Cassar Nicolas

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  • Article
    Can we estimate air‐sea flux of biological O2 from total dissolved oxygen?
    (American Geophysical Union, 2022-08-14) Huang, Yibin ; Eveleth, Rachel ; Nicholson, David P. ; Cassar, Nicolas
    In this study, we compare mechanistic and empirical approaches to reconstruct the air-sea flux of biological oxygen (F[O2]bio-as) by parameterizing the physical oxygen saturation anomaly (ΔO2[phy]) in order to separate the biological contribution from total oxygen. The first approach matches ΔO2[phy] to the monthly climatology of the argon saturation anomaly from a global ocean circulation model's output. The second approach derives ΔO2[phy] from an iterative mass balance model forced by satellite-based physical drivers of ΔO2[phy] prior to the sampling day by assuming that air-sea interactions are the dominant factors driving the surface ΔO2[phy]. The final approach leverages the machine-learning technique of Genetic Programming (GP) to search for the functional relationship between ΔO2[phy] and biophysicochemical parameters. We compile simultaneous measurements of O2/Ar and O2 concentration from 14 cruises to train the GP algorithm and test the validity and applicability of our modeled ΔO2[phy] and F[O2]bio-as. Among the approaches, the GP approach, which incorporates ship-based measurements and historical records of physical parameters from the reanalysis products, provides the most robust predictions (R2 = 0.74 for ΔO2[phy] and 0.72 for F[O2]bio-as; RMSE = 1.4% for ΔO2[phy] and 7.1 mmol O2 m−2 d−1 for F[O2]bio-as). We use the empirical formulation derived from GP approach to reconstruct regional, inter-annual, and decadal variability of F[O2]bio-as based on historical oxygen records. Overall, our study represents a first attempt at deriving F[O2]bio-as from snapshot measurements of oxygen, thereby paving the way toward using historical O2 data and a rapidly growing number of O2 measurements on autonomous platforms for independent insight into the biological pump.
  • Article
    Impact of variable air-sea O2 and CO2 fluxes on atmospheric potential oxygen (APO) and land-ocean carbon sink partitioning
    (Copernicus Publications on behalf of the European Geosciences Union, 2008-06-02) Nevison, Cynthia D. ; Mahowald, Natalie M. ; Doney, Scott C. ; Lima, Ivan D. ; Cassar, Nicolas
    A three dimensional, time-evolving field of atmospheric potential oxygen (APO ~O2/N2+CO2) was estimated using surface O2, N2 and CO2 fluxes from the WHOI ocean ecosystem model to force the MATCH atmospheric transport model. Land and fossil carbon fluxes were also run in MATCH and translated into O2 tracers using assumed O2:CO2 stoichiometries. The modeled seasonal cycles in APO agree well with the observed cycles at 13 global monitoring stations, with agreement helped by including oceanic CO2 in the APO calculation. The modeled latitudinal gradient in APO is strongly influenced by seasonal rectifier effects in atmospheric transport. An analysis of the APO-vs.-CO2 mass-balance method for partitioning land and ocean carbon sinks was performed in the controlled context of the MATCH simulation, in which the true surface carbon and oxygen fluxes were known exactly. This analysis suggests uncertainty of up to ±0.2 PgC in the inferred sinks due to variability associated with sparse atmospheric sampling. It also shows that interannual variability in oceanic O2 fluxes can cause large errors in the sink partitioning when the method is applied over short timescales. However, when decadal or longer averages are used, the variability in the oceanic O2 flux is relatively small, allowing carbon sinks to be partitioned to within a standard deviation of 0.1 Pg C/yr of the true values, provided one has an accurate estimate of long-term mean O2 outgassing.
  • Article
    Expanding Tara oceans protocols for underway, ecosystemic sampling of the ocean-atmosphere interface during Tara Pacific expedition (2016-2018)
    (Frontiers Media, 2019-12-11) Gorsky, Gabriel ; Bourdin, Guillaume ; Lombard, Fabien ; Pedrotti, Maria Luiza ; Audrain, Samuel ; Bin, Nicolas ; Boss, Emmanuel S. ; Bowler, Chris ; Cassar, Nicolas ; Caudan, Loic ; Chabot, Genevieve ; Cohen, Natalie R. ; Cron, Daniel ; De Vargas, Colomban ; Dolan, John R. ; Douville, Eric ; Elineau, Amanda ; Flores, J. Michel ; Ghiglione, Jean-Francois ; Haëntjens, Nils ; Hertau, Martin ; John, Seth G. ; Kelly, Rachel L. ; Koren, Ilan ; Lin, Yajuan ; Marie, Dominique ; Moulin, Clémentine ; Moucherie, Yohann ; Pesant, Stephane ; Picheral, Marc ; Poulain, Julie ; Pujo-Pay, Mireille ; Reverdin, Gilles ; Romac, Sarah ; Sullivan, Mathew B. ; Trainic, Miri ; Tressol, Marc ; Troublé, Romain ; Vardi, Assaf ; Voolstra, Christian R. ; Wincker, Patrick ; Agostini, Sylvain ; Banaigs, Bernard ; Boissin, Emilie ; Forcioli, Didier ; Furla, Paola ; Galand, Pierre E. ; Gilson, Eric ; Reynaud, Stephanie ; Sunagawa, Shinichi ; Thomas, Olivier P. ; Vega Thurber, Rebecca ; Zoccola, Didier ; Planes, Serge ; Allemand, Denis ; Karsenti, Eric
    Interactions between the ocean and the atmosphere occur at the air-sea interface through the transfer of momentum, heat, gases and particulate matter, and through the impact of the upper-ocean biology on the composition and radiative properties of this boundary layer. The Tara Pacific expedition, launched in May 2016 aboard the schooner Tara, was a 29-month exploration with the dual goals to study the ecology of reef ecosystems along ecological gradients in the Pacific Ocean and to assess inter-island and open ocean surface plankton and neuston community structures. In addition, key atmospheric properties were measured to study links between the two boundary layer properties. A major challenge for the open ocean sampling was the lack of ship-time available for work at “stations”. The time constraint led us to develop new underway sampling approaches to optimize physical, chemical, optical, and genomic methods to capture the entire community structure of the surface layers, from viruses to metazoans in their oceanographic and atmospheric physicochemical context. An international scientific consortium was put together to analyze the samples, generate data, and develop datasets in coherence with the existing Tara Oceans database. Beyond adapting the extensive Tara Oceans sampling protocols for high-resolution underway sampling, the key novelties compared to Tara Oceans’ global assessment of plankton include the measurement of (i) surface plankton and neuston biogeography and functional diversity; (ii) bioactive trace metals distribution at the ocean surface and metal-dependent ecosystem structures; (iii) marine aerosols, including biological entities; (iv) geography, nature and colonization of microplastic; and (v) high-resolution underway assessment of net community production via equilibrator inlet mass spectrometry. We are committed to share the data collected during this expedition, making it an important resource important resource to address a variety of scientific questions.
  • Article
    Linking Southern Ocean mixed-layer dynamics to net community production on various timescales
    (American Geophysical Union, 2021-09-21) Li, Zuchuan ; Lozier, M. Susan ; Cassar, Nicolas
    Mixed-layer dynamics exert a first order control on nutrient and light availability for phytoplankton. In this study, we examine the influence of mixed-layer dynamics on net community production (NCP) in the Southern Ocean on intra-seasonal, seasonal, interannual, and decadal timescales, using biogeochemical Argo floats and satellite-derived NCP estimates during the period from 1997 to 2020. On intraseasonal timescales, the shoaling of the mixed layer is more likely to enhance NCP in austral spring and winter, suggesting an alleviation of light limitation. As expected, NCP generally increases with light availability on seasonal timescales. On interannual timescales, NCP is correlated with mixed layer depth (MLD) and mixed-layer-averaged photosynthetically active radiation (PAR) in austral spring and winter, especially in regions with deeper mixed layers. Though recent studies have argued that winter MLD controls the subsequent growing season's iron and light availability, the limited number of Argo float observations contemporaneous with our satellite observations do not show a significant correlation between NCP and the previous-winter's MLD on interannual timescales. Over the 1997–2020 period, we observe regional trends in NCP (e.g., increasing around S. America), but no trend for the entire Southern Ocean. Overall, our results show that the dependence of NCP on MLD is a complex function of timescales.
  • Article
    Export production and its regulating factors in the West Antarctica Peninsula region of the Southern Ocean
    (American Geophysical Union, 2012-04-24) Huang, Kuan ; Ducklow, Hugh W. ; Vernet, Maria ; Cassar, Nicolas ; Bender, Michael L.
    In connection with the Palmer LTER program, mixed layer water samples were collected during the cruise of the L.M. Gould in Jan., 2008 at 49 stations on a 20 × 100 km grid in the West Antarctica Peninsula (WAP) region of the Southern Ocean. In this study, [O2]/[Ar] ratios and the triple isotope composition of dissolved O2 were measured, and were used to estimate net community O2 production (NCP) and gross primary O2 production (GPP), respectively. These estimates are further converted to carbon export production, primary production and the f-ratio. Our measurements give NCP ranging from −3 to 76 mmol O2 m−2 day−1 (−25 to 650 mg C m−2 day−1), and GPP from 40 to 220 mmol O2 m−2 day−1 (180 to 1010 mg C m−2 day−1). The O2 NCP/GPP ratios range from −0.04 to 0.43, corresponding to f-ratios of −0.08 to 0.83. NCP and the NCP/GPP ratio are highest in the northern coastal areas, and decrease to lower values toward the southern coastal area and the open ocean. The inshore-offshore gradient appears to be regulated primarily by iron availability, as supported by the positive correlation between NCP and Fv/Fm ratios (r2 = 0.22, p < 0.05). Mixed layer depth (MLD) is inversely correlated with NCP (r2 = 0.21, p < 0.002) and NCP/GPP (r2 = 0.21, p < 0.02), and highest NCP occurred in the fresh water lenses probably formed from melted coastal glaciers. These results suggest that export production and the f-ratio increase where water stratification is intensified by input of fresh meltwater, and that mixed layer stratification is the major factor regulating NCP in the inner-shelf and coastal regions. Along-shelf variability of phytoplankton community composition is highly correlated with NCP, i.e., NCP increases when the diatom-dominated community in the south transitions to the cryptophyte-dominated one in the north. A high correlation is also observed between NCP and the logarithm of the surface chlorophyll concentration (r2 = 0.72, p < 0.0001) , which makes it possible to estimate carbon export as a function of Chl a concentration in this region.
  • Article
    Theoretical considerations on factors confounding the interpretation of the oceanic carbon export ratio
    (American Geophysical Union, 2018-10-13) Li, Zuchuan ; Cassar, Nicolas
    The fraction of primary production exported out of the surface ocean, known as the export ratio (ef ratio), is often used to assess how various factors, including temperature, primary production, phytoplankton size, and community structure, affect the export efficiency of an ecosystem. To investigate possible causes for reported discrepancies in the dominant factors influencing the export efficiency, we develop a metabolism‐based mechanistic model of the ef ratio. Consistent with earlier studies, we find based on theoretical considerations that the ef ratio is a negative function of temperature. We show that the ef ratio depends on the optical depth, defined as the physical depth times the light attenuation coefficient. As a result, varying light attenuation may confound the interpretation of ef ratio when measured at a fixed depth (e.g., 100 m) or at the base of the mixed layer. Finally, we decompose the contribution of individual factors on the seasonality of the ef ratio. Our results show that at high latitudes, the ef ratio at the base of mixed layer is strongly influenced by mixed layer depth and surface irradiation on seasonal time scales. Future studies should report the ef ratio at the base of the euphotic layer or account for the effect of varying light attenuation if measured at a different depth. Overall, our modeling study highlights the large number of factors confounding the interpretation of field observations of the ef ratio.
  • Article
    Net community production and gross primary production rates in the western equatorial Pacific
    (American Geophysical Union, 2010-10-12) Stanley, Rachel H. R. ; Kirkpatrick, John B. ; Cassar, Nicolas ; Barnett, Bruce A. ; Bender, Michael L.
    Net community production (NCP) and gross primary production (GPP) are two key metrics for quantifying the biological carbon cycle. In this study, we present a detailed characterization of NCP and GPP in the western equatorial Pacific during August and September 2006. We use continuous measurements of dissolved gases (O2 and Ar) in the surface water in order to quantify NCP at subkilometer scale resolution. We constrain GPP in discrete samples using the triple isotopic composition of O2. We find the average NCP in the western equatorial Pacific is 5.9 ± 0.9 mmol O2 m−2 d−1 (equivalent to 1.5 ± 0.2 mol C m−2 yr−1 with error estimates reflecting 1σ confidence levels) and the average GPP is 121 ± 34 mmol O2 m−2 d−1 (equivalent to 32 ± 9 mol C m−2 yr−1). The measurements reveal significant spatial variability on length scales as small as 50 km. The NCP/GPP ratio is 5.7% ± 1.8%. We also present results for NCP and GPP in the coastal area off Papua New Guinea and for GPP in the central Pacific along the equator.
  • Working Paper
    EXPORTS Measurements and Protocols for the NE Pacific Campaign
    (NASA STI Program and Woods Hole Oceanographic Institution, 2021-02) Behrenfeld, Michael J. ; Benitez-Nelson, Claudia R. ; Boss, Emmanuel S. ; Brzezinski, Mark A. ; Buck, Kristen N. ; Buesseler, Ken O. ; Burd, Adrian B. ; Carlson, Craig A. ; Cassar, Nicolas ; Cetinić, Ivona ; Close, Hilary G. ; Craig, Susanne E. ; D'Asaro, Eric A. ; Durkin, Colleen A. ; Estapa, Margaret L. ; Fassbender, Andrea ; Fox, James ; Freeman, Scott ; Gifford, Scott M. ; Gong, Weida ; Graff, Jason R. ; Gray, Deric ; Guidi, Lionel ; Halsey, Kim ; Hansell, Dennis A. ; Haëntjens, Nils ; Horner, Tristan J. ; Jenkins, Bethany D. ; Jones, Janice L. ; Karp-Boss, Lee ; Kramer, Sasha J. ; Lam, Phoebe J. ; Lee, Craig M. ; Lee, Jong-Mi ; Liu, Shuting ; Mannino, Antonio ; Maas, Amy E. ; Marchal, Olivier ; Marchetti, Adrian ; McDonnell, Andrew M. P. ; McNair, Heather ; Menden-Deuer, Susanne ; Morison, Francoise ; Nelson, Norman B. ; Nicholson, David P. ; Niebergall, Alexandria K. ; Omand, Melissa M. ; Passow, Uta ; Perry, Mary J. ; Popp, Brian N. ; Proctor, Chris ; Rafter, Patrick ; Roca-Martí, Montserrat ; Roesler, Collin S. ; Rubin, Edwina ; Rynearson, Tatiana A. ; Santoro, Alyson E. ; Siegel, David A. ; Sosik, Heidi M. ; Soto Ramos, Inia ; Stamieszkin, Karen ; Steinberg, Deborah K. ; Stephens, Brandon M. ; Thompson, Andrew F. ; Van Mooy, Benjamin A. S. ; Zhang, Xiaodong
    EXport Processes in the Ocean from Remote Sensing (EXPORTS) is a large-scale NASA-led and NSF co-funded field campaign that will provide critical information for quantifying the export and fate of upper ocean net primary production (NPP) using satellite information and state of the art technology.
  • Article
    Global estimates of marine gross primary production based on machine learning upscaling of field observations
    (American Geophysical Union, 2021-01-13) Huang, Yibin ; Nicholson, David P. ; Huang, Bangqin ; Cassar, Nicolas
    Approximately half of global primary production occurs in the ocean. While the large-scale variability in net primary production (NPP) has been extensively studied, ocean gross primary production (GPP) has thus far received less attention. In this study, we derived two satellite-based GPP models by training machine learning algorithms (Random Forest) with light-dark bottle incubations (GPPLD) and the triple isotopes of dissolved oxygen (GPP17Δ). The two algorithms predict global GPPs of 9.2 ± 1.3 × 1015 and 15.1 ± 1.05 × 1015 mol O2 yr−1 for GPPLD and GPP17Δ, respectively. The projected GPP distributions agree with our understanding of the mechanisms regulating primary production. Global GPP17Δ was higher than GPPLD by an average factor of 1.6 which varied meridionally. The discrepancy between GPP17Δ and GPPLD simulations can be partly explained by the known biases of each methodology. After accounting for some of these biases, the GPP17Δ and GPPLD converge to 9.5 ∼ 12.6 × 1015 mol O2 yr−1, equivalent to 103 ∼ 150 Pg C yr−1. Our results suggest that global oceanic GPP is 1.5–2.2 fold larger than oceanic NPP and comparable to GPP on land.
  • Article
    Decline in plankton diversity and carbon flux with reduced sea ice extent along the Western Antarctic Peninsula
    (Nature Research, 2021-08-16) Lin, Yajuan ; Moreno, Carly ; Marchetti, Adrian ; Ducklow, Hugh W. ; Schofield, Oscar M. E. ; Delage, Erwan ; Meredith, Michael M. ; Li, Zuchuan ; Eveillard, Damien ; Chaffron, Samuel ; Cassar, Nicolas
    Since the middle of the past century, the Western Antarctic Peninsula has warmed rapidly with a significant loss of sea ice but the impacts on plankton biodiversity and carbon cycling remain an open question. Here, using a 5-year dataset of eukaryotic plankton DNA metabarcoding, we assess changes in biodiversity and net community production in this region. Our results show that sea-ice extent is a dominant factor influencing eukaryotic plankton community composition, biodiversity, and net community production. Species richness and evenness decline with an increase in sea surface temperature (SST). In regions with low SST and shallow mixed layers, the community was dominated by a diverse assemblage of diatoms and dinoflagellates. Conversely, less diverse plankton assemblages were observed in waters with higher SST and/or deep mixed layers when sea ice extent was lower. A genetic programming machine-learning model explained up to 80% of the net community production variability at the Western Antarctic Peninsula. Among the biological explanatory variables, the sea-ice environment associated plankton assemblage is the best predictor of net community production. We conclude that eukaryotic plankton diversity and carbon cycling at the Western Antarctic Peninsula are strongly linked to sea-ice conditions.
  • Article
    Decomposing the oxygen signal in the ocean interior: beyond decomposing organic matter
    (American Geophysical Union, 2021-09-13) Cassar, Nicolas ; Nicholson, David P. ; Khatiwala, Samar ; Cliff, Ellen
    In the subsurface ocean, O2 depleted because of organic matter remineralization is generally estimated based on apparent oxygen utilization (AOU). However, AOU is an imperfect measure of oxygen utilization because of O2 air-sea disequilibrium at the site of deepwater formation. Recent methodological and instrumental advances have paved the way to further deconvolve the processes driving the O2 signature. Using numerical model simulations of the global ocean, we show that the measurements of the dissolved O2/Ar ratio, which so far have been confined to the ocean surface, can provide improved estimates of oxygen utilization, especially in regions where the disequilibrium at the site of deepwater formation is associated with physical processes. We discuss applications of this new approach and implications for the current tracers relying on O2 such as remineralization ratios, respiratory quotients, and preformed nutrients. Finally, we propose a new composite geochemical tracer, [O2]bio combining dissolved O2/Ar and phosphate concentration. Being insensitive to photosynthesis and respiration, the change in this new tracer reflects gas exchange at the air-sea interface at the sites of deepwater formation.
  • Article
    An operational overview of the EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) Northeast Pacific field deployment
    (University of California Press, 2021-07-07) Siegel, David A. ; Cetinić, Ivona ; Graff, Jason R. ; Lee, Craig M. ; Nelson, Norman B. ; Perry, Mary J. ; Soto Ramos, Inia ; Steinberg, Deborah K. ; Buesseler, Ken O. ; Hamme, Roberta C. ; Fassbender, Andrea ; Nicholson, David P. ; Omand, Melissa M. ; Robert, Marie ; Thompson, Andrew F. ; Amaral, Vinicius ; Behrenfeld, Michael J. ; Benitez-Nelson, Claudia R. ; Bisson, Kelsey ; Boss, Emmanuel S. ; Boyd, Philip ; Brzezinski, Mark A. ; Buck, Kristen N. ; Burd, Adrian B. ; Burns, Shannon ; Caprara, Salvatore ; Carlson, Craig A. ; Cassar, Nicolas ; Close, Hilary G. ; D'Asaro, Eric A. ; Durkin, Colleen A. ; Erickson, Zachary K. ; Estapa, Margaret L. ; Fields, Erik ; Fox, James ; Freeman, Scott ; Gifford, Scott M. ; Gong, Weida ; Gray, Deric ; Guidi, Lionel ; Haëntjens, Nils ; Halsey, Kim ; Huot, Yannick ; Hansell, Dennis A. ; Jenkins, Bethany D. ; Karp-Boss, Lee ; Kramer, Sasha J. ; Lam, Phoebe J. ; Lee, Jong-Mi ; Maas, Amy E. ; Marchal, Olivier ; Marchetti, Adrian ; McDonnell, Andrew M. P. ; McNair, Heather ; Menden-Deuer, Susanne ; Morison, Francoise ; Niebergall, Alexandria K. ; Passow, Uta ; Popp, Brian N. ; Potvin, Geneviève ; Resplandy, Laure ; Roca-Martí, Montserrat ; Roesler, Collin S. ; Rynearson, Tatiana A. ; Traylor, Shawnee ; Santoro, Alyson E. ; Seraphin, Kanesa ; Sosik, Heidi M. ; Stamieszkin, Karen ; Stephens, Brandon M. ; Tang, Weiyi ; Van Mooy, Benjamin ; Xiong, Yuanheng ; Zhang, Xiaodong
    The goal of the EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) field campaign is to develop a predictive understanding of the export, fate, and carbon cycle impacts of global ocean net primary production. To accomplish this goal, observations of export flux pathways, plankton community composition, food web processes, and optical, physical, and biogeochemical (BGC) properties are needed over a range of ecosystem states. Here we introduce the first EXPORTS field deployment to Ocean Station Papa in the Northeast Pacific Ocean during summer of 2018, providing context for other papers in this special collection. The experiment was conducted with two ships: a Process Ship, focused on ecological rates, BGC fluxes, temporal changes in food web, and BGC and optical properties, that followed an instrumented Lagrangian float; and a Survey Ship that sampled BGC and optical properties in spatial patterns around the Process Ship. An array of autonomous underwater assets provided measurements over a range of spatial and temporal scales, and partnering programs and remote sensing observations provided additional observational context. The oceanographic setting was typical of late-summer conditions at Ocean Station Papa: a shallow mixed layer, strong vertical and weak horizontal gradients in hydrographic properties, sluggish sub-inertial currents, elevated macronutrient concentrations and low phytoplankton abundances. Although nutrient concentrations were consistent with previous observations, mixed layer chlorophyll was lower than typically observed, resulting in a deeper euphotic zone. Analyses of surface layer temperature and salinity found three distinct surface water types, allowing for diagnosis of whether observed changes were spatial or temporal. The 2018 EXPORTS field deployment is among the most comprehensive biological pump studies ever conducted. A second deployment to the North Atlantic Ocean occurred in spring 2021, which will be followed by focused work on data synthesis and modeling using the entire EXPORTS data set.