scholarly journals Sensitivity of phytoplankton distributions to vertical mixing along a North Atlantic transect

2014 ◽  
Vol 11 (2) ◽  
pp. 839-893
Author(s):  
L. Hahn-Woernle ◽  
H. A. Dijkstra ◽  
H. J. Van der Woerd
Keyword(s):  
North Atlantic ◽  
Light Attenuation ◽  
Vertical Mixing ◽  
Clear Correlation ◽  
Optical Parameters ◽  
Chl A ◽  
Phytoplankton Concentration ◽  
In Situ Data ◽  
The North ◽  
Using Data

Abstract. Using in situ data of upper ocean vertical mixing profiles along a transect in the North Atlantic and an idealised phytoplankton growth model (PGM), we study the sensitivity of the surface phytoplankton concentration to vertical mixing distributions. Optical parameters in the PGM are calibrated with observations of light attenuation. The calibration of the biological parameters in the PGM is carried out at three different referent stations with observed vertical profiles of chlorophyll a (Chl a) and nutrient concentration. Vertical mixing profiles at all other stations are next used at the reference stations to study the sensitivity of modelled phytoplankton distributions to vertical mixing. We find that shifts in vertical mixing are able to induce a transition from an upper chlorophyll maximum to a deep one and vice-versa. Furthermore, a clear correlation between the surface phytoplankton concentration and the mixing induced nutrient flux is found in nutrient limited regions. This may open up the possibility to extract characteristics of vertical mixing from satellite ocean colour data using data-assimilation methods.

scholarly journals Sensitivity of phytoplankton distributions to vertical mixing along a North Atlantic transect

Ocean Science ◽  
2014 ◽  
Vol 10 (6) ◽  
pp. 993-1011 ◽  
Author(s):  
L. Hahn-Woernle ◽  
H. A. Dijkstra ◽  
H. J. Van der Woerd
Keyword(s):  
North Atlantic ◽  
Light Attenuation ◽  
Vertical Mixing ◽  
Clear Correlation ◽  
Model Parameters ◽  
Chl A ◽  
Phytoplankton Concentration ◽  
In Situ Data ◽  
The North ◽  
The North Atlantic

Abstract. Using in situ data of upper ocean vertical mixing along a transect in the North Atlantic and a one-dimensional phytoplankton growth model, we study the sensitivity of the surface phytoplankton concentration to vertical mixing distributions. The study is divided into two parts. In the first part, the model is calibrated to the observations. The optical model parameters are determined from measurements of the light attenuation. The biological parameters are calibrated to three different reference stations with observed vertical profiles of the chlorophyll a (Chl a) concentration and the nutrient concentration. In the second part, the sensitivity of the three model calibrations to the vertical mixing is studied. Therefore measured vertical mixing profiles are applied to the model. These mixing profiles are based on the measurements along the transect and are treated as a set of possible mixing situations of the North Atlantic. Results show that shifts in vertical mixing are able to induce a transition from an upper chlorophyll maximum to a deep one and vice versa. Furthermore, a clear correlation between the surface phytoplankton concentration and the mixing induced nutrient flux is found for nutrient-limited cases. This may open up the possibility to extract characteristics of vertical mixing from satellite ocean colour data using data-assimilation methods.

scholarly journals Organic nutrients and excess nitrogen in the North Atlantic subtropical gyre

2008 ◽  
Vol 5 (5) ◽  
pp. 1199-1213 ◽  
Author(s):  
A. Landolfi ◽  
A. Oschlies ◽  
R. Sanders
Keyword(s):  
Vertical Integration ◽  
North Atlantic ◽  
Accurate Estimate ◽  
Nitrogen Accumulation ◽  
Excess Nitrogen ◽  
The North ◽  
The North Atlantic ◽  
Organic Nutrients ◽  
Using Data ◽  
Nitrogen Excess

Abstract. To enable an accurate estimate of total excess nitrogen (N) in the North Atlantic, a new tracer TNxs is defined, which includes the contribution of organic nutrients to the assessment of N:P stoichiometric anomalies. We measured the spatial distribution of TNxs within the subtropical North Atlantic using data from a trans-Atlantic section across 24.5° N occupied in 2004. We then employ three different approaches to infer rates of total excess nitrogen accumulation using pCFC-12 derived ventilation ages (a TNxs vertical integration, a one end-member and a two-end member mixing model). Despite some variability among the different methods the dissolved organic nutrient fraction always contributes to about half of the TNxs accumulation, which is in the order of 9.38±4.18×1011 mol N y−1. We suggest that neglecting organic nutrients in stoichiometric balances of the marine N and P inventories can lead to systematic errors when estimating deviations of nitrogen excess or deficit relative to the Redfield ratio in the oceans. For the North Atlantic the inclusion of the organic fraction to the excess nitrogen pool leads to an upward revision of the N supply by N2 fixation to 10.2±6.9×1011 mol N y−1.

scholarly journals Characterization of mixing errors in a coupled physical biogeochemical model of the North Atlantic: implications for nonlinear estimation using Gaussian anamorphosis

Ocean Science ◽  
2010 ◽  
Vol 6 (1) ◽  
pp. 247-262 ◽  
Author(s):  
D. Béal ◽  
P. Brasseur ◽  
J.-M. Brankart ◽  
Y. Ourmières ◽  
J. Verron
Keyword(s):  
North Atlantic ◽  
Chlorophyll Concentration ◽  
Vertical Mixing ◽  
Wind Forcing ◽  
Observation System ◽  
Biogeochemical Model ◽  
Significant Information ◽  
The North ◽  
Biogeochemical Models ◽  
The North Atlantic

Abstract. In biogeochemical models coupled to ocean circulation models, vertical mixing is an important physical process which governs the nutrient supply and the plankton residence in the euphotic layer. However, vertical mixing is often poorly represented in numerical simulations because of approximate parameterizations of sub-grid scale turbulence, wind forcing errors and other mis-represented processes such as restratification by mesoscale eddies. Getting a sufficient knowledge of the nature and structure of these errors is necessary to implement appropriate data assimilation methods and to evaluate if they can be controlled by a given observation system. In this paper, Monte Carlo simulations are conducted to study mixing errors induced by approximate wind forcings in a three-dimensional coupled physical-biogeochemical model of the North Atlantic with a 1/4° horizontal resolution. An ensemble forecast involving 200 members is performed during the 1998 spring bloom, by prescribing perturbations of the wind forcing to generate mixing errors. The biogeochemical response is shown to be rather complex because of nonlinearities and threshold effects in the coupled model. The response of the surface phytoplankton depends on the region of interest and is particularly sensitive to the local stratification. In addition, the statistical relationships computed between the various physical and biogeochemical variables reflect the signature of the non-Gaussian behaviour of the system. It is shown that significant information on the ecosystem can be retrieved from observations of chlorophyll concentration or sea surface temperature if a simple nonlinear change of variables (anamorphosis) is performed by mapping separately and locally the ensemble percentiles of the distributions of each state variable on the Gaussian percentiles. The results of idealized observational updates (performed with perfect observations and neglecting horizontal correlations) indicate that the implementation of this anamorphosis method into sequential assimilation schemes can substantially improve the accuracy of the estimation with respect to classical computations based on the Gaussian assumption.

scholarly journals Assessment of a global climatology of oceanic dimethylsulfide (DMS) concentrations based on SeaWiFS imagery (1998-2001)

2004 ◽  
Vol 61 (5) ◽  
pp. 804-816 ◽  
Author(s):  
S Belviso ◽  
C Moulin ◽  
L Bopp ◽  
J Stefels
Keyword(s):  
North Atlantic ◽  
Surface Waters ◽  
Sea Surface ◽  
Wide Field ◽  
Chl A ◽  
The North ◽  
Wide Field Of View ◽  
Lyase Activity ◽  
Mixed Populations

A method is developed to estimate sea-surface particulate dimethylsulfoniopropionate (DMSPp) and dimethylsulfide (DMS) concentrations from sea-surface concentrations of chlorophyll a (Chl a). When compared with previous studies, the 1° × 1° global climatology of oceanic DMS concentrations computed from 4 years (1998–2001) of Chl a measurements derived from SeaWiFS (satellite-based, sea-viewing wide field of view sensor) exhibits lower seasonal variability in the southern hemisphere than in the northern hemisphere. A first evaluation of the method shows that it reasonably well represents DMSPp and DMS in the North Atlantic subtropical gyre, in large blooms of mixed populations of diatoms and Phaeocystis spp., and in massive blooms of Phaeocystis spp. but fails for large, almost pure blooms of diatoms. DMSPp and DMS concentrations derived from SeaWiFS were also compared with spatially and temporally coincident in situ measurements acquired independently in the Atlantic between 39°N and 45°N and in subtropical and subantarctic Indian Ocean surface waters. Moderate spring and summer phytoplankton blooms there exhibited similar trends in DMSPp and DMS levels vs. moderate blooms of mixed populations of prymnesiophytes and dinoflagellates investigated by others. Measured DMS largely exceeded simulated DMS concentrations, whereas measured and simulated DMSPp levels were in close agreement. DMS accumulation is tentatively attributed to dinoflagellate DMSP lyase activity.

scholarly journals Grazing behavior and winter phytoplankton accumulation

2021 ◽  
Vol 18 (20) ◽  
pp. 5595-5607
Author(s):  
Mara Freilich ◽  
Alexandre Mignot ◽  
Glenn Flierl ◽  
Raffaele Ferrari
Keyword(s):  
North Atlantic ◽  
Phytoplankton Biomass ◽  
Phytoplankton Bloom ◽  
Biomass Accumulation ◽  
Grazing Pressure ◽  
Mathematical Framework ◽  
Grazing Behavior ◽  
Phytoplankton Concentration ◽  
The North ◽  
The North Atlantic

Abstract. Recent observations have shown that phytoplankton biomass increases in the North Atlantic during winter, even when the mixed layer is deepening and light is limited. Current theories suggest that this is due to a release from grazing pressure. Here we demonstrate that the often-used grazing models that are linear at low phytoplankton concentration do not allow for a wintertime increase in phytoplankton biomass. However, mathematical formulations of grazing as a function of phytoplankton concentration that are quadratic at low concentrations (or more generally decrease faster than linearly as phytoplankton concentration decreases) can reproduce the fall to spring transition in phytoplankton, including wintertime biomass accumulation. We illustrate this point with a minimal model for the annual cycle of North Atlantic phytoplankton designed to simulate phytoplankton concentration as observed by BioGeoChemical-Argo (BGC-Argo) floats in the North Atlantic. This analysis provides a mathematical framework for assessing hypotheses of phytoplankton bloom formation.

scholarly journals Western Boundary of the North Atlantic Subtropical Gyre: Decadal Change

2021 ◽  
Author(s):  
Daniel Santana-Toscano ◽  
M. Dolores Pérez-Hernández ◽  
Verónica Caínzos ◽  
Melania Cubas Armas ◽  
Cristina Arumí-Planas ◽  
...  
Keyword(s):  
North Atlantic ◽  
Meridional Overturning Circulation ◽  
Subtropical Gyre ◽  
North Equatorial Current ◽  
Western Boundary ◽  
Boundary Current ◽  
The North ◽  
North Brazil ◽  
Different Seasons ◽  
Using Data

<p>The A20 is a meridional hydrographic section located at 52ºW on the western North Atlantic Subtropical Gyre that encloses the path of the water masses of the Atlantic Meridional Overturning Circulation (AMOC). Using data from three A20 hydrographic cruises carried out in 1997, 2003 and 2012 together with LADCP-SADCP data and the velocities from an inverse box model, the circulation of the western North Atlantic Subtropical Gyre is estimated. The main poleward current of the AMOC is the Gulf Stream (GS) which carries 129.0±10.5 Sv in 2003 and 110.4±12.2 Sv in 2012. Due to the seasonality, the GS position is shifted southward in 2012 - relative to that of 2003 - as both cruises took place in different seasons. In opposite direction, the Deep Western Boundary Current (DWBC) crosses the section twice, first at 39.3-43.2ºN (-34.9±7.5 Sv in 2003 and -25.3±9.4 Sv in 2012) and then at 7.0-11.7ºN (42.0±8.0 Sv in 2003 and 48.0±8.1 Sv in 2012). Additionally, two zonal currents contribute with westward transport below 20ºN: the North Equatorial Current and the North Brazil Current; with a net value of -28.0±4.1 Sv in 2003 and -36.7±3.6 Sv in 2012.</p>

scholarly journals An investigation of grazing behaviors that result in winter phytoplankton biomass accumulation

2020 ◽  
Author(s):  
Mara Freilich ◽  
Alexandre Mignot ◽  
Glenn Flierl ◽  
Raffaele Ferrari
Keyword(s):  
North Atlantic ◽  
Phytoplankton Biomass ◽  
Phytoplankton Bloom ◽  
Biomass Accumulation ◽  
Grazing Pressure ◽  
Mathematical Framework ◽  
Phytoplankton Concentration ◽  
The North ◽  
Bloom Formation ◽  
The North Atlantic

Abstract. Recent observations have shown that phytoplankton biomass increases in the North Atlantic during winter, even when the mixed layer is deepening and light is limited. Current theories suggest that this is due to a release from grazing pressure. Here we demonstrate that the often-used grazing models that are linear at low phytoplankton concentration do not allow for a wintertime increase in phytoplankton biomass. However, certain mathematical formulations of grazing that are quadratic (or more generally non-linear) in phytoplankton concentration at low concentrations can reproduce the fall to spring transition in phytoplankton, including wintertime biomass accumulation. We illustrate this point with a minimal model for the annual cycle of North Atlantic phytoplankton designed to simulate phytoplankton concentration as observed by BioGeoChemical-Argo (BGC-Argo) floats in the North Atlantic. This analysis provides a mathematical framework for assessing hypotheses of phytoplankton bloom formation.

scholarly journals Spring bloom onset in the Nordic Seas

2016 ◽  
Vol 13 (11) ◽  
pp. 3485-3502 ◽  
Author(s):  
Alexandre Mignot ◽  
Raffaele Ferrari ◽  
Kjell Arne Mork
Keyword(s):  
North Atlantic ◽  
Spring Bloom ◽  
Large Fraction ◽  
Biomass Accumulation ◽  
Marine Phytoplankton ◽  
The Arctic ◽  
Nordic Seas ◽  
In Situ Data ◽  
The North ◽  
The North Atlantic

Abstract. The North Atlantic spring bloom is a massive annual growth event of marine phytoplankton, tiny free-floating algae that form the base of the ocean's food web and generates a large fraction of the global primary production of organic matter. The conditions that trigger the onset of the spring bloom in the Nordic Seas, at the northern edge of the North Atlantic, are studied using in situ data from six bio-optical floats released north of the Arctic Circle. It is often assumed that spring blooms start as soon as phytoplankton cells daily irradiance is sufficiently abundant that division rates exceed losses. The bio-optical float data instead suggest the tantalizing hypothesis that Nordic Seas blooms start when the photoperiod, the number of daily light hours experienced by phytoplankton, exceeds a critical value, independently of division rates. The photoperiod trigger may have developed at high latitudes where photosynthesis is impossible during polar nights and phytoplankton enters into a dormant stage in winter. While the first accumulation of biomass recorded by the bio-optical floats is consistent with the photoperiod hypothesis, it is possible that some biomass accumulation started before the critical photoperiod but at levels too low to be detected by the fluorometers. More precise observations are needed to test the photoperiod hypothesis.

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