Sources of stream sulfate at the Hubbard Brook Experimental Forest: Long-term analyses using stable isotopes

Sulfur deposition in the northeastern U.S. has been decreasing since the 1970s and there has been a concomitant decrease in the SO42– lost from drainage waters from forest catchments of this region. It has been established previously that the SO42– lost from drainage waters exceeds SO42– inputs in bulk precipitation, but the cause for this imbalance has not been resolved. The use of stable S isotopes and the availability of archived bulk precipitation and stream water samples at the Hubbard Brook Experimental Forest (HBEF) in New Hampshire provided a unique opportunity to evaluate potential sources and sinks of S by analyzing the long-term patterns (1966–1994) of the 34S values of SO42–. In bulk precipitation adjacent to the Ecosystem Laboratory and near Watershed 6 the 34S values were greater (mean: 4.5 and 4.2l, respectively) and showed more variation (variance: 0.49 and 0.30) than stream samples from Watersheds 5 (W5) and 6 (W6) (mean: 3.2 and 3.7j; variance: 0.09 and 0.08, respectively). These results are consistent with other studies in forest catchments that have combined results for mass balances with stable S isotopes. These results indicate that for those sites, including the HBEF, where atmospheric inputs are 10 kg S ha–1 yr–1, most of the deposited SO42– cycles through the biomass before it is released to stream water. Results from W5, which had a whole-tree harvest in 1983–1984 showed that adsorption/desorption processes play an important role in regulating net SO42– retention for this watershed-ecosystem. Although the isotopic results suggest the importance of S mineralization, conclusive evidence that there is net mineralization has not yet been shown. However, S mass balances and the isotopic result are consistent with the mineralization of organic S being a major contributor to the SO42– in stream waters at the HBEF.     

Spatial and temporal trends in stable carbon and oxygen isotope ratios of juvenile winter flounder otoliths

Isotopic ratios of fish otoliths have been used in numerous studies as natural tags or markers to aid in the study of connectivity among fish populations. We investigated the use of spatial and temporal changes in the stable carbon and oxygen isotope ratios of otoliths to differentiate juvenile habitats of winter flounder (Pseudopleuronectes americanus). Young-of-the-year (YOY) juvenile winter flounder were collected annually over a three-year period from 18 stations along the coast of Rhode Island, USA. Sagittal otoliths were removed from fish and analyzed for stable carbon (¹³C/¹²C or δ¹³C) and oxygen (¹⁸O/¹⁶O or δ¹⁸O) isotope ratios using continuous flow isotope ratio mass spectrometry. Differences in isotope ratios were observed among stations and along salinity gradients in the Narragansett Bay estuary and an estuarine river system (Narrow River). Overall, the isotope ratio patterns observed among stations were consistent over the three sampling years; however, differences were noted in isotope ratios and the magnitude of the isotope ratio gradients among years. Significant positive correlations were noted between salinity and δ¹³C for two of the three years. For each of the three years sampled there was a highly significant positive correlation (2002, r = 0.93, P < 0.01; 2003, r = 0.85, P < 0.01; 2004, r = 0.97, P < 0.01) between δ¹⁸O and the salinity of the collection site. Also, there was a significant negative correlation between the number of months of above average river flow and δ¹⁸O for the three sampling years (r = 0.99, P < 0.05). These findings suggest that yearly changes in the volume of freshwater inputs to these estuarine habitats may be related to the differences observed in otolith δ¹⁸O isotope ratios. Because of these year-to-year differences, sampling of each cohort may be necessary in order to use this isotopic technique for winter flounder connectivity studies.     

The biogeochemistry of sulfur at Hubbard Brook

A synthesis of the biogeochemistry of S was done during 34 yr(1964–1965 to 1997–1998) in reference and human-manipulated forestecosystems of the Hubbard Brook Experimental Forest (HBEF), NH. There have beensignificant declines in concentration (–0.44µmol/liter-yr) and input (–5.44mol/ha-yr)of SO₄ ^2– in atmospheric bulk wet deposition, and inconcentration(–0.64 µmol/liter-yr) an d output (–3.74mol/ha-yr) of SO₄ ^2– in stream water ofthe HBEF since 1964. These changes arestrongly correlated with concurrent decreases in emissions of SO₂from the source area for the HBEF. The concentration and input ofSO₄ ^2– in bulk deposition ranged from a low of 13.1µmol/liter (1983–1984) and 211 mol/ha-yr(1997–1998) to a high of 34.7 µmol/liter(1965–1966) and 479 mol/ha-yr (1967–1968), with along-term mean of 23.9 µmol/liter and 336mol/ha-yr during 1964–1965 to 1997–1998. Despiterecentdeclines in concentrations, SO₄ ^2– is the dominantanion in both bulk deposition and streamwater at HBEF. Dry deposition is difficult to measure, especially inmountainousterrain, but was estimated at 21% of bulk deposition. Thus, average totalatmospheric deposition was 491 and 323 mol/ha-yr during1964–1969 and 1993–1998, respectively. Based on the long-term³⁴S pattern associated with anthropogenic emissions,SO₄ ^2– deposition at HBEF is influenced by numerousSO₂sources, but biogenic sources appear to be small. Annual throughfall plusstemflow in 1993–1994 was estimated at 346 molSO₄ ^2–/ha. Aboveground litterfall, for thewatershed-ecosystemaveraged about 180 mol S/ha-yr, with highest inputs (190 molS/ha-yr) in the lower elevation, more deciduous forest zone. Weatheringrelease was calculated at a maximum of 50 mol S/ha-yr. Theconcentration and output of SO₄ ^2– in stream waterranged from a low of 42.3µmol/liter (1996–1997) and 309 mol/ha-yr(1964–1965), to a high of 66.1 µmol/liter(1970–1971) and 849 mol/ha-yr (1973–1974), with along-term mean of 55.5 µmol/liter and 496mol/ha-yr during the 34 yrs of study. Gross outputs ofSO₄ ^2– in stream water consistently exceeded inputsin bulkdeposition and were positively and significantly related to annualprecipitationand streamflow. The relation between gross SO₄ ^2–output and annual streamflow changed with time asatmospheric inputs declined. In contrast to the pattern for bulk depositionconcentration, there was no seasonal pattern for streamSO₄ ^2– concentration. Nevertheless, stream outputs ofSO₄ ^2– were highly seasonal, peaking during springsnowmelt, andproducing a monthly cross-over pattern where net hydrologic flux (NHF) ispositive during summer and negative during the remainder of the year. Nosignificant elevational pattern in streamwaterSO₄ ^2– concentration was observed. Mean annual,volume-weightedsoil water SO₄ ^2– concentrations were relativelyuniform by soil horizon andacross landscape position. Based upon isotopic evidence, much of theSO₄ ^2– entering HBEF in atmospheric depositioncycles throughvegetation and microbial biomass before being released to the soil solution andstream water. Gaseous emissions of S from watershed-ecosystems at HBEF areunquantified, but estimated to be very small. Organic S (carbon bonded andestersulfates) represents some 89% of the total S in soil at HBEF. Some 6% exists asphosphate extractable SO₄ ^2– (PSO₄).About 73% of the total S in the soilprofile at HBEF occurs in the Bs2 horizon, and some 9% occurs in the forestfloor. The residence time for S in the soil was calculated to be 9 yr, butonly a small portion of the total organic soil pool turns over relativelyquickly. The S content of above- and belowground biomass is about 2885mol/ha, of which some 3–5% is in standing dead trees. Yellowbirch, American beech and sugar maple accounted for 89% of the S in trees, with31% in branches, 27% in roots and 25% in the lightwood of boles. The pool of Sin living biomass increased from 1965 to 1982 due to biomass accretion, andremained relatively constant thereafter. Of current inputs to the availablenutrient compartment of the forest ecosystem, 50% is from atmospheric bulkdeposition, 24% from net soil release, 11% from dry deposition, 11% from rootexudates and 4% is from canopy leaching. Comparing ecosystem processes for Sfrom 1964–1969 to 1993–1998, atmospheric bulk deposition decreasedby 34%, stream output decreased by 10%, net annual biomass storage decreased by92%, and net soil release increased by 184% compared to the 1964–1969values. These changes are correlated with decreased emissions of SO₂from the source area for the HBEF. Average, annual bulk deposition inputsexceeded streamwater outputs by 160.0 ± 75.3 SD molS/ha-yr,but average annual net ecosystem fluxes (NEF) were much smaller, mostlynegativeand highly variable during the 34 yr period (–54.3 ± 72.9 SDmol S/ha-yr; NEF range, +86.8 to –229.5). While severalmechanisms may explain this small discrepancy, the most likely are netdesorption of S and net mineralization of organic S largely associated with theforest floor. Our best estimates indicate that additional S from dry depositionand weathering release is probably small and that desorption accounts for about37% of the NEF imbalance and net mineralization probably accounts for theremainder (60%). Additional inputs from dry deposition would result fromunmeasured inputs of gaseous and particulate deposition directly to the forestfloor. The source of any unmeasured S input has important implications for therecovery of soils and streams in response to decreases in inputs of acidicdeposition. Sulfate is a dominant contributor to acid deposition at HBEF,seriously degrading aquatic and terrestrial ecosystems. Because of the strongrelation between SO₂ emissions and concentrations ofSO₄ ^2– in both atmospheric deposition and streamwater at HBEF,further reductions in SO₂ emissions will be required to allowsignificant ecosystem recovery from the effects of acidic deposition. Thedestruction or removal of vegetation on experimental watershed-ecosystems atHBEF resulted in increased rates of organic matter decomposition andnitrification, a lowering of soil and streamwater pH, enhancedSO₄ ^2– adsorption on mineral soil and smallerconcentrations andlosses of SO₄ ^2– in stream water. With vegetationregrowth, this adsorbedSO₄ ^2– is released from the soil, increasingconcentrations andfluxes of SO₄ ^2– in drainage water. Streamwaterconcentration ofSO₄ ^2– and gross annual output ofSO₄ ^2–/ha are essentially the same throughout theHubbard BrookValley in watersheds varying in size by about 4 orders of magnitude, from 3 to3000 ha.     

The biogeochemistry of potassium at Hubbard Brook

A synthesis of the biogeochemistry of K was conducted during 1963–1992 in the reference and human-manipulated watershed-ecosystems of the Hubbard Brook Experimental Forest (HBEF), NH. Results showed that during the first two years of the study (1963–65), which coincided with a drought period, the reference watershed was a net sink for atmospheric inputs of K. During the remaining years, this watershed has been a net source of K for downstream ecosystems. There have been long-term declines in volume-weighted concentration and flux of K at the HBEF; however, this pattern appears to be controlled by the relatively large inputs during the initial drought years. Net ecosystem loss (atmospheric deposition minus stream outflow) showed an increasing trend of net loss, peaking during the mid-1970s and declining thereafter. This pattern of net K loss coincides with trends in the drainage efflux of SO₄ ^2– and NO₃ ^–, indicating that concentrations of strong acid anions may be important controls of dissolved K loss from the site. There were no long-term trends in streamwater concentration or flux of K. A distinct pattern in pools and fluxes of K was evident based on biotic controls in the upper ecosystem strata (canopy, boles, forest floor) and abiotic controls in lower strata of the ecosystem (mineral soil, glacial till). This biological control was manifested through higher concentrations and fluxes of K in vegetation, aboveground litter, throughfall and forest floor pools and soil water in the northern hardwood vegetation within the lower reaches of the watershedecosystem, when compared with patterns in the high-elevation spruce-fir zone. Abiotic control mechanisms were evident through longitudinal variations in soil cation exchange capacity (related to soil organic matter) and soil/till depth, and temporal and disturbance-related variations in inputs of strong-acid anions. Marked differences in the K cycle were evident at the HBEF for the periods 1964–69 and 1987–92. These changes included decreases in biomass storage, net mineralization and throughfall fluxes and increased resorption in the latter period. These patterns seem to reflect an ecosystem response to decreasing rates of biomass accretion during the study. Clearcutting disturbance resulted in large losses of K in stream water and from the removal of harvest products. Stream losses occur from release from slash, decomposition of soil organic matter and displacement from cation exchange sites. Elevated concentrations of K persist in stream water for many years after clearcutting. Of the major elements, K shows the slowest recovery from clearcutting disturbance.     

The Biogeochemistry of Carbon at Hubbard Brook

The biogeochemical behavior of carbon in the forested watersheds of the Hubbard Brook Experimental Forest (HBEF) was analyzed in long-term studies. The largest pools of C in the reference watershed (W6) reside in mineral soil organic matter (43% of total ecosystem C) and living biomass (40.5%), with the remainder in surface detritus (14.5%). Repeated sampling indicated that none of these pools was changing significantly in the late-1990s, although high spatial variability precluded the detection of small changes in the soil organic matter pools, which are large; hence, net ecosystem productivity (NEP) in this 2nd growth forest was near zero (± about 20 g C/m²-yr) and probably similar in magnitude to fluvial export of organic C. Aboveground net primary productivity (ANPP) of the forest declined by 24% between the late-1950s (462 g C/m²-yr) and the late-1990s (354 g C/m²-yr), illustrating age-related decline in forest NPP, effects of multiple stresses and unusual tree mortality, or both. Application of the simulation model PnET-II predicted 14% higher ANPP than was observed for 1996–1997, probably reflecting some unknown stresses. Fine litterfall flux (171 g C/m²-yr) has not changed much since the late-1960s. Because of high annual variation, C flux in woody litterfall (including tree mortality) was not tightly constrained but averaged about 90 g C/m²-yr. Carbon flux to soil organic matter in root turnover (128 g C/m²-yr) was only about half as large as aboveground detritus. Balancing the soil C budget requires that large amounts of C (80 g C/m²-yr) were transported from roots to rhizosphere carbon flux. Total soil respiration (TSR) ranged from 540 to 800 g C/m²-yr across eight stands and decreased with increasing elevation within the northern hardwood forest near W6. The watershed-wide TSR was estimated as 660 g C/m²-yr. Empirical measurements indicated that 58% of TSR occurred in the surface organic horizons and that root respiration comprised about 40% of TSR, most of the rest being microbial. Carbon flux directly associated with other heterotrophs in the HBEF was minor; for example, we estimated respiration of soil microarthropods, rodents, birds and moose at about 3, 5, 1 and 0.8 g C/m²-yr, respectively, or in total less than 2% of NPP. Hence, the effects of other heterotrophs on C flux were primarily indirect, with the exception of occasional irruptions of folivorous insects. Hydrologic fluxes of C were significant in the watershed C budget, especially in comparison with NEP. Although atmospheric inputs (1.7 g C/m²-yr) and streamflow outputs (2.7 g C/m²-yr) were small, larger quantities of C were transported within the ecosystem and a more substantial fraction of dissolved C was transported from the soil as inorganic C and evaded from the stream as CO₂ (4.0 g C/m²-yr). Carbon pools and fluxes change rapidly in response to catastrophic disturbances such as forest harvest or major windthrow events. These changes are dominated by living vegetation and dead wood pools, including roots. If biomass removal does not accompany large-scale disturbance, the ecosystem is a large net source of C to the atmosphere (500–1200 g C/m²-yr) for about a decade following disturbance and becomes a net sink about 15–20 years after disturbance; it remains a net sink of about 200–300 g C/m²-yr for about 40 years before rapidly approaching steady state. Shifts in NPP and NEP associated with common small-scale or diffuse forest disturbances (e.g., forest declines, pathogen irruptions, ice storms) are brief and much less dramatic. Spatial and temporal patterns in C pools and fluxes in the mature forest at the HBEF reflect variation in environmental factors. Temperature and growing-season length undoubtedly constrain C fluxes at the HBEF; however, temperature effects on leaf respiration may largely offset the effects of growing season length on photosynthesis. Occasional severe droughts also affect C flux by reducing both photosynthesis and soil respiration. In younger stands nutrient availability strongly limits NPP, but the role of soil nutrient availability in limiting C flux in the mature forest is not known. A portion of the elevational variation of ANPP within the HBEF probably is associated with soil resource limitation; moreover, sites on more fertile soils exhibit 20–25% higher biomass and ANPP than the forest-wide average. Several prominent biotic influences on C pools and fluxes also are clear. Biomass and NPP of both the young and mature forest depend upon tree species composition as well as environment. Similarly, litter decay differs among tree species and forest types, and forest floor C accumulation is twice as great in the spruce–fir–birch forests at higher elevations than in the northern hardwood forests, partly because of inherently slow litter decay and partly because of cold temperatures. This contributes to spatial patterns in soil solution and streamwater dissolved organic carbon across the Hubbard Brook Valley. Wood decay varies markedly both among species and within species because of biochemical differences and probably differences in the decay fungi colonizing wood. Although C biogeochemistry at the HBEF is representative of mountainous terrain in the region, other sites will depart from the patterns described at the HBEF, due to differences in site history, especially agricultural use and fires during earlier logging periods. Our understanding of the C cycle in northern hardwood forests is most limited in the area of soil pool size changes, woody litter deposition and rhizosphere C flux processes.     

Stable isotope ratios and forensic analysis of microorganisms

In the aftermath of the anthrax letters of 2001, researchers have been exploring various analytical signatures for the purpose of characterizing the production environment of microorganisms. One such signature is stable isotope ratios, which in heterotrophs, are a function of nutrient and water sources. Here we discuss the use of stable isotope ratios in microbial forensics, using as a database the carbon, nitrogen, oxygen, and hydrogen stable isotope ratios of 247 separate cultures of Bacillus subtilis 6051 spores produced on a total of 32 different culture media. In the context of using stable isotope ratios as a signature for sample matching, we present an analysis of variations between individual samples, between cultures produced in tandem, and between cultures produced in the same medium but at different times. Additionally, we correlate the stable isotope ratios of carbon, nitrogen, oxygen, and hydrogen for growth medium nutrients or water with those of spores and show examples of how these relationships can be used to exclude nutrient or water samples as possible growth substrates for specific cultures.     

Global dataset for carbon and nitrogen stable isotope ratios of lotic periphyton

Carbon and nitrogen stable isotope ratios (δ¹³C and δ¹⁵N) have been widely employed in food web analysis. In lotic environments, periphyton is a major primary producer that makes a large contribution to food web production as well as carbon and nitrogen cycling. While the δ¹³C and δ¹⁵N values have many advantages as a natural tracer, the controls over their high spatial and temporal variability in stream periphyton are not well known. Here, we present the global dataset of δ¹³C and δ¹⁵N values of lotic periphyton from 54 published and two unpublished sources, including 978 observations from 148 streams/rivers in 38 regions around the world, from arctic to tropical sites. The 54 published sources were articles recorded during the period of 1994–2016 in 25 academic journals. The two unpublished sources were from the authors’ own data. The dataset showed that δ¹³C and δ¹⁵N values of periphyton ranged from ?47.3 to ?9.3‰ and from ?5.6 to + 22.6‰, respectively. The dataset also includes physicochemical factors (altitude, coordinates, catchment area, width, depth, geology, vegetation, canopy coverage, biome, season, presence of anadromous salmon, temperature, pH, current velocity, and discharge), nutrient data (nitrate and ammonium concentrations), and algal attributes (chlorophyll a concentration, algal species compositions, and carbonates removal) in streams/rivers studied, all of which may help interpret the δ¹³C and δ¹⁵N values of periphyton. The metadata file outlines structure of all the data and with references for data sources, providing a resource for future food web studies in stream and river ecosystems.     

Disentangling effects of growth and nutritional status on seabird stable isotope ratios

A growing number of studies suggest that an individual’s physiology affects its carbon and nitrogen stable isotope signatures, obscuring a signal often assumed to be only a reflection of diet and foraging location. We examined effects of growth and moderate food restriction on red blood cell (RBC) and feather δ¹⁵N and δ¹³C in rhinoceros auklet chicks (Cerorhinca monocerata), a piscivorous seabird. Chicks were reared in captivity and fed either control (75 g/day; n = 7) or ~40% restricted (40 g/day; n = 6) amounts of high quality forage fish. We quantified effects of growth on isotopic fractionation by comparing δ¹⁵N and δ¹³C in control chicks to those of captive, non-growing subadult auklets (n = 11) fed the same diet. To estimate natural levels of isotopic variation, we also collected blood from a random sample of free-living rhinoceros auklet adults and chicks in the Gulf of Alaska (n = 15 for each), as well as adult feather samples (n = 13). In the captive experiment, moderate food restriction caused significant depletion in δ¹⁵N of both RBCs and feathers in treatment chicks compared to control chicks. Growth also induced depletion in RBC δ¹⁵N, with chicks exhibiting lower δ¹⁵N when they were growing the fastest. As growth slowed, δ¹⁵N increased, resulting in an overall pattern of enrichment over the course of the nestling period. Combined effects of growth and restriction depleted δ¹⁵N in chick RBCs by 0.92‰. We propose that increased nitrogen-use efficiency is responsible for ¹⁵N depletion in both growing and food-restricted chicks. δ¹⁵N values in RBCs of free-ranging auklets fell within a range of only 1.03‰, while feather δ¹⁵N varied widely. Together, our captive and field results suggest that both growth and moderate food restriction can affect stable isotope ratios in an ecologically meaningful way in RBCs although not feathers due to greater natural variability in this tissue.     

Structure of tropical river food webs revealed by stable isotope ratios

Fish assemblages in tropical river food webs are characterized by high taxonomic diversity, diverse foraging modes, omnivory, and an abundance of detritivores. Feeding links are complex and modified by hydrologic seasonality and system productivity. These properties make it difficult to generalize about feeding relationships and to identify dominant linkages of energy flow. We analyzed the stable carbon and nitrogen isotope ratios of 276 fishes and other food web components living in four Venezuelan rivers that differed in basal food resources to determine 1) whether fish trophic guilds integrated food resources in a predictable fashion, thereby providing similar trophic resolution as individual species, 2) whether food chain length differed with system productivity, and 3) how omnivory and detritivory influenced trophic structure within these food webs. Fishes were grouped into four trophic guilds (herbivores, detritivores/algivores, omnivores, piscivores) based on literature reports and external morphological characteristics. Results of discriminant function analyses showed that isotope data were effective at reclassifying individual fish into their pre-identified trophic category. Nutrient-poor, black-water rivers showed greater compartmentalization in isotope values than more productive rivers, leading to greater reclassification success. In three out of four food webs, omnivores were more often misclassified than other trophic groups, reflecting the diverse food sources they assimilated. When fish δ¹⁵N values were used to estimate species position in the trophic hierarchy, top piscivores in nutrient-poor rivers had higher trophic positions than those in more productive rivers. This was in contrast to our expectation that productive systems would promote longer food chains. Although isotope ratios could not resolve species-level feeding pathways, they did reveal how top consumers integrate isotopic variability occurring lower in the food web. Top piscivores, regardless of species, had carbon and nitrogen profiles less variable than other trophic groups.     

On estimating the precision of stable isotope ratios in processed tree-rings

Stable isotope dendrochronology is a well-developed field of research, but improvements to methodologies are on-going. We propose an improved method for estimating the precision of stable isotope ratios (δ) of tree-ring samples that are processed from whole wood to various end products such as cellulose-nitrate, α-cellulose, or cellulose intermediates. The status quo method for estimating the δ precision of organic solids is to characterise the long-term 2-sigma range of δ values for a ready-made Quality Assurance (QA) standard that is included in each analysis run of samples. While the status quo method is appropriate for characterising analytical uncertainties associated with the mass spectrometer, combustion or pyrolysis system, and analyte specifics, it does not reflect uncertainties associated with sample processing from inadvertent and unrealised operator error (e.g., contamination by airborne particles, incomplete chemical processing, sample storage issues, and other unforeseen errors), although such errors would probably be rare with an experienced operator. The proposed method improves upon the status quo method as it respects the Identical Treatment principle by subjecting QA standards to the same processing steps that samples undergo. As such, analytical uncertainties associated with sample processing would be integrated into the QA standard's δ value and precision estimate. In effect, the proposed method is a system to monitor inter-batch reproducibility and, by the same token, can be used to identify batches that were potentially compromised during processing. A pilot study example is used to demonstrate the proposed method for δ¹⁸O analysis of α-cellulose samples.     

Ecological attributes recorded in stable isotope ratios of arboreal prosimian hair

Carbon and nitrogen stable isotope ratios were measured in hair samples from two species of Galago from Gedi Ruins National Monument in eastern Kenya and from Lepilemur leucopus from Beza Mahafaly Special Reserve in southern Madagascar. Forest structure was generally similar in the two areas but average rainfall was lower in Madagascar. Species average ¹³C values varied with feeding height in the forest canopy and with average rainfall level as expected from reported variation in plant ¹³C values. G. garnettii, which feeds higher in the forest canopy, had less negative ¹³C values than G. zanzibaricus, which spends more time below 5 m. L. leucopus, from a drought-afflicted forest, had less negative hair ¹³C values than the two galago species. The values within the Lepilemur sample showed a positive linear relation with percent dependence on a CAM tree species and with xeric conditions within the species reserve. Nitrogen stable isotope ratios varied with trophic level of feeding and with time spent feeding on leguminous plants. The insectivorous galagos had significantly more positive ¹⁵N values than the folivorous L. leucopus. Within the Lepilemur sample, ¹⁵N values varied inversely with the percent of feeding time spent on leguminous plants. The range of ¹⁵N and ¹³C values in each of the prosimian species is larger than reported for animals fed monotonous diets and for New World monkey species. The monkey species feed as groups of individuals whereas the prosimians have solitary feeding habits. The ranges in the prosimian species apparently reflect the greater variation in diet among individual prosimians compared to individual monkeys. The isotope data reported here are equivalent, on average, to those reported for other arboreal species from similar forest habitats and with similar dietary habits. This supports the use of such data for paleoecological reconstruction of forest and woodland systems and diet reconstruction of extinct primate populations and species. Received: 18 April 1997 / Accepted: 11 August 1997     

Food source of riparian spiders analyzed by using stable isotope ratios

We analyzed the food source of riparian spiders in a middle reach of the Chikuma River, Japan, by using stable isotope ratios of carbon and nitrogen. The carbon and nitrogen isotope ratios of attached algae were higher than those of terrestrial plants, reflecting a large carbon isotope fractionation in terrestrial plants and a difference in nitrogen sources. The carbon isotope ratios of terrestrial insects were similar to those of the terrestrial plants, and the ratios of aquatic insects were scattered between those of the terrestrial plants and the attached algae. The carbon and nitrogen isotope ratios of spiders were intermediate between those of the terrestrial and aquatic insects. The two-source mixing model using the carbon isotope ratio showed that the web-building spiders utilized both the terrestrial and aquatic insects, with large contribution by the aquatic insects (54% on average with a maximum of 92% among spiders taxa collected in each zone), in the riparian area in a middle reach of the Chikuma River. The large contribution of the aquatic insects was often observed for the spiders collected near river channel (<5m) and for the horizontal web-building spiders collected across the riparian area. The relative contribution of the aquatic insects might be related with food availability (distance from river channel) and spiders food preference reflected in their web types (horizontal vs. vertical). Our results showed that organic materials produced in the river channel, in the riparian area, and in the terrestrial area surrounding the riparian area were mixed at the carnivorous trophic level of riparian spiders.     

Are sulfur isotope ratios sufficient to determine the antiquity of sulfate reduction

Sulfur isotope fractionation values have been measured in sedimentary sulfides of varying ages, The Antiquity and evolutionary status of bacterial sulfate reduction... has been inferred from these measurements by Schidlowski (1979). However, under experimental conditions, the isotope values vary widely due to inadequately controlled variables. Thus the direct extrapolation of sulfur isotope fractionation values measured in the laboratory to those measured in sedimentary rocks is unwarranted. New sulfur transforming microbes have been described and recent measurements indicate that inorganic processes affect sulfur isotope fractionation values. This information is summarized here; at present sulfur isotope fractionation values are insufficient to determine the antiquity of sulfate reduction.     

Effect of lipid removal on carbon and nitrogen stable isotope ratios in crustacean tissues

The analysis of tissue's naturally occurring stable carbon and nitrogen isotope ratios is a useful tool to delineate trophic relationships. However, the interpretation of δ¹³C and δ¹⁵N is complicated by the influence of multiple factors such as the tissue-specific lipid content. The aim of this work was to evaluate the effects of lipid extraction on δ¹³C and δ¹⁵N compositions in muscle, hepatopancreas and gonads of a marine decapod crustacean, the spider crab Maja brachydactyla. Samples were analyzed for stable isotopes before and after lipid removal, using a derived Soxhlet extraction method. Differences in δ¹³C and δ¹⁵N were measured among tissues before and after treatment. Lipid extraction of muscle did not have a significant effect on either δ¹³C or δ¹⁵N. By contrast, ecologically significant shifts for both carbon and nitrogen stable isotopes ratios (+ 2.9 ± 0.8‰ for δ¹³C, and + 1.2 ± 0.7‰ for δ¹⁵N) were noticed in the hepatopancreas. In regard to gonads, lipid extraction led to a shift only on δ¹³C (+ 1.3 ± 0.3‰). Finally, the derived Soxhlet extraction method removed the lipid influence for δ¹³C, and had an effect on δ¹⁵N composition for lipid-rich samples. We recommend this treatment for carbon stable isotope studies on decapod crustacean lipid-rich tissues.     

Preservation effects on stable isotope ratios and consequences for the reconstruction of energetic pathways

Stable isotope analysis provides a powerful tool for describing the energetic pathways in a variety of ecosystems. However, isotope ratios of animal tissues can be altered by preservation methods, potentially leading to biased estimates of energy pathways when they are not taken into account. Here, we investigated the direct preservation effects of formalin, ethanol, NaCl, and drying on the δ¹³C and δ¹⁵N of fish muscle tissues, as well as the ultimate effects on the reconstruction of the energy pathways. All preservation methods, except drying, had significant impacts on δ¹³C and δ¹⁵N values. The effects of preservation appear to be highly taxa-specific and no significant time-dependent variations in nearly 2-year duration of preservation. δ¹³C and δ¹⁵N values were generally changed dramatically within the early stage of the preservation process and became stable over a relatively long-term preservation. Using an isotopic balance mixing model, the isotope-based food web reconstruction reveals that, without preservation correction, the importance of the pelagic energetic pathways for the fishes could be misestimated, except for the drying preservation. These results highlight that preservation can bias the interpretation of food web reconstruction results.     

Spatial variability of stable carbon and nitrogen isotope ratios in a Mediterranean coastal lagoon

Exploring the trophic pathway of organic matter within the Mauguio lagoon (southern France, western Mediterranean), we found spatial differences in the isotopic composition (both δ¹³C and δ¹⁵N values) of organic matter sources (primary producers, particulate and sedimentary organic matter), which were mirrored in the upper trophic levels (invertebrates and fish). On average, δ¹³C was heavier by about 1.5–2‰ in the location under marine influence than in the sites influenced by freshwater discharge. The opposite trend was found for δ¹⁵N, which attained maximum values in the north-central zone influenced by freshwater delivery. For both C and N stable isotope ratios, the highest spatial variability was found in organic matter sources (2–3‰), while invertebrates and fish exhibited less variability (\~1–2‰). The differences observed may be related to both anthropogenic (wastewater input) and natural (marine vs. terrestrial inputs) factors. Discharge of wastewater, which affects the innermost location, generally determines an increase in the relative abundance of ¹⁵N. In addition, terrestrially derived nutrients and organic matter, which also affect the innermost location, are known to determine a shift towards ¹³C-depleted values. Our results substantiate the finding that the analysis of carbon and nitrogen stable isotopes can help in elucidating origin and fate of organic matter in coastal lagoons, which are characterised by a great spatial variability and complexity.     

Hydrogen and oxygen isotope ratios of tree ring cellulose for field-grown riparian trees

The isotopic composition of tree ring cellulose was obtained over a 2-year period from small-diameter riparian-zone trees at field sites that differed in source water isotopic composition and humidity. The sites were located in Utah (cool and low humidity), Oregon (cool and high humidity), and Arizona (warm and low humidity) with source water isotope ratio values of –125/–15‰ (δD/δ¹⁸O), –48/–6‰, and –67/–7‰, respectively. Monthly environmental measurements included temperature and humidity along with measurements of the isotope ratios in atmospheric water vapor, stream, stem, and leaf water. Small riparian trees used only stream water (both δD and δ¹⁸O of stem and stream water did not differ), but δ values of both atmospheric water vapor and leaf water varied substantially between months. Differences in ambient temperature and humidity conditions between sites contributed to substantial differences in leaf water evaporative enrichment. These leaf water differences resulted in differences in the δD and δ¹⁸O values of tree ring cellulose, indicating that humidity information was recorded in the annual rings of trees. These environmental and isotopic measurements were used to test a mechanistic model of the factors contributing to δD and δ¹⁸O values in tree ring cellulose. The model was tested in two parts: (a) a leaf water model using environmental information to predict leaf water evaporative enrichment and (b) a model describing biochemical fractionation events and isotopic exchange with medium water. The models adequately accounted for field observations of both leaf water and tree ring cellulose, indicating that the model parameterization from controlled experiments was robust even under uncontrolled and variable field conditions. Received: 7 April 1999 / Accepted: 8 December 1999     

Effect of competition on stable carbon isotope ratios of two tussock grass species

Summary Previous studies have shown that plant carbon isotope composition varies when plants experience differences in water and nutrient availability. However, none have addressed the effect of root interactions, including competition for these soil resources, on carbon isotope ratios. We studied the effect of interspecific root interactions on the productivity and carbon isotope ratios of two Great Basin tussock grass species (Agropyron desertorum and Pseudoroegneria spicata). We compared grasses grown in mixture with sagebrush (Artemisia tridentara) to grasses in similar mixtures but where root interactions with sagebrush were limited by fiberglass partitions. During both years of the study, tussocks growing in competition with sagebrush produced tissue with more negative ¹³C values than grasses experiencing limited root interaction with sagebrush. The magnitude of this difference (0.5 to 0.9%) is similar to that found in other studies when soil fertility and moisture availability were altered.