Stable Isotope Analysis In R Research Articles

Resource Utilization of Three Copepods Along Salinity Gradient in the Seomjin River Estuary, Revealed by Stable Isotope Analysis

Estuaries are one of the most productive regions in ecosystems and serve as important nursery and recruitment habitats for various living resources. However, information on the trophic dynamics of estuarine copepods along the salinity gradient is not well known. Here, we investigated resource partitioning among three copepods, Pseudodiaptomus japonicus, Sinocalanus tenellus, and Tortanus dextrilobatus in the Seomjin River estuary of the Korea using stable carbon and nitrogen isotope analyses. Among the three copepods, S. tenellus showed the lowest δ13C values, while the highest δ13C values were observed in T. dextrilobatus. Stable Isotope Analysis in R (SIAR) showed that S. tenellus fed mainly on terrestrial particulate organic matter (POM), while the contribution of marine POM was relatively high in the diet of T. dextilobatus. In contrast, P. japonicus showed relatively high trophic flexibility between terrestrial and marine POM. Thus, the Stable Isotope Bayesian Ellipses in R (SIBER) showed no dietary niche overlap among the three copepods. Therefore, we suggest that the coexistence of S. tenellus, P. japonicus, and T. dextrilobatus in the Seomjin River estuary can be closely associated with species-specific differences in resource utilization along the salinity gradient.

Validation of Stable Isotope Analysis in R (SIAR) Model for Source Appointment of Agricultural Water Pollution

Validation of Stable Isotope Analysis in R (SIAR) Model for Source Appointment of Agricultural Water Pollution

Identification and apportionment of nitrate sources in the phreatic aquifers in Northern Jordan using a dual isotope method (δ15N and δ18O of NO3-)

Contamination of groundwater resources from nitrates is a worldwide problem, and Jordan is not an exception. Source identification of contamination is a prerequisite for the protection of water resources and human health. Moreover, investigating the controlling processes of groundwater quality is essential for groundwater sustainability, especially in water-poor countries like Jordan. Environmental isotopes of nitrate (15N and 18O of NO3-) have been applied for the first time in Jordan to assess the origin and cycling of nitrogen pollution in two phreatic aquifer systems: Shallala aquifer (B5/B4) and Amman/Wadi Sir aquifer (B2/A7), hosted in multiple land use area in northern Jordan. The nitrate concentration exceeded the natural background (5–10 mg/L) in all samples, ranging from 14 to 251 mg/l as NO3- in both groundwater systems. About 46% of the samples showed NO3- concentration exceeding the WHO threshold of 50 mg/l in drinking water. Isotope values of nitrate (15N and 18O of NO3-) indicated that manure, urban wastewaters, and nitrified synthetic fertilizers were the main sources of nitrate in the groundwater resources in the study area. SIAR (Stable Isotope Analysis in R) model showed that synthetic fertilizers contributed least to nitrate in groundwater in the study area (average = 3.3%). Nitrified synthetic fertilizers contribution was higher in the B2/A7 aquifer (61%) than in B5/B4 aquifer (49%), whereas anthropogenic organic matter contribution in the B5/B4 (46%) is higher than in the B2/A7 aquifer (36%). Anthropogenic organic matter contribution in the B5/B4 aquifer varied from 20% in the west to 85% in east. Nitrified synthetic fertilizers contribution varied from 10% in the east to 74% in the west. Sustainable management of groundwater resources in the study area necessitates application of best management practices and proper land use planning.

Quantifying major NOx sources of aerosol nitrate in Hangzhou, China, by using stable isotopes and a Bayesian isotope mixing model

Nitrogen oxides (NOx) from anthropogenic emissions oxidized to form nitrate (NO3−) and play a key role in air pollution. The aim of this study is to identify the NOx sources and their seasonal variations in Hangzhou, a typical city in East China. To achieve this, the chemical characteristics of water-soluble inorganic ions (WSIIs) and the stable isotopes of NO3− (δ15N–NO3- and δ18O–NO3-) were measured in fine particulate matter (PM2.5) and total suspended particle (TSP) aerosol samples that were collected between December 2018 and November 2019. It was found that the annual average concentration of PM2.5 (69.28 ± 34.69 μg/m3) in Hangzhou exceeded the Chinese air quality limit of 35.00 μg/m3. The results showed that NO3− accounted for the largest proportion of the WSIIs in both PM2.5 and TSP, and that major ions were mainly in the form of NH4NO3, (NH4)2SO4, Ca(NO3)2, CaSO4 in aerosol particles. High ratios of NO3−/SO42− in PM2.5 and TSP indicated that vehicle emissions became increasingly important source of aerosol pollution in Hangzhou during the study period. The δ15N–NO3- values ranged from +1.1‰ to +9.2‰ in PM2.5 and from −1.0‰ to +8.5‰ in TSP in Hangzhou. A Bayesian isotopic mixing model (stable isotope analysis in R (SIAR)) was applied to quantify the NOx sources in Hangzhou. This revealed that vehicle emissions, coal combustion and biomass burning were the main NOx sources of PM2.5 and TSP. The seasonal variation of the δ15N–NO3- values in PM2.5 and TSP in the study area were due to seasonal changes in the NOx contributions from different sources. The SIAR model results indicated that coal combustion increased in association with heating in north China during the winter (winter: 38.4% for PM2.5 and 36.4% for TSP, summer: 30.6% for PM2.5 and 29.8% for TSP) and the microbial N cycle increased with a higher application of fertilizer to crops during the summer (winter: 5.3% for PM2.5 and 5.6% for TSP, summer: 10.7% for PM2.5 and 11.6% for TSP). The improved SIAR model combined with the equilibrium fractionation reported the lower contributions from coal combustion (15.5–20.2%) and the higher contributions from Microbial N cycle (22.9–29.3%), as compared to the SIAR model. The seasonal variation of the δ18O–NO3- values was influenced by differences in the oxidation of NO2 via O3 (forming HNO3), whereby the N2O5 pathway dominated during the winter and the OH pathway dominated during the summer.

Confirming the Source of High-Sulfate Concentrations in Dead Horse Creek, Winkler, Manitoba, Canada, Using a Dual-Isotope Bayesian Probability Mixing Model

Sulfate (SO42−) is an essential nutrient for the growth of several agricultural crops. However, higher than normal SO42− runoff into rivers and streams can cause environmental impacts including acidification, heavy metal release, phosphorus remobilization, and methylmercury release in aquatic systems. The city of Winkler, Manitoba, Canada, uses groundwater for its municipal needs. Some of the wells have a high SO42− content that is stripped off and discarded into the municipal sewage lagoons which are biannually emptied into an adjacent creek (Dead Horse Creek, DHC). However, sewage lagoon effluent is not the only input source of SO42−. Culvert drainage and soil runoff from the surrounding agricultural fields also contribute SO42− to DHC. During the summers of 2011, 2012, and 2018, sewage lagoons, culverts, wells, precipitation, soil, and fertilizers were sampled from the DHC area near Winker, Manitoba. Isotopic compositions (sulfur and oxygen) for all potential SO42− inputs to DHC have been determined. In this study, a dual-isotope approach was employed in combination with Stable Isotope Analysis in R (SIAR) Bayesian probability analysis to estimate the contributions of all potential SO42− sources to DHC. The Winkler sewage lagoons were found to be the primary SO42− contributor, an outcome which is directly related to the disposal of concentrated SO42− waste from the City of Winkler’s water treatment plant. In addition, runoff of soils (through culverts) was also determined to be a significant contributor of SO42− to DHC. Based on the results of this study, it is recommended that the City of Winkler’s Waste Water Treatment Plant incorporate a process to remove the high SO42− concentrations from the lagoons prior to the release of sewage effluent into DHC.

Source Apportionment of Atmospheric Ammonia: Sensitivity Test Based on Stable Isotope Analysis in R Language

Ammonia (NH3) is an important precursor of fine particles and nitrogen deposition. It is critical to identify and quantify the sources of NH3 before the implementation of a mitigation strategy. Stable isotope analysis in R (SIAR) has potential with regard to the source apportionment of NH3, but its reliability is closely related to the signatures (δ15N-NH3) of emission sources. Based on SIAR, we found that the agricultural contribution varied significantly with mean δ15N-NH3 values of endmember input. In contrast, both the contributions of fossil fuel and NH3 slip showed low sensitivity against the change of endmember input. Moreover, the agricultural contribution changed by about 20% due to the variations in agricultural endmember mean values. Such a change is five times that due to the variations in endmember standard deviation values. Notably, regardless of the number of input sources tested, "non-agricultural source" was the dominant source of NH3 during hazy days in January 2013 in Beijing. Since various agricultural sources showed large variations in δ15N-NH3, future studies should focus on the endmember signatures of agricultural sources to further reduce the uncertainty in SIAR-based NH3 source apportionment.

Connecting land use−land cover and precipitation with organic matter biogeochemistry in a tropical river–estuary system of western peninsular India

Organic matter (OM) composition changed due to land use ─ land cover (LULC) and hydrology modification, has distinctive linkage towards sustainable environment management in tropical river systems. It is crucial in small river systems, which experience delay of freshwater flow to the estuaries due to headwater damming, also LULC alteration along the entire basin. In order to understand this fundamental linkage in tropical Zuari river-estuary (ZRE), we analyzed multi-proxy data of organic carbon to total nitrogen ratio (Corg/N), stable organic carbon isotope (δ13Corg) and lignin phenols measured in seasonally collected suspended particulate matter (SPM) and sediment samples. Results highlighted about moderate seasonality of OM tracers, with a significant effect of LULC alteration, which nevertheless a striking feature in monsoon-fed river-estuaries of peninsular India. Particulate Corg export from ZRE estimated to be 20 × 103 kg yr−1, was much lower as compared to tropical river-estuary systems elsewhere. OM fraction from vascular plant (mangroves) contributed to SPM and sediment was 15% and 40%, respectively, calculated using a Bayesian mixing calculation through Stable isotope analysis in R (SIAR). Presence of mudflat LULC in the estuarine region notably caused 20% decrease in Corg and 60% increase in lignin phenol (Λ8) as compared to their limits in upstream. This is although mudflat accounts only 3% of ZRE catchment. The degree of shifts in OM tracers highlights towards efficient entrapment, transformation and/or utilization of riverine OM in the mudflats of ZRE. Accelerated human induced LULC dampens the seasonality of OM characteristics and flow is highlighted through this study, which is essential towards sustainable environmental management practice in small rivers of India and World.

Tracing the sources of sediment and associated particulate nitrogen from different land uses in the Johnstone River catchment, Wet Tropics, north-eastern Australia

While the ecosystem of the Great Barrier Reef (GBR), north-eastern Australia, is being threatened by the elevated levels of sediments and nutrients discharged from adjacent coastal river systems, the source of these detrimental pollutants are not well understood. Here we used a combined isotopic (δ13C, δ15N) and geochemical (Zn, Pt and S) signatures and stable isotope analysis in R (SIAR) mixing model to estimate the contribution of different land uses to the sediment and associated particulate nitrogen delivered to the Johnstone River. Results showed that rainforest was the largest contributor of suspended and bed sediments in the river estuary (both 33.1%), followed by banana (26.7%, 20.4%), sugarcane (21.5%, 21.4%) and grazing (18.7%, 25.1%). However, bananas and sugarcane land uses had the highest contribution to sediments delivered to the coast per unit of area. This will help land managers to prioritise on-ground activities to improve water quality in the GBR lagoon.

Identifying Nitrate Sources in a Typical Karst Underground River Basin

Underground rivers are an important source of groundwater in karst area. Recently, nitrate pollution of underground rivers has become a serious issue. To identify the sources of nitrate in Guancun typical karst underground river basin, stable isotope techniques (δ15N-NO3-, δ18O-NO3-, and δ18O-H2O) were applied in this study. The contribution rates of different nitrate sources in groundwater were quantitatively identified based on the stable isotope analysis in R (SIAR) model, and the influence of land use type on nitrate distribution and source in watershed water was clarified. The results showed that ① nitrate mainly came from fertilizers, soil organic nitrogen, and manure/sewage based to the isotopic composition of nitrate nitrogen and oxygen isotopes. It was revealed that non-point sources significantly contributed to nitrate in waters of the Guancun underground river basin. ② Nitrification dominated the formation process of nitrate in groundwater, and the initial values of nitrogen and oxygen isotopes were not affected by fractionation. ③ Based on SIAR, the contribution of different sources to nitrate in water in the basin varied seasonally, and the contributions of fertilizer, soil organic nitrogen, and manure/sewage to nitrate were 57.07%, 34.06%, and 8.87% in the wet season and 34.14%, 33.02%, and 32.84% in the dry season, respectively. Overall, the present study quantitatively evaluated the temporal variations of nitrate sources in a typical karst groundwater river basin and provided a theoretical foundation for prevention and control management of non-point source pollution and watershed management in karst areas.

Identifying the sources of nitrate in a small watershed using δ15N-δ18O isotopes of nitrate in the Kelan Reservoir, Guangxi, China

China has one of the largest areas of agriculture and one of the highest application rates of chemical fertilizer in the world. Agricultural non-point source pollution has became important source of nitrogen (N) pollution of surface water. In this study, the δ15N and δ18O isotopes of nitrate as a tool were used to reveal the sources of nitrate in water bodies in the Nala watershed, at a sugarcane planted area in Guangxi, China. According to the analyses based on a stable isotope analysis in R (SIAR), the mean contributions of different nitrate sources in the Nala watershed could be ranked as follows: manure (39.90 %) > soil N (SN) (34.94 %) > chemical fertilizer (CF) (17.77 %) > rainfall (7.39 %) in July; manure (42.30 %) > SN (33.54 %) > CF (24.16 %) in December. The CF and SN collectively mean contributing > 50 % of nitrate both in July and in December in the watershed, therefore, reducing the use of CF is the key to control nitrate pollution in the Nala watershed. Manure also composed a substantial contribution (mean 39.68 %, in July and December) to nitrate in the small watershed, it is necessary to provide recycle treatment for manure for the purpose of alleviating the pollution. The δ15N and δ18O isotopes of nitrate can accurately identify various sources of nitrate and SIAR offers a useful method to quantify the contribution rates of different N sources to the N pollution of watersheds.

Testing the sediment fingerprinting technique using the SIAR model with artificial sediment mixtures

The accurate identification of primary sediment sources in a watershed is necessary to implement targeted management practices that will reduce erosion and restore water quality. Sediment fingerprinting is a commonly used tool to accomplish this task. However, the accuracy and precision of different procedures to select tracers for un-mixing sediment sources are still a largely uninvestigated area in relation to sediment fingerprinting. The goal of this research was to validate a sediment fingerprinting methodology by applying it to the Oostanaula Creek watershed in southeast Tennessee, USA. We assessed three method protocols (soil digestion procedure, objective source grouping, and tracer selection) that are utilized for assessing the performance of fingerprinting in terms of apportionment outputs. The major and trace elemental composition of sediment source and suspended sediment were determined by total dissolution and nitric acid extraction followed by analysis with inductively coupled plasma-optical emission spectrometry (ICP-OES). The Kruskal-Wallis (KW) test as well as stepwise discriminant function analysis (DFA) was utilized during tracer selection. The source un-mixing model utilized was a Bayesian mathematical model within Stable Isotope Analysis in R (SIAR). Sediment fingerprinting in the Oostanaula watershed proved to be difficult due to the chemical and mineralogical similarities of the potentially erodible source material. Upon analysis, it was found that the sediment tracers identified as those with low misclassification during cluster analysis would not guarantee a high degree of accuracy during source apportionment. However, there are certain outputs with low errors as compared with the real proportional contributions in artificial mixtures, for example, findings showed that bank erosion is a primary source of suspended sediment in the Oostanaula Creek. Source apportionment from sediment fingerprinting was sensitive to the digestion procedure, objective source groupings, and the tracer selection. Our research provides a quantitative approach for assessing the validity of the sediment fingerprinting technique.

Triple oxygen isotopic evidence for atmospheric nitrate and its application in source identification for river systems in the Qinghai-Tibetan Plateau

Nitrate source identification in river systems is important for water quality management. Recently, the oxygen isotopic anomaly of nitrate in atmospheric deposition (Δ17Oatm) is used to identify unprocessed atmospheric nitrate in river systems to reduce the uncertainty caused by the wide range of δ18O. In high-elevation regions, such as the Qinghai-Tibetan Plateau (QTP) featured with lower temperature and pressure as well as strong radiation, the Δ17Oatm might be different from that in low-elevation regions, but no relevant studies have been reported. In this work, Δ17Oatm in the QTP was studied, and the fingerprints of nitrate isotopes in synthetic fertilizer, livestock manure, domestic sewage, and soil organic nitrogen (SON) were identified and used to quantify various source contributions to riverine nitrate in the Yellow River and Changjiang River source regions located in the QTP during 2016–2017. The results showed that the average of Δ17Oatm in the QTP was 16.4‰, lower than the range (19–30‰) reported for the low-elevation regions. The possible mechanism is decreased O3 as well as increased hydroxyl and peroxy radical levels in the troposphere caused by the climate condition and ozone valley in the QTP will affect the production pathways of atmospheric nitrate. By combining the sewage discharge data with the output results of the SIAR (stable isotope analysis in R) model based on the stable isotope data, manure was determined to be one of the major sources to riverine nitrate for both rivers. The contributions of various sources to riverine nitrate were 47 ± 10% for manure, 30 ± 5% for SON, 10 ± 4% for atmospheric precipitation, 9 ± 2% for synthetic fertilizer, and 4 ± 0% for sewage in the Yellow River source region. This study indicates that the unique atmospheric conditions in the QTP have led to a lower Δ17Oatm value, and atmospheric source makes a considerable contribution to riverine nitrate in the QTP.

Seasonal fluctuation in food sources of herbivorous gastropods in a subtropical seagrass bed estimated by stable isotope analysis

AbstractVarious herbivorous invertebrates in seagrass beds are considered to be generalists in food use and their diets may temporally fluctuate according to the availability of food sources. We assessed whether food sources of herbivorous gastropods vary in a subtropical seagrass bed in Nagura Bay, Ishigaki Island, where coexisting seaweeds grow densely in spring but minimally in summer. Abundant gastropods and their possible food sources were collected in spring and summer of 2013 and 2015, and their stable carbon and nitrogen isotope ratios were measured. Between the two seasons, each possible food source had similar isotopic values, but all the herbivorous gastropod species in summer were more enriched in13C than the gastropod samples in spring. The mixing models in SIAR (Stable Isotope Analysis in R) showed that the total contribution rates of seaweeds, i.e. rhodophytes, phaeophytes and chlorophytes, for all herbivorous gastropod species decreased from spring to summer; in contrast, the contribution rate of seagrasses increased. Linear Mixed Models showed that the seasonal variation in δ13C of the herbivorous gastropods was larger than that of the possible food sources, adding further evidence to the seasonal change in food sources of the herbivorous gastropods. This seasonal change in food use appears to correspond to the change in seaweed biomass, suggesting that herbivorous gastropods flexibly change their diets depending on food availability.

Particulate Organic Carbon Composition in Temperature Fronts of the Northeastern Arabian Sea during Winter

AbstractIn order to understand the major sources of particulate organic carbon (POC) in the frontal zones and to examine their variability with space and time, a total of five temperature fronts of different ages was sampled in the northeastern Arabian Sea during winter. Compared to the nonfrontal regions, POC and chlorophyll‐a were higher within the coastal fronts, whereas chlorophyll‐a was lower within the open ocean front (T1). The variation of POC between coastal and open ocean fronts is attributed to the combined influence of variable vertical mixing, heterotrophic transformation and age of the front. Relatively depleted δ13CPOC and δ15NPN were observed within the fronts, suggesting that POC pool is contributed by in situ production supported by upwelling of nutrient‐rich water and zooplankton biomass. Elemental C:N ratios, POC:Chl‐a, δ13CPOC, and δ15NPN suggest that POC is mainly contributed from primary producers and heterotrophs in the study region. However, relative contributions from these two sources vary spatially from coastal to open ocean and with the age of the front. Stable Isotope Analysis in R (SIAR) model revealed that zooplankton biomass largely contributed to POC in the open ocean (60–80%) than phytoplankton (20–40%) and nearly equal contribution was observed in the coastal fronts (50–60% and 40–50%, respectively). This study, thus, demonstrates that dominant heterotrophy and autotrophy in the open ocean and coastal fronts and it is consistent with their biomasses. Predominant heterotrophy in the open ocean is attributed to deeper mixed layer resulting in upwelling of bacteria‐rich and phytoplankton‐poor water to surface leading to existence of microbial loop.

Risk assessment and source identification of coastal groundwater nitrate in northern China using dual nitrate isotopes combined with Bayesian mixing model

ABSTRACTDue to the intensive and complicated human activities, the identification of nitrate pollution source of coastal aquifer is usually a challenge. This study firstly adopted stable isotope technique and stable isotope analysis in R (SIAR) model to identify the nitrate sources and contribution proportions of different sources in typical coastal groundwater of northern China. The results showed that about 91.5% of the groundwater samples illustrated significantly high nitrate concentrations exceeding the maximum WHO drinking water standard (50 mg/l), reflecting the high risk of groundwater nitrate pollution in the coastal area. A total of 57 sampling sites were classified into three groups according to hierarchical cluster analysis (HCA). The δ15N-NO3− and δ18O-NO3− values of groundwater samples from Group C (including nine samples) were much higher than those from Group A (including 40 samples) and Group B (including 8 samples). SIAR results showed that NH4+ fertilizer was the dominant nitrate source for groundwater samples of Groups A and B while manure and sewage (M&S) served as dominant source for Group C. This study provided essential information on the high risk and pollution sources of coastal groundwater nitrate of northern China.

Food Web Structure and Trophic Relations in a Riverine Mangrove System of the Tropical Eastern Pacific, Central Coast of Colombia

The isotopic composition of carbon and nitrogen of sources and consumers and the mixing model stable isotope analysis in R (SIAR) were used to understand the trophic structure to identify the main carbon sources in the food web of a highly humid riverine mangrove system on the Pacific coast of Colombia. The δ13C values of carbon sources varied from − 30.6 to − 26.9‰. Consumers were separated into invertebrates (with δ13C mean values from − 24.8 ± 0.3‰ to − 15.7 ± 0.7‰) and fish (with δ13C mean values from − 21.2 ± 1.3‰ to − 17.2 ± 0.9‰). Benthic mobile invertebrates generally had depleted δ13C values related to different trophic pathways to those of filter feeders and fish (with more enriched δ13C values). In agreement with similar studies in estuarine macro-tidal areas (e.g., Brazil and Panamanian Pacific), the large range in δ13C values for consumers may indicate that different carbon sources are important for the analyzed food web. In general, fish exhibited greater nitrogen values than invertebrates, indicating higher trophic levels. Among all consumers, with some exceptions, δ15N values correctly reflected the trophic levels, as species from lower trophic levels had lower δ15N than species from higher trophic levels. The trophic length of the food web was estimated from producers to higher consumers based on δ15N values. Taking into account a nitrogen trophic fractionation value of 3.4‰, the studied system exhibited a food web with four trophic levels. Based on the distribution of consumer trophic guilds in the δ13C-δ15N plane and the mixing model results, we recognized that at least two main pathways are present in the system to support the community.

Characterization of food web structure of the upper continental slope of the Celtic Sea highlighting the trophic ecology of five deep-sea fishes

Summary In marine ecosystems, the study of trophic relationships has extensively benefited from the development of stable isotope analyses (SIA) as dietary tracers. SIA are particularly useful in elucidating the structure of deep sea food webs given the constraints involved in obtaining gut-content data from deep trawling. We used carbon and nitrogen stable isotope analyses and Stable Isotope Bayesian Ellipses in R (SIBER) and Stable Isotope Analysis in R (SIAR) routines, to determine the trophic ecology of five deep-sea fishes from the upper continental slope of the Celtic Sea. SIA made it possible to deduce some general tendencies in food-web structure and species trophic interactions and confirmed diet determined by gut-content analysis for the same species, in other ecoregions. More specifically, mixing models revealed that the deep sea species considered are omnivorous and are able to feed on all the sampled taxa. Based on isotopic ratio, no clear differences in fish diet could be detected from one species to another except for rabbit fish, which has benthic affinities. Three species, blackbelly rosefish, greater forkbeard and softhead grenadier showed overlapping isotopic niches. This study is the first attempt to describe the trophic ecology of deep sea species on the Celtic Sea upper continental slope. In the context of the development of ecosystem integrated modeling approaches for managing fisheries in the Celtic sea, and considering the vulnerability of deep-water species, improving the knowledge on the trophic ecology of these local species is of importance in order to allow their sustainable exploitation.

Dietary map of Nile tilapia using stable isotopes in three tropical lakes, Ethiopia

AbstractComprehensive analysis of food webs requires identifying dietary sources that support the production of all major organisms within the food web/food chain. Here, we use stable isotope ratios (δ13C, δ34S and δ15N) to assess the relative contribution of different basal carbon sources to the diet of Nile tilapia (Oreochromis niloticus L.) in three tropical lakes Hawassa (also Awasa in literature), Ziway and Koka (Ethiopia). Computations were carried out with Stable Isotope Analysis in R (SIAR) model to quantify the dietary proportion of each prey for the tilapia fish. Basal food sources were distinguishable based on their δ13C, δ34S and δ15N values. In Lake Ziway, macrophytes (64%) were the dominant assimilated diet of tilapia while particulate organic matter (POM) and zooplankton contributed only 20% and 16%, respectively. In parallel, Nile tilapia in Lake Hawassa assimilated macrophytes (35%), POM (33%) and zooplankton (32%) at comparatively equal proportion. The dietary sources of the fish in Lake Koka were POM (49%) and zooplankton (51%). In contrast with earlier studies based on gut content analysis, the present results reveal that macrophytes contributed more and phytoplankton less than previously reported especially in macrophyte‐dominated lakes Ziway and Hawassa. The ecological condition of the lakes might have been predominantly accountable for the diet change of the tilapia. As dietary data are prerequisite for food web/food chain analysis and aquaculture industry, re‐evaluating the diet of aquatic organisms appear relevant.

Nitrate Source Identification and Nitrification-denitrification at the Sediment-water Interface

Identifying nitrate sources and its transformation mechanisms are important for nitrate pollution control in surface water. The columnar core sediment samples in West Lake were taken in different seasons. The transformation of nitrogen at the sediment-water interface was studied using nitrogen and oxygen isotopes, stable isotope analysis in R (SIAR) and acetylene inhibition method in the West Lake, Hangzhou. The results showed that the concentration gradient of both NO3-and NH4+ existed at the sediment-water interface. NO3- concentrations decreased from bottom water to pore water and NO3- was accumulated in sediments. NH4+ concentrations increased from bottom water to pore water and NH4+ was released from sediments. Nitrate sources in bottom water where nitrification exited were sewage (manure), soil nitrogen, chemical fertilizer and precipitation. Sewage (manure) was the major nitrogen contributor (60.8%) in summer. Particularly high δ15 N values in pore water indicated that there was strong denitrification at the sediment-water interface in West Lake. The average nitrification rate and denitrification rate at the sediment-water interface were 2.85 mmol·(m2·d)-1 and 23.51 μmol·(m2·d)-1, respectively. The sediment-water interface played a role in nitrogen removal process in aquatic environment. Seasonal and spatial variations of nitrification rates and denitrification rates were found in this study. Temperature and dissolved oxygen were the main influential factors for the transformation of nitrogen at the sediment-water interface in West Lake.

Larval Anuran Stable Isotope Signatures and Stoichiometry Across Multiple Geographically Isolated Wetlands in the Southeastern United States

Abstract Animals live in complex environments that vary spatially and temporally. This heterogeneity strongly influences the availability and quality of food resources and has strong impacts on growth and survival of consumers. Geographically isolated wetlands provide an interesting system to study trophic relationships because they vary spatially and temporally in hydrology and vegetation. Larval anurans play an important role in these wetland systems because they are often the most abundant consumers. Yet, little is known about larval anuran diet. Here we assessed the diet of 3 larval anurans (Acris gryllus [Southern Cricket Frog], Hyla gratiosa [Barking Treefrog], and Lithobates sphenocephalus [Southern Leopard Frog]), across 2 isolated wetland types (marsh and cypress savanna) using a stable isotope mixing model, stable isotope analysis in R (SIAR). Furthermore, we assessed variation in basal resource and anuran tissue stoichiometry (C:N). Our analyses suggested that larvae of these 3 species primaril...