Eddy-driven stratification initiates North Atlantic spring phytoplankton blooms.
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.
- SourceAvailable from: Emmanuel S Boss[Show abstract] [Hide abstract]
ABSTRACT: Satellite measurements allow global assessments of phytoplankton concentrations and, from observed temporal changes in biomass, direct access to net biomass accumulation rates (r). For the subarctic Atlantic basin, analysis of annual cycles in r reveal that initiation of the annual blooming-phase does not occur in spring after stratification surpasses a critical threshold, but rather in early winter when growth conditions for phytoplankton are deteriorating. This finding has been confirmed with in situ profiling float data. The objective of the current study was to test whether satellite-based annual cycles in r are reproduced by the Biogeochemical Element Cycling - Community Climate System Model and, if so, to use the additional ecosystem properties resolved by the model to better understand factors controlling phytoplankton blooms. We find that the model gives a similar early onset time for the blooming phase, that this initiation is largely due to the physical disruption of phytoplankton-grazer interactions during mixed layer deepening, and that parallel increases in phytoplankton specific division and loss rates during spring maintain the subtle disruption in food web equilibrium that ultimately yields the spring bloom climax. The link between winter mixing and bloom dynamics is illustrated by contrasting annual plankton cycles between regions with deeper and shallower mixing. We show that maximum water column inventories of phytoplankton vary in proportion to maximum winter mixing depth, implying that future reductions in winter mixing may dampen plankton cycles in the subarctic Atlantic. We propose that ecosystem disturbance-recovery sequences are a unifying property of global ocean plankton blooms.Global Biogeochemical Cycles 05/2013; · 4.68 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: In the last decade, the rapid advancements in computational power have favored the development of high-resolution numerical models capable of directly resolving small scale structures such as fronts and filaments. Such models have greatly improved our understanding of submesoscale dynamics. At the same time, the small dimensions and short duration of these structures still pose major challenges for small-scale dedicated field experiments. For this reason, submesoscale studies from in-situ observations are still relatively scarce and quantitative estimates of key physical parameters for high-resolution numerical models, such as horizontal eddy diffusivity, are still lacking. This study presents a novel approach for computing in-situ horizontal eddy diffusivity associated with frontal structures by combining cross-front widths derived from surface thermosalinograph sections with stirring rates estimated from Lagrangian drifter trajectories. The method is applied to the measurements collected across a frontal structure observed in the western part of the Gulf of Lion during the Latex10 campaign (LAgrangian Transport EXperiment, September 1-24, 2010). A total of 76 estimates of eddy diffusivity were obtained for strain rates of 0.70 and 1.21 day-1 and front widths (horizontal scales) ranging between 1 and 4 km. The estimates are log-normally distributed, with 70% of the values ranging between 0.4 and 5 m2 s-1. Further analysis based on high-resolution simulations and remote sensed observations, as well as dedicated field experiments will help to assess the robustness of some the assumptions at the base of the proposed approach, and to extend the results to different ocean regions.Journal of Geophysical Research: Oceans. 12/2013;
- [Show abstract] [Hide abstract]
ABSTRACT: Observed phytoplankton interannual variability has been commonly related to atmospheric variables and climate indices. Here we showed that such relation is highly hampered by internal variability associated with oceanic mesoscale turbulence at middle and high latitudes. We used a 1/54° idealized biogeochemical model with a seasonally repeating atmospheric forcing such that there was no external source of interannual variability. At the scale of moorings, our experiment suggested that internal variability was responsible for interannual fluctuations of the subpolar phytoplankton bloom reaching 80% in amplitude and 2 weeks in timing. Over broader scales, the largest impact occurred in the subtropics with interannual variations of 20% in new production. The full strength of this variability could not be captured with the same model run at coarser resolution, suggesting that submesoscale resolving models are needed to fully disentangle the major drivers of biogeochemical variability at interannual time scales. Submesoscales generate interannual variations in phytoplankton bloomsThe variability occurs at local scale and translates to larger scaleHigh horizontal resolution is needed to fully capture itGeophysical Research Letters 01/2014; 41(7). · 3.98 Impact Factor