Article

Eddy-Driven Stratification Initiates North Atlantic Spring Phytoplankton Blooms

Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA.
Science (Impact Factor: 31.48). 07/2012; 337(6090):54-8. DOI: 10.1126/science.1218740
Source: PubMed

ABSTRACT Springtime phytoplankton blooms photosynthetically fix carbon and export it from the surface ocean at globally important rates. These blooms are triggered by increased light exposure of the phytoplankton due to both seasonal light increase and the development of a near-surface vertical density gradient (stratification) that inhibits vertical mixing of the phytoplankton. Classically and in current climate models, that stratification is ascribed to a springtime warming of the sea surface. Here, using observations from the subpolar North Atlantic and a three-dimensional biophysical model, we show that the initial stratification and resulting bloom are instead caused by eddy-driven slumping of the basin-scale north-south density gradient, resulting in a patchy bloom beginning 20 to 30 days earlier than would occur by warming.

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    • "The s m a l ld i f f e r e n c ei nt h eM-Chl data for F96 and F97 during WES (0.33 ± 0.045 mg m −3 and 0.31 ± 0.026 mg m −3 , respectively) suggests , however, that horizontal advection was not a primary factor controlling phytoplankton concentrations in this region. Furthermore, the increasing I-Chl and I-Tur with increasing PLD cannot simply be explained by eddy-driven stratification that was proposed by Mahadevan et al. (2012). "
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    ABSTRACT: Variability in the chlorophyll a concentration (Chl) in relation to fluctuations in the mixed layer (ML) was investigated together with turbidity (Tur) in the Kuroshio-Oyashio Extension region, using profiling floats. A particular focus was the validity of two hypotheses concerning the spring bloom: the critical depth hypothesis (CDH) and the recently proposed alternative, the disturbance- recovery hypothesis (DRH). During the period from winter to early spring, Chl and Tur integrated over the photosynthetically active layer (PL; defined as the greatest depth of the ML and the euphotic layer) increased with increasing PL depth (PLD), indicating an increase in the phytoplankton biomass. This result is partly consistent with the DRH in that the observed increase in biomass was not explained by an increase in production. Instead, it was more likely attributable to a reduction in the loss rate. However, theoretical analyses revealed that grazer dilution alone could not cause this increase in biomass because such an increase in the ML in the real ocean (as opposed to a dilution experiment within a bottle) would cause a reduction in the mean light intensity. Despite the loss-controlled fluctuation in biomass during the period of low light, a production-driven fluctuation in biomass was also revealed. This occurred when the light intensity was elevated, particularly after late spring, and was consistent with the CDH. Thus, the present study suggests that both the production-driven and loss-driven hypotheses are responsible for the dynamics of the phytoplankton dynamics from winter to spring in the Kuroshio-Oyashio Extension region.
    Journal of Marine Systems 07/2015; 91. DOI:10.1016/j.jmarsys.2015.06.004 · 2.48 Impact Factor
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    • "Energetic mesoscale eddies play a significant role in inducing phytoplankton blooms and redistributing biomass in the ocean [McGillicuddy et al., 1998; Chelton et al., 2011a; Godø et al., 2012; Mahadevan et al., 2012]. Upward nutrient flux driven by the upwelling in cyclonic eddies enhances primary production in the nutrient-limited upper ocean [e.g., Falkowski et al., 1991; Klein and Lapeyre, 2009]. "
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    ABSTRACT: Mesoscale eddies play a significant role in supplying the nutrients required for phytoplankton blooms and redistributing biomass in the ocean. However, how eddies influence nutrient flux and biomass distribution remains unclear. Here we reveal two important dynamical processes (radial displacement and vertical fluctuations) within an anticyclonic eddy by analyzing observations from Argo floats. The Argo floats in the eddy were displaced toward the eddy edge due to the imbalance of radial momentum. Vertical fluctuations below the mixed layer resulted in alternating upwelling and downwelling in the inner and outer parts of the eddy. High salinity deep water was uplifted tens of meters by the upwelling and further extended to the surface with the aid of wind effects. Vertical motions associated with the fluctuations penetrated to the depths of several hundred meters. These processes represent crucial dynamical mechanisms for the motion of particles in mature anticyclonic eddies.
    04/2015; 42:2342-2350. DOI:10.1002/2015GL063120
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    • "The surface physical and biogeochemical setting is described in detail by Alkire et al. (2012), Briggs et al. (2011), Martin et al. (2011) and Mahadevan et al. (2012). Briefly, the onset of the bloom appears to have been triggered by eddy-induced stratification, with chl-a concentrations starting to rise around ten days prior to arrival of the R/V Knorr. "
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