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.
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ABSTRACT: A three dimensional biophysical model was employed to illustrate the biological impacts of a meandering frontal jet, in terms of efficiency and persistency of the autotrophic frontal production, in marginal and semi-enclosed seas. We used the Alboran Sea of the Western Mediterranean as a case study. Here, a frontal jet with a width of 15-20 km, characterized by the relatively low density Atlantic water mass, flows eastward within the upper 100 m as a marked meandering current around the western and the eastern anticyclonic gyres prior to its attachment to the North African shelf/slope topography of the Algerian basin. Its inherent nonlinearity leads to the development of a strong ageostrophic cross-frontal circulation that supplies nutrients into the nutrient-starved euphotic layer and stimulates phytoplankton growth along the jet. Biological production is larger in the western part of the basin and decreases eastwards with the gradual weakening of the jet. The higher production at the subsurface levels suggests that the Alboran Sea is likely more productive than predicted by the satellite chlorophyll data. The Mediterranean water mass away from the jet and the interiors of the western and eastern anticyclonic gyres remain unproductive.PLoS ONE 11/2014; 9(11):e111482. · 3.53 Impact Factor
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ABSTRACT: In marine ecosystems, like most natural systems, patchiness is the rule. A characteristic of pelagic ecosystems is that their 'substrate' consists of constantly moving water masses, where ocean surface turbulence creates ephemeral oases. Identifying where and when hotspots occur and how predators manage those vagaries in their preyscape is challenging because wide-ranging observations are lacking. Here we use a unique data set, gathering high-resolution and wide-range acoustic and GPS-tracking data. We show that the upper ocean dynamics at scales less than 10 km play the foremost role in shaping the seascape from zooplankton to seabirds. Short internal waves (100 m-1 km) play a major role, while submesoscale (~1-20 km) and mesoscale (~20-100 km) turbulence have a comparatively modest effect. Predicted changes in surface stratification due to global change are expected to have an impact on the number and intensity of physical structures and thus biological interactions from plankton to top predators.Nature Communications 01/2014; 5:5239. · 10.74 Impact Factor
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ABSTRACT: Here we examine the impact of mesoscale processes on the global marine biogeochemical system by performing simulations at two different resolutions, 2◦ (LO-res) and 1/4◦ resolution (HI-res) using the PELAGOS model. Both the LO-res and HI-res simulations are set up with the same forcings and biogeochemical parameterizations, while the initial conditions are provided by a spinup of the LO-res simulation. This allows us to perform a direct inter- comparison of the two cases with a view to understanding how the introduction of mesoscale features affects the biogeochemical system, specifically how differences in the resolved horizontal and vertical motions are reflected in the plankton biomass and the nutrient availability. While the global large-scale oceanographic features (fronts, gyres, etc) are captured in both the LO-res and HI-res simulations, differences in the mesoscale flow structures, and in particular the resolved vertical physics in the HI-res simulation generate very different behaviour in the biogeochemical system. These differences in the physics drives what is a spun-up biogeochemical system in the LO-res simulation into a new regime in the HI-res simulation with significant reduction of typical low resolution biases. Coastal features are well reproduced due to stronger Ekman upwelling at the continental margins and increased eddy kinetic energy in the Southern Ocean significantly reduces the winter overestimation. These biases in the LO-res model are a result of inadequate vertical dynamics. The enhancement of surface chlorophyll can be attributed to improvements in the winter mixed layer in some regions such as the North Atlantic, while it is overall the difference in the Ekman vertical velocity which improves surface production allowing to simulate more realistic deep chlorophyll maxima as well. While the HI-res is better than the LO-res at capturing the timing of the spring bloom in the Southern Ocean, it still overestimates the peak of the bloom, hinting at the need to better understand the driving forces of the seasonal cycle of sub-Antarctic plankton dynamics.Journal of Marine Systems 10/2014; · 2.48 Impact Factor