[Show abstract][Hide abstract] ABSTRACT: Analysis of individual amino acid (AA) δ15N values is increasingly common in studies of food web architecture, movement ecology, and biogeochemical cycling. However, observations that nitrogen isotope fractionation of AAs may vary across species and trophic positions (TP) complicate the application of compound-specific stable isotope analysis (CSIA) in studies of aquatic food webs. We reared a common estuarine fish (Fundulus heteroclitus) on four diets to test the hypothesis that diet composition significantly affects AA trophic discrimination factors (TDFs) derived from normalizing fractionation of trophic AAs to the canonical source AA phenylalanine. Phenylalanine showed little trophic fractionation regardless of diet composition, confirming its use as a proxy for δ15N values at the base of food webs. However, TDF values showed significant negative relationships with two important measures of diet quality, protein content and AA imbalance between diet and consumer. As a result, consumers feeding on high protein diets with AA compositions matching their own had significantly lower TDF values than previous literature values, while consumers feeding on contrasting low quality diets had significantly higher TDF values than previously reported. Our results provide an explanation for the apparent relationship between TDF value and TP, and highlight a significant limitation when using a single TDF value among all trophic transfers within a food web. Accurate determination of TPs using CSIA will likely require multiple TDF values across trophic levels or further exploration of other trophic AAs, such as proline, that may show less variation in TDF with diet composition than glutamic acid.
[Show abstract][Hide abstract] ABSTRACT: Compound-specific stable isotope analysis (CSIA) of amino acids (AA) has rapidly become a powerful tool in studies of food web architecture, resource use, and biogeochemical cycling. However, applications to avian ecology have been limited because no controlled studies have examined the patterns in AA isotope fractionation in birds. We conducted a controlled CSIA feeding experiment on an avian species, the gentoo penguin (Pygoscelis papua), to examine patterns in individual AA carbon and nitrogen stable isotope fractionation between diet (D) and consumer (C) (D 13 C C-D and D 15 N C-D , respectively). We found that essential AA d 13 C values and source AA d 15 N values in feathers showed minimal trophic fractionation between diet and consumer, providing independent but complimentary archival proxies for primary producers and nitrogen sources respectively, at the base of food webs supporting penguins. Variations in nonessential AA D 13 C C-D values reflected differences in macro-molecule sources used for biosynthesis (e.g., protein vs. lipids) and provided a metric to assess resource utilization. The avian-specific nitrogen trophic discrimination factor (TDF Glu-Phe = 3.5 +/- 0.4 permil) that we calculated from the difference in trophic fractionation (D15N C-D) of glutamic acid and phenylala-nine was significantly lower than the conventional literature value of 7.6permil. Trophic positions of five species of wild penguins calculated using a multi-TDF Glu-Phe equation with the avian-specific TDF Glu-Phe value from our experiment provided estimates that were more ecologically realistic than estimates using a single TDF Glu-Phe of 7.6permil from the previous literature. Our results provide a quantitative, mechanistic framework for the use of CSIA in nonle-thal, archival feathers to study the movement and foraging ecology of avian consumers.
Ecology and Evolution 02/2015; 5(6). DOI:10.1002/ece3.1437 · 2.32 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Rationale:
For bulk carbon and nitrogen isotope analysis of dentin, samples are typically decalcified. Since the non-protein carbon in dentin is low, whole sample analysis may produce reliable data. Compound-specific isotope analysis (CSIA) of bone and tooth dentin protein is a powerful tool for reconstructing the flow of carbon and nitrogen in modern and past food webs. Decalcification has also been used to prepare bone and dentin samples for CSIA, but the effects of this process on bulk dentin, amino acid composition, and their specific isotope values are not known.
The bulk isotope values of raw and decalcified dentin from a sperm whale tooth were measured to determine the effects of decalcification and the accuracy of untreated dentin results. CSIA was also performed on decalcified and raw dentin to examine differences in the amino acid isotope values and molar composition between these two approaches.
Analysis of raw dentin yields precise and accurate bulk isotope measurements for this animal. The isotopic values of decalcified samples and raw dentin for individual amino acids were similar, but the average of the isotope value offsets between the two sample types was significant. The presence of inorganic material complicated raw sample processing for individual amino acid isotope values, and may have contributed to the isotopic differences between decalcified and raw samples.
Decalcification is not needed to measure bulk isotope values in dentin from this modern odontocete, probably because the lipid and carbonate concentrations are low and the carbon isotope values of dentin protein and carbonate are similar. This method should not be applied in some cases (e.g., with fossil dentin and modern bone). Decalcification should still be used prior to CSIA since significant matrix issues occur with raw dentin processing and decalcification does not alter the amino acid molar composition or isotopic values of dentin.
Rapid Communications in Mass Spectrometry 12/2014; 28(24):2744-2752. DOI:10.1002/rcm.7073 · 2.25 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Compound specific isotope analysis of amino acids (CSI-AA) has emerged as an important new method for investigating trophic dynamics in both aquatic and terrestrial systems. Multiple studies have shown that δ15N values of glutamic acid (Glu) and phenylalanine (Phe) can be coupled to provide precise estimates of trophic position (TP), while simultaneously decoupling baseline δ15N values from the effects of trophic transfer. However, the current standard gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) approach is limited by high expense, limited availability, and relatively low precision. We present a new method for making TP estimates in biological samples by CSI-AA (TPCSIA), based on a high-pressure liquid chromatography (HPLC) purification of underivatized amino acids, followed by offline elemental analysis-isotope ratio mass spectrometry (EA-IRMS). We compare results from our new HPLC/EA-IRMS method versus GC-C-IRMS in both standard and natural materials. Nitrogen isotopic values of purified Glu and Phe standards were identical within error for both methods. In five widely different marine organisms, the δ15N values of Glu and Phe were also indistinguishable within error between the two approaches; however, the δ15N values produced by the HPLC/EA-IRMS approach had higher average precision (average SD = 0.3 ± 0.2 ‰) than the GC-C-IRMS measurements (average SD = 0.45 ± 0.15 ‰). The resulting TPCSIA estimates were statistically indistinguishable (t < 1.2, df = 6, P > 0.3) between the two methods for all organisms examined. Our HPLC/EA-IRMS method may therefore allow significant expansion of TPCSIA applications, requiring only commonly available instrumentation to produce high precision TPCSIA values.
[Show abstract][Hide abstract] ABSTRACT: Deep-sea proteinaceous corals represent high-resolution paleoarchives, extending biogeochemical time series far beyond recent instrumental data. While recent studies have applied compound specific amino acid δ15N (δ15N-AA) measurements of their organic skeletal layers to investigate Holocene nitrogen cycling, potential applications of amino acid δ13C (δ13C-AA) in proteinaceous corals have not yet been examined. Here we developed δ13C-AA analysis in deep-sea bamboo coral (Isidella sp.) from the Monterey Canyon to reconstruct exported primary production over an ~ 80 year record. Preserved deep-sea coral essential amino acid δ13C-AA patterns (δ13C-EAA) closely matched those expected from natural and cultured phytoplankton, supporting the hypothesis that deep-sea coral δ13C-EAA values represent unaltered signatures of exported primary production sources. The coral bulk δ13C record showed cyclic 0.5‰ variations over the last century, with a shift to lower δ13C values in the early 1960s. Variations in coral δ13C-EAA values closely followed bulk δ13C signatures, although both the range and the magnitude of change in the bulk δ13C record were highly attenuated compared to the δ13C-EAA record. Our results indicate that δ13C-EAA in proteinaceous corals represent a new, direct proxy for δ13C in primary production that is more sensitive and accurate than bulk δ13C. To test this idea, we used existing phytoplankton δ13C-AA data to calculate an offset between bulk δ13C and δ13C-EAA. When applied to our data, a reconstructed record of δ13C values for exported organic matter was consistent with regional phytoplankton dynamics and expected trophic transfer effects, suggesting significant AA resynthesis only in the non-essential AA pool. Together, these results indicate that δ13C-EAA in deep-sea proteinaceous corals provide a powerful new long-term, high resolution tool for investigating variations in exported primary production and biogeochemistry.
[Show abstract][Hide abstract] ABSTRACT: Climatic variation alters biochemical and ecological processes, but it is difficult both to quantify the magnitude of such changes, and to differentiate long-term shifts from inter-annual variability. Here, we simultaneously quantify decade-scale isotopic variability at the lowest and highest trophic positions in the offshore California Current System (CCS) by measuring δ15N and δ13C values of amino acids in a top predator, the sperm whale (Physeter macrocephalus). Using a time series of skin tissue samples as a biological archive, isotopic records from individual amino acids (AAs) can reveal the proximate factors driving a temporal decline we observed in bulk isotope values (a decline of ≥1 ‰) by decoupling changes in primary producer isotope values from those linked to the trophic position of this toothed whale. A continuous decline in baseline (i.e., primary producer) δ15N and δ13C values was observed from 1993 to 2005 (a decrease of ∼4‰ for δ15N source-AAs and 3‰ for δ13C essential-AAs), while the trophic position of whales was variable over time and it did not exhibit directional trends. The baseline δ15N and δ13C shifts suggest rapid ongoing changes in the carbon and nitrogen biogeochemical cycling in the offshore CCS, potentially occurring at faster rates than long-term shifts observed elsewhere in the Pacific. While the mechanisms forcing these biogeochemical shifts remain to be determined, our data suggest possible links to natural climate variability, and also corresponding shifts in surface nutrient availability. Our study demonstrates that isotopic analysis of individual amino acids from a top marine mammal predator can be a powerful new approach to reconstructing temporal variation in both biochemical cycling and trophic structure.
PLoS ONE 10/2014; 9(10):e110355. DOI:10.1371/journal.pone.0110355 · 3.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The nitrogen (N) isotopic composition (δ15N) of bulk sedimentary N (δ15Nbulk) is a common tool for studying past biogeochemical cycling in the paleoceanographic record. Empirical evidence suggests that natural fluctuations in the δ15N of surface nutrient N are reflected in the δ15N of exported planktonic biomass and in sedimentary δ15Nbulk. However, δ15Nbulk is an analysis of total combustible sedimentary N, and therefore also includes mixtures of N sources and/or selective removal or preservation of N-containing compounds. Compound-specific nitrogen isotope analyses of individual amino acids (δ15NAA) are novel measurements with the potential to decouple δ15N changes in nutrient N from trophic effects, two main processes that can influence δ15Nbulk records. As a proof of concept study to examine how δ15NAA can be applied in marine sedimentary systems, we compare the δ15NAA signatures of surface and sinking POM sources with shallow surface sediments from the Santa Barbara Basin, a sub-oxic depositional environmental that exhibits excellent preservation of sedimentary organic matter. Our results demonstrate that δ15NAA signatures of both planktonic biomass and sinking POM are well preserved in such surface sediments. However, we also observed an unexpected inverse correlation between δ15N value of phenylalanine (δ15NPhe; the best AA proxy for N isotopic value at the base of the food web) and calculated trophic position. We used a simple N isotope mass balance model to confirm that over long time scales, δ15NPhe values should in fact be directly dependent on shifts in ecosystem trophic position. While this result may appear incongruent with current applications of δ15NAA in food webs, it is consistent with expectations that paleoarchives will integrate N dynamics over much longer timescales. We therefore propose that for paleoceanographic applications, key δ15NAA parameters are ecosystem trophic position, which determines relative partitioning of 15N into source AA versus trophic AA pools, and the integrated δ15NAA of all common protein AA (δ15NTHAA), which serves as a proxy for the δ15N of nutrient N. Together, we suggest that these can provide a coupled picture of regime shifts in planktonic ecosystem structure, δ15N at the base of food webs, and possibly additional information about nutrient dynamics.
[Show abstract][Hide abstract] ABSTRACT: We explored δ15N compound-specific amino acid isotope data (CSI-AA) in filter-feeding intertidal mussels (Mytilus californianus) as a new approach to construct integrated isoscapes of coastal primary production. We examined spatial δ15N gradients in the California Upwelling Ecosystem (CUE), determining bulk δ15N values of mussel tissue from 28 sites between Port Orford, Oregon and La Jolla, California, and applying CSI-AA at selected sites to decouple trophic effects from isotopic values at the base of the food web. Bulk δ15N values showed a strong linear trend with latitude, increasing from North to South (from ∼7‰ to ∼12‰, R2 = 0.759). In contrast, CSI-AA trophic position estimates showed no correlation with latitude. The δ15N trend is therefore most consistent with a baseline δ15N gradient, likely due to the mixing of two source waters: low δ15N nitrate from the southward flowing surface California Current, and the northward transport of the California Undercurrent (CUC), with15N-enriched nitrate. This interpretation is strongly supported by a similar linear gradient in δ15N values of phenylalanine (δ15NPhe), the best AA proxy for baseline δ15N values. We hypothesize δ15NPhe values in intertidal mussels can approximate annual integrated δ15N values of coastal phytoplankton primary production. We therefore used δ15NPhe values to generate the first compound-specific nitrogen isoscape for the coastal Northeast Pacific, which indicates a remarkably linear gradient in coastal primary production δ15N values. We propose that δ15NPhe isoscapes derived from filter feeders can directly characterize baseline δ15N values across major biochemical provinces, with potential applications for understanding migratory and feeding patterns of top predators, monitoring effects of climate change, and study of paleo- archives.
PLoS ONE 06/2014; 9(6):e98087. DOI:10.1371/journal.pone.0098087 · 3.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT:  One of the greatest drivers of historical nutrient and sediment transport into the Gulf of Mexico is the unprecedented scale and intensity of land-use change in the Mississippi River Basin. These landscape changes are linked to enhanced fluxes of carbon and nitrogen pollution from the Mississippi River, and persistent eutrophication and hypoxia in the northern Gulf of Mexico. Increased terrestrial runoff is one hypothesis for recent enrichment in bulk nitrogen isotope (δ15N) values, a tracer for nutrient source, observed in a Gulf of Mexico deep-sea coral record. However, unambiguously linking anthropogenic land-use change to whole scale shifts in downstream Gulf of Mexico biogeochemical cycles is difficult. Here we present a novel approach, coupling a new tracer of agro-industrialization to a multi-proxy record of nutrient loading in long-lived deep-sea corals collected in the Gulf of Mexico. We found that coral bulk δ15N values are enriched over the last 150-200 years relative to the last millennia, and compound-specific amino acid δ15N data indicates a strong increase in baseline δ15N of nitrate as the primary cause. Coral rhenium (Re) values are also strongly elevated during this period, suggesting 34% of Re is of anthropogenic origin, consistent with Re enrichment in major world rivers. However, there are no pre-anthropogenic measurements of Re to confirm this observation. For the first time, an unprecedented record of natural and anthropogenic Re variability is documented through coral Re records. Taken together, these novel proxies link upstream changes in water quality to impacts on the deep-sea coral ecosystem.
[Show abstract][Hide abstract] ABSTRACT: The North Pacific subtropical gyre (NPSG) plays a major part in the export of carbon and other nutrients to the deep ocean. Primary production in the NPSG has increased in recent decades despite a reduction in nutrient supply to surface waters. It is thought that this apparent paradox can be explained by a shift in plankton community structure from mostly eukaryotes to mostly nitrogen-fixing prokaryotes. It remains uncertain, however, whether the plankton community domain shift can be linked to cyclical climate variability or a long-term global warming trend. Here we analyse records of bulk and amino-acid-specific (15)N/(14)N isotopic ratios (δ(15)N) preserved in the skeletons of long-lived deep-sea proteinaceous corals collected from the Hawaiian archipelago; these isotopic records serve as a proxy for the source of nitrogen-supported export production through time. We find that the recent increase in nitrogen fixation is the continuation of a much larger, centennial-scale trend. After a millennium of relatively minor fluctuation, δ(15)N decreases between 1850 and the present. The total shift in δ(15)N of -2 per mil over this period is comparable to the total change in global mean sedimentary δ(15)N across the Pleistocene-Holocene transition, but it is happening an order of magnitude faster. We use a steady-state model and find that the isotopic mass balance between nitrate and nitrogen fixation implies a 17 to 27 per cent increase in nitrogen fixation over this time period. A comparison with independent records suggests that the increase in nitrogen fixation might be linked to Northern Hemisphere climate change since the end of the Little Ice Age.
[Show abstract][Hide abstract] ABSTRACT: Compound-specific isotope analysis of individual amino acids (CSI-AA) is a powerful new tool for tracing nitrogen (N) source and transformation in biogeochemical cycles. Specifically, the δ15N value of phenylalanine (δ15NPhe) represents an increasingly used proxy for source δ15N signatures, with particular promise for paleoceanographic applications. However, current derivatization/gas chromatography methods require expensive and relatively uncommon instrumentation, and have relatively low precision, making many potential applications impractical.
A new offline approach has been developed for high-precision δ15N measurements of amino acids (δ15NAA), optimized for δ15NPhe values. Amino acids (AAs) are first purified via high-pressure liquid chromatography (HPLC), using a mixed-phase column and automated fraction collection. The δ15N values are determined via offline elemental analyzer-isotope ratio mass spectrometry (EA-IRMS).
The combined HPLC/EA-IRMS method separated most protein AAs with sufficient resolution to obtain accurate δ15N values, despite significant intra-peak isotopic fractionation. For δ15NPhe values, the precision was ±0.16‰ for standards, 4× better than gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS; ±0.64‰). We also compared a δ15NPhe paleo-record from a deep-sea bamboo coral from Monterey Bay, CA, USA, using our method versus GC/C/IRMS. The two methods produced equivalent δ15NPhe values within error; however, the δ15NPhe values from HPLC/EA-IRMS had approximately twice the precision of GC/C/IRMS (average stdev of 0.27‰ ± 0.14‰ vs 0.60‰ ± 0.20‰, respectively).
These results demonstrate that offline HPLC represents a viable alternative to traditional GC/C/IMRS for δ15NAA measurement. HPLC/EA-IRMS is more precise and widely available, and therefore useful in applications requiring increased precision for data interpretation (e.g. δ15N paleoproxies). Copyright
Rapid Communications in Mass Spectrometry 11/2013; 27(21):2327-2337. DOI:10.1002/rcm.6695 · 2.25 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Tracing the origin of nutrients is a fundamental goal of food web research but methodological issues associated with current research techniques such as using stable isotope ratios of bulk tissue can lead to confounding results. We investigated whether naturally occurring δ(13)C patterns among amino acids (δ(13)CAA) could distinguish between multiple aquatic and terrestrial primary production sources. We found that δ(13)CAA patterns in contrast to bulk δ(13)C values distinguished between carbon derived from algae, seagrass, terrestrial plants, bacteria and fungi. Furthermore, we showed for two aquatic producers that their δ(13)CAA patterns were largely unaffected by different environmental conditions despite substantial shifts in bulk δ(13)C values. The potential of assessing the major carbon sources at the base of the food web was demonstrated for freshwater, pelagic, and estuarine consumers; consumer δ(13)C patterns of essential amino acids largely matched those of the dominant primary producers in each system. Since amino acids make up about half of organismal carbon, source diagnostic isotope fingerprints can be used as a new complementary approach to overcome some of the limitations of variable source bulk isotope values commonly encountered in estuarine areas and other complex environments with mixed aquatic and terrestrial inputs.
PLoS ONE 09/2013; 8(9):e73441. DOI:10.1371/journal.pone.0073441 · 3.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Compound-specific isotope analysis of individual amino acids (AA) is a rapidly growing tool in ecological studies to assess diet and trophic position (TP) in both modern and ancient foodwebs. We conducted the first controlled feeding study examining d15N values in AAs in a marine mammal (harbor seal Phoca vitulina). The pattern of d15N variation among AAs in seals was similar to that observed in other heterotrophs, although exceptions were found with proline and threonine. However, many d15N changes with trophic transfer were very different than those reported for zooplankton and other lower TP marine consumers. In particular the measured trophic enrichment factor (TEF) now broadly used for TP estimation (TEFGlu-Phe) was much lower in harbor seals (similar to 4.3%) than the current commonly applied value (similar to 7.5%). Recently published data on wild marine birds (penguins) and elasmobranchs (stingrays) suggests that similar, low TEF values may also be characteristic of these taxa. Together, these data imply that marine mammals and other higher animals have different, but also diagnostic, changes in delta N-15-AA with trophic transfer vs. organisms examined in previous feeding studies (e.g. zooplankton, bony fish and mollusks), possibly due to dietary protein content, trophic position, and/or form of nitrogen excretion (urea vs. ammonia). Therefore, we propose that for marine mammals, a multi-TEF calculation is required to account for variations of TEF between animals within a food web, and we demonstrate that this approach can predict accurate TP estimates for harbor seals. These results also have significant implication for the application of compound-specific isotope analysis of AAs on terrestrial ecology and trophic structure.
[Show abstract][Hide abstract] ABSTRACT: Coincident with climate shifts and anthropogenic perturbations, the highly voracious jumbo squid Dosidicus gigas reached unprecedented northern latitudes along the NE Pacific margin post 1997-98. The physical or biological drivers of this expansion, as well as its ecological consequences remain unknown. Here, novel analysis from both bulk tissues and individual amino acids (Phenylalanine; Phe and Glutamic acid; Glu) in both gladii and muscle of D. gigas captured in the Northern California Current System (NCCS) documents for the first time multiple geographic origins and migration. Phe δ(15)N values, a proxy for habitat baseline δ(15)N values, confirm at least three different geographic origins that were initially detected by highly variable bulk δ(15)N values in gladii for squid at small sizes (<30 cm gladii length). In contrast, bulk δ(15)N values from gladii of large squid (>60 cm) converged, indicating feeding in a common ecosystem. The strong latitudinal gradient in Phe δ(15)N values from composite muscle samples further confirmed residency at a point in time for large squid in the NCCS. These results contrast with previous ideas, and indicate that small squid are highly migratory, move into the NCCS from two or more distinct geographic origins, and use this ecosystem mainly for feeding. These results represent the first direct information on the origins, immigration and habitat use of this key "invasive" predator in the NCCS, with wide implications for understanding both the mechanisms of periodic D. gigas population range expansions, and effects on ecosystem trophic structure.
PLoS ONE 03/2013; 8(3):e59651. DOI:10.1371/journal.pone.0059651 · 3.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Stable nitrogen isotopic analysis of individual amino acids (δ15N-AA) has unique potential to elucidate the complexities of food webs, track heterotrophic transformations, and understand diagenesis of organic nitrogen (ON). While δ15N-AA patterns of autotrophs have been shown to be generally similar, prior work has also suggested that differences may exist between cyanobacteria and eukaryotic algae. However, δ15N-AA patterns in differing oceanic algal groups have never been closely examined. The overarching goals of this study were first to establish a more quantitative understanding of algal δ15N-AA patterns, and second to examine whether δ15N-AA patterns have potential as a new tracer for distinguishing prokaryotic vs. eukaryotic N sources. We measured δ15N-AA from prokaryotic and eukaryotic phytoplankton cultures and used a complementary set of statistical approaches (simple normalization, regression-derived fractionation factors, and multivariate analyses) to test for variations. A generally similar δ15N-AA pattern was confirmed for all algae, however significant AA-specific variation was also consistently identified between the two groups. The relative δ15N fractionation of Glx (glutamine + glutamic acid combined) vs. total proteinaceous N appeared substantially different, which we hypothesize could be related to differing enzymatic forms. In addition, the several other AA (most notably glycine and leucine) appeared to have strong biomarker potential. Finally, we observed that overall patterns of δ15N values in algae correspond well with the Trophic vs. Source-AA division now commonly used to describe variable AA δ15N changes with trophic transfer, suggesting a common mechanistic basis. Overall, these results show that autotrophic δ15N-AA patterns can differ between major algal evolutionary groupings for many AA. The statistically significant multivariate results represent a first approach for testing ideas about relative eukaryotic vs. prokaryotic ON sources in the sea.
[Show abstract][Hide abstract] ABSTRACT: We report the interseasonal variation in bulk elemental and stable isotopic (δ13C, δ15N) composition of dissolved and particulate organic matter (DOM, POM) during a high-resolution time series (2007–2009) on the Big Sur coast. In addition to interseasonal variations, we explore the relationships between physical size and reactivity (i.e., composition) of exportable organic matter (OM) pools, and characterize the elemental and isotopic composition of size-fractionated POM and DOM pools within this potentially high-nutrient, low-chlorophyll (HNLC) California upwelling region. Average POM concentrations were low (< 5.2 mmol C L-1 and < 0.7 mmol N L-1), and all OM pools had low C : N ratios (DOM average = 15.0 and POM < 7.8)—indicating that this upwelling center may represent an important source of N-rich material to offshore environments. Seasonal ‘‘excess’’ dissolved organic carbon (DOC) production was significant (~ 30–80 mmol L-1 DOC); however, dissolved organic nitrogen (DON) production and cycling was largely decoupled from both DOC and physical processes. Overall, our results are consistent with this region representing an ‘‘HNLC upwelling system.’’ The distinct bulk elemental and isotopic OM composition and seasonality vs. that of high-productivity upwelling regions highlights the need to better understand the biogeochemical diversity of upwelling systems. Finally, we observed a quantifiable size–composition relationship across both POM and DOM size classes, perhaps representing a powerful new tool for modeling N vs. C fluxes in ocean biogeochemical cycles.
Limnology and Oceanography 11/2012; 57(6):1757–1774. DOI:10.4319/lo.2012.57.6.1757 · 3.79 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Despite the importance of the nitrogen (N) cycle on marine productivity, little is known about variability in N sources and cycling in the ocean in relation to natural and anthropogenic climate change. Beyond the last few decades of scientific observation, knowledge depends largely on proxy records derived from nitrogen stable isotopes (δ(15)N) preserved in sediments and other bioarchives. Traditional bulk δ(15)N measurements, however, represent the combined influence of N source and subsequent trophic transfers, often confounding environmental interpretation. Recently, compound-specific analysis of individual amino acids (δ(15)N-AA) has been shown as a means to deconvolve trophic level versus N source effects on the δ(15)N variability of bulk organic matter. Here, we demonstrate the first use of δ(15)N-AA in a paleoceanographic study, through analysis of annually secreted growth rings preserved in the organic endoskeletons of deep-sea gorgonian corals. In the Northwest Atlantic off Nova Scotia, coral δ(15)N is correlated with increasing presence of subtropical versus subpolar slope waters over the twentieth century. By using the new δ(15)N-AA approach to control for variable trophic processing, we are able to interpret coral bulk δ(15)N values as a proxy for nitrate source and, hence, slope water source partitioning. We conclude that the persistence of the warm, nutrient-rich regime since the early 1970s is largely unique in the context of the last approximately 1,800 yr. This evidence suggests that nutrient variability in this region is coordinated with recent changes in global climate and underscores the broad potential of δ(15)N-AA for paleoceanographic studies of the marine N cycle.
Proceedings of the National Academy of Sciences 01/2011; 108(3):1011-5. DOI:10.1073/pnas.1004904108 · 9.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Stable carbon and nitrogen isotope ratios (δ13C and δ15N) of serum, red blood cells (RBC), muscle, and blubber were measured in captive and wild northeast Pacific harbor seals (Phoca vitulina richardii) at three coastal California sites (San Francisco Bay, Tomales Bay, and Channel Islands). Trophic discrimination factors (ΔTissue-Diet) were calculated for captive seals and then applied in wild counterparts in each habitat to estimate trophic position and feeding behavior. Trophic discrimination factors for δ15N of serum (+3.8‰), lipid-extracted muscle (+1.6‰), and lipid-blubber (+6.5‰) are proposed to determine trophic position. An offset between RBC and serum of +0.3‰ for δ13C and −0.6‰ for δ15N was observed, which is consistent with previous research. Specifically, weaner seals (<1 yr) had large offsets, suggesting strong trophic position shifts during this life stage. Isotopic values indicated an average trophic position of 3.6 at both San Francisco Bay and Tomales Bay and 4.2 at Channel Islands. Isotopic means were strongly dependent on age class and also suggested that mean diet composition varies considerably between all locations. Together, these data indicate that isotopic composition of blood fractions can be an effective approach to estimate trophic position and dietary behavior in wild pinnipeds.
[Show abstract][Hide abstract] ABSTRACT: Hydrothermal fluids circulate through extensive areas of the upper
oceanic crust. Most hydrothermal circulation occurs on ridge flanks,
where low-temperature fluids flow through porous basalts. These fluids
contain variable levels of dissolved organic carbon, but the source and
composition of this carbon are uncertain. Here, we report
Δ14C and δ13C measurements of
dissolved organic carbon in ridge-flank and on-axis hydrothermal fluids
sampled from the Juan de Fuca Ridge. Dissolved organic carbon from two
independent ridge-flank sites was characterized by low
δ13C and Δ14C values. The
δ13C values ranged from -26 to -35‰, and were
consistent with a chemoautotrophic origin. The 14C ages of
the dissolved organic carbon ranged from 11,800 to 14,400 years before
present, revealing that the carbon was around three times older than
dissolved organics in the deep ocean. The Δ14C values
of the ridge-flank dissolved organic matter also corresponded closely to
those of dissolved inorganic carbon in the same fluid samples. Taken
together, the data suggest that chemosynthetic crustal microbial
communities synthesize dissolved organic carbon from inorganic carbon in
ridge-flank fluids. We suggest that ridge-flank circulation may support
an indigenous biosphere extensive enough to export substantial fixed
carbon, with distinct isotopic and probably compositional character, to
the overlying ocean.