Effects of HCl pretreatment, drying, and storage on the stable isotope ratios of soil and sediment samples

Innovative approach to reveal source contribution of dissolved organic matter in a complex river watershed using end-member mixing analysis based on spectroscopic proxies and multi-isotopes

Dissolved organic matter (DOM) in river watersheds dynamically changes based on its source during a monsoon period with storm event. However, the variations in DOM in urban and rural river watersheds that are dominated by point and non-point sources have not been adequately explored to date. We developed an innovative approach to reveal DOM sources in complex river watershed systems during pre-monsoon, monsoon, and post-monsoon periods using end-member mixing analysis (EMMA) by combining multi-isotope values (δ¹³C-DOC, δ¹⁵N-NO₃ and δ¹⁸O-NO₃) and spectroscopic indices (fluorescence index [FI], biological index [BIX], humification index [HIX], and specific UV absorbance [SUVA]). Several potential end-members of DOM sources were collected from watersheds, including top-soils, groundwater, plant group (fallen leaves, riparian plants, suspended algae), and different effluents (cattle and pig livestock, agricultural land, urban, industry facility, swine treatment facility and wastewater treatment facility). Concentrations of dissolved organic carbon, dissolved organic nitrogen, NO₃-N, and NH₄-N increased during the monsoon period with an increase in the input of anthropogenic DOM, which have higher HIX values owing to the flushing effect. The results of EMMA indicate that soil and agricultural effluents accounted for a substantial contribution of anthropogenic DOM at varying rates based on seasons. We also found that results of EMMA based on combining spectroscopic indices and δ¹³C-DOC isotope values were more accurate in tracing DOM sources with respect to land-use characteristics compared to applying only spectroscopic indices. The positive relationship between FI, BIX and δ¹⁵N-NO₃ were revealed that nitrate would be decomposed from DOM affected by intensive agricultural activities. In addition, consistent with the EMMA results, the molecular composition of the DOM was clearly evidenced by a large number of CHON formulas, accounting for over 50% of the total characterized compounds, including pesticides and pharmaceuticals used in agriculture farmland and livestock. Our results clearly demonstrated that EMMA based on combing multi-stable isotopes and spectroscopic indices could be trace the DOM source, which is important for understanding changes in the DOM quality, and application of nitrate isotopes and molecular analysis supports in-depth interpretation. This study provides easy and intuitive techniques for the estimation of the relative impacts of DOM sources in complex river watersheds, which can be verified in various ways rather than relying on a single technique approach.

No evidence for effects of mill-grinding on δ13 C, δ15 N and δ34 S values in different marine taxa

RATIONALE

The analysis of natural variation in light stable isotopes such as carbon (δ¹³ C), nitrogen (δ¹⁵ N) and sulfur (δ³⁴ S) plays an important role in deepening our understanding of ecosystems. To avoid misinterpretation, robust results are required, where pre-treatment steps such as sample homogenization are crucial to guarantee representative samples. The grinding of samples using stainless steel balls in polypropylene tubes (e.g., laboratory bead-beater) has been identified as a potential source of bias.

METHODS

We tested possible effects of mill-grinding (e.g., contamination) of samples of coastal marine taxa including primary producers, primary consumers and higher trophic level fish. We compared potential impacts of homogenization by mill-grinding with hand-grinding over an extended time on δ¹³ C, δ¹⁵ N and δ³⁴ S values.

RESULTS

One-way Welch's analysis of variance (ANOVA) showed that there were no statistical differences between methods for all the studied taxa. Also, repeated measures ANOVA showed no evidence of effects of grinding for extended times (from 30 to 120 seconds) for δ¹³ C, δ¹⁵ N and δ³⁴ S values.

CONCLUSIONS

We found no evidence that grinding samples in polypropylene tubes in a bead-beater resulted in any marked alteration of the isotopic composition on the studied samples, e.g., through contamination by plastic. As such, we consider mill-grinding as an appropriate method for the homogenization of samples from a range of different marine taxa, which under controlled conditions did not affect δ¹³ C, δ¹⁵ N and δ³⁴ S analysis.

Distributions and potential sources of traditional and emerging polycyclic aromatic hydrocarbons in sediments from the lower reach of the Yangtze River, China

This study investigated the current contamination status and potential sources of traditional and emerging polycyclic aromatic hydrocarbons (t-PAHs and e-PAHs) in the sediments across a wide area of the Yangtze River, spanning nine cities. Fifty-seven sediment samples were collected in 2019, from which 15 t-PAHs and 11 e-PAHs were analyzed using GC-MSD. In addition, organic carbon (OC), total nitrogen (TN), and carbon and nitrogen stable isotope ratios (δ¹³C and δ¹⁵N) in sediments were measured to evaluate associations with PAHs contamination. OC, TN, and their stable isotope ratios showed a wide range of site-specific contents and values, indicating high variation in contamination and sources. Concentrations of t-PAHs and e-PAHs in sediments ranged from 0.6 to 200,000 ng g^-1 dry weight (dw) and 1.1 to 20,000 ng g^-1 dw, respectively. Hotspot sites located in Nanjing (PuKou), Taizhou (JingJiang), and Suzhou (ZhangJiaGang). PAHs contamination reflected land use type and human activity in the surrounding area. Positive matrix factorization (PMF) modeling showed that, on average (n = 57), vehicle emissions were the most dominant contribution (57%), followed by petroleum (28%) and fossil fuel combustion (15%). Sites with high PAHs contamination in sediments were of severe ecological risk. Contributions to the potential risks of PAHs were most significant in the order of dibenz[a,h]anthracene, benzo[b]fluoranthene, and benzo[a]pyrene. The primary origin of these compounds appeared to be fossil fuel combustion. The results of this study are expected to provide useful baseline data on the current contamination status and potential sources of traditional and emerging pollutants in the sediments of the Yangtze River, China.

Assessment of Pollution Sources and Contribution in Urban Dust Using Metal Concentrations and Multi-Isotope Ratios (13C, 207/206Pb) in a Complex Industrial Port Area, Korea

The metal concentrations and isotopic compositions (13C, 207/206Pb) of urban dust, topsoil, and PM10 samples were analyzed in a residential area near Donghae port, Korea, which is surrounded by various types of industrial factories and raw material stockpiled on empty land, to determine the contributions of the main pollution sources (i.e., Mn ore, Zn ore, cement, coal, coke, and topsoil). The metal concentrations of urban dust in the port and residential area were approximately 85~112 times higher (EF > 100) in comparison with the control area (EF < 2), especially the Mn and Zn ions, indicating they were mainly derived from anthropogenic source. These ions have been accumulating in urban dust for decades; furthermore, the concentration of PM10 is seven times higher than that of the control area, which means that contamination is even present. The isotopic (13C, 207/206Pb) values of the pollution sources were highly different, depending on the characteristics of each source: cement (−19.6‰, 0.8594‰), Zn ore (−24.3‰, 0.9175‰), coal (−23.6‰, 0.8369‰), coke (−27.0‰, 0.8739‰), Mn ore (−24.9‰, 0.9117‰), soil (−25.2‰, 0.7743‰). As a result of the evaluated contributions of pollution source on urban dust through the Iso-source and SIAR models using stable isotope ratios (13C, 207/206Pb), we found that the largest contribution of Mn (20.4%) and Zn (20.3%) ions are derived from industrial factories and ore stockpiles on empty land (Mn and Zn). It is suggested that there is a significant influence of dust scattered by wind from raw material stockpiles, which are stacked near ports or factories. Therefore, there is evidence to support the idea that port activities affect the air quality of residence areas in a city. Our results may indicate that metal concentrations and their stable isotope compositions can predict environmental changes and act as a powerful tool to trace the past and present pollution history in complex contexts associated with peri-urban regions.

Trophic Ecology of Endangered Gold-Spotted Pond Frog in Ecological Wetland Park and Rice Paddy Habitats

Simple Summary

Gaining information about the habitat environment and biological interactions is important for conserving gold-spotted pond frogs, which are faced with a threat of local population extinction in Korea due to artificial habitat changes. Based on stable isotope ratios, we estimated the ecological niche space (ENS) of gold-spotted pond frogs in an ecological wetland park and a rice paddy differing in habitat patch connectivity and analyzed the possibility of their ENS overlapping that of competitive and predatory frogs. Gold-spotted pond frogs showed a broader ENS in the ecological wetland park, wherein predation was relatively easy, than in the rice paddy. However, the ENS of the gold-spotted pond frogs was highly probably overlapped with that of other competing frog species that shared some of the food sources. Nevertheless, since the stable isotope analysis showed that gold-spotted pond frogs fed on more diverse prey than their competitors, it would remain relatively easy to procure alternative food sources, which are less affected by the competition in an environment with abundant food. Therefore, for stable settling of gold-spotted pond frogs into habitats and preserving their population, establishing habitat environments with highly diverse food sources is crucial, following consideration of their feeding behavior.

Abstract

The gold-spotted pond frog (Pelophylax chosenicus) is an endangered amphibian species in South Korea. In order to obtain ecological information regarding the gold-spotted pond frog’s habitat environment and biological interactions, we applied stable isotope analysis to quantify the ecological niche space (ENS) of frogs including black-spotted pond frogs (P. nigromaculatus) and bullfrogs (Lithobates catesbeianus) within the food web of two different habitats—an ecological wetland park and a rice paddy. The gold-spotted pond frog population exhibited a broader ENS in the ecological wetland park than in the rice paddy. According to the carbon stable isotope ratios, gold-spotted pond frogs mainly fed on insects, regardless of habitat type. However, the results comparing the range of both carbon and nitrogen stable isotopes showed that gold-spotted pond frogs living in the rice paddy showed limited feeding behavior, while those living in the ecological wetland park fed on various food sources located in more varied trophic positions. Although the ENS of the gold-spotted pond frog was generally less likely to be overlapped by that of other frog species, it was predicted to overlap with a high probability of 87.3% in the ecological wetland park. Nevertheless, gold-spotted pond frogs in the ecological wetland park were not significantly affected by the prey competition with competitive species by feeding on other prey for which other species’ preference was low. Since these results show that a habitats’ food diversity has an effect on securing the ENS of gold-spotted pond frogs and prey competition, we recommend that the establishment of a food environment that considers the feeding behavior of gold-spotted pond frogs is important for the sustainable preservation of gold-spotted pond frogs and their settlement in alternative habitats.

Synthesis of mycosporine-like amino acids by a size-fractionated marine phytoplankton community of the arctic beaufort sea

During the RV-ARAON cruise, a comparative study on the biosynthesis of mycosporine-like amino acids (MAAs) was conducted for the size-fractionated phytoplankton of the Beaufort Sea (Arctic). The MAAs contents in the micro-phytoplankton community (>20 μm size) is considerably higher than that observed in the nano- (20-2 μm size) and pico-phytoplankton (<2 μm size) communities. The micro-phytoplankton of the Mackenzie Shelf had a relatively higher Chlorophyll a (Chl a) concentration. Considering the total phytoplankton community, the MAAs concentration as well as net production of individual MAAs (such as shinorine and palythine) were higher at the Mackenzie Shelf rather than at the sites located beyond the Beaufort Sea; precisely, the highest net production rates of shinorine and palythine were 0.211 (±0.02) ng C L^-1 d^-1 and 0.136 (±0.001) ng C L^-1 d^-1 respectively (No other MAAs were detected). The micro-phytoplankton used around 0.5% of the total carbon uptake for the synthesis of MAAs. Compared to the smaller phytoplankton community, the micro-phytoplankton utilized more of their energy for the biosynthesis of MAAs; on the other hand, nano- and pico-phytoplankton focused on cellular activity and had poor biosynthesis of MAAs. This clearly indicates the phytoplankton size-dependent variation in the biosynthesis of MAA in the natural phytoplankton community. This study revealed the environmental adaptation of the various sizes of phytoplankton community as well as their physiological response in the Arctic Beaufort Sea.

Assessing environmental changes in Lake Shihwa, South Korea, based on distributions and stable carbon isotopic compositions of n-alkanes

We investigate historical environmental changes in an artificial lake, Lake Shihwa in South Korea, based on bulk (TOC, TN, C/N ratio, δ¹³C_TOC, and δ¹⁵N_TN) and molecular (concentrations and δ¹³C of n-alkanes) parameters, by analyzing riverbank sediments (n = 12), lake surface sediments (n = 9), and lake core sediments (n = 108). Although the bulk organic parameters showed similar characteristics for all lake surface sediment samples, the distribution pattern and δ¹³C of n-alkanes revealed distinct differences between 2009 samples and 2012/2016 samples. This change of sedimentary organic matter characteristics can be attributed to operation of the tidal power plant that began in 2011, which improved lake water circulation and thus changed the lake sedimentary environment from anoxic to more oxic conditions. The vertical profiles of bulk and molecular lake sediment core records collected in 2009, especially at the site closest to the dike, showed a drastic shift around 1987, indicating that stronger anoxic sedimentary conditions prevailed after 1987. This is linked to sea dike construction in 1987, which prohibited sea-lake water exchange and thus deteriorated water quality in Lake Shihwa. We conclude that Lake Shihwa has experienced severe environmental changes due to human activities.

Characterizing isotopic compositions of TC-C, NO3--N, and NH4+-N in PM2.5 in South Korea: Impact of China's winter heating

The origin of PM_2.5 has long been the subject of debate and stable isotopic tools have been applied to decipher. In this study, weekly PM_2.5 samples were simultaneously collected at an urban (Seoul) and rural (Baengnyeong Island) site in Korea from January 2014 through February 2016. The seasonal variation of isotopic species showed significant seasonal differences with sinusoidal variation. The isotopic results implied that isotope species from Baengnyeong were mostly originated from coal combustion during China's winter heating seasons, whereas in summer, the isotopic patterns observed that were more likely to be from marine. In Seoul, coal combustion related isotopic patterns increased during China's winter heating period while vehicle related isotopic patterns were dominated whole seasons by default. Therefore, aerosol formation was originated from long-range transported coal combustion-related NO_x by vehicle-related NH₃ in Seoul. δN-NH₄⁺ in Seoul showed highly enriched ¹⁵N compositions in all seasons, indicating that NH₃ from vehicle emission is the important source of NH₄⁺ in PM_2.5 in Seoul. In addition, Baengnyeong should be consistently considered as a key region for observing the changes of isotopic features depend on the contribution of individual emissions to the atmospheric as a result of the reduction of coal consumption in China.

Precise carbon isotopic ratio analyses of micro amounts of carbonate and non-carbonate in basalt using continuous-flow isotope ratio mass spectrometry

RATIONALE

Continuous-flow isotope ratio mass spectrometry (CF-IRMS) is a specialized technique used to quickly analyze very small amounts of sample. We have used CF-IRMS to assess the influences of sample weight and relative carbon content on the accuracy and precision of the δ¹³ C values of micro amounts of carbonate and non-carbonate in silicate rocks.

METHODS

The analytical work was performed on a Gasbench II (GB) sample preparation device and on an Elemental Analyzer (EA), which were both interfaced to CF-IRMS instruments. Potential silicate matrix effects on the carbon isotopic analyses were investigated by measuring mixtures of calibrated carbon reference materials and quartz powder. The calibration lines, established by the measured raw values and the known values of three reference materials mixed with quartz powder, were used to calibrate the δ¹³ C values of basalt samples from eastern China.

RESULTS

The δ¹³ C values measured by GB-CF-IRMS of one national carbonate reference material, GBW04416, deviate slightly from the known value for approximately 20-70 μg of carbonate contained in 4.5-mL vials; the smaller the sample size, the lower the measured δ¹³ C values. External precision better than 0.1‰ (1σ, n = 26) is achieved at a signal intensity for mass 44 of between 868 and 1614 mV, corresponding to a sample weight of 30.8-50.2 μg, whereas it is reduced to 0.27‰ (1σ, n = 34) at a signal intensity between 519 and 1614 mV, corresponding to a sample weight of 21.1-50.2 μg. In the EA-CF-IRMS experiments for non-carbonate carbon, at high carbon concentration (greater than 800 ppm) and at optimum sample weights, the accuracy and precision are both better than 0.2‰. For carbon concentrations less than 500 ppm, the measured δ¹³ C values deviate from the average by up to -1.2‰ and the precision is 0.74‰.

CONCLUSIONS

The measured δ¹³ C values decrease substantially at lower carbon concentration and higher sample weights, and poorer precision is obtained. Suggestions are made to measure repeatedly the same carbon concentration of sample and reference materials in order to obtain not only reproducible, but also accurate carbon isotope ratios.

Accumulation of Microcystin (LR, RR and YR) in Three Freshwater Bivalves in Microcystis aeruginosa Bloom Using Dual Isotope Tracer

Abstract: Stable isotope tracers were first applied to evaluate the Microcystis cell assimilation efficiency of Sinanodonta bivalves, since the past identification method has been limited to tracking the changes of each chl-a, clearity, and nutrient. The toxicity profile and accumulation of MC-LR, -RR and -YR in different organs (foot and digestive organs) from the three filter-feeders (Sinanodonta woodina, Sinanodonta arcaeformis, and Unio douglasiae) were assessed under the condition of toxigenic cyanobacteria (Microcystis aeruginosa) blooms through an in situ pond experiment using ¹³C and ¹⁵N dual isotope tracers. Chl-a concentration in the manipulated pond was dramatically decreased after the beginning of the second day, ranging from 217.5 to 15.6 μg·L^-1. The highest amount of MCs was incorporated into muscle and gland tissues in U. douglasiae during the study period, at nearly 2 or 3 times higher than in S.woodiana and S. arcaeformis. In addition, the incorporated ¹³C and ¹⁵N atom % in the U. douglasiae bivalve showed lower values than in other bivalves. The results demonstrate that U. douglasiae has less capacity to assimilate toxic cyanobacteria derived from diet. However, the incorporated ¹³C and ¹⁵N atom % of S. arcaeformis showed a larger feeding capacity than U. douglasiae and S. wodiana. Our results therefore also indicate that S. arcaeformis can eliminate the toxin more rapidly than U. douglasiae, having a larger detoxification capacity.