Warming of the Arctic has resulted in environmental and ecological changes, termed borealization, leading to the northward shift of temperate species. Borealization has occurred across all trophic ...levels, altering the structure of the food web. The onset and rate of borealization likely varies with latitude, depending on local warming and advection of warmer water into the Arctic. In order to assess latitudinal trends in food web structure in the Arctic, we analyzed stable nitrogen isotopes of specific amino acids alongside bulk stable carbon isotopes in ringed seal muscle tissue from the Canadian Arctic Archipelago (high-Arctic) and Southern Baffin Bay (mid-Arctic) from 1990 to 2016. Our results indicate a shift in food web structure in the high-Arctic that has occurred more recently when compared with the mid-Arctic. Specifically, over the past 25 years, the trophic position of ringed seals from the mid-Arctic was largely constant, whereas the trophic position of ringed seals decreased in the high-Arctic, reaching similar values observed in the mid-Arctic in 2015–2016. This suggests a potential shortening of the food chain length in the high-Arctic, possibly driven by changes in zooplankton communities feeding complexity in association with sea ice decline. This study identifies a temporal offset in the timing of borealization in the Canadian Arctic, resulting in different response of food webs to ecological changes, depending on latitude.
Nano‐ and picophytoplankton are a major component of open‐ocean ecosystems and one of the main plankton functional types in biogeochemical models, yet little is known about their trace metal ...contents. In cultures of the picoeukaryote Ostreococcus lucimarinus, iron limitation reduced iron quotas by 68%, a fraction of the plasticity known in diatoms. In contrast, a commonly co‐occurring cyanobacterium, Prochlorococcus, showed variable iron contents with iron availability in culture. Synchrotron X‐ray fluorescence was used to measure single‐cell metal (Mn, Fe, Co, Ni, Zn) quotas of autotrophic flagellates (1.4–16.8‐μm diameter) collected from four ocean regions. Iron quotas were tightly constrained and showed little response to iron availability, similar to cultured Ostreococcus. Zinc quotas also did not vary with zinc availability but appeared to vary with phosphorus availability. These results suggest that macronutrient and metal availability may be equally important for controlling metal contents of small eukaryotic open‐ocean phytoplankton.
A simplified cycling and transport model for inorganic nutrients and dissolved organic nitrogen (DON) and phosphorus (DOP) is applied to the Atlantic, on the basis of recent measurements along a ...meridional transect. The DON and DOP are separated into semilabile pools with a lifetime of 6 months and a refractory pool which is conserved in the interior, but photochemically breaks down in the euphotic zone with a lifetime of 6–12 years. The semilabile pool typically only makes up 10% of the surface DON, but up to 95% of DOP. DON and DOP are preferentially produced over upwelling zones in the euphotic zone. Elevated surface concentrations of DON and DOP generally occur over the tropics where the mixed layer is thin, but become diluted at higher latitudes where the mixed layer is thick. There is a northward transport of DON and DOP from the tropics into the northern subtropical gyre, as part of the Ekman wind‐driven circulation and overturning circulation. At the same time, there is a depth‐integrated southward transport of nitrate and phosphate leading to a loss of nitrate and phosphate over the subtropics of the North Atlantic, which is partly compensated by the supply of DON and DOP. Inputs of DON enhance the particle export over the subtropical gyres by typically 0.05 mol N m−2 yr−1, compared with a background model particle export of typically 0.2 mol N m−2 yr−1. Inputs of DOP appear to be more significant (owing to their smaller refractory content) in increasing particle export by typically 12 mmol P m−2 yr−1 over the subtropical gyre, which is sufficiently large to supply half of the phosphorus requirements for particle export.
Fixed nitrogen is a key nutrient involved in regulating global marine productivity and hence the global oceanic carbon cycle. Oceanic nitrogen (N
2) fixation is estimated to supply 8×10
12
moles
N
y
...−1 to the ocean, approximately equal to current riverine and the atmospheric inputs of fixed N, and between 50 and 100% of current estimates of oceanic denitrification. However, the spatial and temporal variability of N
2 fixation remains uncertain, mostly because of the normal low resolution sampling for diazotroph distribution and fixation rates. It is well established that N
2 fixation, mediated by the enzyme nitrogenase, is a source of hydrogen (H
2), but the extent to which it leads to supersaturation of H
2 in oceanic waters is unresolved. Here, we present simultaneous measurements of upper ocean dissolved H
2 concentration (nmol
L
−1), and rates of N
2 fixation (μmol
N
m
−3
d
−1), determined using
15N
2 tracer techniques (at 7 or 15
m), on a transect from Fiji to Hawaii. We find a significant correlation (
r=0.98) between dissolved H
2 and rates of N
2 fixation, with the greatest supersaturation of H
2 and highest rates of N
2 fixation being observed in the subtropical gyres at the southern (∼18°S) and northern (18°N) reaches of the transect. The lowest H
2 saturation and N
2 fixation were observed in the equatorial region between 8°S and 14°N. We propose that an empirical relationship between H
2 supersaturations and N
2 fixation measurements could be used to guide sampling for
15N fixation measurements or to aid the spatial interpolation of such measurements.
Mechanisms supplying nitrogen (N) to phytoplankton, and thus constraining the levels of export production, over the oligotrophic subtropical Atlantic are assessed along a meridional transect. Stable ...nitrogen isotope signals reveal a localized region of N2 fixation over the northern subtropical gyre. Elsewhere, particulate organic nitrogen was isotopically enriched and there was no widespread evidence of a trophic bias. Thus phytoplankton are utilizing an enriched source of N along the transect through much of the oligotrophic Atlantic, which may reflect utilization of nitrate from the deep ocean or, possibly, a supply of dissolved organic nitrogen (DON) from a non‐N2 fixing source. While there is a significant supply of DON over the subtropical gyres, reaching 0.15 mol Nm−2 yr−1, less than 10% of the DON is semilabile and thus only implies a relatively small contribution to the nitrogen supply required for export production. Over the central part of the subtropical gyres, the supply of N to phytoplankton is probably from nitrate in the underlying thermocline, possibly from convection and diapycnic transfer, or more likely, from finescale upwelling by mesoscale eddies and frontal circulations. The lateral supply of dissolved organic phosphorus (DOP) appears to be a factor of 2–3 times more important than the lateral supply of semilabile DON, and thus might play a role in contributing to the phosphorus (P) supply for phytoplankton. The lateral supply of DON and DOP might also be important in closing the N and P budgets over the North Atlantic.
Shelf seas are an important global carbon sink. In the seasonal thermocline, the subsurface chlorophyll maximum (SCM) supports almost half of summer shelf production. Using observations from the ...seasonally stratified Celtic Sea (June 2010), we identify wind‐driven inertial oscillations as a mechanism for supplying the SCM with the nitrate needed for phytoplankton growth and carbon fixation. Analysis of wind, currents, and turbulent dissipation indicates that inertial oscillations are triggered by a change in the wind velocity. High magnitude, short‐lived dissipation spikes occur when the shear and wind vectors align, increasing the daily nitrate flux to the SCM by a factor of at least 17. However, it is likely that the sampling resolution of turbulent dissipation does not always capture the maximum wind‐driven peak in mixing. We estimate that wind‐driven inertial oscillations supply the SCM with ~33% to 71% of the nitrate required for new production in shelf seas during summer.
Key Points
A change in the wind forcing causes inertially driven short‐lived mixing events
These are difficult to resolve but increase the daily nitrate flux 17‐fold
These events may support 33–71% of summer new production in shelf seas
Marine nitrogen fixation is a major source of new nitrogen to the ocean, which interacts with climate driven changes to physical nutrient supply to regulate the response of ocean primary production ...in the oligotrophic tropical ocean. Warming and changes in nutrient supply may alter the ecological niche of nitrogen‐fixing organisms, or ‘diazotrophs’, however, impacts of warming on diazotroph physiology may also be important. Lab‐based studies reveal that warming increases the nitrogen fixation‐specific elemental use efficiency (EUE) of two prevalent marine diazotrophs, Crocosphaera and Trichodesmium, thus reducing their requirements for the limiting nutrients iron and phosphorus. Here, we coupled a new diazotroph model based upon observed diazotroph energetics of growth and resource limitation to a state‐of‐the‐art global model of phytoplankton physiology and ocean biogeochemistry. Our model is able to address the integrated response of nitrogen fixation by Trichodesmium and Crocosphaera to warming under the IPCC high emission RCP8.5 scenario for the first time. Our results project a global decline in nitrogen fixation over the coming century. However, the regional response of nitrogen fixation to climate change is modulated by the diazotroph‐specific thermal performance curves and EUE, particularly in the Pacific Ocean, which shapes global trends. Spatially, the response of both diazotrophs is similar with expansion towards higher latitudes and reduced rates of nitrogen fixation in the lower latitudes. Overall, 95%–97% of the nitrogen fixation climate signal can be attributed to the combined effect of temperature on the niche and physiology of marine diazotrophs, with decreases being associated with a reduced niche and increases resulting due to a combination of expanding niche and temperature driven changes to EUE. Climate change impacts on both the niche and physiology of marine diazotrophs interact to shape patterns of marine nitrogen fixation, which will have important implications for ocean productivity in the future.
To explore the impact of global change on marine nitrogen fixation, thermal performance curves of growth and resource limitation for two prevalent marine nitrogen‐fixing cyanobacteria were incorporated into a state‐of‐the‐art Earth System Model for the first time. Marine nitrogen fixation is predicted to decrease globally over the coming century, with the combined effect of temperature on the niche and physiology of marine diazotrophs explaining 95%–97% of the nitrogen fixation climate signal. Climate change impacts on both the niche and physiology on marine diazotrophs interact to shape patterns of marine nitrogen fixation, with important implications for ocean productivity in the future.
•Seasonality and cross shelf distribution of DOM and its optical properties were examined in the Celtic Sea.•Strong cross shelf gradients in humic DOM components with significant negative ...correlations with salinity and DOC.•Variability in labile DOM protein components linked to biological productivity.
The Celtic Sea is a productive temperate sea located on the Northwest European Shelf. It is an important pathway for the delivery of land-derived material to the North Atlantic Ocean, including dissolved organic matter (DOM). The aim of this study was to determine the seasonal and spatial variability in the magnitude, source and composition of DOM at three sites representing on shelf, central shelf and shelf edge regions in the Celtic Sea, using observations collected during the UK Shelf Sea Biogeochemistry (SSB) research programme (November 2014 – August 2015). The concentration of dissolved organic carbon (DOC) alongside DOM absorbance and fluorescence indices were measured and fluorescence Excitation and Emission Matrices (EEMs) combined with Parallel Factor Analysis (PARAFAC) were used to assess DOM composition and lability. The PARAFAC model identified four unique fluorescent components for autumn (November 2014), winter (March 2015), spring (April 2015) and summer (July 2015) consisting of two humic-like components attributed to terrestrial (C1) and marine sources (C2), and two protein components identified as tyrosine-like (C3) and tryptophan-like (C4) attributed to in situ production. DOC varied seasonally and there were strong cross shelf trends. The protein components (C3 and C4) exhibited large seasonal and within season variability particularly during productive periods. In contrast, there were persistent cross shelf gradients in the CDOM absorption coefficient at 305 nm (a305), the UV specific absorbance at 280 nm (SUVA280), the humification index (HIX), and the humic-like fluorescent components (C1 and C2), which were higher in the on shelf region and decreased towards the shelf edge. The humic-like components and the slope ratio (SR) were significantly correlated with salinity throughout all seasons, indicating a strong influence of terrestrially-derived organic matter in the Celtic Sea, with potentially up to 35% of DOC in the central shelf during winter originating from terrestrial inputs. Results from this study illustrate the importance of monitoring DOM quantitatively and qualitatively for a better understanding of the supply, production, cycling and export of this dynamic organic carbon pool in shelf seas.
•Seasonal dissolved and particulate organic matter pools examined in the Celtic Sea.•Organic carbon (C) concentrations highest in spring and lowest in autumn.•Nutrient (C, N and P) dynamics decoupled ...with seasons and during bloom events.•Dissolved organic matter always carbon-rich relative to particulates.•DOC dominates downward C flux in summer, POC dominates downward flux in spring.
Organic matter (OM) plays an important role in productive shelf seas and their contribution to global carbon (C) and nutrient cycles. We investigated dissolved and particulate OM (DOM and POM, respectively) dynamics over a seasonal cycle in the Celtic Sea. The quantity of OC was largest during the spring bloom and lowest in autumn. DOM was always C rich relative to the POM pool and the Redfield ratio (106C:16N:P). There was clear decoupling between C, N and P and the response of OM composition to different seasons and nutrient statuses of the microbial community. The C:P stoichiometry was much more variable than the C:N stoichiometry, which was near constant. Downward OC fluxes were dominated by POM during bloom events and DOM during the stratified summer. In terms of partitioning, 92–96% of OC was in the DOM pool throughout sampling, which given its high C:N (12.4–17) suggests it was an efficient vehicle for potential off-shelf export of C during winter mixing.
Nitrogen stable isotopes (δ
15
N) are used to study food web and foraging dynamics due to the step-wise enrichment of tissues with increasing trophic level, but they rely on the isoscape baseline ...that varies markedly in the Arctic due to the interplay between Atlantic- and Pacific-origin waters. Using a hierarchy of simulations with a state-of-the-art ocean-biogeochemical model, we demonstrate that the canonical isotopic gradient of 2–3‰ between the Pacific and Atlantic sectors of the Arctic Ocean has grown to 3–4‰ and will continue to expand under a high emissions climate change scenario by the end of the twenty-first century. δ
15
N increases in the Pacific-influenced high Arctic due to increased primary production, while Atlantic sector decreases result from the integrated effects of Atlantic inflow and anthropogenic inputs. While these trends will complicate longitudinal food web studies using δ
15
N, they may aid those focussed on movement as the Arctic isoscape becomes more regionally distinct.
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