北海道大学・化学物質循環学分野・地球環境・炭素循環・地球温暖化

Distribution of picoplankton in the northeastern South China Sea with special reference to the effects of the Kuroshio intrusion and the associated mesoscale eddies. Sci. Total Environ. 589, 1−10 (2017).
J. Li^1,3,4, X. Jiang^1,3,4 , G. Li^1,4 , Z. Jing² , L. Zhou^1,4 , Z. Ke^1,4 and Y. Tan^{1, 4} (¹LMB, SCSIO, CAS, ²LTO, SCSIO, CAS, ³UCAS, ⁴LAMB)

 We investigated picoplankton distribution patterns and environmental variables along an east-to-west transect in the northeastern South China Sea (SCS) during late winter 2016, giving us the opportunity to examine the impacts of the Kuroshio intrusion and the associated eddies. The results indicated that the subsurface (50–75 m) phytoplankton biomass chlorophyll (Chl a) maximum (SCM) disappeared and was replaced by higher Chl a in the middle part of the transect due to the impacts of the Kuroshio intrusion and mesoscale eddies. Both flow cytometry and pyrosequencing data revealed that picoplankton abundance and community structure were significantly influenced by perturbations in complex physical processes. Picoeukaryotes represented most of the total phytoplankton biomass, and their maximum abundance (> 10⁴ cells mL⁻¹ ) occurred within cyclonic eddy-affected regions (Stations 11 and 12), whereas the abundance of Prochlorococcus was the lowest in these regions. Prochlorococcus showed a higher abundance in the Kuroshio-affected area, while Synechococcus was mostly distributed at the upper well-lit depths, with its maximum abundance observed in surface waters (0–30 m) adjacent to the cyclonic eddy center. Heterotrophic bacteria (HBA) displayed high abundance along the transect, consistent with the total phytoplankton biomass. Phylogenetic analysis revealed 26 bacterial phyla, with major components belonging to Proteobacteria, Cyanobacteria, Actinobacteria, and Bacteroidetes, as well as SAR406. Notably, relatively more Rhodobacterales, Flavobacteriales, Alteromonadales, and Vibrionales that were distributed in surface waters of the cyclonic eddy center were specifically associated with the phytoplankton (mainly picoeukaryotes) bloom. Our study highlights the impacts of the Kuroshio intrusion in regulating the microbial ecology of the northeastern SCS and the potential coupling between phytoplankton and bacteria.

Widespread Anthropogenic Nitrogen in Northwestern Pacific Ocean Sediment. Environ. Sci. Technol., 51, 6044–6052 (2017).
H. Kim¹, K. Lee², D. Lim², S. Nam³, T. Kim⁴, J. T. Yang¹, Y. H. Ko¹, K. Shin⁵ and E. Lee⁶ (¹DESE, Pohang University of Science and Technology, ²SSRI, Korea Institute of Ocean Science and Technology, ³ARC, Korea Polar Research Institute, ⁴Department of Marine Science, Incheon National University, ⁵Department of Marine Sciences and Convergent Technology, Hanyang University, ⁶Ocean Research Division, Korea Hydrographic and Oceanographic Agency)

 Sediment samples from the East China and Yellow seas collected adjacent to continental China were found to have lower δ¹⁵N values (expressed as δ¹⁵N = [¹⁵N:¹⁴N_sample/¹⁵N:¹⁴N_air –1] × 1000‰; the sediment ¹⁵N:¹⁴N ratio relative to the air nitrogen ¹⁵N:¹⁴N ratio). In contrast, the Arctic sediments from the Chukchi Sea, the sampling region furthest from China, showed higher δ¹⁵N values (2–3‰ higher than those representing the East China and the Yellow sea sediments). Across the sites sampled, the levels of sediment δ¹⁵N increased with increasing distance from China, which is broadly consistent with the decreasing influence of anthropogenic nitrogen (N^ANTH) resulting from fossil fuel combustion and fertilizer use. We concluded that, of several processes, the input of N^ANTH appears to be emerging as a new driver of change in the sediment δ¹⁵N value in marginal seas adjacent to China. The present results indicate that the effect of N^ANTH has extended beyond the ocean water column into the deep sedimentary environment, presumably via biological assimilation of N^ANTH followed by deposition. Further, the findings indicate that N^ANTH is taking over from the conventional paradigm of nitrate flux from nitrate-rich deep water as the primary driver of biological export production in this region of the Pacific Ocean.

Potential use of the N₂/Ar ratio as a constraint on the oceanic fixed nitrogen loss, Global Biogeochem. Cycles, 30, 576–594 (2016).
M. Shigemitsu^1,2, N. Gruber³, A. Oka⁴ and Y. Yamanaka¹ ( ¹EES, Hokkaido University, ²Global Chemical and Physical Oceanography Group, RCGC, JAMSTEC, ³Environmental Physics Group, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, ⁴AORI, The University of Tokyo)

 Using a global ocean biogeochemical model, we investigate the suitability of the N₂/Ar supersaturation ratio (ΔN₂/Ar) as a tracer of marine nitrogen fixation and denitrification, i.e., the main biological processes that add or remove fixed nitrogen to or from the ocean. In a series of factorial simulations, we demonstrate that, in regions away from the oxygen minimum zones (OMZs), the ΔN₂/Ar characteristics are mostly determined by benthic denitrification occurring in the deep ocean with minor contributions from benthic and water column denitrification at shallower depths. In the OMZs, the subsurface maxima of ΔN₂/Ar are mainly determined by water column denitrification. In contrast, nitrogen fixation has little impact on ΔN₂/Ar owing to the rapid loss of the N₂ supersaturation signal through air–sea exchange. We thus conclude that ΔN₂/Ar can act as a powerful constraint on water column and benthic denitrification occurring in intermediate to deep waters, but it cannot be used to estimate nitrogen fixation. A comparison between the currently very limited observations of the ΔN₂/Ar with our model results shows an acceptable level of agreement, suggesting that the model's prescribed rates and distributions of benthic and water column denitrification (i.e., 140 and 52 Tg N yr⁻¹ , respectively) are reasonable and confirm the results derived from other constraints.

Tracing denitrification in the Canada Basin: N₂ loss to the atmosphere on the Chukchi Shelf and benthic inputs in deep waters. Deep-Sea Res. Part I, in press. https://doi.org/10.1016/j.dsr.2018.11.003
J. L. Reeve^1,2, R. C Hamme¹, W. J Williams³ (¹School of Earth and Ocean Sciences, University of Victoria, ²Department of Geological Sciences, University of Colorado, ³Fisheries and Oceans Canada, Institute of Ocean Sciences)

 The global marine fixed nitrogen budget acts as a strong control on oceanic primary productivity, and the Arctic plays a disproportionately large role in the sink terms for this budget. This paper aims to quantify the impact of nitrogen cycling on the Canada Basin, utilizing two tracers of denitrification: N₂/Ar, a dissolved gas tracer, and N^*, a nutrient ratio tracer. In the Pacific Winter Water (PWW), which forms in the Chukchi Sea, we observe a disconnect between N₂/Ar and N^*, where the excess N₂ expected from N^* observations is far larger than the N₂ excess we measure. We show that loss of N₂ to the atmosphere through ventilation on the Chukchi Shelf likely accounts for this disparity, highlighting the importance of using N₂/Ar as a denitrification tracer only in isolated water masses. We additionally observe increasing N₂/Ar and decreasing N^* in the old deep waters of the Canada Basin, suggesting benthic denitrification has been operating in the deep sediments over the 500-year age of this water mass. We use a one-dimensional vertical reaction-diffusion model to estimate denitrification rates of 0.0053–0.0130 mmol N m⁻² d ⁻¹ , or 0.04–0.1 Tg N y^-1 integrated over the whole basin, which is about half the rates estimated for other deep basins, in-line with lower remineralization rates in the deep Canada Basin. Further measurements of these tracers in the Arctic, particularly directly in the Chukchi Sea, will help constrain the relative importance of physical vs. biological processes on N₂ in this region.

Variability in timing and magnitude of spring bloom in the Oyashio region, the western subarctic Pacific off Hokkaido, Japan. Fish. Oceanogr. 6, 118–129 (1997).
H. Kasai¹, H. Saito¹, A. Yoshimori² and S. Taguchi^1,3 (¹HNF, ²Department of Physics, Nagoya University, ³Faculty of Engineering, Soka University)

 The spring bloom of phytoplankton is a well-established, regular, seasonal event in the western subarctic Pacific and is considered one of the most important conditions of massive production of pelagic fishes. A series of 12 cruises was conducted from 1990 to 1992 to examine the timing and magnitude of the spring phytoplankton bloom in the Oyashio region, the western subarctic Pacific off Hokkaido, Japan. An interannual variability in the bloom events was also analysed. On the basis of hydrographical characteristics, the study area was divided into three water masses: the Oyashio Water Mass, the Mixed Water Mass, and the Coastal Water Mass. Spring blooms were observed first in April in the Oyashio and the Coastal Water Masses, and continued to May in 1991 and 1992. However, no bloom was recorded in the Mixed Water Mass. High nutrient supply into the surface mixed layer during winter is likely to be one of the factors supporting an intense spring bloom in the Oyashio Water Mass. A significant positive relationship between log-transformed surface chlorophyll a concentration and maximum density gradient (MDG) within the euphotic layer was obtained in April, indicating the importance of vertical stability of the water column in the initiation of spring blooms in the Oyashio and the Coastal Water Masses. The spring blooms in 1991 were much more extensive and lasted longer than in 1990. It is suggested that meteorological conditions and abundance of grazers were responsible for this interannual difference.

Temporal variability of Trichodesmium spp. and diatom-diazotroph assemblages in the North Pacific subtropical gyre. Front. Mar. Sci., 5, 27 (2018).
A. E. White¹, K. S. Watkins-Brandt¹ and M. J. Church² (¹CEOAS, Oregon State University, ²FLBS, University of Montana)

  In oligotrophic ocean regions such as the North Pacific Subtropical Gyre (NPSG), N₂ fixation (i.e., diazotrophy) by a diverse consortium of microorganisms has been shown to contribute significantly to new production and particle export. In 2015 and 2016, we measured near-monthly abundances of the large cell-sized (> 10 µm) diazotrophic genera Trichodesmium and diatom-associated Richelia and Calothrix spp. in the NPSG via microscopy and quantitative PCR of nifH genes. Of these genera, we find Trichodesmium to be the more abundant over our study period, with cell concentrations in the upper water column (0–45 m) ranging from 1 to 5,988 cells L⁻¹ , while the sum of Richelia and Calothrix spp. abundances ranged from 4 to 157 heterocysts L⁻¹ . Significant discrepancies between absolute abundances were noted between cell and gene-based approaches to biomass determination (nifH copies L⁻¹ were up to 10²–10³ higher than cell concentrations). Potential explanations for these striking discrepancies are discussed. Using the maximum N fixation rates per cell found in the existing literature for these genera, we estimate potential N₂ fixation rates via these large diazotroph communities to be between 0.01 and 1.5 nmol N L⁻¹ d⁻¹. When comparing these rates to available ¹⁵N₂ tracer measurements, we conclude that large diazotrophs were generally minor (<10 %) contributors to bulk N₂ fixation in the surface ocean during our study period. Conversely, high concentrations of Trichodesmium observed in fall-winter of 2015 and 2016 were estimated to drive >50 % of measured N₂ fixation rates. While these large cell-sized and heterogeneously distributed organisms may still disproportionately contribute to export, cell-abundance based rate estimates suggests that other diazotrophs are largely responsible for N₂ fixation rates measured in bottle-based incubations.

Effects of an iron-light co-limitation on the elemental composition (Si, C, N) of the marine diatoms Thalassiosira oceanica and Ditylum brightwellii. Biogeosciences, 7, 657-669 (2010).
E. Bucciarelli, P. Pondaven, and G. Sarthou (Université Européenne de Bretagne, Université de Brest, Technôpole Brest Iroise)

  We examined the effect of iron (Fe) and Fe-light (Fe-L) co-limitation on cellular silica (BSi), carbon (C) and nitrogen (N) in two marine diatoms, the small oceanic diatom Thalassiosira oceanica and the large coastal species Ditylum brightwellii. We showed that C and N per cell tend to decrease with increasing Fe limitation (i.e. decreasing growth rate), both under high light (HL) and low light (LL). We observed an increase (T. oceanica, LL), no change (T. oceanica, HL) and a decrease (D. brightwellii, HL and LL) in BSi per cell with increasing degree of limitation. The comparison with literature data showed that the trend in C and N per cell for other Fe limited diatoms was similar to ours. Interspecific differences in C and N quotas of Fe limited diatoms observed in the literature seem thus to be mostly due to variations in cell volume. On the contrary, there was no global trend in BSi per cell or per cell volume, which suggests that other interspecific differences than Fe-induced variations in cell volume influence the degree of silicification. The relative variations in C:N, Si:C and Si:N versus the relative variation in specific growth rate (i.e. µ:µ_max) followed the same patterns for T. oceanica and D. brightwellii, whatever the irradiance level. However, the variations of C:N under Fe limitation reported in the literature for other diatoms are contrasted, which may thus be more related to growth conditions than to interspecific differences. As observed in other studies, Si:C and Si:N ratios increased by more than 2-fold between 100% and 40% of µ_max. Under more severe limitation (HL and LL), we observed for the first time a decrease in these ratios. These results may have important biogeochemical implications on the understanding and the modelling of the oceanic biogeochemical cycles, e.g. carbon and silica export.

An integrated study of photochemical function and expression of a key photochemical gene (psbA) in photosynthetic communities of Lake Bonney (McMurdo Dry Valleys, Antarctica). FEMS Microbiol. Ecol. 89, 293-302 (2014).
W. Kong¹, W. Li¹, I. Romancova², O. Prášil² and R. Morgan-Kiss¹ (¹MBI, Miami University, ²Laboratory of Photosynthesis, Algatech, Institute of Microbiology ASCR)

  Lake Bonney is one of several permanently ice-covered lakes in the McMurdo Dry Valleys, Antarctica, which maintain the only year-round biological activity on the Antarctic continent. Vertically stratified populations of autotrophic microorganisms occupying the water columns are adapted to numerous extreme conditions, including very low light, hypersalinity, ultra-oligotrophy and low temperatures. In this study, we integrated molecular biology, microscopy, flow cytometry, and functional photochemical analyses of the photosynthetic communities residing in the east and west basins of dry valley Lake Bonney. Diversity and abundance of the psbA gene encoding a major protein of the photosystem II reaction center were monitored during the seasonal transition between Antarctic summer (24-h daylight) to winter (24-h darkness). Vertical trends through the photic zone in psbA abundance (DNA and mRNA) closely matched that of primary production in both lobes. Seasonal trends in psbA transcripts differed between the two lobes, with psbA expression in the west basin exhibiting a transient rise in early Fall. Last, using spectroscopic and flow cytometric analyses, we provide the first evidence that the Lake Bonney photosynthetic community is dominated by picophytoplankton that possess photosynthetic apparatus adapted to extreme shade.

Detecting phytoplankton diatom fraction based on the spectral shape of satellite-derived algal light absorption coefficient. Limnol. Oceanogr. 63(S1), S85–S98 (2018).
G. Zheng^1,2 and P. DiGiacomo¹ (¹NOAA/NESDIS, STAR, ²GST)

  Knowledge about phytoplankton composition is important for biological and biogeochemical research as well as for ecological applications (e.g., water quality) in coastal and inland waters. Satellite remote sensing can potentially map the baseline patterns, anomalies, and trends of phytoplankton composition on a synoptic basis. A prominent challenge is the attribution of the total optical signal to phytoplankton amid interference from minerals and humus. Here, we obtained the phytoplankton light absorption coefficient, a_ph(λ), in the Chesapeake Bay by partitioning satellite-derived total light absorption coefficient of water using the generalized stacked-constraints model (GSCM). We show that the red-to-blue band ratio of GSCM-derived a_ph(670)/a_ph(440) can be associated with diatom fraction in Chesapeake Bay. Further, the spatial-temporal patterns shown in the satellite-derived diatom fraction data agree well with field studies conducted previously around this region, including low diatom dominance in summer, high diatom dominance in the lower bay in winter, diatom-dominated spring blooms in coastal waters outside of the bay, and increasing seasonal variability of diatom fraction from the upper to the lower bay. We also found that in the middle bay the summer diatom fraction correlates strongly with spring streamflow on an annual basis, which can be explained because sediment deposited by spring freshets is the main source of silicate supply during summer. These results suggest that the satellite-derived diatom fraction maps can serve as a baseline for detecting phytoplankton composition anomalies, and highlight the effectiveness of using absorption-based approach to extract phytoplankton composition information for optically complex waters.

Increase of dissolved inorganic carbon and decrease in pH in near-surface waters in the Mediterranean Sea during the past two decades. Biogeosciences, 15, 5653-5662 (2018).
L. Merlivat¹, J. Boutin¹, D. Antoine^2,3, L. Beaumont4, M. Golbol³, and V. Vellucci³ (¹Sorbonne Université-CNRS-IRD-MNHN, LOCEAN, ²RSSRG, School of Earth and Planetary Sciences, Curtin University, ³Sorbonne Université-CNRS, LOV, ⁴DT INSU-CNRS)

  Two 3-year time series of hourly measurements of the fugacity of CO₂ (f CO₂) in the upper 10 m of the surface layer of the northwestern Mediterranean Sea have been recorded by CARIOCA sensors almost two decades apart, in 1995–1997 and 2013–2015. By combining them with the alkalinity derived from measured temperature and salinity, we calculate changes in pH and dissolved inorganic carbon (DIC). DIC increased in surface seawater by ∼25 µmol kg⁻¹ and f CO₂ by 40 µatm, whereas seawater pH decreased by ∼0.04(0.0022 yr⁻¹). The DIC increase is about 15% larger than expected from the equilibrium with atmospheric CO₂. This could result from natural variability, e.g. the increase between the two periods in the frequency and intensity of winter convection events. Likewise, it could be the signature of the contribution of the Atlantic Ocean as a source of anthropogenic carbon to the Mediterranean Sea through the Strait of Gibraltar. We then estimate that the part of DIC accumulated over the last 18 years represents ∼30% of the total inventory of anthropogenic carbon in the Mediterranean Sea.

Very low isotope ratio of iron in fine aerosols related to its contribution of the surface ocean. J. Geophys. Res. Atmos., 121, 119-136 (2016).
M. Kurisu¹, Y. Takahashi¹, T. Iizuka¹ and M. Uematsu² (¹Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, ²AORI, University of Tokyo)

  Seven size-fractionated aerosol samples were collected from Hiroshima, Japan, and were analyzed in terms of chemical composition, soluble fraction of iron (Fe), Fe species, and Fe isotope ratios. The results suggested that Fe in fine particles contained a larger fraction of anthropogenic aerosols than coarse particles did. Iron in the fine particles was more soluble in simulated seawater (up to 25 %) than that in the coarse particles and was in the form of Fe (hydr)oxide species, such as ferrihydrite or hematite. The Fe isotope ratios (δ⁵⁶ Fe) of the coarse particles (+0.04 ‰ to +0.30 ‰) were close to the crustal mean value (0.0 ‰). By contrast, the δ⁵⁶Fe values of fine particles were much lower and ranged from -2.01 ‰ to -0.56 ‰. δ⁵⁶Fe values of the soluble Fe fraction in the fine particles were remarkably low (-3.91 to -1.87 ‰), suggesting that anthropogenic aerosols yield soluble Fe with low δ⁵⁶Fe values. Such low values could be explained by kinetic isotope fractionation during evaporation of Fe at high temperatures, coupled with the refractory characteristics of Fe. Marine aerosols from the Northwest Pacific were also analyzed. The δ⁵⁶Fe values in the fine particles were also lower than those in the coarse particles. These results may be important to quantitatively estimate the contribution of anthropogenic Fe deposited on the surface ocean on the basis of the Fe isotopes.

Pigment composition and photoprotection of Arctic sea ice algae during spring. Mar. Ecol. Prog. Ser., 585, 49–69 (2017).
V. Galindo¹, M. Gosselin², J. Lavaud³, C. J. Mundy¹, B. Else⁴, J. Ehn¹, M. Babin³, S. Rysgaard^1,5,6 (¹CEOS, Faculty of Environment, Earth and Resources, University of Manitoba, ²ISMER, Université du Québec à Rimouski, ³L’UMI TAKUVIK, Département de biologie, CNRS/Université Laval, Québec-Océan, ⁴Department of Geography, University of Calgary, 5ARC, Aarhus University, 6GCRC, Greenland Institute of Natural Resources

  From the beginning of spring to the melt period, ice algae in the bottom of Arctic sea ice experience a large irradiance range, varying from <0.1% up to 25–30% of the incoming visible radiation. The increase in spring is usually rapid, with a varying photoacclimative response by bottom ice algae to protect themselves against excess light, such as changes in cellular pigment composition. This study focused on the temporal variation in pigment composition of bottom ice algae under 2 contrasting snow depths (thin and thick) during spring. Controlled experiments were also carried out to investigate the photoprotective capacity of ice algae to relatively high irradiances during a short-term period (<6h). Ice algae were able to photoacclimate rapidly and effectively to irradiance ranging from 10 to 100 µmol photons m^-2 s^-1. However, we observed contrasting responses in photoacclimation depending on the ice algal community composition and their light history. Our experimental results suggest that the xanthophyll cycle (diadinoxanthin to diatoxanthin conversion) and D1-protein recycling play an important role in stabilizing photoprotection in ice algae. In addition, bottom ice algae likely employed a ‘cellular light-exposure memory’ strategy in order to improve their photoacclimative response to changing light exposure. According to our data, this process could be maintained over at least 2 wk. Hence, ice algae may be more resilient to varying light conditions than previously thought and may be well-adapted for the expected future light regime changes associated with variability in snow and sea ice cover.

Multi-decadal variations in the oceanic CO₂ uptake and biogeochemical parameters over the northern and southern high latitudes. Polar Sci, doi:10.1016/j.polar.2018.05.008 (2018, in press).
V. Pant¹, J. Moher¹ and V. Seelanki¹ (¹CAS, IIT Delhi)

 The Community Earth System Model with the biogeochemistry module (CESM1-BGC) was used with the historical and RCP8.5 scenario to examine the spatial and temporal variability in the oceanic CO₂ flux and biogeochemical parameters for the years 1850–2100. The 10-year periods of 1850–1860, 2010–2020, and 2090–2100 were used to represent the oceanic conditions in the past (historical), present, and future climate, respectively. The model simulations showed interesting differences between CO₂ flux and biogeochemical parameters in the northern (NH) and southern (SH) high latitudes oceanic regions. The sea surface temperature increases monotonically during 1960–2100 in both the hemispheres. The enhanced CO₂ concentrations in the air lead to an increase in CO₂ flux into the SH high-latitudes resulting into increasing oceanic acidification. However, over the NH high latitudes, the increase in CO₂ flux ceased by the year 2050 and decreased during 2080–2100. This decrease in the CO₂ flux of Arctic waters could be associated with the freshwater addition due to ice-melt that reduces the mixed layer depth in northern high latitudes and decreases CO₂ uptake in future climate. The biological productivity (Chl concentrations) in the NH high latitudes found to decrease rapidly (2.5 mg m^-3 lower than present) during 2000–2100. The time-series analysis of biogeochemical parameters at three regions in each hemisphere reveals marked differences over different oceanic regions within the same hemisphere in addition to the inter-hemispheric differences. The changes in large-scale overturning circulation and enhanced stratification leads to a reduction in nutrient supply to surface waters from deeper layers and decreases Chl concentrations in the southern ocean. The multi-decadal spatiotemporal variability in physical and biogeochemical parameters are discussed in terms of their inter-dependence, oceanic processes, air-sea exchange in the warming climate.

Expansion of denitrification and anoxia in the eastern tropical North Pacific from 1972 to 2012. Geophys. Res. Lett., 43, 5252–5260 (2016).
R. E. A. Horak¹, W. Ruef¹, B. B. Ward² and A. H. Devol¹ (¹School of Oceanography, University of Washington, ²Geosciences Department, Princeton University)

 The eastern tropical North Pacific (ETNP) is a large region of anoxic water that hosts widespread water column N loss (denitrification). There is some disagreement about the long-term trends of denitrification and anoxia and long-term studies of water column denitrification within the anoxic zone are lacking. In this study, we compared ETNP water column nitrite, N*, and O₂ data along the same transect for four studies ranging from 1972 to 2012. Anoxic water volume increased, and lowoxygen conditions expanded into shallower isopycnals from 1972 to 2012. A geochemical marker for cumulative N loss indicates that denitrification was highest in 2012 and the upper oxygen-deficient zone (ODZ) experienced the most change. Oxygen and N loss changes in the world’s largest ODZ for 2012 could not be explained by the Pacific Decadal Oscillation, and decreased O₂ in supply currents and increased wind-driven upwelling are likely mechanisms contributing to increased N loss and anoxia.

A reevaluation of the magnitude and impacts of anthropogenic atmospheric nitrogen inputs on the ocean, Global Biogeochem. Cycles, 31, (2), 289–305 (2017).
T. D. Jickells¹, E. Buitenhuis¹, K. Altieri², A. R. Baker¹, D. Capone³, R. A. Duce⁴, F. Dentener⁵, K.Fennel⁶, M. Kanakidou⁷, J. LaRoche⁸, K. Lee⁹, P. Liss¹, J. J. Middelburg¹⁰, J. K.Moore¹¹, G. Okin¹², A. Oschlies¹³, M. Sarin¹⁴, S. Seitzinger¹⁵, J. Sharples¹⁶, A. Singh¹⁴, P. Suntharalingam¹, M.Uematsu¹⁷ and L. M. Zamora^18,19 (¹School of Environmental Science, UEA, ²ERC, UCT, ³Department of Biological Sciences, USC, ⁴Departments of Oceanography and Atmospheric Sciences, TAMU, ⁵DGJRC, ⁶Department of Oceanography, Dalhousie University, ⁷Department of Chemistry, University of Crete, ⁸Department of Biology, Dalhousie University, ⁹SEE, POSTECH, ¹⁰Faculty of Geosciences, University of Utrecht, ¹¹ESS, University of California, ¹²Department of Geography, University of California, ¹³GEOMAR, ¹⁴Geosciences Division, PRL, ¹⁵Department of Environmental Studies, UVic, ¹⁶School of Environmental Sciences, University of Liverpool, ¹⁷AORI, University of Tokyo, ¹⁸GSFC, NASA, ¹⁹USRA)

 We report a new synthesis of best estimates of the inputs of fixed nitrogen to the world ocean via atmospheric deposition and compare this to fluvial inputs and dinitrogen fixation. We evaluate the scale of human perturbation of these fluxes. Fluvial inputs dominate inputs to the continental shelf, and we estimate that about 75% of this fluvial nitrogen escapes from the shelf to the open ocean. Biological dinitrogen fixation is the main external source of nitrogen to the open ocean, i.e., beyond the continental shelf. Atmospheric deposition is the primary mechanism by which land-based nitrogen inputs, and hence human perturbations of the nitrogen cycle, reach the open ocean. We estimate that anthropogenic inputs are currently leading to an increase in overall ocean carbon sequestration of similar to 0.4% (equivalent to an uptake of 0.15 Pg Cyr^-1 and less than the Duce et al. (2008) estimate). The resulting reduction in climate change forcing from this ocean CO₂ uptake is offset to a small extent by an increase in ocean N₂O emissions. We identify four important feedbacks in the ocean atmosphere nitrogen system that need to be better quantified to improve our understanding of the perturbation of ocean biogeochemistry by atmospheric nitrogen inputs. These feedbacks are recycling of (1) ammonia and (2) organic nitrogen from the ocean to the atmosphere and back, (3) the suppression of nitrogen fixation by increased nitrogen concentrations in surface waters from atmospheric deposition, and (4) increased loss of nitrogen from the ocean by denitrification due to increased productivity stimulated by atmospheric inputs.

Light absorption by phytoplankton in the North Pacific Subtropical Gyre, Limnol. Oceanogr. 62, 1526-1540 (2017).
R. M. Letelier^1,2, A. E. White^1,2, R. R. Bidigare², B. Barone^2,3, M. J. Church^2,3,4, D. M. Karl^2,3 (¹ CEOAS, OSU, ²Daniel K. Inouye Center for Microbial Oceanography: Research and Education, ³Department of Oceanography, UH, ⁴FLBS, UM)

 To constrain the energy fueling photosynthesis in the North Pacific Subtropical Gyre (NPSG) we characterize the variability of phytoplankton absorption spectra in conjunction with that of the light field at Station ALOHA (22°45’N, 158°00’W). Furthermore, we decompose the phytoplankton absorption into photosynthetic and photoprotective components based on high-performance liquid chromatography pigment analysis. Between January 2006 and December 2012 the variability in chlorophyll-specific absorption (a^*_Φ) above the deep chlorophyll maximum (DCM) layer was driven by changes in photoprotective carotenoid concentrations while the chlorophyll-specific absorption of photosynthetic pigments (a^*_Φ(PSP) ) remained nearly constant with a mean (± SD) value of 0.008± 0.001 m² (mg chl a) ^-1 . In contrast, below the DCM layer changes in a^*_Φ resulted from increases in the relative contribution of photosynthetic pigments with depth, suggesting that the constancy in a^*_Φ(PSP) above the DCM layer is controlled by nutrient limitation. While the daily photon fluxes absorbed by photosynthetic pigments in the upper 45 m did not vary at a seasonal scale, averaging 0.45± 0.12 mol quanta m^-2 d^-1 in winter and 0.46± 0.10 mol quanta m^-2 d^-1 in summer, when integrated over the upper 200 m these fluxes ranged from 0.64± 0.16 to 0.79± 0.19 mol quanta m^-2 d^-1 in winter and summer, respectively. Based on the rate of photons trapped by the photosynthetic pigments and on the seasonal euphotic zone depth integrated gross O₂ evolution rates derived from H₂¹⁸O in situ incubations we estimate a mean photosynthetic yield of ~0.1 mol O₂ evolved per mol quanta absorbed by photosynthetic pigments.

Nutrient limitation suppresses the temperature dependence of phytoplankton metabolic rates, ISME J., 12, 1836-1845 (2018).
E. Marañón¹, M. P. Lorenzo¹, P. Cermeño² and B. Mouriño-Carballido¹ (¹Departamento de Ecologíay Biología Animal, Universidade de Vigo, ²ICM, CSIC)

 Climate warming has the potential to alter ecosystem function through temperature-dependent changes in individual metabolic rates. The temperature sensitivity of phytoplankton metabolism is especially relevant, since these microorganisms sustain marine food webs and are major drivers of biogeochemical cycling. Phytoplankton metabolic rates increase with temperature when nutrients are abundant, but it is unknown if the same pattern applies under nutrient-limited growth conditions, which prevail over most of the ocean. Here we use continuous cultures of three cosmopolitan and biogeochemically relevant species (Synechococcus sp., Skeletonema costatum and Emiliania huxleyi) to determine the temperature dependence (activation energy, E_a) of metabolism under different degrees of nitrogen (N) limitation. We show that both CO₂ fixation and respiration rates increase with N supply but are largely insensitive to temperature. E_a of photosynthesis (0.11±0.06 eV, mean±SE) and respiration (0.04±0.17 eV) under N-limited growth is significantly smaller than E_a of growth rate under nutrient-replete conditions (0.77±0.06 eV). The reduced temperature dependence of metabolic rates under nutrient limitation can be explained in terms of enzyme kinetics, because both maximum reaction rates and halfsaturation constants increase with temperature. Our results suggest that the direct, stimulating effect of rising temperatures upon phytoplankton metabolic rates will be circumscribed to ecosystems with high-nutrient availability.

Diversity and activity of nitrogen-fixing communities across ocean basins, Limnol. Oceanogr., 62, 1805-1909 (2017).
M. R. Gradoville¹, D. Bombar², B. C. Crump¹, R. M. Letelier¹, J. P. Zehr², A. E. White¹ (¹CEOAS, Oregon State University, ²Ocean Sciences Department, University of California Santa Cruz)

 Recent observations of N₂ fixation rates (NFR) and the presence of nitrogenase (nifH) genes from heterotrophic N₂-fixing (diazotrophic) prokaryotes in unusual habitats challenge the paradigm that pelagic marine N₂ fixation is constrained to cyanobacteria in warm, oligotrophic, surface waters. Here, we compare NFR and diazotrophic diversity (assessed via high-throughput nifH sequencing) from a region known to be dominated by cyanobacterial diazotrophs (the North Pacific Subtropical Gyre, NPSG) to two regions dominated by heterotrophic diazotrophs: the Eastern South Pacific (ESP, from the Chilean upwelling system to the subtropical gyre) and the Pacific Northwest coastal upwelling system (PNW). We observed distinct biogeographical patterns among the three regions. Diazotrophic community structure differed strongly between the NPSG, dominated by cyanobacterium UCYN-A, and the ESP, dominated by heterotrophic nifH group 1 J/1 K, yet surface NFR were similar in magnitude (up to 5.1 nmol N L^-1 d^-1). However, while diverse, predominantly heterotrophic nifH genes were recovered from the PNW and the mesopelagic of the NPSG, NFR were undetectable in both of these environments (although glucose amendments stimulated low rates in the deep NPSG). Our work suggests that while diazotrophs may be nearly omnipresent in marine waters, the activity of this functional group is regionally restricted. Further, we show that the detection limits of the ¹⁵N₂ fixation assay suggest that many of the low NFR reported for the mesopelagic (often < 0.1 nmol N L^-1 d^-1 in the literature) are not indicative of active diazotrophy, highlighting the challenges of assessing the ecosystem significance of heterotrophic diazotrophs.

Insights Into the Biogeochemical Cycling of Iron, Nitrate, and Phosphate Across a 5,300 km South Pacific Zonal Section (153°E–150°W), Global Biogeochemical Cycles, 122, 32, 187-207 (2018).
M. J. Elwood¹, A. R. Bowie^2,3, A. Baker⁴, Melanie Gault-Ringold^2,5, C. Hassler⁶, C. S. Law^5,7 , W. A. Maher⁸, A. Marriner⁷, S. Nodder⁷, S. Sander⁵, C. Stevens^7,9, A. Townsend¹⁰, P. Merwe², E. M. S. Woodward¹¹, K. Wuttig² , and P. W. Boyd^5,7,12 (¹RSES, ANU, ²ACECRC, ³IMAS, University of Tasmania, ⁴Centre for Ocean and Atmospheric Sciences, School of Environmental Sciences, UEA, ⁵Department of Chemistry, University of Otago, ⁶DEFSE, University of Geneva, ⁷NIWA, ⁸Institute for Applied Ecology, University of Canberra, ⁹Department of Physics, University of Auckland, ¹⁰CSL, University of Tasmania, ¹¹PML, ¹²Now at IMAS, University of Tasmania)

 Iron, phosphate, and nitrate are essential nutrients for phytoplankton growth, and hence, their supply into the surface ocean controls oceanic primary production. Here we present a GEOTRACES zonal section (GP13; 30–33°S, 153°E–150°W) extending eastward from Australia to the oligotrophic South Pacific Ocean gyre outlining the concentrations of these key nutrients. Surface dissolved iron concentrations are elevated at >0.4 nmol L^-1 near continental Australia (west of 165°E) and decreased eastward to ≤0.2 nmol L^-1 (170°W–150°W). The supply of dissolved iron into the upper ocean (<100m) from the atmosphere and vertical diffusivity averaged 11 ± 10 nmol m^-2 d^-1 . In the remote South Pacific Ocean (170°W–150°W), atmospherically sourced iron is a significant contributor to the surface dissolved iron pool with average supply contribution of 23 ± 17% (range 3% to 55%). Surface water nitrate concentrations averaged 5 ± 4 nmol L^-1 between 170°W and 150°W, while surface water phosphate concentrations averaged 58 ± 30 nmol L^-1 . The supply of nitrogen into the upper ocean is primarily from deeper waters (24–1647 µmol m^-2 d ^-1 ) with atmospheric deposition and nitrogen fixation contributing <1% to the overall flux along the eastern part of the transect. The deep water N:P ratio averaged 14.5 ± 0.5 but declined to <1 above the deep chlorophyll maximum (DCM) indicating a high N:P assimilation ratio by phytoplankton leading to almost quantitative removal of nitrate. The supply stoichiometry for iron and nitrogen relative to phosphate at and above the DCM declines eastward leading to two biogeographical provinces: one with diazotroph production and the other without diazotroph production.

Water column iron dynamics in the subarctic North Pacific Ocean and the Bering Sea. J. Geophys. Res. Oceans, 118, 1257-1271 (2013).
R. Uchida¹, K. Kuma², A. Omata¹, S. Ishikawa¹, N. Hioki², H. Ueno², Y. Isoda², K. Sakaoka², Y. Kamei² and S. Takagi² (¹Graduate School of Environmental Science, Hokkaido University, ²Faculty of Fisheries, Hokkaido University)

 We measured water column iron concentrations from west to east along 47°N in the subarctic North Pacific, and in the Bering Sea. In the North Pacific dissolved Fe (D‐Fe) showed surface depletion, mid‐depth maxima at 1000–1500 m (west, 1.3–1.6 nM; east, 0.9–1.1 nM), and a gradual decrease with depth below 3500–4000 m depth (west, 1.1–1.4 nM; east, 0.6–0.7 nM). D‐Fe and total soluble Fe (T‐Fe) in deep water showed a decreasing trend eastward. The higher iron concentrations in western deep waters probably result from higher inputs of dissolved Fe through atmospheric deposition or lateral transport. In contrast, D‐Fe throughout the Bering Sea showed a consistent depth regime characterized by a rapid increase with depth to mid‐depths, a gradual increase with depth in intermediate water to a maximum of 1.6–1.7 nM at 1500–2250 m, and a gradual decrease with depth to 1.3–1.4 nM at 3700 m. Higher iron concentrations and deeper D‐Fe maxima in the Bering Sea are likely due to higher biological productivity and greater and deeper D‐Fe input from the decomposition of sinking particulate organic matter in deep water. We suggest that the higher concentrations and deeper input of D‐Fe as well as PO4 and humic‐type fluorescent dissolved organic matter in the Bering Sea probably results from the longer time for the accumulation of decomposition products resulting from iron supply from the organic‐rich downslope sediment along the steep continental slopes and slow replacement of the deep water in the Bering Sea Basin.

The observed evolution of oceanic pCO₂ and its drivers over the last two decades, Global Biogeochem. Cycles, 26, GB2021, doi:10.1029/2011GB004095, (2012).
A. Lenton¹, N. Metzl², T. Takahashi³, M. Kuchinke¹, R. J. Matear¹, T. Roy², S. C. Sutherland³, C. Sweeney⁴, and B. Tilbrook¹ (¹Wealth from Ocean Flagship, CSIRO Maine and Atmospheric Research, Hobart, Tasmania, ²LOCEAN-IPSL, CNRS, Université Pierre et Marie Curie, Paris, ³LDEO, Earth Institute, Columbia University, Palisades, New York, ⁴Global Monitoring Division, ESRL, NOAA, Boulder, Colorado)

 This study uses a database of more than 4.4 million observations of ocean pCO₂ to investigate oceanic pCO₂ growth rates. This study uses pCO₂ measurements, with corresponding sea surface temperature and salinity measurements, to reconstruct alkalinity and dissolved inorganic carbon to understand what is driving these growth rates in different ocean regions. If the oceanic pCO₂ growth rate is faster (slower) than the atmospheric CO₂ growth rate, the region can be interpreted as having a decreasing (increasing) atmospheric CO₂ uptake. Only the Western subpolar and subtropical North Pacific, and the Southern Ocean are found to have sufficient spatial and temporal observations to calculate the growth rates of oceanic pCO₂ in different seasons. Based on these regions, we find the strength of the ocean carbon sink has declined over the last two decades due to a combination of regional drivers (physical and biological). In the subpolar North Pacific reduced atmospheric CO₂ uptake in the summer is associated with changes in the biological production, while in the subtropical North Pacific enhanced uptake in winter is associated with enhanced biological production. In the Indian and Pacific sectors of the Southern Ocean a reduced winter atmospheric CO₂ uptake is associated with a positive of the Southern Annular Mode (SAM) response. Conversely in the more stratified Atlantic Ocean sector enhanced summer uptake is associated with increased biological production and reduced vertical supply. This study is not able to separate climate variability and change as the calculated growth rates are at the limit of detection and are associated with large uncertainties. Ongoing sustained observations of global oceanic pCO₂ and its drivers, including dissolved inorganic carbon and alkalinity, are key to detecting and understanding how the ocean carbon sink will evolve in future and what processes are driving this change.

Western Pacific atmospheric nutrient deposition fluxes, their impact on surface ocean productivity. Global Biogeochem. Cycles, 28, 712–728 (2014).
M. Martino¹, D. Hamilton¹, A. R. Baker¹, T. D. Jickells¹, T. Bromley², Y. Nojiri³, B. Quack⁴, P. W. Boyd^5,6(¹COAS, UEA, Norwich, UK, ²NIWA, Wellington, New Zealand, ³NIES, Tsukuba, Japan, ⁴Helmholtz-Zentrum für Ozeanforschung Kiel (GEOMAR), Marine Biogeochemie/Chemische Ozeanographie, Kiel, Germany, ⁵NIWA Centre for Chemical and Physical Oceanography, Department of Chemistry, University of Otago, Dunedin, New Zealand, ⁶Now at IMAS, University of Tasmania, Hobart, Tasmania, Australia)

 The atmospheric deposition of both macronutrients and micronutrients plays an important role in driving primary productivity, particularly in the low-latitude ocean. We report aerosol major ion measurements for five ship-based sampling campaigns in the western Pacific from ~25°N to 20°S and compare the results with those from Atlantic meridional transects (~50°N to 50°S) with aerosols collected and analyzed in the same laboratory, allowing full incomparability. We discuss sources of the main nutrient species (nitrogen (N), phosphorus (P), and iron (Fe)) in the aerosols and their stoichiometry. Striking north–south gradients are evident over both basins with the Northern Hemisphere more impacted by terrestrial dust sources and anthropogenic emissions and the North Atlantic apparently more impacted than the North Pacific. We estimate the atmospheric supply rates of these nutrients and the potential impact of the atmospheric deposition on the tropical western Pacific. Our results suggest that the atmospheric deposition is P deficient relative to the needs of the resident phytoplankton. These findings suggest that atmospheric supply of N, Fe, and P increases primary productivity utilizing some of the residual excess phosphorus (P^*) in the surface waters to compensate for aerosol P deficiency. Regional primary productivity is further enhanced via the stimulation of nitrogen fixation fuelled by the residual atmospheric iron and P^*. Our stoichiometric calculations reveal that a P^* of 0.1 µmol L^-1 can offset the P deficiency in atmospheric supply for many months. This study suggests that atmospheric deposition may sustain ~10% of primary production in both the western tropical Pacific.

The seeding of ice algal blooms in Arctic pack ice: The multiyear ice seed repository hypothesis. J. Geophys. Res. Biogeosci. 122, 1529–1548 (2017).
L. M. Olsen¹, S. R. Laney², P. Duarte¹, H. M. Kauko¹, M. Fernández-Méndez¹, C. J. Mundy³, A. Rösel¹, A. Meyer¹, P. Itkin¹, L. Cohen¹, I. Peeken⁴, A. Tatarek⁵, M. Róźańska-Pluta⁵, J. Wiktor⁵, T. Taskjelle⁶, A. K. Pavlov¹, S.R. Hudson¹, M. A. Granskog¹, H. Hop^1,7, and P. Assmy¹ (¹NPI, Fram Centre, ²Biology Department, WHOI, ³CEOS, University of Manitoba, ⁴AWI Helmholtz Center for Polar and Marine Research, ⁵IOPAS, ⁶Department of Physics and Technology, University of Bergen, ⁷Department of Arctic and Marine Biology, Faculty of Biosciences, Fisheries and Economics, UiT, Arctic University of Norway)

 During the Norwegian young sea ICE expedition (N-ICE2015) from January to June 2015 the pack ice in the Arctic Ocean north of Svalbard was studied during four drifts between 83° and 80°N. This pack ice consisted of a mix of second year, first year, and young ice. The physical properties and ice algal community composition was investigated in the three different ice types during the winter-spring-summer transition. Our results indicate that algae remaining in sea ice that survived the summer melt season are subsequently trapped in the upper layers of the ice column during winter and may function as an algal seed repository. Once the connectivity in the entire ice column is established, as a result of temperature-driven increase in ice porosity during spring, algae in the upper parts of the ice are able to migrate toward the bottom and initiate the ice algal spring bloom. Furthermore, this algal repository might seed the bloom in younger ice formed in adjacent leads. This mechanism was studied in detail for the dominant ice diatom Nitzschia frigida. The proposed seeding mechanism may be compromised due to the disappearance of older ice in the anticipated regime shift toward a seasonally ice-free Arctic Ocean.

The effect of organic carbon on fixed nitrogen loss in the eastern tropical South Pacific and Arabian Sea oxygen deficient zones. Limnol. Oceanogr., 59, 1267–1274 (2014).
B. X. Chang^1,2, J. R. Rich³, A. Jayakumar², H. Naik⁴, A. K. Pratihary⁴, R. G. Keil¹, B. B. Ward², and A. H. Devol¹ (¹School of Oceanography, University of Washington, ²Department of Geosciences, Princeton University, ³Department of Ecology & Environmental Biology, Center for Environmental Studies, Brown University, ⁴CSIR Centre for Excellence in Aquatic Biogeochemistry, Chemical Oceanography Division, NIO)

 The three major oxygen deficient zones (ODZs) of the world oceans (eastern tropical North and South Pacific (ETNP and ETSP, respectively), and Arabian Sea (AS) host the vast majority of pelagic fixed nitrogen (N) loss and up to half of total marine N loss. The input of organic matter is an important control on the absolute and relative importance of the two main pathways of N removal (denitrification and anammox). We investigated the response of N loss in the ETSP and AS ODZs to additions of organic matter in the form of glucose and naturally derived dissolved and particulate organic matter (DOM and POM, respectively). In the ETSP ODZ, the addition of glucose stimulated denitrification (1.6-fold increase after 5 d) but not anammox (14-fold decrease after 5 d). In the AS ODZ, only POM, not DOM, significantly increased rates of denitrification at the base of the oxycline (5.4–6.4-fold increase after 2 d), but not at the secondary nitrite maximum. These results suggest that denitrification was generally limited by organic matter supply at the time of this study in both the ETSP and AS ODZs, although the lability of the organic matter supplied was important. Interestingly, ¹⁵N₂ produced in ETSP and AS incubations was not binomially distributed relative to the reactants after the influence of anammox was taken into account, suggesting an unknown production mechanism or pathway of N removal.

A survey of carbon monoxide and non-methane hydrocarbons in the Arctic Ocean during summer 2010. Biogeosciences, 10, 1909-1935 (2013).
S. Tran¹, B. Bonsang¹, V. Gros¹, I. Peeken^2,3, R. Sarda-Esteve¹, A. Bernhardt², and S. Belviso¹ (¹LSCE,UVSQ, ²AWI, ³MARUM)

 During the ARK XXV 1+2 expedition in the Arctic Ocean carried out in June–July 2010 aboard the R/V Polarstern, we measured carbon monoxide (CO), nonmethane hydrocarbons (NMHC) and phytoplankton pigments at the sea surface and down to a depth of 100 m. The CO and NMHC seasurface concentrations were highly variable; CO, propene and isoprene levels ranged from 0.6 to 17.5 nmol L^-1, 1 to 322 pmol L^-1 and 1 to 541 pmol L^-1, respectively. The CO and alkene concentrations as well as their sea–air fluxes were enhanced in polar waters off of Greenland, which were more stratified because of ice melting and richer in chromophoric dissolved organic matter (CDOM) than typical North Atlantic waters. The spatial distribution of the surface concentrations of CO was consistent with our current understanding of CO-induced UV photoproduction in the sea. The vertical distributions of the CO and alkenes were comparable and followed the trend of light penetration, with the concentrations displaying a relatively regular exponential decrease down to non-measurable values below 50 m. However, no diurnal variations of CO or alkene concentrations were observed in the stratified and irradiated surface layers. On several occasions, we observed the existence of subsurface CO maxima at the level of the deep chlorophyll maximum. This finding suggests the existence of a nonphotochemical CO production pathway, most likely of phytoplanktonic origin. The corresponding production rates normalized to the chlorophyll content were in the range of those estimated from laboratory experiments. In general, the vertical distributions of isoprene followed that of the phytoplankton biomass. These data support the existence of a dominant photochemical source of CO and light alkenes enhanced in polar waters of the Arctic Ocean, with a minor contribution of a biological source of CO. The biological source of isoprene is observed in the different water masses but significantly increases in the warmer Atlantic waters.

Behavior of trace metals in the sediment pore waters of intertidal mudflats of a tropical wetland. Environ. Toxicol. Chem, 19, 535-542 (2000).
K.-T. Yu¹, M. H-W. Lam¹, Y.-F. Yen², and A. P. K. Leung² (¹Centre for Coastal Pollution and Conservation, City University of Hong Kong, ²Department of Chemistry, Hong Kong University of Science and Technology)

 Vertical profiles of dissolved Cd, Cr, Cu, Pb, Zn, Fe, and Mn in the sediment pore waters of the intertidal mudflats of the Mai Po and Inner Deep Bay Ramsar Site of Hong Kong, People’s Republic of China, were measured using the polyacrylamide gel diffusive equilibration thin film (DET) technique. The ranges of concentrations of dissolved Cd, Cr, Cu, Pb, Zn, Fe, and Mn in the pore water of the top 0 to 20 cm of sediment were 2.2 to 10.0 nM, 346.0 to 950.0 nM 243.8 to 454.8 nM, 23.2 to 51.2 nM, 39.8 to 249.5 mM, and 13.4 to 20.7 mM, respectively. Enrichment of these trace metals was observed in the upper 0 to 7 cm layer. Profiles of conditional distribution coefficient, log(K_D), of the trace metals and results of multiple regression analysis have revealed that reduction of Mn (hydrous) oxides was the major remobilization mechanism for Cd, Cr, Cu, Pb, and Zn in the mudflats. Benthic diffusive fluxes of these trace metals from the mudflats were also estimated on the basis of the concentration gradients of trace metals between surface sediments and the overlying water column. The magnitude of the estimated diffusive fluxes followed the order Zn > Cr > Cu > Pb > Cd.

Diurnal variation in the coupling of photosynthetic electron transport and carbon fixation in iron-limited phytoplankton in the NE subarctic Pacific. Biogeosciences, 13, 1091-1035 (2016).
N. Schuback¹, M. Flecken², M. T. Maldonado¹, and P. D. Tortell^1,3 (¹EOAS, University of British Colombia, ²RWTH Aachen University, ³Department of Botany, University of British Colombia)

 Active chlorophyll a fluorescence approaches, including fast repetition rate fluorometry (FRRF), have the potential to provide estimates of phytoplankton primary productivity at an unprecedented spatial and temporal resolution. FRRF-derived productivity rates are based on estimates of charge separation in reaction center II (ETR_RCII), which must be converted into ecologically relevant units of carbon fixation. Understanding sources of variability in the coupling of ETR_RCII and carbon fixation provides physiological insight into phytoplankton photosynthesis and is critical for the application of FRRF as a primary productivity measurement tool. In the present study, we simultaneously measured phytoplankton carbon fixation and ETR_RCII in the iron-limited NE subarctic Pacific over the course of a diurnal cycle. We show that rates of ETR_RCII are closely tied to the diurnal cycle in light availability, whereas rates of carbon fixation appear to be influenced by endogenous changes in metabolic energy allocation under iron-limited conditions. Unsynchronized diurnal oscillations of the two rates led to 3.5-fold changes in the conversion factor between ETR_RCII and carbon fixation (K_c / n_PSII). Consequently, diurnal variability in phytoplankton carbon fixation cannot be adequately captured with FRRF approaches if a constant conversion factor is applied. Utilizing several auxiliary photophysiological measurements, we observed that a high conversion factor is associated with conditions of excess light and correlates with the increased expression of non-photochemical quenching (NPQ) in the pigment antenna, as derived from FRRF measurements. The observed correlation between NPQ and K_c / n_PSII requires further validation but has the potential to improve estimates of phytoplankton carbon fixation rates from FRRF measurements alone.

The anthropogenic perturbation of the marine nitrogen cycle by atmospheric deposition: Nitrogen cycle feedbacks and the ¹⁵N Haber-Bosch effect. Global Biogeochem. Cycles, 30, 1418-1440 (2016).
S. Yang¹ and N. Gruber¹ (¹Environmental Physics, IBP, ETH Zurich)

 Over the last 100 years, anthropogenic emissions have led to a strong increase of atmospheric nitrogen deposition over the ocean, yet the resulting impacts and feedbacks are neither well understood nor quantified. To this end, we run a suite of simulations with the ocean component of the Community Earth System Model v1.2 forced with five scenarios of nitrogen deposition over the period from 1850 through 2100, while keeping all other forcings unchanged. Even though global oceanic net primary production increases little in response to this fertilization, the higher export and the resulting expansion of the oxygen minimum zones cause an increase in pelagic and benthic denitrification and burial by about 5%. In addition, the enhanced availability of fixed nitrogen in the surface ocean reduces global ocean N₂ fixation by more than 10%. Despite the compensating effects through these negative feedbacks that eliminate by the year 2000 about 60% of the deposited nitrogen, the anthropogenic nitrogen input forced the upper ocean N budget into an imbalance of between 9 and 22 Tg N yr^-1 depending on the deposition scenario. The excess nitrogen accumulates to highly detectable levels and causes in most areas a distinct negative trend in the 𝛿¹⁵N of the oceanic fixed nitrogen pools—a trend we refer to as the ¹⁵N Haber-Bosch effect. Changes in surface nitrate utilization and the nitrogen feedbacks induce further changes in the 𝛿¹⁵N of NO₃^- , making it a good but complex recorder of the overall impact of the changes in atmospheric deposition.

Photoprotection and recovery of photosystem II in the Southern Ocean phytoplankton, Polar Sci., 12, 5–11 (2017).
T. Katayama¹, R. Makabe^{2, 3}, M. Sampei⁴, H. Hattori⁵, H. Sasaki⁶ and S. Taguchi¹ (¹Faculty of Science and Engineering, Soka University, ²National Institute of Polar Research, ³SOKENDAI, ⁴Faculty of Fisheries Sciences, Hokkaido University, ⁵Faculty of Biology, Tokai University, ⁶Faculty of Science and Engineering, Ishinomaki Senshu University)

 The future shoaling of surface mixed layer depth due to global warming will expose natural assemblages of phytoplankton to increased mean light. Under these conditions, photoprotective acclimation against high light can determine ecological success. We investigated photoprotective responses to sunlight and recovery from photodamage of photosystem II (PSII) in natural assemblages north and south of the Polar Front (PF). The decrease in the maximum quantum yield (F_v/F_m) of PSII during direct sunlight exposure for 2 h was moderated progressively by the enhancement of diatoxanthin synthesis. When the light-exposed cells were incubated under three reduced light conditions, F_v/F_m recovered to more than the initial values north of the PF but did not reach initial values south of the PF. Temperatures higher in the north than the south of the PF could have induced the faster recovery from photodamage of PSII in assemblages north of the PF. These northern assemblages may be able to acclimate to fast-changing light conditions.

Phytoplankton pigment absorption: A strong predictor of primary productivity in the surface ocean. Deep-Sea Res. Pt I, 54, 155-163 (2007).
Marra, J¹, C. C. Trees², J. E. O’Reilly³ (¹Lamont-Doherty Earth Observatory of Columbia University, ²Center for Hydrologic Optics and Remote Sensing, San Diego State University, ³Northeast Fisheries Science Center, NOAA)

 Over a range of trophic conditions in the ocean, we argue that variations in productivity are more closely related to variations in phytoplankton absorption than to variations in the chlorophyll-a (Chl-a) concentration. Our analysis suggests that environmental variability is expressed through the absorption properties of phytoplankton pigments rather than their quantity, and that productivity normalized to absorption is relatively invariant in the world ocean. The relationship between primary productivity and phytoplankton absorption makes possible a more direct approach to the estimation of ocean productivity from satellite sensors.

Temporal variability of nitrogen fixation and particulate nitrogen export at Station ALOHA, Limnol. Oceanogr., 62, 200–216 (2017).
D. Böttjer^1,2, J. E. Dore³, D. M. Karl^1,2, R. M. Letelier^2,4, C. Mahaffey⁵, S. T. Wilson^1,2, J. Zehr^2,6, and M. J. Church^1,2 (¹Department of Oceanography, SOEST, ²Daniel K. Inouye C-MORE, ³Department of LRES, ⁴CEOAS, OSU, ⁵Department of Earth, Ocean, and Ecological Sciences, ⁶Ocean Sciences Department)

 We present nearly 9 yrs (June 2005–December 2013) of measurements of upper-ocean (0 m to 125 m) dinitrogen (N₂) fixation rates, coupled with particulate nitrogen (PN) export at 150 m, from Station ALOHA (22°45‘N, 158°W) in the North Pacific Subtropical Gyre. Between June 2005 and June 2012, N₂ fixation rates were measured based on adding the ¹⁵N₂ tracer as a gas bubble. Beginning in August 2012, ¹⁵N₂ was first dissolved into filtered seawater and the ¹⁵N₂-enriched water was subsequently added to N₂ fixation incubations. Direct comparisons between methodologies revealed a robust relationship, with the addition of ¹⁵N₂-enriched seawater resulting in twofold greater depth-integrated rates than those derived from adding a ¹⁵N₂ gas bubble. Based on this relationship, we corrected the initial period of measurements, and the resulting rates of N₂ fixation averaged 230±136 µmol N m^-2 d^-1 for the full time series(n=71). Analysis of the 15N isotopic composition of sinking PN, together with an isotope mass balance model, revealed that N₂ fixation supported 26–47 % of PN export during calendar years 2006–2013. The N export derived from these fractional contributions and measured N₂ fixation rates ranged between 502 and 919 µmolN m^-2 d^-1, which are equivalent to rates of net community production (NCP) of 1.5 to 2.7 mol C m^-2 yr^-1, consistent with previous independent estimates of NCP at this site.

The influence of mesoscale physical structure in the phytoplankton taxonomic composition of the subsurface chlorophyll maximum off western Baja California. D. S. Res. Ocgraph., 70, 91-102 (2012).
A. Almazan-Becerril¹, D. Rivas², E. Garcie-Mendoza² (¹Unidad de Ciencias del Agua, ²Departamento de Oceanografıa Biologica)

 The distribution of the subsurface chlorophyll maximum (SCM) layer, its taxonomic phytoplankton composition, and the maximum quantum efficiency of charge separation of PSII (Fv/Fm) was investigated in the west coast off Baja California during October 2003. SCM characteristics were described and related to the hydrographic regime and the mesoscale physical structures present during this period. Seven groups of phytoplankton were detected in the SCM based on chemotaxonomic analysis of pigment fingerprints: diatoms, haptophytes, pelagophytes, prasinophytes, cryptophytes, Prochlorococcus and cyanobacteria. The distribution of these groups was heterogeneous and closely related to the circulation patterns characterized by the interaction of subarctic and tropical water. Eddies and meanders were detected in the study area and these structures exerted a direct response in the depth, chlorophyll concentration, and photosynthetic competence of phytoplankton in the SCM. A cyclonic eddy characterized by a high chlorophyll concentration (1.6 mg m^-3) and high values of Fv/Fm, (0.52) was detected in the northern zone of the study area. In the central zone, a cyclonic eddy (1.2 mg m3); and other structure resembling a mode-water eddy was located northwest of Cedros Island. This structure presented the highest chlorophyll concentration (1.8 mg m^-3) and high Fv/Fm (0.5). Chlorophyll concentration and the photosynthetic performance of the phytoplankton commu- nity was lower outside of these eddies. Cyanobacteria dominated the phytoplankton SCM community in these areas.

Both soluble and colloidal iron phases control dissolved iron variability in the tropical North Atlantic Ocean. Geochim. Cosmochim. Acta Res. 125, 539-550 (2014).
J. N. Fitzsimmons¹, E. A. Boyle² (¹MIT/WHOI Joint Program in Chemical Oceanography, ²MIT)

 The size partitioning of dissolved iron (dFe, ＜0.4 µm) into soluble (sFe, ＜0.02 µm) and colloidal (0.02 µm ＜ cFe ＜ 0.4 µm) phases was investigated at seven stations in the tropical North Atlantic Ocean, and the results are compared to the dFe size fractionation study of Bergquist et al. (2007) in the same region. Downwind of the North African dust plumes, cFe comprised 80 ± 7% of the surface dFe pool at six stations, supporting the hypothesis that atmospherically-derived Fe is maintained in the colloidal size fraction. At the deep chlorophyll maximum, colloidal Fe had minimum concentrations or was completely absent, suggesting that cFe was either preferentially taken up by microbes and/or scavenged/aggregated at these depths. At remineralization depths, sFe was the dominant fraction both in the subtropical gyre-like stations (76% sFe; [sFe] = 0.42 ± 0.03 nmol/kg) and in the oxygen minimum zone (56% sFe; [sFe] = 0.65 ± 0.03 nmol/kg). Only at remineralization depths of stations with intermediate oxygen concentrations (100–110 µmol/kg) did colloidal Fe dominate (contributing 58% of dFe) , indicating that cFe may be serving as a conduit of dFe loss during mixing of high-Fe OMZ and low-Fe gyre waters. North Atlantic Deep Water (NADW) had a typical sFe concentration of 0.34 ± 0.05 nmol/kg. In the deepest samples composed of a NADW/Antarctic Bottom Water mixture where the bottom water may have attained a ∼0.1 nmol/kg hydrothermal Fe input during transit past the Mid-Atlantic Ridge, sFe did not increase coincidentally with dFe, indicating that any potential hydrothermal Fe contribution was colloidal. In general, the results of this study counter the previous hypothesis of Bergquist et al. (2007) that the colloidal Fe fraction predominately controls dFe variability, instead suggesting that both soluble and colloidal Fe are variable, and both contribute to the observed dFe variability throughout the North Atlantic. The nearly 50–50% dFe partitioning into soluble and colloidal phases below the DCM suggest one of two partitioning mechanisms persists: (1) soluble and colloidal Fe exchange rates reach a “steady state,” over which regional, uniquely-partitioned Fe sources can be overlain, or (2) the partitioning of Fe-binding ligands between the two size fractions is variable in the open ocean and directly controls dFe partitioning.

Estimating carbonate parameters from hydrographic data for the intermediate and deep waters of the Southern Hemisphere oceans, Biogeosciences, 10, 6199–6213, (2013).
H. C. Bostock¹, S. E. Mikaloff Fletcher¹ and M. J. M. Williams¹ (¹National Institute of Water and Atmospheric Research Ltd.)

 Using ocean carbon data from global datasets, we have developed several multiple linear regression (MLR) algorithms to estimate alkalinity and dissolved inorganic carbon (DIC) in the intermediate and deep waters of the Southern Hemisphere (south of 25◦S) from only hydrographic data (temperature, salinity and dissolved oxygen). A Monte Carlo experiment was used to identify a potential density (σ_θ) of 27.5 as an optimal break point between the two regimes with different MLR algorithms. The algorithms provide a good estimate of DIC (R² =0.98) and alkalinity (R² =0.91), and excellent agreement for aragonite and calcite saturation states (R² =0.99). Combining the algorithms with the CSIRO Atlas of Regional Seas (CARS), we have mapped the calcite saturation horizon (CSH) and aragonite saturation horizon (ASH) for the Southern Ocean at a spatial resolution of 0.5◦. These maps are more detailed and more consistent with the oceanography than the previously gridded GLODAP data. The high-resolution ASH map reveals a dramatic circumpolar shoaling at the polar front. North of 40◦S the CSH is deepest in the Atlantic (∼4000m) and shallower in the Pacific Ocean (∼2750 m), while the CSH sits between 3200 and 3400m in the Indian Ocean. The uptake of anthropogenic carbon by the ocean will alter the relationships between DIC and hydrographic data in the intermediate and deep waters over time. Thus continued sampling will be required, and the MLR algorithms will need to be adjusted in the future to account for these changes.

Slow acidification of the winter mixed layer in the subarctic western North Pacific, J. Geophys. Res. Oceans, 122, 6923-6935 (2017).
M. Wakita¹, A. Nagano², T. Fujiki² and S. Watanabe¹ (¹MIO, JAMSTEC, ²Research and Development Center for Global Change, JAMSTEC)

 This study used carbon dioxide (CO₂) system data collected during 1999–2015 to investigate ocean acidification at time series sites in the western subarctic region of the North Pacific Ocean. The annual mean pH at station K2 decreased at a rate of 0.0025 ± 0.0010 year⁻¹ mostly in response to oceanic uptake of anthropogenic CO₂. The Revelle factor increased rapidly (0.046 ± 0.022 year⁻¹), an indication that the buffering capacity of this region of the ocean has declined faster than at other time series sites. In the western subarctic region, the pH during the winter decline at a slower rate of 0.0008 ± 0.0004 year⁻¹. This was attributed to a reduced rate of increase of dissolved inorganic carbon (DIC) and an increase of total alkalinity (TA). The reduction of DIC increase was caused by the decline of surface water density associated with the pycnocline depression and the reduction of vertical diffusion flux from the upper pycnocline. These physical changes were probably caused by northward shrinkage of the western subarctic gyre and global warming. Meanwhile, the contribution of the density decline to the TA increase is canceled out by that of the reduced vertical diffusive flux. This study speculated that the winter TA increase is caused mainly by the accumulation of TA due to the weakened calcification by organisms during the winter.

Annual sea-air CO₂ fluxes in the Bering Sea: Insights from new autumn and winter observations of a seasonally ice-covered continental shelf. J. Geophys. Res. Oceans, 119, 6693-6708 (2014)
J. N. Cross^{1, 2}, J. T. Mathis^{1, 2}, K. E. Frey³, C. E. Cosca¹, S. L. Danielson², N. R. Bates⁴, R. A. Feely¹, T. Takahashi⁵ and W. Evans^{1, 2} (¹NOAA, PMEL, ²OARC, University of Alaska, ³Graduate School of Geography, Clark University, ⁴BIOS, ⁵LDEO)

 High-resolution data collected from several programs have greatly increased the spatiotemporal resolution of pCO₂(sw) data in the Bering Sea, and provided the first autumn and winter observations. Using data from 2008 to 2012, monthly climatologies of sea-air CO₂ fluxes for the Bering Sea shelf area from April to December were calculated, and contributions of physical and biological processes to observed monthly sea-air pCO₂ gradients (ΔpCO₂) were investigated. Net efflux of CO₂ was observed during November, December, and April, despite the impact of sea surface cooling on ΔpCO₂. Although the Bering Sea was believed to be a moderate to strong atmospheric CO₂ sink, we found that autumn and winter CO₂ effluxes balanced 65% of spring and summer CO₂ uptake. Ice cover reduced sea-air CO₂ fluxes in December, April, and May. Our estimate for ice-cover corrected fluxes suggests the mechanical inhibition of CO₂ flux by sea-ice cover has only a small impact on the annual scale (＜2%). An important data gap still exists for January to March, the period of peak ice cover and the highest expected retardation of the fluxes. By interpolating between December and April using assumptions of the described autumn and winter conditions, we estimate the Bering Sea shelf area is an annual CO₂ sink of ∼ 6.8 Tg C yr⁻¹. With changing climate, we expect warming sea surface temperatures, reduced ice cover, and greater wind speeds with enhanced gas exchange to decrease the size of this CO₂ sink by augmenting conditions favorable for greater wintertime outgassing.

Okhotsk Sea intermediate water formation deduced from oxygen isotope systematics. J. Geophys. Res., 106, 31.075－31,084, (2001).
M. Yamamoto¹, N. Tanaka¹ and S. Tsunogai¹ (¹Graduate School of Environment Earth Science, Hokkaido University)

 To clarify the mechanisms of the Okhotsk Sea intermediate water formation, water samples collected in the Okhotsk Sea and neighboring western North Pacific were analyzed for δ¹⁸O as well as salinity and other routinely measured components. The δ¹⁸O－salinity relation in the Okhotsk Sea was markedly different from that in the western subarctic Pacific feeding the Okhotsk Sea. The intermediate water of the Okhotsk Sea was less saline than that of the western subarctic water at the same density but was more saline at the same δ¹⁸O value, especially in the density range 26.5 ＜ σ _θ ＜ 27.0. This could be due to mixing of dense shelf water formed during sea-ice formation with neighboring water. The amount of fresh water removed by the sea ice formation was estimated from the salinity anomaly of the Okhotsk Sea intermediate water. The calculated amounts of fresh water removed from the Okhotsk Sea intermediate water corresponded to the amount of sea ice formed for 0.6－ 3.8 years in the Okhotsk Sea. This suggests that the residence time of the Okhotsk Sea intermediate water is a few years.

Increasing anthropogenic nitrogen in the North Pacific Ocean. Science, 346, 1102-1106 (2014).
Il-N. Kim¹, K. Lee¹, N. Gruber², D. M. Karl³, J. L. Bullister⁴, S. Yang² and T-W. Kim⁵ (¹School of Environmental Sciences and Engineering, POSTECH. ²Environmental Physics Group, IBP. ³Daniel K. Inouye C-MORE, University of Hawaii at Manoa. ⁴PMEL, NOAA. ⁵Ocean Circulation and Climate Research Division, KIOST.)

 The recent increase in anthropogenic emissions of reactive nitrogen from northeastern Asia and the subsequent enhanced deposition over the extensive regions of the North Pacific Ocean (NPO) have led to a detectable increase in the nitrate (N) concentration of the upper ocean. The rate of increase of excess N relative to phosphate (P) was found to be highest (∼0.24 micromoles per kilogram per year) in the vicinity of the Asian source continent, with rates decreasing eastward across the NPO, consistent with the magnitude and distribution of atmospheric nitrogen deposition. This anthropogenically driven increase in the N content of the upper NPO may enhance primary production in this N-limited region, potentially leading to a long-term change of the NPO from being N-limited to P-limited.

A significant methane source over the Chukchi Sea shelf and its sources. Cont. Shelf Res. 148, 150-158 (2017).
Y. Li¹, L. Zhan¹, J. Zhang¹, L. Chen¹, J. Chen², Y. Zhuang² (¹ Key Laboratory of GCMAC, Xiamen, China, ² Laboratory of Marine Ecosystem and Biogeochemistry, Hangzhou, China)

  Dissolved methane (CH₄) was measured at various depths in the western Arctic Ocean. The CH₄ concentrations at the surface show an increasing trend northward toward stations at the shelf break and a decreasing trend toward stations in the Canada Basin. The mean sea-to-air flux is estimated to be 10.08 µmol/m²/d, indicates that the Chukchi Sea shelf (CSS) is an active site of CH₄. Methane concentrations at the shelf stations increase from the surface to the bottom, and the maximum nutrient concentrations occur in the bottom layer. Strong correlations exist between CH₄ and PO₄^3-, SiO₄^2-, or NO₂^-, suggesting that the production of CH₄ is likely related to the degradation of organic matter in the sediment, supporting a biogenic source. At the slope and basin stations, the maximum values were observed in the subsurface of the upper halocline layer (UHL), and the concentrations decrease with increasing depth. The CH₄ concentrations are elevated by ~7.9 nmol/L in the UHL compared with the homogeneous CH₄ concentrations observed in the deep water. The elevated values in the UHL result primarily from northward spreading of CH₄-rich water from the shelf. A mass balance model was used to calculate the CH₄ budget in the CSS. The results show that effluxes of CH₄ from the sediment-water interface and the in situ production of CH₄ represent the major sources of CH₄ over the CSS (95%). The main outputs for CH₄ in the CSS are the sea-to-air flux and oxidation of CH₄ in the water column, which account for 95% of the CH₄ exports.

Sediment transport by sea ice in the Chukchi and Beaufort Seas: Increasing importance due to changing ice conditions? Deep-Sea Res. 52, 3281-3302 (2005).
H. Eicken¹, R. Gradinger², A. Gaylord³, A. Mahoney¹, I. Rigor⁴, and H. Melling⁵. (¹Geophysical Institute, UAF. ²Institute of Marine Science, UAF. ³Nuna Technologies. ⁴APL-UW. ⁵Institute of Ocean Sciences.)

 Sediment-laden sea ice is widespread over the shallow, wide Siberian Arctic shelves, with off-shelf export from the Laptev and East Siberian Seas contributing substantially to the Arctic Ocean’s sediment budget. By contrast, the North American shelves, owing to their narrow width and greater water depths, have not been deemed as important for basin-wide sediment transport by sea ice. Observations over the Chukchi and Beaufort shelves in 2001/02 revealed the widespread occurrence of sediment-laden ice over an area of more than 100,000 km² between 68 and 74˚N and 155 and 170˚W. Ice stratigraphic studies indicate that sediment inclusions were associated with entrainment of frazil ice into deformed, multiple layers of rafted nilas, indicative of a flaw-lead environment adjacent to the landfast ice of the Chukchi and Beaufort Seas. This is corroborated by buoy trajectories and satellite imagery indicating entrainment in a coastal polynya in the eastern Chukchi Sea in February of 2002 as well as formation of sediment-laden ice along the Beaufort Sea coast as far eastward as the Mackenzie shelf. Moored upward-looking sonar on the Mackenzie shelf provides further insight into the ice growth and deformation regime governing sediment entrainment. Analysis of Radarsat Synthetic Aperture (SAR) imagery in conjunction with bathymetric data help constrain the water depth of sediment resuspension and subsequent ice entrainment (>20m for the Chukchi Sea). Sediment loads averaged at 128 t km^-2, with sediment occurring in layers of roughly 0.5m thickness, mostly in the lower ice layers. The total amount of sediment transported by sea ice (mostly out of the narrow zone between the landfast ice edge and waters too deep for resuspension and entrainment) is at minimum 4 x 10⁶ t in the sampling area and is estimated at 5–8 x 10⁶ t over the entire Chukchi and Beaufort shelves in 2001/02, representing a significant term in the sediment budget of the western Arctic Ocean. Recent changes in the Chukchi and Beaufort Sea ice regimes (reduced summer minimum ice extent, ice thinning, reduction in multi-year ice extent, altered drift paths and mid-winter landfast ice break-out events) have likely resulted in an increase of sediment-laden ice in the area. Apart from contributing substantially to along- and across-shelf particulate flow, an increase in the amount of dirty ice significantly impacts (sub-) ice algal production and may enhance the dispersal of pollutants.

Fixed nitrogen loss from the eastern tropical North Pacific and Arabian Sea oxygen deficient zones determined from measurements of N₂:Ar. Global Biogeochem. Cycles, 26, GB3030, doi:10.1029 (2012). B. X. Chang^1,2, A. H. Devol¹ and S. R. Emerson¹ (¹School of Oceanography, University of Washington, ²Now at Department of Geosciences, Princeton University)

 Previous work estimating the N₂ excess above background due to denitrification has suggested that nitrate deficit-type methods may be an underestimate of fixed nitrogen (N) loss in the major oxygen deficient zones of the ocean. The N₂ excess approach has the advantage over nitrate deficit-type methods in that it does not depend on stoichiometric assumptions of fixed N to phosphate or oxygen utilization and avoids any uncertainly regarding the pathway of N loss. Measurements of N₂:Ar from two stations within the eastern tropical North Pacific and from one station within the Arabian Sea oxygen deficient zones were used to determine the N₂ excess due to denitrification. In both of the regions, the N₂ excess was comparable in shape and magnitude to the concurrent fixed nitrogen deficit. In the eastern tropical North Pacific oxygen deficient zone, the N₂ excess was near zero at the surface and rose to maxima of 13.7 ± 1.8 and 10.8 ± 1.9 µM N, compared to maximum fixed N deficits of 13.5 ± 1.5 and 12.3 ± 1.5 µM N, respectively. In the Arabian Sea oxygen deficient zone, the maximum N₂ excess was 11.1 ± 1.5 µM N, compared to a maximum deficit of 12.5 ± 1.0 µM N. These results suggests that previous estimates of fixed N loss based on fixed N deficit calculations in these regions are likely reasonable, given the same considerations of volume and residence time of the water of the oxygen deficient zone.

Empirical algorithms to estimate water column pH in the Southern Ocean. Geophys. Res. Lett, 43, 3415–3422 (2016). N. L. Williams¹, L. W. Juranek¹, K. S. Johnson², R. A. Feely³, S. C. Riser⁴, L. D. Talley⁵, J. L. Russell⁶, J. L. Sarmiento⁷, and R. Wanninkhof⁸　(¹College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, ²Monterey Bay　Aquarium Research Institute, ³Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, ⁴School of Oceanography, University of Washington, ⁵Scripps Institution of Oceanography, University of California, ⁶Department of Geosciences, University of Arizona, ⁷Program in Atmospheric and Oceanic Sciences, Princeton University, ⁸Atlantic Oceanographic and Meteorological, Laboratory, National Oceanic and Atmospheric Administration

 Empirical algorithms are developed using high-quality GO-SHIP hydrographic measurements of commonly measured parameters (temperature, salinity, pressure, nitrate, and oxygen) that estimate pH in the Pacific sector of the Southern Ocean. The coefficients of determination, R², are 0.98 for pH from nitrate (pH^N) and 0.97 for pH from oxygen (pH^Ox) with RMS errors of 0.010 and 0.008, respectively. These algorithms are applied to Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) biogeochemical profiling floats, which include novel sensors (pH, nitrate, oxygen, fluorescence, and backscatter). These algorithms are used to estimate pH on floats with no pH sensors and to validate and adjust pH sensor data from floats with pH sensors. The adjusted float data provide, for the first time, seasonal cycles in surface pH on weekly resolution that range from 0.05 to 0.08 on weekly resolution for the Pacific sector of the Southern Ocean.

Basin scale variability of active diazotrophs and nitrogen fixation in the North Pacific, from the tropics to the subarctic Bering Sea. Global Biogeochem. Cycles, 31, 996–1009 (2017). T. Shiozaki^1,2, D. Bombar³, L. Riemann³, F. Hashihama⁴, S. Takeda⁵, T. Yamaguchi⁶, M. Ehama⁴, K. Hamasaki¹, and K. Furuya⁶ (¹Atmosphere and Ocean Research Institute, University of Tokyo, ²Now at Research and Development Center for Global Change, JAMSTEC, ³Marine Biological Section, Department of Biology, University of Copenhagen, ⁴Department of Ocean Sciences, Tokyo University of Marine Science and Technology, ⁵Faculty of Fisheries, Nagasaki University, ⁶Department of Aquatic Bioscience, Graduate School of Agricultural and Life Sciences, University of Tokyo)

 Nitrogen-fixing microorganisms (diazotrophs) provide biologically available nitrogen to plankton communities and thereby greatly influence the productivity in many marine regions. Various cyanobacterial groups have traditionally been considered the major oceanic diazotrophs, but later noncyanobacterial and presumably heterotrophic diazotrophs were also found to be widespread and potentially important in nitrogen fixation. However, the distribution and activity of different diazotroph groups is still poorly constrained for most oceanic ecosystems. Here we examined diazotroph community structure and activity along a 7500 km south-north transect between the central equatorial Pacific and the Bering Sea. Nitrogen fixation contributed up to 84% of new production in the upper waters of the subtropical gyre, where the diazotroph community included the gammaproteobacterium γ-24774A11 and highly active cyanobacterial phylotypes (＞50% of total nifH transcript abundance). Nitrogen fixation was sometimes detectable down to 150 m depth and extended horizontally to the edge of the gyre at around 35°N. Nitrogen fixation was even detected far north on the Bering Sea shelf. In the Alaskan Coastal Waters on the Bering Sea shelf, low nitrate together with high dissolved iron concentrations seemed to foster diazotroph growth, including a prominent role of UCYN-A2, which was abundant near the surface (1.2×10⁵ nifH gene copies L⁻¹). Our study provides evidence for nitrogen fixation in the Bering Sea and suggests a clear contrast in the composition of diazotrophs between the tropical/subtropical gyre and the separate waters in the cold northern regions of the North Pacific.

Vertical material flux under seasonal sea ice in the Okhotsk Sea north of Hokkaido. Polar Science, 2, 41－54, (2008). T. Hiwatari¹, K. Shirasawa³, Y. Fukamachi³, R. Nagata⁴, T. Koizumi⁵, H. Koshikawa¹, K. Kohata² (¹Asian Environmental Group, NIES, ²Water and Soil Environmental Division, NIES, ³ILTS, Hokkaido University, ⁴Okhotsk Garinko Tower, ⁵Mikuniya Co)

 Downward material fluxes under seasonal sea ice were measured using a time-series sediment trap installed at an offshore site in the Okhotsk Sea north of Hokkaido, Japan, from 13 January to 23 March 2005. The maximum fluxes of lithogenic material (753 mg m^-2 day^-1) and organic matter (mainly detritus; 333 mg m^-2 day^-1) were recorded during the period in which sea ice drifted ashore and increased in extent, from 13 January to 9 February. Organic matter as fecal pellets (81－93 mg m^-2 day^-1) and opal as biosilica (51－67 mg m^-2 day^-1 ) , representing diatom fluxes, were abundant in sediment trap samples obtained during the period of full sea ice coverage from 10 February to 9 March. Microscopic observation revealed that fecal pellets were largely diatom frustules, suggesting that zooplankton actively grazed on ice algae during the period of full sea ice coverage. During the period of retreating sea ice, from 10 to 23 March, the phytoplankton flux showed a rapid increase (from 9.5 to 22.5×10⁶ cells m^-2 day^-1), reflecting their release into the water column as the sea ice melted. Our results demonstrate that the quantity and quality of sinking biogenic and lithogenic materials vary with the seasonal extent of sea ice in mid-winter.

Influences of riverine and upwelling waters on the coastal carbonate system off Central Chile and their ocean acidification implications. J. Geophys. Res. Biogeosci., 121, 1468-1483 (2016). C. A. Vargas^1,2,3, P. Y. Contreras^1,2, C. A. Pérez¹, M. Sobarzo⁴, G. S. Saldías⁵ and J. Salisbury⁶ (¹Aquatic Ecosystem Functioning Lab, Department of Aquatic System, Faculty of Environmental Sciences and Environmental Sciences Center EULA Chile, Universidad de Concepción, ²Center for the Study of Multiple Drivers on Marine Socio-Ecological Systems, Universidad de Concepción, ³IMO, Universidad de Concepción, ⁴DOCE and Interdisciplinary Center for Aquaculture Research, Faculty of Natural and Oceanographics Sciences, Universidad de Concepción, ⁵CEOAS, Oregon State University, 6Institute for the Study of EOS, UNH)

 A combined data set, combining data from field campaigns and oceanographic cruises, was used to ascertain the influence of both river discharges and upwelling processes, covering spatial and temporal variation in dissolved inorganic carbon (DIC) and aragonite saturation state. This work was conducted in one of the most productive river-influenced upwelling areas in the South Pacific coasts (36°S). Additionally, further work was also conducted to ascertain the contribution of different DIC sources, influencing the dynamics of DIC along the land-ocean range. Six sampling campaigns were conducted across seven stations at the Biobío River basin, covering approximately 200 km. Three research cruises were undertaken simultaneously, covering the adjacent continental shelf, including 12 sampling stations for hydrographic measurements. Additionally, six stations were also sampled for chemical analyses, covering summer, winter, and spring conditions over 2010 and 2011. Our results evidenced that seaward extent of the river plume was more evident during the winter field campaign, when highest riverine DIC fluxes were observed. The carbonate system along the river-ocean continuum was very heterogeneous varying over spatial and temporal scales. High DIC and pCO₂ were observed in river areas with larger anthropogenic effects. CO₂ supersaturation at the river plume was observed during all campaigns due to the influence of low pH river waters in winter/spring and high-pCO₂ upwelling waters in summer. δ¹³C_DIC evidenced that main DIC sources along the river and river plume corresponded to the respiration of terrestrial organic matter. We have linked this natural process to the carbonate saturation on the adjacent river-influenced coastal area, suggesting that Ω_aragonite undersaturation in surface/subsurface waters is largely modulated by the influence of both river discharge and coastal upwelling events in this productive coastal area. Conditions of low Ω_aragonite might impact negatively physiological traits for marine organisms, such as bivalves, gastropods, and crustaceans. Therefore, local populations from river-influenced sites could be inherently more tolerant to ocean acidification than organisms living in regions with lower Ω_aragonite variability.

Dynamic biogeochemical provinces in the global ocean. Global Biogeochem. Cycles, 27, 1046–1058 (2013). G. Reygondeau^1,2,3, A. Longhurst⁴, E. Martinez^2,5, G. Beaugrand⁶, D. Antoine^2,7, and O. Maury¹ (¹IRD, UMR EME 212, CRMHT, ²LOV, CNRS–Université Pierre et Marie Curie, ³CEES, Department of Biosciences, University of Oslo, ⁴Cajarc, ⁵MIO, Aix-Marseille University, IRD UMR 235, CNRS/INSU UMR 7294, ⁶LOG, UMR LOG CNRS 8187, Station Marine, CNRS, Université des Sciences et Technologies de Lille, ⁷Department of Imaging and Applied Physics, RSSRG, Curtin University)

  In recent decades, it has been found useful to partition the pelagic environment using the concept of biogeochemical provinces, or BGCPs, within each of which it is assumed that environmental conditions are distinguishable and unique at global scale. The boundaries between provinces respond to features of physical oceanography and, ideally, should follow seasonal and interannual changes in ocean dynamics. But this ideal has not been fulfilled except for small regions of the oceans. Moreover, BGCPs have been used only as static entities having boundaries that were originally established to compute global primary production. In the present study, a new statistical methodology based on non-parametric procedures is implemented to capture the environmental characteristics within 56 BGCPs. Four main environmental parameters (bathymetry, chlorophyll a concentration, surface temperature, and salinity) are used to infer the spatial distribution of each BGCP over 1997–2007. The resulting dynamic partition allows us to integrate changes in the distribution of BGCPs at seasonal and interannual timescales, and so introduces the possibility of detecting spatial shifts in environmental conditions.

Iron in the southeastern Bering Sea: Elevated leachable particulate Fe in shelf bottom waters as an important source for surface waters. Cont. Shelf Res., 30, (2010). M. P. Hurst¹, A. M. Aguilar-Islas² and K. W. Bruland³ (¹Department of Chemistry, Humboldt State University, ²IARC, University of Alaska, ³Department of Ocean Sciences, University of California Santa Cruz)

 Surface transects and vertical profiles of total and leachable particulate Fe, Mn, Al and P, along with dissolved and soluble Fe were obtained during August 2003 in the southeastern Bering Sea. High concentrations of leachable particulate Fe were observed in the bottom waters over the Bering Sea shelf with an unusually high percentage of the suspended particulate Fe being leachable. Leachable particulate Fe averaged 81% of total particulate Fe, and existed at elevated concentrations that averaged 23 times greater than dissolved Fe in the isolated cool pool waters over the mid shelf where substantial influence of sedimentary denitrification was apparent. The elevated leachable particulate Fe is suggested to be a result of sedimentary Fe reduction in surficial sediments, diffusion of Fe(II) from the sediments to the bottom waters, and subsequent oxidation and precipitation of reduced Fe in the overlying bottom waters. Eddies and meanders of the Bering Slope Current can mix this Fe-rich water into the Green Belt at the outer shelf-break front. Elevated levels of leachable particulate Fe were observed in surface waters near the Pribilof Islands where enhanced vertical mixing exists. Storm events and/or cooling during fall/winter with the resultant destruction of the thermally stratified two-layer system can also mix the subsurface water into surface waters where the elevated leachable particulate Fe is a substantial source of biologically available Fe.

Controls on iron distributions in the deep water column of the North Pacific Ocean: Iron(III) hydroxide solubility and marine humic-type dissolved organic matter. J. Geophys. Res., 114, (2009). S. Kitayama¹, K. Kuma^1,2 E. Manabe¹, K. Sugie¹, H. Takata², Y. Isoda², K. Toya², S. Saitoh², S. Takagi², Y. Kamei², and K. Sakaoka² (¹Graduate School of Environmental Science Hokkaido University, ²Faculty of Fisheries Sciences, Hokkaido University)

 Dissolved Fe in the western and central North Pacific Ocean was characterized by surface depletion, middepth maxima and, below that, a slight decrease with depth similar to the vertical distributions of nutrients, apparent oxygen utilization, Fe(III) hydroxide solubility, and humic-type fluorescence (H-flu) intensity. Dissolved Fe concentrations ([D-Fe], ＜0.22-µm fraction) in the deep water column were one-half lower in the central region (0.3–0.6 nM) than the western region (0.5–1.2 nM) although the Fe(III) solubility ([Fe(III)sol], ＜0.025-µm fraction) levels and distributions in deep waters were almost the same between both regions with middepth maxima (∼0.6 nM) at 500–1500-m depth range and then a gradual decrease to ∼0.3 nM at 5000-m depth. Higher [D-Fe] than [Fe(III)sol] in the deep water column of the western region results from the higher production of dissolved Fe from the decomposition of sinking particulate organic matter in the western region than the central region because of the high atmospheric and/or lateral Fe inputs in the western region. Similarity between [D-Fe] level and [Fe(III)sol] value at each deep water depth in the central region may be attributed to [D-Fe] being nearly in the solubility equilibrium with Fe(III) hydroxide in seawater. Strong linear correlation between [D-Fe] and H-flu intensity in the central region and relatively similar linear relationships between [Fe(III)sol] and H-flu intensity in the western and central regions are the first confirmation that humic-type fluorescent dissolved organic matter may be responsible for [D-Fe] in the deep water column as natural organic ligands complexing with Fe(III).

Interactive influence of iron and light limitation on phytoplankton at subsurface chlorophyll maxima in the eastern North Pacific. Limnol. Oceanogr., 53, 1303-1318 (2008). B. M. Hopkinson¹, K. A. Barbeau¹ (¹Scripps Institution of Oceanography, UC San Diego)

 The roles of iron and light as limiting and colimiting factors for phytoplankton growth in subsurface chlorophyll maxima (SCMs) were investigated in mesotrophic to oligotrophic waters of the Southern California Bight and the eastern tropical North Pacific using microcosm manipulation experiments. Phytoplankton responses indicative of iron–light colimitation were found at several SCMs underlying macronutrient-limited surface waters in the eastern Pacific. Iron additions led to a shift in the size and taxonomic structure of the phytoplankton community, where large diatoms dominated what was formerly a diverse community of relatively small phytoplankton. The strongest and most ubiquitous responses of diatoms to iron addition were found under elevated light conditions, indicating that iron availability may have the greatest potential to affect SCM phytoplankton communities when light levels increase rapidly, such as during eddy events or with strong internal waves. The results show that iron influences phytoplankton community structure at SCMs, which would have consequences for nutrient cycling and carbon export within the lower euphotic zone.

Impact of an unusually large warm‐core eddy on distributions of nutrients and phytoplankton in the southwestern Canada Basin during late summer/early fall 2010. Geophysical Research Letters, 38, L16602, doi:10.1029/2011GL047885 (2011). S. Nishino¹, M. Itoh¹, Y. Kawaguchi¹, T. Kikuchi¹ and M. Aoyama² (1 Research Institute for Global Change, JAMSTEC. 2MRI, Geochemical Research Department)

 Recent freshening of the Arctic Ocean due to melting of sea ice and enhanced Ekman pumping has deepened the nutricline over the Canada Basin and reduced nutrient concentrations in the euphotic zone. Cold‐core eddies frequently transport nutrient‐rich shelf water to the Canada Basin, but the eddies are much deeper than the euphotic zone. Because warm‐core eddies appear near the surface or at a depth range shallower than that of the cold‐core eddies, they may play a crucial role in determining nutrient distributions in the euphotic zone and hence may affect primary production. During late summer/early fall 2010, we conducted detailed surveys of a warm core eddy, which was unusually large (∼100 km in diameter). We suggest that this warm‐core eddy which contained high ammonium shelf water could supply ammonium to the euphotic zone in the southwestern Canada Basin and may sustain ∼30% higher biomass of picophytoplankton (＜ 2 mm) than that in the surrounding water in the basin. The role of warm‐core eddies in supplying nutrients to the euphotic zone and controlling phytoplankton distributions seems to be more important than previously because the recent deepening of the nutricline in the Canada Basin has decreased the nutrient supply to the euphotic zone.

Decadal acidification in the water masses of the Atlantic Ocean. PNAS, 112, 9950-9955 (2015)
A. F. Ríosa¹, L. Resplandy², M. I. García-Ibáñez¹, N. M. Fajar¹, A. Velo¹, X. A. Padin¹, R. Wanninkhof³, R. Steinfeldt⁴, G. Rosón⁵ and F. F. Pérez¹ (¹Marine Research Institute, IIM-CSIC, ²Scripps Institution of Oceanography, University of California, ³Atlantic Oceanographic and Meteorological Laboratory, NOAA, ⁴Oceanography Department, IUP, University of Bremen, ⁵Faculty of Marine Sciences, University of Vigo).

 Global ocean acidification is caused primarily by the ocean’s uptake of CO₂ as a consequence of increasing atmospheric CO₂ levels. We present observations of the oceanic decrease in pH at the basin scale (50°S–36°N) for the Atlantic Ocean over two decades (1993–2013). Changes in pH associated with the uptake of anthropogenic CO₂ (ΔpHCant) and with variations caused by biological activity and ocean circulation (ΔpHNat) are evaluated for different water masses. Output from an Institut Pierre Simon Laplace climate model is used to place the results into a longer-term perspective and to elucidate the mechanisms responsible for pH change. The largest decreases in pH (∆pH) were observed in central, mode, and intermediate waters, with a maximum ΔpH value in South Atlantic Central Waters of −0.042 ± 0.003. The ΔpH trended toward zero in deep and bottom waters. Observations and model results show that pH changes generally are dominated by the anthropogenic component, which accounts for rates between −0.0015 and −0.0020/y in the central waters. The anthropogenic and natural components are of the same order of magnitude and reinforce one another in mode and intermediate waters over the time period. Large negative ΔpHNat values observed in mode and intermediate waters are driven primarily by changes in CO₂ content and are consistent with (i) a poleward shift of the formation region during the positive phase of the Southern Annular Mode in the South Atlantic and (ii) an increase in the rate of the water mass formation in the North Atlantic.