scholarly journals Upper ocean mixing controls the seasonality of planktonic foraminifer fluxes and associated strength of the carbonate pump in the oligotrophic North Atlantic

2014 ◽  
Vol 11 (8) ◽  
pp. 12223-12254 ◽  
Author(s):  
K. H. Salmon ◽  
P. Anand ◽  
P. F. Sexton ◽  
M. Conte
Keyword(s):  
Mixed Layer ◽  
North Atlantic ◽  
Community Dynamics ◽  
Phytoplankton Bloom ◽  
North Atlantic Ocean ◽  
Planktonic Foraminifera ◽  
Upper Ocean ◽  
Late Winter ◽  
Planktonic Foraminifer ◽  
Spring Phytoplankton Bloom

Abstract. Oligotrophic regions represent up to 75% of Earth's open-ocean environments, and are typically characterized by nutrient-limited upper-ocean mixed layers. They are thus areas of major importance in understanding the plankton community dynamics and biogeochemical fluxes. Here we present fluxes of total planktonic foraminifera and eleven planktonic foraminifer species from a bi-weekly sediment trap time series in the oligotrophic Sargasso Sea, subtropical western North Atlantic Ocean at 1500 m water depth, over two ∼2.5 year intervals, 1998–2000 and 2007–2010. Foraminifera flux was closely correlated with total mass flux and with carbonate and organic carbon fluxes. We show that the planktonic foraminifera flux increases approximately five-fold during the winter–spring, contributing up to ∼40% of the total carbonate flux, driven primarily by increased fluxes of deeper dwelling ("globorotaliid") species. Interannual variability in total foraminifera flux, and in particular fluxes of the deep dwelling Globorotalia truncatulinoides, Globorotalia hirsuta, Globorotalia inflata, were related to differences in seasonal mixed layer dynamics affecting the strength of the spring phytoplankton bloom and export flux, and by the passage of mesoscale eddies. The heavily calcified, dense carbonate tests of deeper dwelling species (3 times denser than surface dwellers) can contribute up to 90% of the foraminiferal-derived carbonate in this region during late winter-early spring, implying a high seasonality of the biological carbonate pump in oligotrophic oceanic regions. Our data suggest that climate cycles, such as the North Atlantic Oscillation, that modulate the depth of the mixed layer, intensity of nutrient upwelling and primary production could also modulate the strength of the biological carbonate pump in the oligotrophic North Atlantic.

scholarly journals Upper ocean mixing controls the seasonality of planktonic foraminifer fluxes and associated strength of the carbonate pump in the oligotrophic North Atlantic

2015 ◽  
Vol 12 (1) ◽  
pp. 223-235 ◽  
Author(s):  
K. H. Salmon ◽  
P. Anand ◽  
P. F. Sexton ◽  
M. Conte
Keyword(s):  
North Atlantic ◽  
Community Dynamics ◽  
Phytoplankton Bloom ◽  
North Atlantic Ocean ◽  
Planktonic Foraminifera ◽  
Euphotic Zone ◽  
Early Spring ◽  
Late Winter ◽  
Planktonic Foraminifer ◽  
Spring Phytoplankton Bloom

Abstract. Oligotrophic regions represent up to 75% of Earth's open-ocean environments. They are thus areas of major importance in understanding the plankton community dynamics and biogeochemical fluxes. Here we present fluxes of total planktonic foraminifera and 11 planktonic foraminifer species measured at the Oceanic Flux Program (OFP) time series site in the oligotrophic Sargasso Sea, subtropical western North Atlantic Ocean. Foraminifera flux was measured at 1500 m water depth, over two ~ 2.5-year intervals: 1998–2000 and 2007–2010. We find that foraminifera flux was closely correlated with total mass flux, carbonate and organic carbon fluxes. We show that the planktonic foraminifera flux increases approximately 5-fold during the winter–spring, contributing up to ~ 40% of the total carbonate flux. This was primarily driven by increased fluxes of deeper-dwelling globorotaliid species, which contributed up to 90% of the foraminiferal-derived carbonate during late winter–early spring. Interannual variability in total foraminifera flux, and in particular fluxes of the deep-dwelling species (Globorotalia truncatulinoides, Globorotalia hirsuta and Globorotalia inflata), was related to differences in seasonal mixed layer dynamics affecting the strength of the spring phytoplankton bloom and export flux, and by the passage of mesoscale eddies. As these heavily calcified, dense carbonate tests of deeper-dwelling species (3 times denser than surface dwellers) have greater sinking rates, this implies a high seasonality of the biological carbonate pump in oligotrophic oceanic regions. Our data suggest that climate cycles, such as the North Atlantic Oscillation, which modulates nutrient supply into the euphotic zone and the strength of the spring bloom, may also in turn modulate the production and flux of these heavily calcified deep-dwelling foraminifera by increasing their food supply, thereby intensifying the biological carbonate pump.

scholarly journals Characterizing upper-ocean mixing and its effect on the spring phytoplankton bloom with in situ data

2015 ◽  
Vol 72 (6) ◽  
pp. 1961-1970 ◽  
Author(s):  
Sarah R. Brody ◽  
M. Susan Lozier
Keyword(s):  
Mixed Layer ◽  
Spring Bloom ◽  
Phytoplankton Bloom ◽  
Upper Ocean ◽  
Late Winter ◽  
Uniform Density ◽  
Spring Phytoplankton Bloom ◽  
In Situ Data ◽  
Active Mixing

Abstract Since publication, the Sverdrup hypothesis, that phytoplankton are uniformly distributed within the ocean mixed layer and bloom once the ocean warms and stratifies in spring, has been the conventional explanation of subpolar phytoplankton spring bloom initiation. Recent studies have sought to differentiate between the actively mixing section of the upper ocean and the uniform-density mixed layer, arguing, as Sverdrup implied, that decreases in active mixing drive the spring bloom. In this study, we use in situ data to investigate the characteristics and depth of active mixing in both buoyancy- and wind-driven regimes and explore the idea that the shift from buoyancy-driven to wind-driven mixing in the late winter or early spring creates the conditions necessary for blooms to begin. We identify the bloom initiation based on net rates of biomass accumulation and relate changes in the depth of active mixing to changes in biomass depth profiles. These analyses support the idea that decreases in the depth of active mixing, a result of the transition from buoyancy-driven to wind-driven mixing, control the timing of the spring bloom.

scholarly journals Interannual Variability of the Mixed Layer Winter Convection and Spice Injection in the Eastern Subtropical North Atlantic

2015 ◽  
Vol 45 (2) ◽  
pp. 504-525 ◽  
Author(s):  
Nicolas Kolodziejczyk ◽  
Gilles Reverdin ◽  
Alban Lazar
Keyword(s):  
Interannual Variability ◽  
Mixed Layer ◽  
North Atlantic ◽  
Mixed Layer Depth ◽  
Buoyancy Flux ◽  
Upper Ocean ◽  
Late Winter ◽  
Layer Depth ◽  
Ocean Layer ◽  
Bulk Model

AbstractThe Argo dataset is used to study the winter upper-ocean conditions in the northeastern subtropical (NEA) Atlantic during 2006–12. During late winter 2010, the mixed layer depth is abnormally shallow and a negative anomaly of density-compensated salinity, the so-called spiciness, is generated in the permanent pycnocline. This is primarily explained by unusual weak air–sea buoyancy flux during the late winter 2010, in contrast with the five other studied winters. Particularly deep mixed layers and strong spiciness anomalies are observed during late winter 2012. The 2010 winter conditions appear to be related to historically low North Atlantic Oscillation (NAO) and high tropical North Atlantic index (TNA). Interannual variability of the eastern subtropical mixed layer is further investigated using a simple 1D bulk model of mean temperature and salinity linear profiles, based on turbulent kinetic energy conservation in the upper-ocean layer, and forced only with seasonal air–sea buoyancy forcing corresponding to fall–winter 2006–12. It suggests that year-to-year variability of the winter convective mixing driven by atmospheric buoyancy flux is able to generate interannual variability of both late winter mixed layer depth and spiciness in a strongly compensated layer at the base of the mixed layer and in the permanent pycnocline.

scholarly journals Iron stable isotopes track pelagic iron cycling during a subtropical phytoplankton bloom

2014 ◽  
Vol 112 (1) ◽  
pp. E15-E20 ◽  
Author(s):  
Michael J. Ellwood ◽  
David A. Hutchins ◽  
Maeve C. Lohan ◽  
Angela Milne ◽  
Philipp Nasemann ◽  
...  
Keyword(s):  
Stable Isotopes ◽  
Mixed Layer ◽  
Phytoplankton Bloom ◽  
Iron Bioavailability ◽  
Global Ocean ◽  
Minor Role ◽  
Surface Mixed Layer ◽  
Spring Phytoplankton Bloom ◽  
Dissolved Iron ◽  
Particulate Iron

The supply and bioavailability of dissolved iron sets the magnitude of surface productivity for ∼40% of the global ocean. The redox state, organic complexation, and phase (dissolved versus particulate) of iron are key determinants of iron bioavailability in the marine realm, although the mechanisms facilitating exchange between iron species (inorganic and organic) and phases are poorly constrained. Here we use the isotope fingerprint of dissolved and particulate iron to reveal distinct isotopic signatures for biological uptake of iron during a GEOTRACES process study focused on a temperate spring phytoplankton bloom in subtropical waters. At the onset of the bloom, dissolved iron within the mixed layer was isotopically light relative to particulate iron. The isotopically light dissolved iron pool likely results from the reduction of particulate iron via photochemical and (to a lesser extent) biologically mediated reduction processes. As the bloom develops, dissolved iron within the surface mixed layer becomes isotopically heavy, reflecting the dominance of biological processing of iron as it is removed from solution, while scavenging appears to play a minor role. As stable isotopes have shown for major elements like nitrogen, iron isotopes offer a new window into our understanding of the biogeochemical cycling of iron, thereby allowing us to disentangle a suite of concurrent biotic and abiotic transformations of this key biolimiting element.

scholarly journals Energetic Submesoscales Maintain Strong Mixed Layer Stratification during an Autumn Storm

2017 ◽  
Vol 47 (10) ◽  
pp. 2419-2427 ◽  
Author(s):  
Daniel B. Whitt ◽  
John R. Taylor
Keyword(s):  
Mixed Layer ◽  
North Atlantic ◽  
Small Scale ◽  
Upper Ocean ◽  
Ocean Mixed Layer ◽  
Ocean Stratification ◽  
The North ◽  
Large Eddy ◽  
The Mean ◽  
Scale Turbulence

AbstractAtmospheric storms are an important driver of changes in upper-ocean stratification and small-scale (1–100 m) turbulence. Yet, the modifying effects of submesoscale (0.1–10 km) motions in the ocean mixed layer on stratification and small-scale turbulence during a storm are not well understood. Here, large-eddy simulations are used to study the coupled response of submesoscale and small-scale turbulence to the passage of an idealized autumn storm, with a wind stress representative of a storm observed in the North Atlantic above the Porcupine Abyssal Plain. Because of a relatively shallow mixed layer and a strong downfront wind, existing scaling theory predicts that submesoscales should be unable to restratify the mixed layer during the storm. In contrast, the simulations reveal a persistent and strong mean stratification in the mixed layer both during and after the storm. In addition, the mean dissipation rate remains elevated throughout the mixed layer during the storm, despite the strong mean stratification. These results are attributed to strong spatial variability in stratification and small-scale turbulence at the submesoscale and have important implications for sampling and modeling submesoscales and their effects on stratification and turbulence in the upper ocean.

Long-term ventilation changes of Subpolar Mode Waters in the North Atlantic Ocean and its impact on the oxygen distribution

2021 ◽  
Author(s):  
Ilaria Stendardo ◽  
Bruno Buongiorno Nardelli ◽  
Sara Durante
Keyword(s):  
Atlantic Ocean ◽  
North Atlantic ◽  
North Atlantic Ocean ◽  
Horizontal Resolution ◽  
Late Winter ◽  
Mode Water ◽  
Mode Waters ◽  
The North ◽  
The North Atlantic ◽  
The Impact

<p>In the subpolar North Atlantic Ocean, Subpolar Mode Waters (SPMWs) are formed during late winter convection following the cyclonic circulation of the subpolar gyre. SPMWs participate in the upper flow of the Atlantic overturning circulation (AMOC) and provide much of the water that is eventually transformed into several components of the North Atlantic deep water (NADW), the cold, deep part of the AMOC. In a warming climate, an increase in upper ocean stratification is expected to lead to a reduced ventilation and a loss of oxygen. Thus, understanding how mode waters are affected by ventilation changes will help us to better understand the variability in the AMOC. In particular, we would like to address how the volume occupied by SPMWs has varied over the last decades due to ventilation changes, and what are the aspects driving the subpolar mode water formation, their interannual variations as well as the impact of the variability in the mixing and subduction and vertical dynamics on ocean deoxygenation. For this purpose, we use two observation-based 3D products from Copernicus Marine Service (CMEMS), the ARMOR3D and the OMEGA3D datasets. The first consists of 3D temperature and salinity fields, from the surface to 1500 m depth, available weekly over a regular grid at 1/4° horizontal resolution from 1993 to present. The second consists of observation-based quasi-geostrophic vertical and horizontal ocean currents with the same temporal and spatial resolution as ARMOR3D.</p>

scholarly journals The Intraseasonal Dynamics of the Mixed Layer Pump in the Subpolar North Atlantic Ocean: A Biogeochemical‐Argo Float Approach

2019 ◽  
Vol 33 (3) ◽  
pp. 266-281 ◽  
Author(s):  
L. Lacour ◽  
N. Briggs ◽  
H. Claustre ◽  
M. Ardyna ◽  
G. Dall'Olmo
Keyword(s):  
Atlantic Ocean ◽  
Mixed Layer ◽  
North Atlantic ◽  
North Atlantic Ocean ◽  
Argo Float

Modelling the Initiation of Spring Phytoplankton Blooms: a Synthesis of Physical and Biological Interannual Variability off Southwest Nova Scotia, 1983–85

1989 ◽  
Vol 46 (S1) ◽  
pp. s183-s199 ◽  
Author(s):  
R. Ian Perry ◽  
Peter C. F. Hurley ◽  
Peter C. Smith ◽  
J. Anthony Koslow ◽  
Robert O. Fournier
Keyword(s):  
Nova Scotia ◽  
Mixed Layer ◽  
Phytoplankton Bloom ◽  
Mixed Layer Depth ◽  
Critical Value ◽  
Zooplankton Biomass ◽  
Phytoplankton Production ◽  
Spring Phytoplankton Bloom ◽  
Turbulent Dissipation ◽  
Differential Advection

Chlorophyll and nitrate data from monthly surveys off southwest Nova Scotia indicate the spring phytoplankton bloom began near the end of March of each year, occurring early (late) in 1984 (1983). The highest chlorophyll biomass(all months) was found in 1985. Using survey data, the Sverdrup hypothesis for the initiation of the bloom was tested by comparing the critical depth, Zcr, for net phytoplankton production to the observed mixed-layer depth, Zmix. Survey median Zcr/Zmix were consistently less than 1 until May, suggesting that observed blooms were initiated by events outside the specific survey periods. Results of a mixed-layer model incorporating surface heating, differential advection and turbulent dissipation by wind and tide showed reasonable agreement with observed mixed depths, and patterns of the mean (modelled) mixed-layer light intensity are significantly correlated with observed chlorophyll biomass. In 1983 and 1984, mean light intensities first exceeded the critical value for a bloom to occur in late March. In 1985, transient periods of stratification in mid-February and early March produced intensities greater than the critical value. These events, together with higher nitrate concentrations and lower Zooplankton biomass, appear to be responsible for the high chlorophyll biomass observed in 1985.

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