Increase In δ13C Research Articles

Effects on soil chemistry of tropical deforestation for agriculture and subsequent reforestation with special reference to changes in carbon and nitrogen

While soils can act either as a source or as a sink for atmospheric carbon dioxide and nitrogen, depending on land cover change and management, there is a great need to improve our understanding of the dynamics of soil structure and chemistry following land conversion in tropical regions. In this study, we investigated various soil structural and chemical variables (bulk density, pH, cation exchange capacity, phosphorus, magnesium, potassium), soil carbon (C), soil nitrogen (N), δ13C, and δ15N in relation to conversion of rain forest to tea cultivation and subsequent changes due to agricultural abandonment and reforestation with Caribbean pine. Although Caribbean pine has shown potential for reforestation throughout Asia and Latin America, effects on soil properties in comparison with other land uses have not been quantified. Our study compared: (1) the original mixed-dipterocarp rain forest, (2) tea plantations, (3) Kekilla fernlands and (4) Caribbean pine plantations. Tea plantations show higher bulk density (BD) as an evidence of compaction. Although soil C concentration in tea plantations were lower than other land uses at 0–10cm, when bulk density was used with C as a composite measure, tea plantation showed the highest value for both soil C and N stocks. Disregarding tea plantations, Caribbean pine plantations had the highest C but showed the lowest N stocks at both soil depths. Soil δ13C and δ15N values of all land uses increase with increasing soil depth; but Caribbean pine plantations showed the greatest increase in δ13C at both soil depths (e.g. −27.82±0.38‰ at 0–10cm to −26.50±0.37‰ at 10–20cm). For both Kekilla fernlands and Caribbean pine plantations the relationship between δ13C and δ15N was strongly linear. By comparing the physical and chemical soil properties of these land uses with undisturbed rain forest, we established baseline data to determine influence of land conversion on soil properties. Based on soil pH, CEC and other major nutrients (P, K, Mg), there are strong legacy effects of land use potentially from both fertilization and fire. Our results also showed strong evidence that δ13C and δ15N was both increased with depth under pine reforestation and fernlands, suggesting soils can recuperate with a consistent input of litter and slower decomposition processes. There is some evidence that recruitment of natural regeneration beneath pine can help facilitate faster litter decomposition that can revert soil structure and fertility to a status similar to that of the original forest.

Strong Coupling of Shoot Assimilation and Soil Respiration during Drought and Recovery Periods in Beech As Indicated by Natural Abundance δ13C Measurements.

Drought down-regulates above- and belowground carbon fluxes, however, the resilience of trees to drought will also depend on the speed and magnitude of recovery of these above- and belowground fluxes after re-wetting. Carbon isotope composition of above- and belowground carbon fluxes at natural abundance provides a methodological approach to study the coupling between photosynthesis and soil respiration (SR) under conditions (such as drought) that influence photosynthetic carbon isotope discrimination. In turn, the direct supply of root respiration with recent photoassimilates will impact on the carbon isotope composition of soil-respired CO2. We independently measured shoot and soil CO2 fluxes of beech saplings (Fagus sylvatica L.) and their respective δ13C continuously with laser spectroscopy at natural abundance. We quantified the speed of recovery of drought stressed trees after re-watering and traced photosynthetic carbon isotope signal in the carbon isotope composition of soil-respired CO2. Stomatal conductance responded strongly to the moderate drought (-65%), induced by reduced soil moisture content as well as increased vapor pressure deficit. Simultaneously, carbon isotope discrimination decreased by 8‰, which in turn caused a significant increase in δ13C of recent metabolites (1.5–2.5‰) and in δ13C of SR (1–1.5‰). Generally, shoot and soil CO2 fluxes and their δ13C were in alignment during drought and subsequent stress release, clearly demonstrating a permanent dependence of root respiration on recently fixed photoassimilates, rather than on older reserves. After re-watering, the drought signal persisted longer in δ13C of the water soluble fraction that integrates multiple metabolites (soluble sugars, amino acids, organic acids) than in the neutral fraction which represents most recently assimilated sugars or in the δ13C of SR. Nevertheless, full recovery of all aboveground physiological variables was reached within 4 days – and within 7 days for SR – indicating high resilience of (young) beech against moderate drought.

Tracing the fluid source of heavy REE mineralisation in carbonatites using a novel method of oxygen-isotope analysis in apatite: The example of Songwe Hill, Malawi

Stable (C and O) isotope data from carbonates are one of the most important methods used to infer genetic processes in carbonatites. However despite their ubiquitous use in geological studies, it is suspected that carbonates are susceptible to dissolution-reprecipitation and isotopic resetting, especially in shallow intrusions, and may not be the best records of either igneous or hydrothermal processes. Apatite, however, should be much less susceptible to these resetting problems but has not been used for O isotope analysis. In this contribution, a novel bulk-carbonatite method for the analysis of O isotopes in the apatite PO4 site demonstrates a more robust record of stable isotope values. Analyses of apatite from five carbonatites with magmatic textures establishes a preliminary Primary Igneous Apatite (PIA) field of δ18O=+2.5 to +6.0‰ (VSMOW), comparable to Primary Igneous Carbonatite (PIC) compositions from carbonates.Carbonate and apatite stable isotope data are compared in 10 carbonatite samples from Songwe Hill, Malawi. Apatite is heavy rare earth element (HREE) enriched at Songwe and, therefore, oxygen isotope analyses of this mineral are ideal for understanding HREE-related mineralisation in carbonatites. Carbonate C and O isotope ratios show a general trend, from early to late in the evolution, towards higher δ18O values (+7.8 to +26.7‰, VSMOW), with a slight increase in δ13C (−4.6 to −0.1‰, VPDB). Oxygen isotope ratios from apatite show a contrary trend, decreasing from a PIA field towards more negative values (+2.5 to −0.7‰, VSMOW). The contrasting results are interpreted as the product of the different minerals recording fluid interaction at different temperatures and compositions. Modelling indicates the possibility of both a CO2 rich fluid and mixing between meteoric and deuteric waters. A model is proposed where brecciation leads to depressurisation and rapid apatite precipitation. Subsequently, a convection cell develops from a carbonatite, interacting with surrounding meteoric water. REE are likely to be transported in this convection cell and precipitate owing to decreasing salinity and/or temperature.

Effects of temperature and food quality on isotopic turnover and discrimination in a cladoceran

Our experimental study was designed to assess the effects of temperature on nitrogen isotope turnover and to measure the effects of temperature and food quality on the stable carbon and nitrogen isotope discrimination factors (Δ13C and Δ15N) in a cladoceran. For the first part of our study, Daphnia were fed with non-enriched or 15N-enriched Scenedesmus obliquus at 12, 15, 20, and 25 °C. For the second part, Daphnia were reared at 15, 20, and 25 °C on Scenedesmus or Cryptomonas sp. There were no clear effects of temperature on turnover rates of the nitrogen isotope of cladocerans. However, a general increase in Δ13C with increasing temperature was measured, regardless of the food source. Δ15N was also affected by temperature, but contrasting results were measured depending on the food source used. There were significant effects of food quality on Δ13C and Δ15N in Daphnia, as values obtained for Daphnia fed Scenedesmus were always higher than those obtained for Daphnia fed Cryptomonas. Our experiments produced discrimination factors that were very different from those usually considered in isotope studies and showed that the values used for isotope model implementation to analyze field data need to be adapted to environmental conditions.

The diet of black bears tracks the human footprint across a rapidly developing landscape

Food subsidies have become a widely available and predictable resource in human-modified landscapes for many vertebrate species. Such resources can alter individual foraging behavior of animals, and induce population-wide changes. Yet, little consensus exists about the relative influence of the availabilities of native and human food subsidies to wildlife foraging throughout altered landscapes. We explored this unresolved question by analyzing the effects of landscape factors on American black bear (Ursus americanus) diet across the state of Colorado, USA. We estimated assimilated diet using stable isotope analysis of harvested black bear tissues to determine the contribution of human-derived foods to bear diets throughout Colorado, as well as how increasing reliance on human-derived food subsidies increases the risk of conflict. We found that bears (n=296) showed strong regional diet variability, but substantial use of human-derived food subsidies in eastern Colorado (>30% assimilated diet). The age-sex class of the bear and housing density of its harvest location were the most influential predictors of 13C enrichment (a tracer of human food subsidies). Furthermore, foraging on subsidies increased risk of conflict; the odds of being a nuisance bear increased by 60% for each ~1‰ increase in δ13C. Our study confirms the efficacy of δ13C as a proxy for human activity, and indicates that while demographic differences play a clear role in the foraging ecology of bears, availability of subsidies coincident with varying levels of human activity appears to be a major driver in predicting black bear diet throughout the western United States.

Surface soil organic carbon in temperate and subtropical oriental oak stands of East China

Forest ecosystems contain large amounts of soil organic carbon (SOC), which is a major component of biogeochemical cycles that may be sensitive to environmental change. We used a combination of nuclear magnetic resonance (NMR) spectroscopy and elemental and isotopic composition to examine the influence of soil properties and climatic factors on the quantity and degree of decomposition of SOC for organic and surface mineral horizons in seven oriental oak (Quercus variabilis Blume) forest sites arranged across a 11o latitudinal gradient in East China. Lacking Oa horizons, the two southernmost sites contained lower amounts of SOC in the forest floor horizon, but otherwise, latitudinal trends were not consistent. The SOC stock in the 0–10 cm mineral horizon exhibited no clear trend along the gradient and had a negative association with clay + silt content. Based on a higher alkyl/O-alkyl (A/O) ratio and alkyl/methoxyl (A/M) ratio, the SOC at the 0–10 cm depth appeared to be relatively more decomposed in three of the four southern subtropical sites. However, the degree of SOC degradation also decreased strongly with increasing soil pH (R2 = 0.90, P = 0.001). Soil organic carbon exhibited increases in δ13C and δ15N and decreases in the C/N ratio with depth for all the seven sites, indicating an increase in its extent of decomposition. Our analysis indicated that the A/M ratio from NMR provided the best indication of the extent of SOC degradation along the latitudinal transect, whereas the elemental and isotopic composition better reflected patterns with soil depth.

Developmental and spatial variations in the diet signatures of hyperbenthic shrimp Nauticaris marionis at the Prince Edward Islands based on stable isotope ratios and fatty acid profiles

The caridean shrimp Nauticaris marionis is an ecologically important species in the benthic community around the sub-Antarctic Prince Edward Islands (PEI) as it represents a key prey item for a variety of top predators breeding on the islands. We hypothesized that the diet of N. marionis shifts during its development, and that spatial variability in food availability results in differentiation in the diet signatures of specimens collected from various locations of the shelf waters around the PEI. Specimens were collected from nine stations (depth range 70 to 240m) around the PEI at inter-island shelf (from west to east: upstream, between and downstream) and nearshore regions during austral autumn 2009. Stable isotope and fatty acid data both revealed spatial and developmental variations in the shrimp diet. Nearshore shrimp were more 13C-enriched than those from the inter-island region, suggesting increased kelp detritus entered the food web in the nearshore regions. The shrimp showed increases in δ13C and δ15N signatures (and trophic position) with an increase in body size, resulting in distinctions between size classes that reflected shifts in their trophic niche through development. The fatty acid profiles similarly indicated distinctions in diet with increased shrimp size (in the deep regions), and spatial variability was evident in relation to region and depth. All shrimp contained large proportions of polyunsaturated and essential fatty acids, indicating that the quality of food consumed was similar between regions despite the diet variability. Our results provide new dietary information about a key species operating near the base of the food web at the highly productive PEI, and show that there were no areas of enhanced nutrition available to the shrimp. As such, there was no nutritional advantage to shrimp inhabiting any specific region around the PEI.

Growth and photosynthetic responses in Jatropha curcas L. seedlings of different provenances to watering regimes

Seedlings from four provenances of Jatropha curcas were subjected to 80, 50, and 30% of soil field capacity in potted experiments in order to study their responses to water availability. Our results showed that with the decline of soil water availability, plant growth, biomass accumulation, net photosynthetic rate, stomatal conductance (gs), and transpiration rate (E) decreased, whereas leaf carbon isotope composition (δ13C), leaf pigment contents, and stomatal limitation value increased, while maximal quantum yield of PSII photochemistry was not affected. Our findings proved that stomatal limitation to photosynthesis dominated in J. curcas under low water availability. The increase of δ13C should be attributed to the decrease in gs and E under the lowest water supply. J. curcas could adapt to low water availability by adjusting its plant size, stomata closure, reduction of E, increasing δ13C, and leaf pigment contents. Moreover, effects of provenance and the interaction with the watering regime were detected in growth and many physiological parameters. The provenance from xeric habitats showed stronger plasticity in the plant size than that from other provenances under drought. The variations may be used as criteria for variety/provenance selection and improvement of J. curcas performance.

The contribution of human foods to honey bee diets in a mid-sized metropolis

Concern for honey bee health has implicated both urbanization and nutritional stress as factors contributing to honey bee declines. The expansion of urban areas has resulted in reduced foraging habitat for bees, while at the same time introducing new food sources, such as foods unintentionally provided by humans as litter or in waste containers. While human foods play an important role in the diets of other urban animals, the extent to which honey bees feed on these resources has not been well characterized. Here, we compared the amount of human foods in honey bee diets across a rural-urban gradient in North Carolina, USA, using stable isotopes of carbon (δ13C). Human-produced sugars—primarily sugarcane and corn syrup—have a characteristic isotope signature that can be used to quantify the relative amount of human foods in animal diets. We predicted that urban bees would have an increase in δ13C if they were feeding on human-produced sugars, but we found no change in δ13C between urban and rural colonies. Instead, we found an increase in δ13C in managed bees in both habitats, which is indicative of supplemental sugar feeding by beekeepers. Rather than switching to human food sources, urban bees seem to be feeding on urban flowers or insect-produced honeydew. This suggests an important role for urban flowers and green spaces in maintaining healthy pollinator populations in cities.

Holocene climate variability and anthropogenic impacts from Lago Paixban, a perennial wetland in Peten, Guatemala

Analyses of an ~6m sediment core from Lago Paixban in Peten, Guatemala, document the complex evolution of a perennial wetland over the last 10,300years. The basal sediment is comprised of alluvial/colluvial fill deposited in the early Holocene. The absence of pollen and gastropods in the basal sediments suggests intermittently dry conditions until ~9000cal yr.BP (henceforth BP) when the basin began to hold water perennially. Lowland tropical forest taxa dominated the local vegetation at this time. A distinct band of carbonate dating to ~8200BP suggests regionally dry conditions, possibly associated with the 8.2ka event. Wetter conditions during the Holocene Thermal Maximum are indicated by evidence of a raised water level and an open water lake. The timing of this interval coincides with strengthening of the Central American Monsoon. An abrupt change at 5500BP involved the development of a sawgrass marsh and onset of peat deposition. The lowest recorded water levels date to 5500–4500BP. Pollen, isotope, geochemical, and sedimentological data indicate that the coring site was near the edge of the marsh during this period. A rise in the water table after 4500BP persisted until around 3500BP. Clay marl deposition from 3500 to 210BP corresponds to the period of Maya settlement. An increase in δ13C, the presence of Zea pollen, and a reduction in the percentage of forest taxa pollen indicate agricultural activity at this time. In contrast to several nearby paleoenvironmental studies, proxy evidence from Lago Paixban indicates human presence through the Classic/Postclassic period transition (~1000BP) and persisting until the arrival of Europeans. Cessation of human activity around 210BP resulted in local afforestation and the re-establishment of the current sawgrass marsh at Lago Paixban.

Holocene records of environmental change in High Plains playa wetlands, Kansas, US

Holocene environmental change is poorly characterized throughout much of the High Plains. Playa wetlands are sites with appreciable potential for reconstructing environmental change because of their environmental sensitivity. Radiocarbon, litho- and magneto-stratigraphic, and stable carbon isotope (δ13C) data for four playas on the Kansas High Plains indicate significant change occurred throughout the Holocene, which had profound impacts on playa ecosystem functions (e.g. groundwater recharge, surface water storage, and habitat). Minimum δ13C values and buried soils observed during the Pleistocene–Holocene transition suggest sufficient moisture to support vegetative cover and promote pedogenesis. Low magnetic susceptibility, rapid increase in δ13C, and weak pedogenic alteration in overlying deposits suggest early Holocene aridity with increased landscape instability and sediment deposition. Early to middle Holocene stratigraphy transitioned from loess with little pedogenic modification into overlying buried soils; δ13C values indicate playa vegetation was composed of as much as 90% C4 plants. Maximum magnetic susceptibility and buried soils in late Holocene deposits indicate sufficient moisture to support vegetative cover and promote pedogenesis. The Pleistocene–Holocene transition was a cool, moist period in which sediment accumulation rates in playas were low and ecosystem functions were optimal. Throughout much of the early to early–middle Holocene, playas were subaerially exposed, sediment accumulation outpaced soil formation, and ecosystem functions were inhibited by lack of moisture. From the middle to late Holocene, the climate returned to more mesic conditions, sediment accumulation and soil formation were in near equilibrium, and the ability of playas to perform ecosystem functions increased.

A nitrogen isotope fractionation factor between diamond and its parental fluid derived from detailed SIMS analysis of a gem diamond and theoretical calculations

To determine the magnitude of N-isotope fractionation between diamond and its parental fluid, detailed C- and N-isotope analyses of a complexly-zoned, eclogitic diamond (JDE-25) were undertaken using secondary ion mass spectrometry. Combined C- and N-isotope and N-abundance measurements were made across four distinct growth zones and show the following range of values: δ13C=−5.7 to −2.1‰; δ15N=−7.0 to +5.5‰; [N]=104 to 5420at.ppm. The core zone displays a continuous, rimward increase in δ13C and δ15N values and decreases in N-abundance, and is interpreted to have formed by fractional crystallization of diamond from a single pulse of fluid (i.e., closed system). Modelling of the isotopic and abundance data from the core zone yields a diamond–fluid nitrogen partition coefficient (KN) of 4.4 and a N-isotope fractionation factor (∆15Ndiam–fluid) of −4.0±1.2‰ (2σ) at ~1100°C, for precipitation from a pure carbonate fluid. Calculated KN and ∆15Ndiam–fluid values would have larger magnitudes if JDE-25 formed from a more complex fluid, in which the carbonate species formed only a minor component.Theoretical calculations of N-isotope fractionation between the principal N-species associated with upper mantle fluids (N2, NH3 or NH4+) and the CN− molecule, as an analogue for the carbon–nitrogen bond in diamond, yield the following ∆15Ndiam (CN)–fluid estimates at 1100°C: −3.6‰ for NH4+, −2.1‰ for N2 and −1.4‰ for NH3. The theoretical calculations provide only minimum estimates of the true diamond–fluid N-isotope fractionation factor, given that the C–N single bond in diamond would have a lower affinity for 15N than the stronger C–N triple bond in the CN− molecule. Accordingly, the theoretical N-isotope fractionation factors are consistent with the empirical fractionation factor derived from diamond JDE-25. As a consequence of the large magnitude of ∆15Ndiam–fluid, intracrystalline N-isotope variations in diamond should provide a sensitive test for fluid-related, fractional crystallization processes. Furthermore, the large magnitude of ∆15Ndiam–fluid could be reflected in the wide range of δ15N values for natural diamonds and the absence of clearly defined modes for the N-isotope compositions of peridotitic and eclogitic diamonds.

Land use change decreases soil carbon stocks in Tibetan grasslands

Land use is an important factor affecting soil organic carbon (SOC) dynamics and can produce positive C climate feedback, but its effects remain unknown for Tibetan ecosystems. Recent land use changes have converted the traditional winter Kobresia pastures of nomads in the northeastern Tibetan Plateau to Elymus pastures or even to cropland. Detailed SOC measurements up to 30-cm depth were combined with analysis of δ13C, δ15N, bulk density, microbial C, and N contents in three land use types. Bulk density was decreased by conversion from Kobresia pasture to cropland but increased by conversion to Elymus pasture. The loss of 1 % of SOC caused by land use change leads to δ13C increase of 0.8 ‰. Conversion to cropland significantly decreased SOC stocks (10 %) and microbial biomass C, but the C loss (1.6 %) was insignificant in Elymus pasture. Land use changes strongly increased soil δ15N in the top 5 cm. Conversion to Elymus pasture did not change the C stocks, but conversion to cropland decreased C stocks by 10 % within 10 years. Soil δ13C and δ15N data indicate acceleration of C and N cycling due to the replacement of Kobresia pasture by Elymus pasture and cropland.

Methane oxidation following submarine permafrost degradation: Measurements from a central Laptev Sea shelf borehole

Submarine permafrost degradation has been invoked as a cause for recent observations of methane emissions from the seabed to the water column and atmosphere of the East Siberian shelf. Sediment drilled 52 m down from the sea ice in Buor Khaya Bay, central Laptev Sea revealed unfrozen sediment overlying ice-bonded permafrost. Methane concentrations in the overlying unfrozen sediment were low (mean 20 µM) but higher in the underlying ice-bonded submarine permafrost (mean 380 µM). In contrast, sulfate concentrations were substantially higher in the unfrozen sediment (mean 2.5 mM) than in the underlying submarine permafrost (mean 0.1 mM). Using deduced permafrost degradation rates, we calculate potential mean methane efflux from degrading permafrost of 120 mg m−2 yr−1 at this site. However, a drop of methane concentrations from 190 µM to 19 µM and a concomitant increase of methane δ13C from −63‰ to −35‰ directly above the ice-bonded permafrost suggest that methane is effectively oxidized within the overlying unfrozen sediment before it reaches the water column. High rates of methane ebullition into the water column observed elsewhere are thus unlikely to have ice-bonded permafrost as their source.

Exploring photosymbiotic ecology of planktic foraminifers from chamber-by-chamber isotopic history of individual foraminifers

AbstractEvolution of photosymbiotic ecology is an important adaptation for planktic foraminifers that enhances the ecological advantage of living in oligotrophic oceans. Therefore, detecting photosymbiotic ecology in fossil species is one of the keys to understanding the paleobiodiversity dynamics of planktic foraminifers. Because foraminiferal tests record the ontogenetic history of ecological information in geochemical signatures, analyzing individual geochemical profiles with growth can reveal a species’ ecology. This study examined chamber-by-chamber stable isotopes (δ13C and δ18O) of foraminiferal individuals to identify photosymbiotic signals. We observed an ontogenetic δ13C increase of up to 2.4‰, accompanied by relatively stable, negative δ18O, in the symbiotic species Globigerinoides conglobatus and Globigerinoides sacculifer. In contrast, δ13C and δ18O showed significant positive correlation during ontogeny in the asymbiotic species Globorotalia truncatulinoides. These two ecological groups produce contrasting isotopic profiles, thereby allowing us to use our ontogenetic isotopic analyses of individual specimens to identify algal photosymbiosis in fossil foraminifers. The chamber-by-chamber isotope analyses with individual ontogeny have great advantages in analyzing rare species because only one individual is required to describe ontogenetic isotopic history. In addition to photosymbiotic identification, our methods hold great potential to provide new insight into species paleoecological studies such as the ontogenetic history of calcification depth.

The intensification of northern component deepwater formation during the mid‐Pleistocene climate transition

AbstractWe reconstruct mid‐Pleistocene (marine isotope stages (MISs) 13–18) deepwater hydrography at Ocean Drilling Program Site 1063 (4583 m water depth, subtropical North Atlantic) using benthic foraminiferal stable isotope records. These new records complete an ~900 kyr long stratigraphy spanning MISs 8–29 (~250–1030 Ka) when combined with previously published records from Site 1063. The results indicate a change in the circulation regime of the abyssal subtropical North Atlantic during MIS 17. Prior to MIS 17, no significant glacial or interglacial δ13C gradients are evident between Site 1063 and the deep South Atlantic. After MIS 17, interglacial intervals at Site 1063 are characterized by δ13C values that consistently approach those recorded in the deep North Atlantic. Comparing Site 1063 δ13C values to 26 additional published records throughout the entire Atlantic basin supports the idea that this δ13C increase is unique to the deep North Atlantic. After MIS 17, the basin‐wide influence of higher δ13C values suggests an increased relative flux of northern sourced bottom waters during interglacial periods. The timing of northern sourced water influence at Site 1063 is consistent with the timing of a shift in the orientation of the Arctic Front. Thus, this shift may signify a link between the northward penetration of relatively warm, saline surface waters into the Norwegian‐Greenland Seas stimulating deep convection. Our findings fit well with the model of Imbrie et al. (1993) for the importance of the Nordic heat pump in establishing strong 100 kyr cyclicity in late Pleistocene glacial cycles.

Effects of lipid extraction and the utility of lipid normalization models on δ13C and δ15N values in Arctic marine mammal tissues

Animals store lipids, which are 13C-depleted, in their tissues that often must be extracted to correctly interpret δ13C data. However, chemical lipid extraction (CLE) can alter δ15N values and lipid normalization (LN) models are not consistent across fauna. We determined whether lipids should be extracted by assessing effects of CLE and validating LN models for liver and muscle from seven and eight marine mammal species, respectively, and skin from one species. In liver, CLE significantly increased δ13C and δ15N values for all species, whereas only a significant increase in δ13C occurred in skin. For muscle, δ13C and δ15N values were generally greater after CLE, but this was not consistent across species. Extracted lipids were depleted by approximately 7 and 5 ‰ for δ13C and δ15N, respectively, in both muscle and liver compared with protein in all species. The reliability of LN models varied between tissues and species; thus, their use is largely dependent on the precision of stable isotope values needed to address the objectives of a study. A decision framework to decide whether CLE or LN models is required for ecological interpretation of stable isotopes based on species, tissue and study objectives is presented.

Stable Isotope Discrimination by Consumers in a Tropical Mangrove Food Web: How Important Are Variations in C/N Ratio?

Stable isotopes are widely used as tracers in food webs and as indicators of the trophic levels (TL) of consumers. The objectives of this study were to verify a possible ecosystem-wide increase in the relative contribution of the heavy isotope with trophic level, for carbon and nitrogen, and to analyze the relationships of C/N ratios and stable isotope ratios of consumers in a tropical mangrove food web. Samples of primary producers and consumers (benthic invertebrates, zooplankton, 31 fish species, and other vertebrates) were collected in the Curuca Estuary, northern Brazil. Our results showed that the relative amount of the heavy isotope significantly increased with TL. Linear models of δ 15N vs TL were highly significant and explained 75 % of the total variability in food web δ 15N. Models for δ 15N yielded a much better fit than the models built for δ 13C, mainly due to a higher variability in source δ 13C and stronger trophic discrimination for 15N than for 13C. Enrichment in δ 15N was 2.3 ± 0.20 ‰ per TL (all data). The increase in δ 13C with TL, in spite of being significant, could not be used to estimate trophic enrichment, since it was affected by the concentration of predatory fish in a 13C-rich algae-based food web. Furthermore, the increase in δ 13C with TL could be fully explained by the change in C/N ratio (i.e., lipid content) with TL. Our results demonstrate the importance of considering TL and C/N ratios in stable isotope analyses of food webs.

Use of Stable Isotopes to Trace Municipal Wastewater Effluents into Food Webs within a Highly Developed River System

Many anthropogenic inputs, such as municipal wastewater effluents (MWWEs), affect stable isotope signatures (δ13C and δ15N) at the base of exposed food webs creating spatial patterns reflecting their incorporation into aquatic food webs. The Grand River in southern Ontario, Canada, is a heavily modified, rapidly urbanizing river that assimilates wastewater from 30 municipal wastewater treatment plants. Stable isotope analysis was applied to resident aquatic invertebrates and fish influenced by three different wastewater outfalls in early, middle, and late summer to determine how values shifted seasonally and with differing effluent quality. There was a slight increase in δ13C in both invertebrates and fish in late summer downstream from the three outfalls, but it is difficult to separate effects of the effluents from downstream gradients. Downstream of two of the three outfalls, the δ15N tended to increase relative to upstream, while the remaining effluent, of the poorest quality, decreased δ15N values of both invertebrates and fish. Spatial trends in stable isotopes became more pronounced as the summer progressed with the greatest between-site differences occurring in late summer. This study reflects the complex nutrient dynamics associated with MWWE inputs to rivers and contributes to our understanding and application of stable isotope analysis in impacted lotic ecosystems. Copyright © 2014 John Wiley & Sons, Ltd.

Modeling the contribution of dissolved organic carbon to carbon sequestration during the last glacial maximum

Dissolved organic carbon (DOC) is a carbon reservoir that is as large as the atmospheric CO2 pool, and its contribution to the global carbon cycle is gaining attention. As DOC is a dissolved tracer, its distribution can serve to trace the mixing of water masses and the pathways of ocean circulation. Published proxy and model reconstructions have revealed that, during the last glacial maximum (LGM), the pattern of deep ocean circulation differed from that of the modern ocean, whereby additional carbon is assumed to have been sequestered in stratified LGM deep water. The aim of this study is to explore the distribution of DOC and its production/removal rate during the LGM using the Grid ENabled Integrated Earth system model (GENIE). Modeled results reveal that increased salinity of bottom waters in the Southern Ocean is associated with stronger stratification and oxygen depletion. The stratified LGM deep ocean traps more nutrients, resulting in a decrease in the DOC reservoir size that, in turn, causes a negative feedback for carbon sequestration. This finding requires an increase in DOC lifetime to compensate for the negative feedback. The upper limit of DOC lifetime is assumed to be 20,000 years. Modeled results derive an increase (decrease) in DOC reservoir by 100 Pg C leading to an atmospheric CO2 decrease (increase) of 9.1 ppm and a dissolved inorganic carbon δ13C increase (decrease) of 0.06‰. The DOC removal rate is estimated to be 39.5 Tg C year–1 in the deep sea during the LGM. The contribution of DOC to the LGM carbon cycle elucidates potential carbon sink-increasing strategies.