isotope Analysis of Atmospheric Ammonium.

The stable nitrogen isotope composition (δ15N) of atmospheric ammonia (NH3) and ammonium (NH4+) has emerged as a potent tool for improving our understanding of the atmospheric burden of reduced nitrogen. However, current chemical oxidation methodologies commonly utilized for characterizing δ15N values of NH4+ samples have been found to lead to low precision for low concentration (i.e., < 5 μmol L-1) samples and often suffer from matrix interferences. Here, we present an analytical methodology to extract and concentrate NH4+ from samples through use of a sample pretreatment step using a solid phase extraction technique involving cation exchange resins. Laboratory control tests indicated that 0.4 g of cation exchange resin (Biorad AG-50W) and 10 mL of 4 M sodium chloride extraction solution enabled the complete capture and removal of NH4+. Using this sample pretreatment methodology, we obtained accurate and precise δ15N values for NH4+ reference materials and an in-house quality control sample at concentrations as low as 1.0 μM. Additionally, the sample pretreatment methodology was evaluated using atmospheric aerosol samples previously measured for δ15N-NH4+ (from Changdao Island, China), which indicated an excellent δ15N-NH4+ match between sample pretreatment and no treatment (y = (0.98 ± 0.05)x + (0.11 ± 0.6), R2 = 0.99). Further, this methodology successfully extracted NH4+ from aerosol samples and separated it from present matrix effects (samples collected from Oahu, Hawaii; pooled standard deviation δ15N-NH4+ = ± 0.5‰,n = 16 paired samples) that without pretreatment originally failed to quantitatively oxidize to nitrite for subsequent δ15N isotope analysis. Thus, we recommend applying this sample pretreatment step for all environmental NH4+ samples to ensure accurate and precise δ15N measurement.

Large Seasonal Variation in Nitrogen Isotopic Abundances of Ammonia Volatilized from a Cropland Ecosystem and Implications for Regional NH3 Source Partitioning.

Ammonia (NH3) volatilization from agricultural lands is a main source of atmospheric reduced nitrogen species (NHx). Accurately quantifying its contribution to regional atmospheric NHx deposition is critical for controlling regional air nitrogen pollution. The stable nitrogen isotope composition (expressed by δ15N) is a promising indicator to trace atmospheric NHx sources, presupposing a reliable nitrogen isotopic signature of NH3 emission sources. To obtain more specific seasonal δ15N values of soil NH3 volatilization for reliable regional seasonal NH3 source partitioning, we utilized an active dynamic sampling technique to measure the δ15N-NH3 values volatilized from maize cropping land in northeast China. These values varied from -38.0 to -0.2‰, with a significantly lower rate-weighted value observed in the early period (May-June, -30.5 ± 6.7‰) as compared with the late period (July-October, -8.5 ± 4.3‰). Seasonal δ15N-NH3 variations were related to the main NH3 production pathway, degree of soil ammonium consumption, and soil environment. Bayesian isotope mixing model analysis revealed that without considering the seasonal δ15N variation in soil-volatilized NH3 could result in an overestimate by up to absolute 38% for agricultural volatile NH3 to regional atmospheric bulk ammonium deposition during July-October, further demonstrating that it is essential to distinguish seasonal δ15N profile of agricultural volatile NH3 in regional source apportionment.

The Possible Use of Stable Carbon and Nitrogen Isotope Signal and Spectral Analysis to Identify Habitat Condition of Aquatic Plants

Many macrophyte species exhibit a high degree of plasticity, enabling them to thrive in various aquatic ecosystems. Identifying the growth conditions of individual aquatic plant species during research or specimen collection is not always possible. In many cases, the nature of the planned research does not necessitate recognizing environmental conditions. However, the scope of identifying the habitat parameters of the collections of submerged aquatic plant herbariums provides an opportunity for further research. This paper explores the possibilities of using isotopic signals of plants, supported by spectral analyses of powdered plant materials, to ascertain the environmental conditions from which the samples were collected. The results obtained from the stable carbon and nitrogen isotope compositions (δ13CORG and δ15NORG) and the analysis of spectral spectra via FTIR-ART (Fourier Transform Infrared Spectroscopy with Attenuated Total Reflectance) of plant material (Elodea canadensis Michx. species) collected from various habitat ecosystems, including rivers and both hard- and softwater lakes, exhibited significant distinctions between these habitats. Particularly high values of δ15NORG were recorded in the material from rivers. The stable carbon and nitrogen isotope compositions did not differentiate between the material collected from softwater and hardwater lakes. Nevertheless, when comparing the isotopic findings with the FTIR-ATR spectral analysis focused on identifying characteristic peaks associated with the presence of calcium carbonate, noticeable differences were observed in the presence and intensity of calcium carbonate peaks in the material. These differences were only evident when nondecarbonated plant material from hardwater lakes was used for the FTIR-ATR analysis. To the best of the author’s knowledge, the combination of methods applied in this study to identify the origin of E. canadensis from various freshwater environments is the first application of its kind that could enable the rapid identification of plant material origin. Such identification could prove useful in environmental, ecological, and paleoenvironmental research. The increased knowledge of macrophytes’ δ13CORG and δ15NORG values might also be essential in further tracking accelerated eutrophication based on aquatic vegetation’s isotopic signals. This might be important due to the assumption that the increased rate of eutrophication influences organic matter sedimentation in aquatic ecosystems, especially lakes.

Digestive system and feeding of deep-sea acorn worm Quatuoralisia malakhovi (Enteropneusta: Torquaratoridae) from the Bering sea

We studied the structure of digestive system of deep-sea torquaratorid Quatuoralisia malakhovi including the food-obtaining apparatus, microanatomy of all regions of digestive tract, and fine structure of various regions of the gut. The mouth of Q. malakhovi is surrounded by two symmetrical triangular lateral collar lips and one midventral collar lip, which may retract into the mouth. The ventral ciliary grooves run along the anterior-ventral edges of lateral lips and lead into the internal canals of the lips. The pharyngeal cavity is divided by left and right parabranchial ridges into the median pharyngeal zone with gill bars and two symmetrical peripheral canals, which fuse behind the site of genesis of new gill bars. The gill pharynx is followed by the hepatic region with metameric hepatic sacculations comprising cells with numerous large phagosomes indicating active intracellular digestion. The intestine is filled with undigested food residues including detritus flakes and remnants of various organisms. The intestine content was examined using SEM and histological studies. We analyzed the ratio of organic and mineral components of the bottom sediment from the habitat of Q. malakhovi, and the stable isotope composition of carbon and nitrogen in the intestine content, in the tissues, and in the sediment from the habitat of the studied acorn worm. Using the obtained data and the photographs and videotapes in situ, we have proposed a hypothesis explaining the feeding mechanism of Q. malakhovi. The width of the ventral ciliary groove of the lateral collar lips (100–200 μm) regulates the maximum size of particles that enter the pharynx of the animal. The minimum size of the sediment particles selected by Q. malakhovi is determined by the width of the gill pores and is 1–3 μm. The selected sediment particles move through the peripheral pharyngeal canals to the hepatic region, where the digestion of the food is carried out. In the posthepatic intestine, weight of the gut contents might serve as an anchor that holds the heavily watered jelly-like torquaratorid body at the bottom during the feeding.

Chemolithoautotrophic Organic Matter Contributions to Subterranean Food Webs Dominated by Filter-feeders

Chemolithoautotrophic Organic Matter Contributions to Subterranean Food Webs Dominated by Filter-feeders

Characterization of the nitrogen stable isotope composition (δ15N) of ship-emitted NOx

Abstract. The nitrogen stable isotope composition (δ15N) of nitrogen oxides (NOx) is a powerful indicator of source apportionment of atmospheric NOx; however, δ15N–NOx values emitted from ships have not been reported, affecting the accuracy of source partitioning of atmospheric NOx in coastal zones with a lot of vessel activity. In addition, δ15N–NOx values from ship emissions could also be important for source apportionment of atmospheric nitrogen deposition in remote ocean regions. This study systemically analysed the δ15N–NOx variability and main influencing factors of ship emissions. The results showed that δ15N–NOx values from ships, which were calculated by weighting the emission values from the main engine and auxiliary engine of the vessel, ranged from −35.8 ‰ to 2.04 ‰ with a mean ± standard deviation of −18.5 ± 10.9 ‰. The δ15N–NOx values increased monotonically with the ongoing tightening of emission regulations, presenting a significantly negative logarithmic relationship with NOx concentrations (p&lt;0.01). The selective catalytic reduction (SCR) system was the most important factor affecting changes in δ15N–NOx values, followed by the ship category, fuel types, and operation states of ships. Based on the relationship between δ15N–NOx values and emission regulations observed in this investigation, a mass-weighted model to compute accurate assessments over time was developed, and the temporal variation in δ15N–NOx values from ship emissions in the international merchant fleet was evaluated. These simulated δ15N–NOx values can be used to select suitable δ15N–NOx values for a more accurate assessment, including the contribution of ship-emitted exhaust to atmospheric NOx and its influence on atmospheric nitrate (NO3-) air quality and nitrogen deposition studies.

Organic Phases in Bivalve (Arctica Islandica) Shells: Their Bulk and Amino Acid Nitrogen Stable Isotope Compositions

AbstractThe stable nitrogen isotope composition of bivalve shell organics serves as a proxy for nitrogen fluxes in modern and past ecosystems. An essential prerequisite to reconstruct environmental variables from δ15N values of bivalve shells is to understand if pristine isotope signals can be retrieved from shell organics after sample pretreatment. δ15N analyses of fossil shells should be limited to the intra‐crystalline organic matrix (intra‐OM), which is trapped within biomineral units and less likely contaminated or diagenetically overprinted than inter‐crystalline organics (inter‐OM). However, it remains unclear whether the different shell organic phases (insoluble/soluble inter‐OM, intra‐OM) are isotopically distinct and whether δ15N values of intra‐OM agree with those of bulk organic matter. These questions were tackled by applying different solvents (H2O, HCl, H2O2, NaOCl) to homogenized shell powder of a modern Arctica islandica. Milli‐Q water did not alter bulk δ15N values indicating the dissolution of the inter‐OM was negligible. Acid‐extracted intra‐OM exhibited a larger isotope variation within replicates and showed a minor but significant fractionation in bulk δ15N values related to the loss of acid‐soluble components. Compared to H2O2, NaOCl oxidative treatment was more effective in cleaning inter‐OM and produced reliable bulk and amino acid (AA)‐specific δ15N data of intra‐OM. Furthermore, differences in the relative abundance and δ15N values of individual AAs suggested that the N isotope composition is not uniform within shells, and the N‐bearing content and AA composition differ between organic phases. Future studies should test the capability of bulk and CSIA‐AA δ15N proxies in fossil shells as paleoenvironmental archives.

Ammonium “nutrient capacitor” model for δ15N signatures associated with marine anoxic events

Abstract Geochemical records of ancient periods of warm climate can be useful to help understand the looming effects of modern anthropogenic warming, including changes to biogeochemical nutrient cycles. Stable nitrogen isotope compositions of marine sediments archive the balance of processes in the global nitrogen cycle. However, the unusual isotopic signals of Mesozoic oceanic anoxic events (OAEs) remain enigmatic, thus hindering our understanding of nitrogen cycle processes and dynamics under conditions of ocean deoxygenation. Here, we present an ammonium “nutrient capacitor” model of the water-column nitrogen cycle to explain the anomalously negative isotopic compositions seen in Mesozoic OAE sediments. Our model applies isotopic inferences derived from high-resolution records of Lake Kivu sediments to show how periodic chemocline overturning of redox-stratified water columns during Mesozoic OAEs may have delivered ammonium to the photic zone in excess of primary producer requirements. Smoothed, stochastic sampling of the changing fluxes within the nitrogen cycle across these events can simulate OAE nitrogen isotope records.

Mesopelagic particulate nitrogen dynamics in the subarctic and subtropical regions of the western North Pacific

Recently, new spatiotemporal-scale particle observations by autonomous profiling floats equipped with bio-optical sensors have revealed that, in addition to gravitational particle sinking, the downward transport of surface particles by physical mixing events, which has been overlooked, contributes to particulate organic carbon export. However, the subsequent behavior of these exported particles in the mesopelagic zone (e.g., particle fragmentation and degradation) remains unclear, although it may influence the efficiency of carbon transport to further depths. This study successfully depicted the new annual mean mesopelagic particulate nitrogen (PN) dynamics with multi-layer, steady-state suspended PN pools by reanalyzing seasonal data on the stable nitrogen isotopic compositions of both suspended and sinking particles, each with different profiles, from subarctic station K2 and subtropical station S1 in the North Pacific, which are both CO2 sinks but in different oceanic settings. As analytical conditions, we assumed that the net loss of sinking PN was entirely due to abiotic fragmentation of particle aggregates to non-sinking particles and that the apparent 15N enrichment associated with heterotrophic degradation in the suspended PN pools was vertically constant. The 15N mass balance for the PN supply to the uppermost mesopelagic pool, derived from such constraints, allowed estimating the PN export by the mixed-layer pump, which was 1.6 times greater at K2 than at S1. However, its contribution to the total export (including gravitational PN sinking) from the surface layer was approximately 20% at both stations. Moreover, the ratio of PN supplied to the uppermost pool by the mixed-layer pump and by the fragmentation of particle aggregates was also similar at both stations, approximately 1:1. Using these ratios, together with separate observations of the mixed-layer pump-driven flux, it may be possible to estimate the efficiency of the particulate organic carbon transport due to the biological gravitational pump responsible for carbon sequestration in the deep sea.

Paleoecology of the mammoth fauna of Southern Siberia during the last glacial period based on stable isotope data

Composition of stable carbon (δ13C) and nitrogen (δ15N) isotopes of bone collagen of large mammals who inhabited Southern Siberia during MIS2 have been analysed. The results suggest significant isotopic differences of those animals from the fauna of other regions. Notably, the differences between animals from neighbouring regions may be more significant than between those from more distant areas. A summary description of the isotopic signatures of the LGP mammoth fauna from the Minusinsk depression has been presented, showing a general pattern of lower δ15N and higher δ13C values as compared to species from other regions. Variations in δ15N values may be explained by strong influence of landscape and climatic factors, in particular, the highly mosaic nature of local landscapes due to its ruggedness and altitudinal zonation. One of the possible mechanisms that could explain the observed δ13C signatures in bone collagen of the mammals is regional geochemistry, namely, the presence of developed system of mineral lakes and saline soils in the area. On saline soils, halophytic plants using the C4 type of photosynthesis and characterized by higher δ13C values, are more common. Furthermore, some halophytes, being C3 plants, are able to adapt to salinity stress and fix carbon in a way similar to C4 plants.

Inferring odontocete life history traits in dentine using a multiproxy approach (δ15 N, δ44/42 Ca and trace elements).

Understanding the interactions between marine mammals and their environment is critical for ecological and conservation purposes. Odontocetes offer a continuous record of their life history from birth as recorded in annual increments of their tooth dentine. Because dentine is not remodeled and contains collagen, nitrogen stable isotope compositions (δ15 N) reflect nursing and weaning events, life history traits that would otherwise be impossible to retrieve in such elusive marine animals. Yet, capturing the magnitude and temporal changes in these events is constrained by tooth size and sampling resolution. Moreover, historical and fossil specimens undergo collagen decay, hence the need to develop the measurements of other proxies. Here, we present a multiproxy approach to investigate the use of Ca isotope compositions (δ44/42 Ca) in relation to δ15 N and laser ablation profiles for different trace metal (Ba, Mg, Sr, Zn) concentrations across the dentine of a single individual of the common bottlenose dolphin Tursiops truncatus. To help interpret the dentine data, we provide milk elemental compositions and δ44/42 Ca values for two odontocete individuals. We discuss the observed changes in δ44/42 Ca across the dentine as potential markers of birth, weaning interval, incidental ingestion of seawater, trophic level and physiology. Incidental ingestion of seawater during nursing induces a positive offset in δ44/42 Ca values recorded in the early formed dentine. Life history parameters of individual marine mammals are extremely difficult to retrieve due to limitations in observing specimens in the wild and the methodology presented here offers new ecological and paleoecological perspectives.

Do marine protected areas influence mercury exposure? Insights from a shark community in the tropical Northeast Pacific

Biomass depletion caused by overfishing is likely to alter the structure of food webs and impact mercury transfer to marine predators. Although marine protected areas (MPAs) are spared from fishing pressure, their influence on biota mercury levels is poorly understood. Here, we used carbon and nitrogen stable isotope compositions as well as mercury concentrations in fin clips to characterize foraging habitat and mercury exposure of a shark community composed of migratory and resident species of the Revillagigedo archipelago, an offshore MPA in the Northeast Pacific off Mexico. We found that the probability of finding migratory sharks in the isotopic niche of Revillagigedo-resident sharks was low, likely reflecting the use of habitats outside the archipelago by highly mobile species. Community-wide variations in mercury were primarily explained by shark length, revealing that bioaccumulation was the main driver of Hg concentrations. We failed to detect a clear effect of foraging habitat on shark mercury exposure, which may be related to migratory species using both exploited and protected areas when moving outside the Revillagigedo MPA. More similar studies on the potential mitigation of Hg contamination by MPAs are needed in the future if fishing pressure increases to satisfy the growing global human population.

Particulate Organic Matter in the Atacama Trench: Tracing Sources and Possible Transport Mechanisms to the Hadal Seafloor

AbstractOceanic trenches are an important sink for organic matter (OM). However, little is known about how much of the OM reaching the hadal region derives from the sunlit surface ocean and other sources. We provide new insight into the OM sources in the Atacama Trench by examining the elemental and stable isotope composition of carbon and nitrogen in bulk OM throughout the entire water column and down to bathyal and hadal sediments. Moreover, we estimated the particulate organic carbon (POC) concentration and downward carbon flux. Our results, based on two‐way variance analysis, showed statistical differences in δ15NPON between the epipelagic zone and the deep zones. However, no statistical differences in δ13CPOC and C:N ratio between hadalpelagic and shallower pelagic zones were found, except for δ13CPOC in the oxygen‐deficient zone. On the contrary, whereas the isotopic signatures of hadal sediments were distinct from those over the entire water column, they were similar to the values in bathyal sediments. Thus, our results suggest that bathyal sediments could contribute more OM to hadal sediments than the different zones of the water column. Indeed, whereas POC flux estimates derived from remote sensing data indicate that ∼16%–27% of POC could evade surface remineralization within the top 200 m and potentially be exported to depths beyond the mesopelagic region, model estimates suggest that ∼3.3% of it could reach hadal depths. Our results provide a quantitative baseline of pelagic‐benthic coupling which can aid in assessment of carbon cycling changes in future climate scenarios.

Physically protected organic matter drives soil carbon sequestration potential of a managed grassland ecosystem in Italian Alps

Mountain managed grasslands are characterized by a high biodiversity and are capable to store a considerable amount of carbon (C) in the soil. In this study, we evaluated the chemical characteristics of physically protected organic matter and its contribution to the soil C stock potential in a subalpine managed grassland ecosystem, combining low-temperature ashing (LTA), stable isotope ratio mass spectrometry (IRMS) and infrared photoacoustic spectroscopy (PAS). The physically protected C fraction, evaluated through LTA by oxygen plasma, amounted ∼74% of the whole soil organic C (SOC) in the first 15 cm depth. This C fraction was closely linked to mineral components in stable aggregates of ∼587 μm mean weight diameter. The increase of both C and nitrogen (N) stable isotope composition (δ13C; δ15N) in the physically protected organic matter, concomitant with a decrease of its C/N ratio, suggests higher microbial transformation degree, age and stability against decomposition of protected organic fraction compared to unprotected one. However, also the younger and less protected C fraction acted as cementing organic labile agent, creating larger but less stable soil aggregates. The infrared photoacoustic spectroscopy showed that the physically protected C fraction was characterized by a more marked presence of phenolic compounds and a reduced amount of proteinaceous and polysaccharides-like materials compared to the more labile and younger organic fraction. These results highlight that managed grasslands can effectively sequester large amount of long-residence C into the soil, promoting aggregate stability and soil fertility, and playing a key role for mitigation strategies against climate change.

An explainable machine learning for geographical origin traceability of mussels Mytilus edulis based on stable isotope ratio and compositions of C, N, O and H

Geographical traceability of marine bivalves has been explored continually, however the traceability results have not been recognized owing to the absence of a reasonable explanation for a certain model prediction. To address this problem, we employed an explainable machine learning to identify the origin of mussels based on stable isotope ratio and compositions of carbon (C), nitrogen (N), oxygen (O), and hydrogen (H). Our findings proved that the Extreme gradient boosting (XGBoost) model may be the best model based upon its high accuracy (93.75 %), precision (93.75 %), recall (94.51 %), F1 score (93.72 %) and AUC (0.990). The feature impact for model out of XGBoost was interpreted by SHapley Additive exPlanations (SHAP) on global and local perspective. This study demonstrated the potential of explainable machine learning to elucidate the complex relationships between desirable geographical information of mussels and the selected features.

A stable isotopic approach for quantifying aquaculture-derived organic matter deposition dynamics in the sediment of a coastal fish farm

The stable carbon and nitrogen isotopic compositions (δ13C and δ15N) of sedimentary organic matter (OM) in fish farms (FFs) were investigated to quantify the aquaculture-derived OM deposition dynamics in the sediment. The dual isotopic compositions of mixed OMs in surface sediments at FFs differed significantly (p < 0.05) from those at reference sites, indicating an increased deposition of fish feces or uneaten feed in the sediments. Furthermore, OM source apportionments revealed that the quantitative contribution of fish feces (23.3 mg g−1 dw) during farming activities was significantly higher than that of other natural OM sources (C3 plants and phytoplankton). After the disassembly of fish cages, the deposited fish feces may be preferentially degraded through processes that require a large amount of oxygen consumption (<0.1 kg C m−2 yr−1). Our isotopic approach may be helpful for assessing the impact of FF wastes and for taking measures to minimize environmental deterioration.

Do stable carbon and nitrogen isotope values of Nitella flexilis differ between softwater and hardwater lakes?

The composition of stable carbon and nitrogen isotopes (δ13C and δ15N) is widely used for tracking the origin of organic matter (OM) present in lacustrine sediments. These isotopes also define the evolution of OM in food web loops. Charophyte species Nitella flexilis C. Agardh, 1824 can be found in different aquatic environments where it contributes significantly to sediment formation and influences biota function. Therefore, it is crucial to study more about δ13C and δ15N in different lake types. Here, we present the results of the first comprehensive study of N. flexilis δ13C and δ15N, which add to the knowledge of the C and N isotope records of charophytes. We obtained the δ13C and δ15N records of N. flexilis OM from hardwater and softwater lakes and checked for differences between these records. We also analyzed the differences in physical and chemical parameters. Finally, we compared the δ13C and δ15N records with physical and chemical parameters to identify the variables that have the highest influence on N. flexilis δ13C and δ15N values. Our study showed that both δ13C and δ15N did not differ significantly in the two types of lakes, although the lakes had significant differences in several physical and chemical parameters (pH, Ca2+, dissolved inorganic carbon, total phosphorus, conductivity). However, we observed that δ13C values were influenced by light conditions (photosynthetic active radiation, depth, dissolved OM), while δ15N values were influenced by the total nitrogen concentration in water.

Parasitic Copepods as Biochemical Tracers of Foraging Patterns and Dietary Shifts in Whale Sharks (Rhincodon typus Smith, 1828)

Understanding the diet of whale sharks (Rhincodon typus) is essential for the development of appropriate conservation strategies for the species. This study evaluated the use of the parasitic copepod (Pandarus rhincodonicus) as a proxy to infer short-term foraging habitats and trophic positions of whale shark hosts. To accomplish this, bulk stable carbon (δ13C) and nitrogen (δ15N) isotope compositions were analysed from 72 paired samples of whale shark skin (dermal) tissues and copepods collected across six years at the Ningaloo Reef aggregation site, Western Australia. This study found that δ15N from parasites and whale shark hosts were strongly correlated. As turn-over times of the parasite and whale shark differ (months vs. years, respectively), the ability of copepods to predict δ15N values indicates that the trophic positions of whale sharks remain consistent across these timeframes. Contrastingly, δ13C in the parasite and host were weakly correlated, likely reflecting differences in the physiology and lifecycle of the copepod parasite compared to the host. Our results suggest δ15N from parasitic copepods provides a reliable proxy of the trophic position of their whale shark hosts, but interpretation of δ13C values as a proxy for the host will require future studies on the lifecycle of P. rhincodonicus.