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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    ABSTRACT: A research cruise in the North Atlantic during the annual diatom bloom provided an ideal platform to study chlorophyll-a (chl-a) transformations associated with a large scale diatom bloom and export below the photic zone. On one deployment, Lagrangian sediment traps captured a significant flux of aggregated diatom cells produced during the termination of the main bloom. We examined the distribution of chl-a transformation products in sinking particles from the sediment traps and in suspended particles from the water column using high-resolution HPLC with multistage mass spectrometry (LC-MSn). There was a dramatic change in the distribution of chl-a and its transformation products between the pre-sinking period, when the average chl-a concentration integrated over the upper 50 m was 68 ± 36 mg m- 2, and the post-sinking period, when it was 30 ± 11 mg m- 2. Before the diatom bloom left the euphotic zone (pre-sinking), suspended particles contained a considerably higher percentage of pheophorbide-a and other chl-a transformation products (27%) than during the post-sinking period (10%). Despite high levels of spatial variability in the chl-a concentration, and despite sampling from both within and outside a main bloom patch, the chl-a transformation products in suspended particles did not exhibit spatial variability. Sinking particles associated with the diatom bloom export had low POC: chl-a ratios (52 - 97), suggesting undegraded phytoplankton cells. However, the samples with especially low POC: chl-a ratios exhibited similar distributions of chl-a transformation products to those with a higher ratio. The proportions of demetalated and de-esterified transformation products increased with depth of suspended particles, although significant levels of these products were also found in the uppermost 20 m during the bloom. This suggests processes in both surface waters and through the water column led to the formation of these products.
    Marine Chemistry 03/2015; 172:23-33. DOI:10.1016/j.marchem.2015.03.005 · 3.20 Impact Factor
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