Increasing subtropical North Pacific Ocean nitrogen fixation since the Little Ice Age

Tropical decadal variability in nutrient supply and phytoplankton community in the Central Equatorial Pacific during the late Holocene

We have reconstructed baseline δ15N and δ13C of export production at Kingman Reef in the Central Equatorial Pacific (CEP) at sub-decadal resolution, nearly continuously over the last 2000 years. The changes in δ15N reflects the strength of the North Equatorial Counter Current (NECC) relative to the South Equatorial Current (SEC), and to a lesser extent, the North Equatorial Current (NEC). Seasonal to multi-decadal variation in the strength of these currents, through the redistribution of heat, have global climate impacts and influence marine and terrestrial ecosystems. We use modern El Niño-La Nina dynamics and the Tropical Pacific Decadal Variability (TPDV) pattern, which is defined in the CEP, as a framework for analyzing the isotopic data. The CEP δ15N and δ13C records exhibit multi-decadal (50-60 year) variability consistent with TPDV. A large multi-centennial feature in the CEP δ15N data, within age-model uncertainties, is consistent with one of the prolonged dry-pluvial sequences in the American west at the end of the Medieval Climate Anomaly, where low TPDV is correlated with drier conditions. This unique record shows that the strength of the NECC, as reflected in baseline δ15N and δ13C, has at quasi-predictable intervals throughout the late Holocene, toggled the phytoplankton community between prokaryotes and picoplankton versus eukaryotes.

Element variation in a clam shell and its implications for cold seep irregular eruptions: Calyptogena sp. in the Haima cold seep

Cold seep is characterized by methane-rich fluids released from subsurface reservoirs, and it sustains the chemosynthetic ecosystems on the seafloor. Previous studies suggest that the activity of cold seep could affect the seawater chemistry and ambient temperature. However, the short-term seep activity was hardly reconstructed due to the focus of studies on carbonate or sediment. Vent macrofauna provide such an opportunity by recording in shells the immediate environment in which they grow. The carbonate skeleton of organisms could theoretically preserve environmental variation. Therefore, high-resolution archives are urgently required to understand the influence of cold seep activity on biogeochemistry. In this study, SEM, EDS mapping, EBSD mapping, and LA-ICP-MS analyses were conducted on a clam (Calyptogena sp.) shell collected alive in 2018 from the Haima cold seep in South China Sea. The CaCO₃, Na, Mg, Sr, and Ba contents and the Sr/Ca, Ba/Ca, and Na/Ca ratios in the hinge plate were measured on LA-ICP-MS by spot analysis and line scanning. The element contents in the hinge are as follows: Mg (38.5-109 μg/g), Na (3117-5246 μg/g), Sr (970-5371 μg/g), and Ba (2.9-11.5 μg/g). The results show that Sr, Re, and Ba content vary synchronously along the direction of growth, but Na has an opposite trend. The element analyses indicate that the eruption of the Haima cold seep was irregular, causing temperature, redox state, and pH changes in the cold seep ecosystem. These findings show that the irregular cold seep activities exert vital influences on the biogeochemistry of the cold seep ecosystem, which shed a light on cold seep biomonitoring.

Insights from Fossil-Bound Nitrogen Isotopes in Diatoms, Foraminifera, and Corals

Nitrogen is a major limiting element for biological productivity, and thus understanding past variations in nitrogen cycling is central to understanding past and future ocean biogeochemical cycling, global climate cycles, and biodiversity. Organic nitrogen encapsulated in fossil biominerals is generally protected from alteration, making it an important archive of the marine nitrogen cycle on seasonal to million-year timescales. The isotopic composition of fossil-bound nitrogen reflects variations in the large-scale nitrogen inventory, local sources and processing, and ecological and physiological traits of organisms. The ability to measure trace amounts of fossil-bound nitrogen has expanded with recent method developments. In this article, we review the foundations and ground truthing for three important fossil-bound proxy types: diatoms, foraminifera, and corals. We highlight their utility with examples of high-resolution evidence for anthropogenic inputs of nitrogen to the oceans, glacial-interglacial-scale assessments of nitrogen inventory change, and evidence for enhanced CO₂ drawdown in the high-latitude ocean. Future directions include expanded method development, characterization of ecological and physiological variation, and exploration of extended timescales to push reconstructions further back in Earth's history.

Natural cycles in South Pacific Gyre strength and the Southern Annular Mode

The South Pacific Gyre (SPG) plays a vital role in regulating Southern Hemisphere climate and ecosystems. The SPG has been intensifying since the twentieth century due to changes in large scale wind forcing. These changes result from variability in the Southern Annular Mode (SAM), causing warming along the eastern SPG which affects local ecosystems. However, our understanding of SPG variability on timescales greater than several decades is poor due to limited observations. Marine sediment cores are traditionally used to determine if recent ocean trends are anomalous, but rarely capture centennial variability in the southwest Pacific and limit our understanding of SPG variability. Here we capture centennial SPG dynamics using a novel high-resolution paleocirculation archive: radiocarbon reservoir ages (R) and local reservoir corrections (∆R) in SPG deep-sea black corals. We find black coral R and ∆R correlates with SAM reconstructions over 0-1000 cal BP and 2000-3000 cal BP. We propose this correlation indicates varying transport of well-ventilated subtropical waters resulting from SPG and SAM interactions. We reconstruct several 'spin up' cycles reminiscent of the recent gyre intensification, which has been attributed to anthropogenic causes. This implies gyre strength and SAM show natural co-variability on anthropogenic timescales which should factor into future climate projections.

New geochemical tools for investigating resource and energy functions at deep-sea cold seeps using amino acid δ15 N in chemosymbiotic mussels (Bathymodiolus childressi)

In order to reconstruct the ecosystem structure of chemosynthetic environments in the fossil record, geochemical proxies must be developed. Here, we present a suite of novel compound-specific isotope parameters for tracing chemosynthetic production with a focus on understanding nitrogen dynamics in deep-sea cold seep environments. We examined the chemosymbiotic bivalve Bathymodiolus childressi from three geographically distinct seep sites on the NE Atlantic Margin and compared isotope data to non-chemosynthetic littoral mussels to test whether water depth, seep activity, and/or mussel bed size are linked to differences in chemosynthetic production. The bulk isotope analysis of carbon (δ¹³ C) and nitrogen (δ¹⁵ N), and δ¹⁵ N values of individual amino acids (δ¹⁵ N_AA ) in both gill and muscle tissues, as well as δ¹⁵ N_AA- derived parameters including trophic level (TL), baseline δ¹⁵ N value (δ¹⁵ N_Phe ), and a microbial resynthesis index (ΣV), were used to investigate specific geochemical signatures of chemosynthesis. Our results show that δ¹⁵ N_AA values provide a number of new proxies for relative reliance on chemosynthesis, including TL, ∑V, and both δ¹⁵ N values and molar percentages (Gly/Glu mol% index) of specific AA. Together, these parameters suggested that relative chemoautotrophy is linked to both degree of venting from seeps and mussel bed size. Finally, we tested a Bayesian mixing model using diagnostic AA δ¹⁵ N values, showing that percent contribution of chemoautotrophic versus heterotrophic production to seep mussel nutrition can be directly estimated from δ¹⁵ N_AA values. Our results demonstrate that δ¹⁵ N_AA analysis can provide a new set of geochemical tools to better understand mixotrophic ecosystem function and energetics, and suggest extension to the study of ancient chemosynthetic environments in the fossil record.

Determination of precise nitrogen stable isotopic baselines from heterotrophic organism in coastal ocean environments using compound specific isotope analysis of amino acids

We sampled mussels (Mytilus edulis) along the coast of the Korean peninsula, and determined their trophic position (TP) using the nitrogen isotope ratio of amino acids (TP_glu/phe). The TP_glu/phe of mussels (2.0 to 2.5) is higher than TP of herbivores, suggesting that TP normalization is required to remove the trophic enrichment effect on basal nitrogen isotope ratio. We found similar trends in spatial variation between nitrogen isotopic baseline based on nitrogen isotope ratio of bulk tissue (δ¹⁵N_base-bulk) and that of phenylalanine (δ¹⁵N_base-phe) in mussels, reflecting different nitrogen sources among sampling sites. The present study suggests that δ¹⁵N_base-bulk and δ¹⁵N_base-phe are powerful tools to trace nitrogen isotopic baselines from primary consumers as well as higher TP organisms.

High-precision measurement of phenylalanine and glutamic acid δ15 N by coupling ion-exchange chromatography and purge-and-trap continuous-flow isotope ratio mass spectrometry

RATIONALE

Nitrogen isotopic compositions (δ¹⁵ N) of source and trophic amino acids (AAs) are crucial tracers of N sources and trophic enrichments in diverse fields, including archeology, astrobiochemistry, ecology, oceanography, and paleo-sciences. The current analytical technique using gas chromatography-combustion-isotope ratio mass spectrometry (GC/C/IRMS) requires derivatization, which is not compatible with some key AAs. Another approach using high-performance liquid chromatography-elemental analyzer-IRMS (HPLC/EA/IRMS) may experience coelution issues with other compounds in certain types of samples, and the highly sensitive nano-EA/IRMS instrumentations are not widely available.

METHODS

We present a method for high-precision δ¹⁵ N measurements of AAs (δ¹⁵ N-AA) optimized for canonical source AA-phenylalanine (Phe) and trophic AA-glutamic acid (Glu). This offline approach entails purification and separation via high-pressure ion-exchange chromatography (IC) with automated fraction collection, the sequential chemical conversion of AA to nitrite and then to nitrous oxide (N₂ O), and the final determination of δ¹⁵ N of the produced N₂ O via purge-and-trap continuous-flow isotope ratio mass spectrometry (PT/CF/IRMS).

RESULTS

The cross-plots of δ¹⁵ N of Glu and Phe standards (four different natural-abundance levels) generated by this method and their accepted values have a linear regression slope of 1 and small intercepts demonstrating high accuracy. The precisions were 0.36‰-0.67‰ for Phe standards and 0.27‰-0.35‰ for Glu standards. Our method and the GC/C/IRMS approach produced equivalent δ¹⁵ N values for two lab standards (McCarthy Lab AA mixture and cyanobacteria) within error. We further tested our method on a wide range of natural sample matrices and obtained reasonable results.

CONCLUSIONS

Our method provides a reliable alternative to the current methods for δ¹⁵ N-AA measurement as IC or HPLC-based techniques that can collect underivatized AAs are widely available. Our chemical approach that converts AA to N₂ O can be easily implemented in laboratories currently analyzing δ¹⁵ N of N₂ O using PT/CF/IRMS. This method will help promote the use of δ¹⁵ N-AA in important studies of N cycling and trophic ecology in a wide range of research areas.

Stable isotope signatures in historic harbor seal bone link food web-assimilated carbon and nitrogen resources to a century of environmental change

Anthropogenic climate change will impact nutrient cycles, primary production, and ecosystem structure in the world's oceans, although considerable uncertainty exists regarding the magnitude and spatial variability of these changes. Understanding how regional-scale ocean conditions control nutrient availability and ultimately nutrient assimilation into food webs will inform how marine resources will change in response to climate. To evaluate how ocean conditions influence the assimilation of nitrogen and carbon into coastal marine food webs, we applied a novel dimension reduction analysis to a century of newly acquired molecular isotope data derived from historic harbor seal bone specimens. By measuring bulk δ¹³ C and δ¹⁵ N values of source amino acids of these top predators from 1928 to 2014, we derive indices of primary production and nitrogen resources that are assimilated into food webs. We determined coastal food webs responded to climate regimes, coastal upwelling, and freshwater discharge, yet the strength of responses to individual drivers varied across the northeast Pacific. Indices of primary production and nitrogen availability in the Gulf of Alaska were dependent on regional climate indices (i.e., North Pacific Gyre Oscillation) and upwelling. In contrast, the coastal Washington and Salish Sea food webs were associated with local indices of freshwater discharge. For some regions (eastern Bering Sea, northern Gulf of Alaska) food web-assimilated production was coupled with nitrogen sources; however, other regions demonstrated no production-nitrogen coupling (Salish Sea). Temporal patterns of environmental indices and isotopic data from Washington state varied about the long-term mean with no directional trend. Data from the Gulf of Alaska, however, showed below average harbor seal δ¹³ C values and above average ocean conditions since 1975, indicating a change in primary production in recent decades. Altogether, these findings demonstrate stable isotope data can provide useful indices of nitrogen resources and phytoplankton dynamics specific to what is assimilated by food webs.

Nitrogen Fixation in a Changing Arctic Ocean: An Overlooked Source of Nitrogen?

The Arctic Ocean is the smallest ocean on Earth, yet estimated to play a substantial role as a global carbon sink. As climate change is rapidly changing fundamental components of the Arctic, it is of local and global importance to understand and predict consequences for its carbon dynamics. Primary production in the Arctic Ocean is often nitrogen-limited, and this is predicted to increase in some regions. It is therefore of critical interest that biological nitrogen fixation, a process where some bacteria and archaea termed diazotrophs convert nitrogen gas to bioavailable ammonia, has now been detected in the Arctic Ocean. Several studies report diverse and active diazotrophs on various temporal and spatial scales across the Arctic Ocean. Their ecology and biogeochemical impact remain poorly known, and nitrogen fixation is so far absent from models of primary production in the Arctic Ocean. The composition of the diazotroph community appears distinct from other oceans – challenging paradigms of function and regulation of nitrogen fixation. There is evidence of both symbiotic cyanobacterial nitrogen fixation and heterotrophic diazotrophy, but large regions are not yet sampled, and the sparse quantitative data hamper conclusive insights. Hence, it remains to be determined to what extent nitrogen fixation represents a hitherto overlooked source of new nitrogen to consider when predicting future productivity of the Arctic Ocean. Here, we discuss current knowledge on diazotroph distribution, composition, and activity in pelagic and sea ice-associated environments of the Arctic Ocean. Based on this, we identify gaps and outline pertinent research questions in the context of a climate change-influenced Arctic Ocean – with the aim of guiding and encouraging future research on nitrogen fixation in this region.

Marine nitrogen fixers mediate a low latitude pathway for atmospheric CO2 drawdown

Roughly a third (~30 ppm) of the carbon dioxide (CO₂) that entered the ocean during ice ages is attributed to biological mechanisms. A leading hypothesis for the biological drawdown of CO₂ is iron (Fe) fertilisation of the high latitudes, but modelling efforts attribute at most 10 ppm to this mechanism, leaving ~20 ppm unexplained. We show that an Fe-induced stimulation of dinitrogen (N₂) fixation can induce a low latitude drawdown of 7–16 ppm CO₂. This mechanism involves a closer coupling between N₂ fixers and denitrifiers that alleviates widespread nitrate limitation. Consequently, phosphate utilisation and carbon export increase near upwelling zones, causing deoxygenation and deeper carbon injection. Furthermore, this low latitude mechanism reproduces the regional patterns of organic δ¹⁵N deposited in glacial sediments. The positive response of marine N₂ fixation to dusty ice age conditions, first proposed twenty years ago, therefore compliments high latitude changes to amplify CO₂ drawdown.

Iron fertilisation of the high latitude oceans is a well-established biological mechanism to explain the ice age drawdown of atmospheric CO₂, yet modelling has so far struggled to account for a sufficient drawdown via this mechanism. Here, the authors propose that N₂ fixers, which inhabit the lower latitude ocean, made a significant contribution to CO₂ drawdown and so amplified the global response to iron fertilisation during ice ages.

Trophic declines and decadal-scale foraging segregation in three pelagic seabirds

We investigated how foraging habits vary among three ecologically distinct wide-ranging seabirds. Using amino acid δ¹⁵N proxies for nutrient regime (δ¹⁵N_Phe) and trophic position (Δδ¹⁵N_Glu-Phe), we compared Newell's shearwater (Puffinus newelli) and Laysan albatross (Phoebastria immutabilis) foraging habits over the past 50-100 years, respectively, to published records for the Hawaiian petrel (Pterodroma sandwichensis). Standard ellipses constructed from the isotope proxies show that inter-population and interspecific foraging segregation have persisted for several decades. We found no evidence of a shift in nutrient regime at the base of the food web for the three species. However, our data identify a trophic decline during the past century for Newell's shearwater and Laysan albatross (probability ≥ 0.97), echoing a similar decline observed in the Hawaiian petrel. During this time, Newell's shearwaters and Hawaiian petrels have experienced population declines and Laysan albatross has experienced range extension and apparent population stability. Counting other recent studies, a pattern of trophic decline over the past century has now been identified in eight species of pelagic seabirds that breed in the Hawaiian Islands. Because our study species forage broadly across the North Pacific Ocean and differ in morphological and behavioral traits and feeding methods, the identified trophic declines suggest a pervasive shift in food web architecture within the past century.

Restructuring of nutrient flows in island ecosystems following human colonization evidenced by isotopic analysis of commensal rats

The role of humans in shaping local ecosystems is an increasing focus of archaeological research, yet researchers often lack an appropriate means of measuring past anthropogenic effects on local food webs and nutrient cycling. Stable isotope analysis of commensal animals provides an effective proxy for local human environments because these species are closely associated with human activities without being under direct human management. Such species are thus central to nutrient flows across a range of socionatural environments and can provide insight into how they intersected and transformed over time. Here we measure and compare stable carbon and nitrogen isotope data from Pacific rat (Rattus exulans) skeletal remains across three Polynesian island systems [Mangareva, Ua Huka (Marquesas), and the Polynesian Outlier of Tikopia] during one of the most significant cases of human migration and commensal introduction in prehistory. The results demonstrate widespread δ15N declines across these islands that are associated with human land use, intensification, and faunal community restructuring. Local comparison of rat stable isotope data also tracks human activities and resource availability at the level of the settlement. Our results highlight the large-scale restructuring of nutrient flows in island ecosystems that resulted from human colonization and ecosystem engineering activities on Pacific islands. They also demonstrate that stable isotope analysis of often-ignored commensal taxa can provide a tool for tracking human land use and environmental effects.

Trophic signatures of seabirds suggest shifts in oceanic ecosystems

Pelagic ecosystems are dynamic ocean regions whose immense natural capital is affected by climate change, pollution, and commercial fisheries. Trophic level-based indicators derived from fishery catch data may reveal the food web status of these systems, but the utility of these metrics has been debated because of targeting bias in fisheries catch. We analyze a unique, fishery-independent data set of North Pacific seabird tissues to inform ecosystem trends over 13 decades (1890s to 2010s). Trophic position declined broadly in five of eight species sampled, indicating a long-term shift from higher-trophic level to lower-trophic level prey. No species increased their trophic position. Given species prey preferences, Bayesian diet reconstructions suggest a shift from fishes to squids, a result consistent with both catch reports and ecosystem models. Machine learning models further reveal that trophic position trends have a complex set of drivers including climate, commercial fisheries, and ecomorphology. Our results show that multiple species of fish-consuming seabirds may track the complex changes occurring in marine ecosystems.

Compound-specific amino acid δ15 N values in archaeological shell: Assessing diagenetic integrity and potential for isotopic baseline reconstruction

RATIONALE

Reconstructing stable isotope (SI) ratios at the base of paleo-food webs is often challenging. For coastal systems, the SI ratios of organic matter in archeological shell represents a possible solution, providing a direct record of primary consumer SI ratios in the littoral zone. However, shell is often porous, with organic compounds susceptible to diagenetic alteration or contamination. If molecular isotopic information is well preserved, compound-specific amino acid isotope analysis (CSI-AA) has the potential to provide direct proxies for baseline SI ratios, bypassing many contamination issues, and to allow assessment of the diagenetic state.

METHODS

We collected shell from both archeological middens and nearby littoral zones in coastal Alaska, and used a simple organic extraction approach based on decalcification with sequential weak HCl additions to liberate organic material. We measured CSI-AA patterns, molar AA distributions, and the CSI-AA degradation parameter (ΣV), in the context of bulk SI ratios in fossil shell, modern shell, and soft tissue from five common taxa (urchin, limpet, mussel, periwinkle, chiton).

RESULTS

CSI-AA patterns in both soft tissue and shell were consistent with primary consumers, and were indistinguishable in most modern and fossil shell pairs, showing that amino acid δ¹⁵ N values can be well preserved in archeological shell. AA molar distributions were also similar, although most fossil shell was enriched in Asx and Gly. Comparison between CSI-AA results from modern specimens confirmed that the source AA group (tracking isotopic baselines) are transferred without substantial modification into the shell record. In contrast, the Trophic AA group had elevated δ¹⁵ N values in shell versus soft tissue for all taxa examined, suggesting that a correction factor will be required for any CSI-AA proxies using these AAs.

CONCLUSIONS

Overall, this new data indicates that the CSI-AA analysis of fossil shell represents a promising new approach to determining isotopic baselines in coastal paleo-ecosystems.

Temporal variation in pelagic food chain length in response to environmental change

Climate variability alters nitrogen cycling, primary productivity, and dissolved oxygen concentration in marine ecosystems. We examined the role of this variability (as measured by six variables) on food chain length (FCL) in the California Current (CC) by reconstructing a time series of amino acid-specific δ¹⁵N values derived from common dolphins, an apex pelagic predator, and using two FCL proxies. Strong declines in FCL were observed after the 1997-1999 El Niño Southern Oscillation (ENSO) event. Bayesian models revealed longer FCLs under intermediate conditions for surface temperature, chlorophyll concentration, multivariate ENSO index, and total plankton volume but not for hypoxic depth and nitrate concentration. Our results challenge the prevalent paradigm that suggested long-term stability in the food web structure in the CC and, instead, reveal that pelagic food webs respond strongly to disturbances associated with ENSO events, local oceanography, and ongoing changes in climate.

The dietary protein paradox and threonine 15 N-depletion: Pyridoxal-5'-phosphate enzyme activity as a mechanism for the δ15 N trophic level effect

RATIONALE

Nitrogen stable isotope ratios (δ¹⁵ N values) are used to reconstruct dietary patterns, but the biochemical mechanism(s) responsible for the diet to tissue trophic level effect and its variability are not fully understood. Here δ¹⁵ N amino acid (AA) values and physiological measurements (nitrogen intake, plasma albumin concentrations, liver-reduced glutathione concentrations and leucine oxidation rates) are used to investigate increased dietary protein consumption and oxidative stress (vitamin E deficiency) in rat total plasma protein.

METHODS

Using gas chromatography/combustion/isotope ratio mass spectrometry, the δ¹⁵ N values from N-pivaloyl-i-propyl esters of 15 AAs are reported for rats (n = 40) fed casein-based diets with: adequate protein (AP, 13.8%; n = 10), medium protein (MP, 25.7%; n = 10), high protein (HP, 51.3%; n = 10) or HP without vitamin E (HP-E; n = 10) for 18 weeks.

RESULTS

Between the HP and AP groups, the δ¹⁵ N_AA values of threonine (-4.0‰), serine (+1.4‰) and glycine (+1.2‰) display the largest differences and show significant correlations with: nitrogen intake, plasma albumin concentrations, liver-reduced glutathione concentrations and leucine oxidation rates. This indicates increased AA catabolism by the dietary induction of shared common metabolic pathways involving the enzymes threonine ammonia-lyase (EC 4.3.1.19), serine hydroxymethyltransferase (EC 2.1.2.1) and the glycine cleavage system (EC 2.1.2.10). The δ¹⁵ N_AA values of the HP-E and HP groups were not found to be significantly different.

CONCLUSIONS

The ¹⁵ N-depleted results of threonine are linked to increased activity of threonine ammonia-lyase, and show potential as a possible biomarker for protein intake and/or gluconeogenesis. We hypothesize that the inverse nitrogen equilibrium isotope effects of Schiff base formation, between AAs and pyridoxal-5'-phosphate cofactor enzymes, play a key role in the bioaccumulation and depletion of ¹⁵ N in the biomolecules of living organisms and contributes to the variability in the nitrogen trophic level effect. Copyright © 2017 John Wiley & Sons, Ltd.

Broad-scale trophic shift in the pelagic North Pacific revealed by an oceanic seabird

Human-induced ecological change in the open oceans appears to be accelerating. Fisheries, climate change and elevated nutrient inputs are variously blamed, at least in part, for altering oceanic ecosystems. Yet it is challenging to assess the extent of anthropogenic change in the open oceans, where historical records of ecological conditions are sparse, and the geographical scale is immense. We developed millennial-scale amino acid nitrogen isotope records preserved in ancient animal remains to understand changes in food web structure and nutrient regimes in the oceanic realm of the North Pacific Ocean (NPO). Our millennial-scale isotope records of amino acids in bone collagen in a wide-ranging oceanic seabird, the Hawaiian petrel (Pterodroma sandwichensis), showed that trophic level declined over time. The amino acid records do not support a broad-scale increase in nitrogen fixation in the North Pacific subtropical gyre, rejecting an earlier interpretation based on bulk and amino acid specific δ¹⁵N chronologies for Hawaiian deep-sea corals and bulk δ¹⁵N chronologies for the Hawaiian petrel. Rather, our work suggests that the food web structure in the NPO has shifted at a broad geographical scale, a phenomenon potentially related to industrial fishing.

More surprises in the global greenhouse: Human health impacts from recent toxic marine aerosol formations, due to centennial alterations of world-wide coastal food webs

Reductions of zooplankton biomasses and grazing pressures were observed during overfishing-induced trophic cascades and concurrent oil spills at global scales. Recent phytoplankton increments followed, once Fe-, P-, and N-nutrient limitations of commensal diazotrophs and dinoflagellates were also eliminated by respective human desertification, deforestation, and eutrophication during climate changes. Si-limitation of diatoms instead ensued during these last anthropogenic perturbations of agricultural effluents and sewage loadings. Consequently, ~15% of total world-wide annual asthma trigger responses, i.e. amounting to ~45 million adjacent humans during 2004, resulted from brevetoxin and palytoxin poisons in aerosol forms of western boundary current origins. They were denoted by greater global harmful algal bloom [HAB] abundances and breathing attacks among sea-side children during prior decadal surveys of asthma prevalence, compiled here in ten paired shelf ecosystems of western and eutrophied boundary currents. Since 1965, such inferred onshore fluxes of aerosolized DOC poisons of HABs may have served as additional wind-borne organic carriers of toxic marine MeHg, phthalate, and DDT/DDE vectors, traced by radio-iodine isotopes to potentially elicit carcinomas. During these exchanges, as much as 40% of mercury poisonings may instead have been effected by inhalation of collateral HAB-carried marine neurotoxic aerosols of MeHg, not just from eating marine fish. Health impacts in some areas were additional asthma and pneumonia episodes, as well as endocrine disruptions among the same adjacent humans, with known large local rates of thyroid cancers, physician-diagnosed pulmonary problems, and ubiquitous high indices of mercury in hair, pesticides in breast milk, and phthalates in urine.

Discovery of symbiotic nitrogen fixation and chemoautotrophy in cold-water corals

Cold-water corals (CWC) are widely distributed around the world forming extensive reefs at par with tropical coral reefs. They are hotspots of biodiversity and organic matter processing in the world’s deep oceans. Living in the dark they lack photosynthetic symbionts and are therefore considered to depend entirely on the limited flux of organic resources from the surface ocean. While symbiotic relations in tropical corals are known to be key to their survival in oligotrophic conditions, the full metabolic capacity of CWC has yet to be revealed. Here we report isotope tracer evidence for efficient nitrogen recycling, including nitrogen assimilation, regeneration, nitrification and denitrification. Moreover, we also discovered chemoautotrophy and nitrogen fixation in CWC and transfer of fixed nitrogen and inorganic carbon into bulk coral tissue and tissue compounds (fatty acids and amino acids). This unrecognized yet versatile metabolic machinery of CWC conserves precious limiting resources and provides access to new nitrogen and organic carbon resources that may be essential for CWC to survive in the resource-depleted dark ocean.

Carbon and nitrogen isotope fractionation of amino acids in an avian marine predator, the gentoo penguin (Pygoscelis papua)

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) (Δ¹³C_C-D and Δ¹⁵N_C-D, respectively). We found that essential AA δ¹³C values and source AA δ¹⁵N 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 Δ¹³C_C-D values reflected differences in macromolecule 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‰) that we calculated from the difference in trophic fractionation (Δ¹⁵N_C-D) of glutamic acid and phenylalanine was significantly lower than the conventional literature value of 7.6‰. 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 7.6‰ from the previous literature. Our results provide a quantitative, mechanistic framework for the use of CSIA in nonlethal, archival feathers to study the movement and foraging ecology of avian consumers.

Carbon and nitrogen isotopes from top predator amino acids reveal rapidly shifting ocean biochemistry in the outer California Current

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.

Microbial oceanography and the Hawaii Ocean Time-series programme

The Hawaii Ocean Time-series (HOT) programme has been tracking microbial and biogeochemical processes in the North Pacific Subtropical Gyre since October 1988. The near-monthly time series observations have revealed previously undocumented phenomena within a temporally dynamic ecosystem that is vulnerable to climate change. Novel microorganisms, genes and unexpected metabolic pathways have been discovered and are being integrated into our evolving ecological paradigms. Continued research, including higher-frequency observations and at-sea experimentation, will help to provide a comprehensive scientific understanding of microbial processes in the largest biome on Earth.

Centennial changes in North Pacific anoxia linked to tropical trade winds

Climate warming is expected to reduce oxygen (O2) supply to the ocean and expand its oxygen minimum zones (OMZs). We reconstructed variations in the extent of North Pacific anoxia since 1850 using a geochemical proxy for denitrification (δ15N) from multiple sediment cores. Increasing δ15N since ~1990 records an expansion of anoxia, consistent with observed O2 trends. However, this was preceded by a longer declining δ15N trend that implies that the anoxic zone was shrinking for most of the 20th century. Both periods can be explained by changes in winds over the tropical Pacific that drive upwelling, biological productivity, and O2 demand within the OMZ. If equatorial Pacific winds resume their predicted weakening trend, the ocean’s largest anoxic zone will contract despite a global O2 decline.

Compound-specific δ15N amino acid measurements in littoral mussels in the California upwelling ecosystem: a new approach to generating baseline δ15N Isoscapes for coastal ecosystems

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 (δ¹⁵N_Phe), the best AA proxy for baseline δ¹⁵N values. We hypothesize δ¹⁵N_Phe values in intertidal mussels can approximate annual integrated δ¹⁵N values of coastal phytoplankton primary production. We therefore used δ¹⁵N_Phe 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 δ¹⁵N_Phe 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.