Agresiones y violencia

In many cases, plankton can be considered passive particles, so ocean current advection has a strong effect on their production and spatial distribution in several ways. Primarily, the flow transports and disperse the main biologi- cal components, such as phytoplankton, zooplankton, bacteria, dissolved and particulate organic matter. Secondly, vertical movements can displace nutrient or phytoplankton within (or out) the euphotic layer and thus modulating the growth rates of the biological dynamics. Also, depending on the encounter rate between nutrients and phytoplankton or phytoplankton and zooplankton their corresponding rate of growth can be affected. The physical mechanisms that affect the marine ecosystems occur over a large range of spatial and temporal scales.

On the planetary scales, and for timescales beyond the year, the transport of nutrients is controlled by the thermohaline and by the wind-driven circulations. Westward propagation associated to Rossby waves is also evident in chlorophyll data derived from satellite measurements of ocean color, although this is due to nonlinear rotating mesoscale coherent structures (Chelton et al., 2011).

At basin scale the production pattern is directly related with the vertical Ekmann pumping. This transport is produced by the constant wind blowing along one direction combined with the Coriolis force. The displacement of surface water mass produce a vertical movement of water, producing the upwelling of rich- nutrients water to the euphotic layer and thereby enhancing phytoplankton growth. This is confirmed by satellite observations. Fig. 1.5 shows a global image of the ocean color, related to phytoplankton, from SeaWIFS. The ocean can be classified in different biogeochemical provinces. The high concentrations of chlorophyll are located in regions of upwelling (eastern boundary currents, equator and subpolar gyres) and minimal in regions of downwelling (in the center of subtropical gyres).

1.5. BIOLOGICAL RESPONSE TO OCEAN TRANSPORT PROCESSES

Figure 1.5: Global view of chlorophyll distribution from SeaWIFS sensor

data generated by the NASA. The image corresponds to one year average. The blue color indicates poor chlorophyll concentrations (i.e. oligotrophic gyres), whereas green to red indicate high chlorophyll concentrations (i.e. eastern boundary upwelling systems, equatorial upwelling, high latitude

spring bloom...).

At mesoscale, frontal and eddy circulations can occur at any depth and affect populations of organisms at all trophic levels, from plankton to top predators (Oschlies and Garçon, 1998; Mahadevan and Archer, 2000; Tew Kai et al., 2009). Fig. 1.3 shows a clear association of plankton variability with mesoscale flow features such as fronts, eddies and filaments. The eddies have two different natures, can increase or reduce the biological production. On one hand, in the biological increasing by eddies there are two mechanism related with the supply of nutrients into the euphotic layer by the deflection of the isopycnal: the eddy-

pumping mechanism associated to vertical transport (McGillicuddy et al., 1998),

and the stretching of water column associated to horizontal transport (Lévy, 2008). These eddies, as cyclonic as anticyclonic, can be formed by baroclinic instabilities of density fronts in the oligotrophic gyres, and thus can be respon- sible of the episodic blooms produced in such poor-nutrients regions (Oschlies and Garçon, 1998; Oschlies, 2002a,b). In this way, mesoscale eddies could be the dominant agents of nutrient transport in the open ocean.

On the other hand, the works by Rossi et al. (2008, 2009); Gruber et al. (2011) show that eddies can suppress phytoplankton productivity in the eastern boundary upwelling, and Williams (2011) propose that the reduction effect of plankton by eddies also occurs in other upwelling areas, such as the Southern Ocean. This will be explained later. Another interpretation is given by Lathuiliere et al. (2011), who used a very simple analytical model to show that the effect of mesoscale activity on plankton production depends on the strength of the

large-scale upwelling. In weak upwelling conditions, phytoplankton abundance increases with the intensity of mesoscale turbulence. In contrast, when the upwelling is stronger than a critical value, phytoplankton abundance decreased due to the intensification of mesoscale turbulence. In this case, the governing process is downward export of phytoplankton below the euphotic layer. In the last years, there are growing evidences that the upper few hundred meters of the oceans are dominated by submesoscale circulations at fronts (Klein and Lapeyre, 2009), and this circulation is important because they can regulate the exchange of properties between the euphotic layer and the ocean interior and thereby the biological production (Lévy et al., 2012). Some preliminary modeling studies to explore the vertical advection at smaller scales (Oschlies and Garçon, 1998; Mahadevan and Archer, 2000; Garçon et al., 2001) suggest that decreasing the resolution of the grid from 10km to 40km can result in errors of 30% in the estimation of PP. Furthermore, some simulations at even higher resolution (2 km) show that the incorrect representation of submesoscale can result in even larger errors, up to 50% (Lévy et al., 2001). This significant increase is mainly due to the resolution of intense vertical velocities, captured within filaments (Klein et al., 1998). In general, the regional flux of nutrients due to submesoscale ver- tical advection depends on how often and how strong is the vertical circulation distribution and on the rate of the exportation of nutrients in upwelling regions by horizontal advection (Martin, 2003; Pasquero et al., 2005). An important difference between submesoscale motions and mesoscale eddies in the biologi- cal production lies in the time scales associated to both processes. Though the vertical velocities associated with mesoscale eddies are much smaller than the submesocale ones, the residence time of nutrient in the euphotic layer is longer for mesoscale eddies than for submesoscale fronts, allowing phytoplankton to uptake nutrients during more time (McGillicuddy et al., 2007).

Strongly non-uniform chaotic mixing was shown to promote coexistence of com- peting species by distributing them along complex manifolds (Tél et al., 2005). Alternatively, the coherent structures characteristics to two-dimensional turbu- lence, like vortices, were shown to contribute to the survival of species with lower fitness. This is supported by recent analysis of satellite data indicating the formation of fluid dynamical niches producing localized transient patches of blooms of different species, which contributes to the maintenance of planktonic biodiversity (d’Ovidio et al., 2010). On the other hand, the structures identified by Lyapunov exponents also influence top marine predators. Trajectories of frigatebirds on top of ridges of Lyapunov structures were found in Tew Kai et al. (2009).

1.6. EFFECT OF MESOSCALE TRANSPORT IN UPWELLING AREAS

1.6

Effect of mesoscale transport in upwelling areas

The Eastern Boundary Upwelling Systems (EBUS) are some of the most produc- tive marine ecosystems of the world oceans. Despite representing less than 1% of the world ocean area, their primary production accounts for about 10% of the oceanic new production (Mackas et al., 2006). EBUSs are characterized by a high abundance of a reduced number of small pelagic fish species that support large fisheries and a number of top predator populations. EBUSs also contribute very significantly to gas exchange between the ocean and the atmosphere, particularly

CO2and N20.

Figure 1.6:A schematic section of coastal upwelling systems and the eddy

formation in the southern hemisphere. Equatorial trade winds are blowing almost all year along the coast, that create westward Ekman drift in the upper layer, and divergence in the coast, that is being compensated by upwelling of deep, cold and nutrient-rich waters. Picture from Williams

(2011).

The four main EBUS, the Canary, California, Humboldt and Benguela Currents, are narrow strips of the ocean that extend latitudinally over several thousands of kilometers and longitudinally to beyond the continental shelves whose widths range from 20 to 200 km. They are located on the western margin of the continents

(eastern part of the oceans), on both hemispheres. In these regions, intense trade winds combined with the earth’s rotation coastal upwelling, bring cold, nutrient- rich water from the deep ocean (of the order of 200 to 300 m) to the surface (see Fig. 1.6). These rich waters reaching the euphotic layer fuel primary production (up to two orders of magnitude higher than in other coastal or open ocean regions), which supports a highly productive food web.

In the past two decades, several multidisciplinary approaches have studied physical and biological processes in these highly dynamic boundary regions (Brink and Cowles, 1991; Barton E. et al., 1998) and more recently (Mackas et al., 2006; Correa-Ramirez et al., 2007; Rossi et al., 2009). It has been shown that (sub)-mesoscale processes are ubiquitous features of these areas, related to the complex coastal circulation and its instabilities, as well as coastline irregularities (Lévy, 2008). This strong (sub)-mesoscale variability also modulates the structure and dynamics of ecosystems. For instance Aristegui et al. (1997) studied the interaction between the Canary island archipelago and the coastal upwelling, showing that both systems present numerous mesoscale structures that strongly interact and influence the biology.

A few academic modelling studies also reported a strong bio-physical interac- tions in upwelling areas. Pasquero et al. (2005) studied the dependence of PP on the spatial and temporal variability of the nutrient flux. Sandulescu et al. (2007, 2008) studied the planktonic biological activity in the wake of an island which is close to an upwelling region. They showed that the interplay between wake structures and biological growth leads to plankton blooms inside mesoscale hy- drodynamic vortices that act as incubators of PP. One of the first high resolution coupled modelling studies of an upwelling area was performed by Koné et al. (2005) over the Benguela area and one year later by Gruber et al. (2006) over the California system. Among other findings, they globally stressed out the impor- tance of the 3-D nature of circulation and mixing in coastal upwelling systems, that strongly influence the biological modelled components.

More recently, Rossi et al. (2008, 2009) from satellite observations, studied the effect of the mesoscale turbulence on the surface phytoplankton distributions. They used the Lyapunov exponents to quantify the mesoscale turbulence of the flow derived from altimetry data, and chlorophyll-a from SeaWIFS data as a proxy of phytoplankton concentration. They observed an anti-correlation between both biological and physical quantities; more mixing less plankton production. Using satellite-based measurements of chlorophyll and eddy kinetic energy, together with model simulations, Gruber et al. (2011) showed that eddies reduce productivity in eastern boundary upwelling zones. Eddies form and provide an opposing circulation (white dashed arrows in Fig. 1.6) to that of the wind blowing along the coast. The overall effect of the winds and eddies is to