Mass Balance Correction Research Articles

Use of Formalin-Preserved Collections to Infer Trophic Indicators of Marine Zooplankton from Stable Isotopes

Formalin preservation affects the stable isotope composition of zooplankton samples, thus limiting the analysis of valuable collections covering large time intervals. Here, we compare different procedures for correcting the bias caused by formalin in δ13C and δ15N of zooplankton community samples. Zooplankton samples representative of seasonal variations in the period 2000–2009 were collected off A Coruña (NW Spain). Part of the sample was immediately dried and analysed for δ13C, δ15N, and elemental composition within 3 years of collection. These values were used as the unpreserved reference. The remaining sample was preserved in 4% formaldehyde and aliquots obtained after a period ranging from 3 years to more than 10 years of storage were analysed as the originally dried samples. Additionally, the copepod fraction of total biomass was determined in the preserved samples. Corrections of formalin effects based on ordinary least squares regression had large uncertainties, while mass balance corrections based on the change in C:N ratio (only possible for δ13C) overestimated reference values. However, either simple corrections based on the mean difference between values in dry and preserved samples or more complex generalised additive models considering seasonality, copepod biomass, and time of sample storage, produced estimations with relatively low uncertainty. Our results highlight the importance of determining specific correction solutions for each preserved collection before reconstructing stable isotope time series. Furthermore, the uncertainties associated with the estimates can be used in sensitivity analysis to assess their potential impact on the interpretation of the series.

Extreme 13C-depletions and organic sulfur content argue for S-fueled anaerobic methane oxidation in 2.72 Ga old stromatolites

The extreme 13C-depletions recorded in Neoarchean organic matter (OM) have commonly been interpreted as markers of methanotrophy. This methane oxidation metabolism could have been performed using oxidants such as dioxygen, sulfate, nitrite/nitrate, and/or Fe3+- and Mn4+-minerals. Acetogenesis using acetyl-CoA metabolism may produce similar 13C-depletions. We investigated δ13C and S/C values of OM in 2.72 Ga old lacustrine stromatolites of the Tumbiana Formation (Australia) using Secondary Ion Mass Spectrometry (SIMS), coupled with X-ray spectroscopy. Type-A OM is embedded in quartz and/or chlorite. We show that mixtures of chlorite bias negatively the δ13Corg of associated OM measured by SIMS, likely through recombination of hydrogen from chlorite with carbon from OM during analysis. Type-A OM, associated with quartz or interleaved with chlorites (but remote enough to avoid H-recombination), displays δ13Corg between −56.1 and −50.6‰ VPDB (mean: −53.5‰, SD ± 1.8‰). The intimate association of Type-A OM with Fe-rich chlorite and its main occurrence in pyritic layers coupled with 13C-depletions is most parsimoniously interpreted as anaerobic methanotrophy using Fe3+-minerals and/or oxidized S. Type-B OM comprises <2-µm organic globules embedded in calcite. A mass balance correction using the δ13C values measured on Type-B globules mixed with calcite and those measured on pure calcite yielded corrected δ13C∗org values for the globules ranging between −65.2 and −52.5‰ (mean −58.8‰, SD ± 3.6‰). In a context where Fe and S reduction could outcompete acetogenesis for a fraction of any available H2, these extremely low δ13Corg values are difficult to explain with chemoautotrophic acetogenesis. Atomic S/C ratios in Type-B globules reach up to 0.042, i.e. up to tenfold the values (0.003–0.004) in Type A-OM. These organic S concentrations, combined with the extremely low δ13C∗org values, support anaerobic oxidation of methane coupled to sulfur oxidation. Finally, Type-C OM, interpreted as migrated pyrobitumen nodules, displays δ13Corg of −48 to −39.1‰. The most extreme low-δ13Corg values of the Precambrian are thus best interpreted as the result of lacustrine methanotrophy fueled by oxidized S and/or Fe species, which likely derived from oxygenic and/or anoxygenic photosynthesis. Photosynthesis, followed by methanogenesis and methanotrophy may have been important in lakes at a time of supercontinent growth and eruption of large subaerial igneous provinces. In this context, anaerobic methanotrophy could have played a part in regulating atmospheric methane.

Effects of preservation methods of muscle tissue from upper-trophic level reef fishes on stable isotope values (δ (13)C and δ (15)N).

Research that uses stable isotope analysis often involves a delay between sample collection in the field and laboratory processing, therefore requiring preservation to prevent or reduce tissue degradation and associated isotopic compositions. Although there is a growing literature describing the effects of various preservation techniques, the results are often contextual, unpredictable and vary among taxa, suggesting the need to treat each species individually. We conducted a controlled experiment to test the effects of four preservation methods of muscle tissue from four species of upper trophic-level reef fish collected from the eastern Gulf of Mexico (Red Grouper Epinephelus morio, Gag Mycteroperca microlepis, Scamp Mycteroperca phenax, and Red Snapper Lutjanus campechanus). We used a paired design to measure the effects on isotopic values for carbon and nitrogen after storage using ice, 95% ethanol, and sodium chloride (table salt), against that in a liquid nitrogen control. Mean offsets for both δ13C and δ15N values from controls were lowest for samples preserved on ice, intermediate for those preserved with salt, and highest with ethanol. Within species, both salt and ethanol significantly enriched the δ15N values in nearly all comparisons. Ethanol also had strong effects on the δ13C values in all three groupers. Conversely, for samples preserved on ice, we did not detect a significant offset in either isotopic ratio for any of the focal species. Previous studies have addressed preservation-induced offsets in isotope values using a mass balance correction that accounts for changes in the isotope value to that in the C/N ratio. We tested the application of standard mass balance corrections for isotope values that were significantly affected by the preservation methods and found generally poor agreement between corrected and control values. The poor performance by the correction may have been due to preferential loss of lighter isotopes and corresponding low levels of mass loss with a substantial change in the isotope value of the sample. Regardless of mechanism, it was evident that accounting for offsets caused by different preservation methods was not possible using the standard correction. Caution is warranted when interpreting the results from specimens stored in either ethanol or salt, especially when using those from multiple preservation techniques. We suggest the use of ice as the preferred preservation technique for muscle tissue when conducting stable isotope analysis as it is widely available, inexpensive, easy to transport and did not impart a significant offset in measured isotopic values. Our results provide additional evidence that preservation effects on stable isotope analysis can be highly contextual, thus requiring their effects to be measured and understood for each species and isotopic ratio of interest before addressing research questions.

Non-lethal sampling of lake sturgeon for stable isotope analysis: Comparing pectoral fin-clip and dorsal muscle for use in trophic studies

When used to study the feeding ecology of fish, stable isotope analyses generally require sacrificing animals to collect tissue samples. Lethal sampling can be undesirable when studying a species at risk such as lake sturgeon (Acipenser fulvescens). We evaluated the feasibility of using pectoral fin clips as an alternative to dorsal muscle tissue in stable isotope studies for adult lake sturgeon. Because lipid content can affect stable isotope ratios, we also determined whether lipid extraction and mathematical normalization affected the relationship between fin-clip and muscle tissue signatures. Dorsal muscle plugs and fin clips were collected from adult lake sturgeon and analyzed for stable isotopes of carbon and nitrogen. Significant, positive relationships were observed between fin and muscle tissues for both δ15N (r2=0.43) and δ13C (r2=0.32). Lipid extraction significantly reduced the among-individual variation, and improved the δ13C relationship significantly (r2=0.74). While general non-linear mathematical lipid normalization did effectively reduce the among-individual variation, a consistent enrichment relative to chemical extraction was observed. An alternative mass balance correction method, however, yielded lake sturgeon specific parameter estimates that facilitated accurate lipid correction. The strong relationships between fin-clip and muscle tissue signatures demonstrate that fin-clips should be considered as good surrogates for muscle tissue, will allow trophic studies to accurately adjust for the effect of differential lipid accumulation and can effectively limit the need for invasive sampling for adult lake sturgeon.

δ13C analysis of amino acids in human hair using trimethylsilyl derivatives and gas chromatography/combustion/isotope ratio mass spectrometry

To provide a simple one-step derivatization procedure for the analysis of a wide variety of amino acids in human hair by gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS). N,O-Bis(trimethylsilyl)trifluoroacetamide (BSTFA) derivatization is already widely used outside the IRMS community, is applicable to a variety of functional groups, and provides products that are common entries in mass spectral databases, thus simplifying compound identification. Method optimization and validation were performed on a mixture of ten standard amino acids found abundantly in human hair. The method was then applied to the analysis of scalp hair from six human subjects. The hair was washed, hydrolyzed with 6 M HCl, derivatized using BSTFA in acetonitrile and analyzed using gas chromatography (GC) with concurrent quadrupole and isotope ratio mass spectrometry (IRMS) detectors. The reproducibility for the δ(13)C measurements, including the derivatization procedure and GC/C/IRMS analysis, on a day-to-day comparison was between 0.19‰ and 0.35‰ (SD, N = 12), with an average standard deviation of 0.26‰. Because trimethylsilylation adds 3N carbon atoms (where N = # reactive protons) to each amino acid, the δ(13)C values for amino acid derivatives were corrected using a mass balance correction and the measured kinetic isotope effect (KIE). The KIE values ranged from 0.984 to 1.020. The procedure gave consistent δ(13)C values with precision similar to other derivatization methods for the range of sample sizes studied: 50-1000 µg of each amino acid. The method gave δ(13)C values consistent with the known literature values when applied to the analysis of amino acids in human hair.

Coupling of kinetic Monte Carlo simulations of surface reactions to transport in a fluid for heterogeneous catalytic reactor modeling

We have developed a method to couple kinetic Monte Carlo simulations of surface reactions at a molecular scale to transport equations at a macroscopic scale. This method is applicable to steady state reactors. We use a finite difference upwinding scheme and a gap-tooth scheme to efficiently use a limited amount of kinetic Monte Carlo simulations. In general the stochastic kinetic Monte Carlo results do not obey mass conservation so that unphysical accumulation of mass could occur in the reactor. We have developed a method to perform mass balance corrections that is based on a stoichiometry matrix and a least-squares problem that is reduced to a non-singular set of linear equations that is applicable to any surface catalyzed reaction. The implementation of these methods is validated by comparing numerical results of a reactor simulation with a unimolecular reaction to an analytical solution. Furthermore, the method is applied to two reaction mechanisms. The first is the ZGB model for CO oxidation in which inevitable poisoning of the catalyst limits the performance of the reactor. The second is a model for the oxidation of NO on a Pt(111) surface, which becomes active due to lateral interaction at high coverages of oxygen. This reaction model is based on ab initio density functional theory calculations from literature.

Pressure Drop Characteristics and Reserves Calculation Method of Gas Reservoir of Multi-Layer Unconsolidated Sandstone

Taking Sebei gas field as an example, this paper sorts hydrostatic pressure test data over the years and summarizes the reasons that the formation pressure drop curve generates a straight line, up-warping and recurvation from the perspective of water invasion, perforation adding, layer adjusting, sand burying, and sand control as well as inter-layer interference and inter-well interference, with typical wells analyzed and explained. On this basis, two kinds of mass balance correction models which have taken the offset of the pressure drop curve into account are proposed to calculate the dynamic reserves of gas wells with different offset shapes of pressure drop curves. Practical result shows the simple concept and strong practicality of this method, which provides an effective method for the accurate calculation of dynamic reserves of such gas reservoir.

Assessing stable isotope dynamics of diapausing Calanus finmarchicus and C. hyperboreus during the overwintering period: a laboratory experiment

This study aimed at describing changes in the stable isotopic composition of late copepodite stage V (CV) subarctic marine copepods (Calanus finmarchicus and C. hyperboreus) during overwintering non-feeding periods. Diapausing stage CVs sampled in deep waters of the Lower St. Lawrence Estuary (Québec, Canada) in late-September 2009 were monitored for 4 months under controlled laboratory conditions. CVs and newly moulted adults were analyzed for δ13C and δ15N signatures as well as lipid, carbon and nitrogen content. Lipids were extracted in half of the samples to compare δ13C of individuals with and without lipids and to evaluate the accuracy of mass balance correction models for δ13C under lipid influence. Lipid content generally decreased with time for both species, which was reflected in an increase of δ13C values of CVs but a constant δ13C in newly moulted adults. Accordingly, lipid extraction resulted in an increase of δ13C in CVs and adults. The mean δ13C signature of lipid-extracted individuals remained constant through the time for CVs of both species and for C. finmarchicus adults. δ15N signatures of individuals increased after lipid extraction, but this did not result in a constant value over time, suggesting that several endogenous metabolic processes affected nitrogen isotopic content. The accuracy of the mass balance model differed between species and stages, suggesting that lipid extraction should always be performed prior to applying mathematical corrections.

Lipid correction for carbon stable isotope analysis of deep-sea fishes

Stable isotope analysis of fish tissue can aid studies of deep-sea food webs because sampling difficulties severely limit sample sizes of fish for traditional diet studies. The carbon stable isotope ratio (δ 13C) is widely used in food web studies, but it must be corrected to remove variability associated with varying lipid content in the tissue. A lipid correction has not been determined for any deep-sea fish. These fishes are ideal for studying lipid correction because lipid content varies widely among species. Our objective was to evaluate an application of a mass balance δ 13C correction to a taxonomically diverse group of deep-sea fishes by determining the effect of lipid extraction on the stable isotope ratios, examining the quality of the model parameters derived for the mass balance correction, and comparing the correction to published results. We measured the lipid extraction effect on the nitrogen stable isotope ratio (δ 15N) and δ 13C of muscle tissue from 30 North Atlantic species. Lipid extraction significantly increased tissue δ 15N (+0.66‰) and δ 13C values, but the treatment effect on δ 13C was dependent on C:N, a proxy for lipid content. We compared the lipid-extracted δ 13C to the δ 13C predicted by the mass balance correction using model variables estimated from either all individuals (pooled) or species-by-species or using published values from other species. The correction using the species-by-species approach performed best; however, all three approaches produced corrected values that were generally within 0.5‰ of the measured lipid-free δ 13C and that had a small over-all bias (<0.5‰). We conclude that a generalized mass balance correction works well for correcting δ 13C in deep-sea fishes, is similar to that developed for other fishes, and recommend caution when applying a generalized correction to fish with high lipid content (C:N >8).

Effects of fixation on freshwater invertebrate carbon and nitrogen isotope composition and its arithmetic correction

Retrospective investigations using carbon and nitrogen stable isotope composition of archived material have a great potential for describing past effects of anthropogenic ecosystem alterations or natural shifts in ecosystems. In this study, we examined the effects of two commonly used preservation substances of freshwater invertebrates, ethanol and lugol, on δ13C and δ15N of various planktonic and benthic taxa. For both isotopes, the average effect of fixation in ethanol was stronger than in lugol, and the effects on δ13C were stronger than on δ15N (average ± SD: 1.18 ± 0.94 and −0.47 ± 0.99 for δ13C ethanol and lugol fixed samples, respectively, and 0.39 ± 0.68 and 0.17 ± 0.77 for δ15N, respectively). The changes in the isotopic composition were not dependent on the initial isotopic composition of each taxon, but were related with concomitant changes in the carbon or nitrogen content. Application of a mass balance correction equation to the fixed samples resulted in a significantly lower average effect of fixation in ethanol (0.01 ± 0.59 and 0.44 ± 0.65 for δ15N and δ13C, respectively), while corrections had little effect for lugol fixed samples (0.24 ± 0.53 and −0.39 ± 0.85, respectively). For both isotopes and fixatives, corrections resulted in linear relationships between fixed vs. control samples, with slopes and intercepts not significantly different from 1 and 0, respectively. Therefore, mass balance correction of stable isotopes in fixed invertebrates is recommended for minimising the effects of fixation.

A standard protocol for stable isotope analysis of zooplankton in aquatic food web research using mass balance correction models

Stable isotope analysis has become a crucial tool for aquatic food web ecologists, but a lack of methodological standardization hinders comparisons between studies. One methodological inconsistency in stable isotope food web research is the decision whether to extract lipids before stable isotope analysis. The depletion in zooplankton stable carbon isotope values (δ13C) due to fatty acid content and the accuracy of mathematical correction models designed to predict this depletion were examined for a range of zooplankton species from nine lakes of diverse size and productivity. Large differences of up to 5% observed between δ13C values of nonextracted and lipid‐extracted zooplankton samples correlated with zooplankton fatty acid content. A mass balance δ13C correction model for fatty acid content using atomic C:N ratios and directly measured δ13C values of fatty acids accurately predicted (R2 = 0.95) lipid‐extracted δ13C values for both copepod and cladoceran zooplankton. Researchers should use mass balance lipid corrections as an efficient method to eliminate bias in comparisons of zooplankton and fish δ13C values and allow their results to be more easily compared with other studies.

Effects of chemical lipid extraction and arithmetic lipid correction on stable isotope ratios of fish tissues

For accurate interpretation of fish trophodynamics from carbon stable isotope data it is necessary to extract tissue lipids. This is because lipid content varies within and among tissues in both space and time, and because lipids are 13C-depleted relative to proteins. However, lipid extraction may affect delta15N, thus requiring costly and time-consuming separation of delta13C and delta15N analyses. These problems have prompted the development of arithmetic correction techniques for delta13C, but the techniques and their underlying assumptions have not been systematically tested. This study compared the effects of lipid extraction and arithmetic correction techniques on delta13C and delta15N of European sea bass (Dicentrarchus labrax) tissues. Following Folch lipid extraction from muscle and liver, there was a mean increase in delta15N of 0.77 per thousand, but enrichment varied with lipid content such that effects on delta15N were hard to predict. Changes in delta13C and C:N between untreated and lipid-extracted samples reflected the quantity of lipid removed. The arithmetic correction techniques of mass balance and lipid correction were sensitive to the C:N of the lipid-extracted tissue and to the assumed depletion of lipid delta13C relative to protein delta13C. However, the mass balance approach was appropriate for the mathematical correction of bulk tissue data in most circumstances, provided that the C:N of lipid-extracted tissue could be determined for a small proportion of samples. Application of mass balance arithmetic correction can lead to significant time and cost savings in trophodynamic studies, because the majority of delta13C and delta15N analyses would not need to be run separately.

The effect of N2O on the isotopic composition of air‐CO2 samples

The ionisation efficiencies of N2O vs. CO2 as well as their ratios were measured in detail introducing clean N2O and CO2 into the electron impact ion source of an isotope ratio mass spectrometer. Changes in the ionisation efficiency ratio (IER) were found for different electron energy settings and compared with the ratios of literature ionisation cross-section values for pure N2O and CO2. To establish the influence of mixtures of N2O and CO2 in a mass spectrometer, artificial air mixtures were prepared by mixing different amounts of N2O and CO2 from well-calibrated spike cylinders with CO2-free air. The mixing ratios varied from 8-512 ppb for N2O and from 328-744 ppm for CO2. With these mixtures the effects of varying N2O concentrations on apparent CO2 isotope ratios in air samples were determined. After applying a mass balance correction the delta13C results were consistent within small error margins. The data seemed almost independent from a particular choice for the IER of N2O vs. CO2 in the correction algorithm. For delta18O a small effect of the ionisation efficiency ratio of N2O vs. CO2 was found. Several sets of calculations were made varying the IER between 0.88 and 0.62. The dependence of delta18O was the smallest with an adopted IER of 0.68-0.72 in the mass balance correction equation for isotopic analysis of CO2 in air. For high-precision measurements of the CO2 stable isotope ratios in air samples a careful assessment of the mass spectrometer performance is necessary. Different ion sources, even different ion source settings, alter the IER of N2O vs. CO2 which is used in the N2O correction algorithm. Preferably, the specific mass spectrometric behaviour should be established with clean N2O/CO2 mixtures or with air mixtures covering a larger range of N2O concentrations.

Investigation of the atmospheric water budget of the BALTEX area using NCEP/NCAR reanalysis data

The Baltic Sea ExperimentBALTEX is the European regional project of GEWEX. The aim ofBALTEXis to improve the knowledge of the water and energy cycle of the Baltic Sea including its catchment area(the BALTEX area). In this study the NCEP/NCAR reanalysis data (Kalnay et al. Bull. Am. Meteorol.Soc., 1996, 77, 437—471) are used to derive the mean atmospheric water budget over the BALTEX area.For this purpose the horizontal water vapour fluxes are calculated. In the long-term mean (here 1948–2000) the vertically integrated divergence of these fluxes must balance evaporation minus precipitation.The latter two are provided by the NCEP/NCAR reanalyses. The water budget over the BALTEX areais, however, not closed. An error estimation is performed. The divergence of the water vapour flux iscorrected due to the mass balance correction technique (Alestalo, Tellus, 1981, 33, 360—371). Largedifferences are obtained between the precipitation given by the NCEP/NCAR reanalysis and by theanalysis of the observations by Rubel and Hantel (Meteorol. Atmos. Phys., 2001, 77, 155—166). Withthese analysed precipitation data and the corrected divergence of the water vapour fluxes the imbalanceof the water vapour budget could be remarkably reduced.

Development of a Mathematical Model for the Simulation of the Biodegradation of Organic Substrates in a High-Solids Anaerobic Digestion Process

A kinetic model is developed to simulate the biodegradation of the organic fraction of municipal solid waste in a high-solids complete-mix continuous-flow anaerobic digestion process. A mass balance correction factor (f) is used to account for (i) the water that is incorporated into the production of biogas, and (ii) the water vapor present in the biogas. With the inclusion of the factor f, expressions are developed to determine the substrate removal efficiency based on first-order and Monod kinetic models. Simulations using first-order and Monod kinetic models are carried out to illustrate the importance of the f factor on the substrate mass removal and microorganism concentration in the high-solids anaerobic digestion process. The effect of the value of f on kinetic constants, and the error involved in considering the high-solids process as a low-solids process are discussed. Finally, experimental results from a pilot-scale high-solids complete-mix batch-fed anaerobic digestion process are used to estimate the first-order rate constant for the biodegradable fraction of municipal solid waste.

A Distributive Mass Balance Correction in Single- and Multigrid Incompressible Flow Calculation

The pressure-velocity coupling in a finite volume method (SIMPLE-type) is enhanced using a distributed mass balance correction method. The mass imbalance generated by the momentum equations is distributed over all control volumes prior to solving the pressure-correction equation. This method considerably enhances the convergence of the SIMPLE method, whether used on either a single staggered grid or a collocated multigrid. Some simple two- and three-dimensional laminar flows are used to test the method.

Dissolved organic carbon and methane in a regional confined aquifer, southern Ontario, Canada: Carbon isotope evidence for associated subsurface sources

Information regarding the origin, composition and transport of natural dissolved organic carbon (DOC) in groundwater is necessary to understand the transport of metals and organic pollutants, as well as for the use of 14C in DOC as an isotopic groundwater dating method. Previous research in several groundwater systems has suggested soil organic C is the predominant source of high molecular weight DOC to the subsurface. Through the use of stable isotopes, 14C and geochemical analyses, this study shows that significant concentrations of DOC and CH 4 in a regional confined aquifer can be generated in situ from subsurface sedimentary organic sources. The DOC and CH 4 produced is a combined result of degradation of buried peats and bacterial action, resulting in high DOC concentrations and strongly methanogenic conditions in the aquifer. The DOC and CH 4 comprise, on average, nearly 50% of the total dissolved C pool in the central part of the aquifer. Methanogenic conditions complicate isotopic groundwater dating by the conventional dissolved inorganic carbon (DIC) method. Estimates of isotopic groundwater residence time using DOC 14C data are proposed by the application of 14C isotope and mass balance corrections.