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High-latitude bloom theories
The strong seasonality of primary production in the Southern Ocean is characteristic of high-latitude oceans. This seasonal cycle is characterised by a sudden increase in the population of phytoplankton (which surface imprint is usually observed in late winter, early spring), called phytoplankton bloom or simply, bloom.
Blooms are typical of high-latitude ocean regions and they have been widely studied, with special emphasis in the North-Atlantic (NA). One of the first (and inspiring) studies about NA blooms was made by Sverdrup in 1953. In it, Sverdrup proposed that blooms initiate in spring when the mixed layer (i.e., the upper-ocean layer which hydrographical properties -T and S- are homoge-neously mixed) reaches a ’critical depth’ at which integrated production overcomes the integrated losses associated with respiration and mortality (figure 1.1 a). The fundamental concepts in Sver-drup’s hypothesis (usually referred to as the Critical depth hypothesis) were based on previous studies by Gran and Braarud [1935] and Riley [1942], but Sverdrup elaborated an apparently easy-testable and quantitative criteria that strongly appealed the oceanographic community. In his theory, seasonality of the mixed layer depth (MLD) has a major role on bloom initiation (figure 1.1.a1 ).
1Caption of figure 1.1: Comparison of bloom hypotheses: (a) the critical depth hypothesis (CDH), (b) the critical turbulence hypothesis (CTH), and (c) the disturbance-recovery hypothesis (DRH). The seasonal cycle in each plot Since the Critical depth hypothesis was published sixty years ago, a number of works have adapted Sverdrup bloom’s conceptual model to experimental data in different high-latitude regions (Nel-son and Smith [1991]; Obata et al. [1996]; Siegel [2002]). At the same time, some others have documented cases where a bloom developed in absence of stratified waters (Townsend et al. [1992]; Dale et al. [1999]). Such apparently controversial results motivated some authors to propose a refinement of Sverdrup’s theory regarding the use of a “mixing depth” rather than a mixed-layer depth: the Critical tubulence hypothesis (Brainerd and Gregg [1995]; Chiswell [2011]; Taylor and Ferrari [2011]). According to these authors, surface blooms appear when vertical turbulence weakens (in late winter, due to weaker winds and net positive heat fluxes) creating stratified upper layers that maintain phytoplankton close to the enlightened zone (i.e., the euphotic layer) of the water-column (figure 1.1.b).
Both the critical depth and the critical turbulence hypotheses emphasised a ‘bottom up’ control (i.e., by light and nutrients) of phytoplankton population. An alternative ‘top down’ view of phytoplankton bloom dynamics (based on the ideas of Cushing [1962]) has also gained prominence in recent years (Banse [1992]; Behrenfeld [2010], Behrenfeld et al. [2013a]). This view suggests that initiation of blooms is due to subtle imbalances in predator–prey relationships that occur over winter (due to deep mixed layer dilution of populations; figure 1.1.c). The bloom theories review by Behrenfeld and Boss [2014] called this ’top-down’ view as the Disturbance – Recovery Hypothesis (hereinafter DRH) and sustained it by experimental and model data. When assuming DRH, bloom onset are always found in winter, much earlier than onset predicted by Sverdrup’s based theories.
In this work, high-latitude phytoplankton blooms were studied not in the North-Atlantic but in Southern Ocean waters. Except for the fact that both regions are situated at similar latitudes (thus, both receive a similar solar radiation throughout the year), these two oceanic regions present many more differences than similarities. At first (satellite) sight, one of the most striking differences, is that bloom variability (in space, timing and magnitude) is much higher in the Southern Ocean. For instance, while in the North-Atlantic spring surface bloom emergence is apparently propagated from south to north in a quasi-zonal pattern (figure 1.2 (a); Siegel [2002] and Henson et al. [2009]), remote-sensing observations in the Southern Ocean reveal strong asymmetries and latitudinal variations (figure 1.2 (b); Thomalla et al. [2011]).
With the aim to understand the whole dynamics (i.e., at surface and at depth) behind Southern Ocean blooms, our approach was based on two assumptions: first, we assumed that North-Atlantic and Southern Ocean blooms were driven by the same fundamental mechanisms; second, we assumed that the strong bloom variability observed in the Southern Ocean was due to the specificities that differentiate this ocean from the North-Atlantic. In the next section, I will
begins with summer on the left. Thick black lines indicate mixed-layer depth (MLD). From Behrenfeld and Boss [2014] resume some of these Southern Ocean specificities, especially those likely to influence bloom dynamics at the present and in the future.
The Southern Ocean
General circulation and frontal dynamics
The Southern Ocean is the only ocean in the world free of latitudinal boundaries. It forms a 21,000km perimeter circumference around the Antarctic continent (i.e., annular) which surface represents the 20% of global ocean surface. Besides this particular geography, powerful and per-sistent westerly winds drive the Antarctic Circumpolar Current (ACC), a strong and voluminous current that flows from west to east enclosing the Antarctic continent (figure 1.3a).
On the northern edge of the ACC, subtropical gyres flow counterclockwise, and their intense and energetic western boundary currents join the northern branches of the ACC in the western Atlantic, Indian, and Pacific basins (figure 1.3a). The ACC and the western boundary currents have a profound influence on the physical and geochemical characteristics of the Southern Ocean (Rintoul et al. [2010]). They form meridional dynamical barriers (Sall´ee et al. [2008]) that receive the name of fronts, and divide the Southern Ocean into four annular regions (figure 1.3b). These zones are, from north to south:
Figure 1.3: Left: A schematic view of the major ocean currents of the Southern Hemisphere oceans south of 201°S. Depths shallower than 3500m are shaded. C, current; G, gyre; F, front; ACC, Antarctic Circumpolar Current. From [Rintoul, 2009]. Right: A schematic view of the ocean frontal system in the Southern Ocean. From H.Grobe, Alfred Wegener Institute.
• The subtropical zone, around 30°S, characterized by stratified surface layers, and relatively weak wind and buoyancy forcing; delimited at south for the sub-tropical front.
• The sub-Antarctic zone, directly north of the ACC, which is characterized by very deep mixed-layers, intense winds, large buoyancy forcing, and the presence of the energetic west-ern boundary currents.
• The ACC (or Polar Frontal) zone, characterized by the top-to-bottom and large circumpolar current.
• The subpolar zone, south of the Polar Front, characterized by the seasonal presence of sea-ice, and a relatively stratified surface layer.
In addition to this upper-ocean circulation, the density gradient associated to ACC together with the influence of strong heat and freshwater fluxes provides a low-resistance pathway from the deep ocean to the surface (upwelling). The combination of both horizontal and vertical water displacements creates a complex circulation in the Southern Ocean that has an important impli-cation for the global ocean circulation. At Weddell Sea, the low buoyancy of the surface waters masses and the bathymetric slope create the appropriate conditions for deep water formation. This surface to bottom transport constitutes the southern limb of the Meridional Overturning Circulation (MOC).
Mixed layer and seasonal variability
In the Southern Ocean, the MLD presents a large spatial variability with some regions where the seasonal cycle is highly marked. Summer MLDs reach about 100m in the vicinity of the ACC. Winter cooling destabilises the water column and increases the MLD. The deepest winter mixed layers (and largest seasonal cycles) are found north of the Antarctic Circumpolar Current, particularly in the eastern Indian and Pacific Oceans. The loss on buoyancy during autumn and winter gradually deepens the mixed layer between January and September. Warming during spring and early summer rapidly re-establishes the shallow summer mixed layer. The amplitude of the seasonal cycle exceeds 400 m in some locations north of the ACC (figure 1.4).
The intra-seasonal and interannual variability of MLD about this large seasonal cycle is sub-stantial, with values exceeding several hundred meters and a standard deviation for the whole Southern Ocean (35–65°S) of 20 m in summer and 60 m in winter.
Figure 1.4: Seasonal cycle of mixed-layer depth. (a) Summer (January) and (b) winter (August) mean depth of the Southern Ocean mixed layer. From Sall´ee et al. [2008].
This estimation of the MLD seasonal cycle and spatial distribution in the Southern Ocean has been extracted from Sall´ee et al. [2008]. In their work, mixed layer was estimated using tempera-ture and salinity measurements of the water-column acquired by autonomous floats (Argo floats). MLD was extracted from these measurements using a density criterion ofΔ fl Æ0.03 kg/m3. Such a criterion has been shown to be the most appropriate for Southern Ocean characteristics (Dong et al. [2008], Sall´ee et al. [2006]) and allows to quantify the upper-ocean layer where the effect of mixing has homogenised the water column. The MLD is a useful concept to address the state of the water-column physical properties however, as shown by Huisman et al. [1999], Chiswell [2011] or Taylor and Ferrari [2011], the MLD fails to correctly represent the processes that control phy-toplankton vertical distribution. To overcome this issue, a new concept, the mixing layer depth, has appeared in recent years in the oceanographic community. The concept of mixing layer depth (which will recurrently appear throughout this manuscript) refers to the upper ocean layer that is actively mixed due to strong turbulence. This magnitude (with m2/s units) is a quantification of the small scale vertical motions that drive phytoplankton inside the water-column. The mix-ing layer depth is much more sensitive to changes in air-sea fluxes (Taylor and Ferrari [2011]) than the MLD. In consequence, seasonal evolution of the mixing depth is strongly perturbed by short intra-seasonal events and can be decoupled from the MLD seasonal cycle (especially in early spring). An important part of the debate on the Critical depth hypothesis comes (as re-cently proposed by Franks [2014] in an excellent work exclusively consecrated to this topic) from the ambiguity of Sverdup’s words and a posterior misinterpretation of his main assumptions. In words of Franks [2014], Sverdrup (1953) wrote about a ”mixed layer”, though he was clearly referring to a ”turbulent layer”, the waters that are kept in motion through turbulence.(…)
Table of contents :
1 General Context
1.1 High-latitude bloom theories
1.2 The Southern Ocean
1.3 Specific objectives and structure
METHODS
2 Observations
2.1 Introduction
2.2 Observations in the Indian Sector of the Southern Ocean
2.2.1 KERFIX station
2.2.2 KEOPS project
2.2.3 Kerguelen elephant seals
2.3 Ocean colour data in the Southern Ocean
2.4 Argo floats
3 Models
3.1 Introduction
3.2 NEMO modelling environment
3.3 Coupled Earth system models
3.4 Regional forced models
3.5 Water-column (1D) biogeochemical model
RESULTS 57
4 Bloom dynamics: mechanisms and phenology
4.1 Introduction
4.2 Onset, intensification and decline of phytoplankton blooms in the Southern Ocean (Article)
4.3 Conclusions
5 Bloom phenology in the Souhtern Ocean
5.1 Introduction
5.2 Characterisation of distinct bloom phenology regimes in the Southern ocean (Article)
5.3 Integrated view of bloom phenology regimes using a regional model
5.4 Conclusions
6 Sensitivity of Southern Ocean primary production to Climate Change
6.1 Introduction
6.2 CMIP5 projections in the Southern Ocean
6.2.1 Projected trends on primary production
6.2.2 Projected changes in the MLD and its influence on #PP
6.3 Mechanics of mixing and iron supply control over primary production
6.3.1 Winter and summer stratification influence on primary production
6.3.2 How vertical iron supply controls community structure?
6.3.3 Winter mixing and vertical iron supply as coupled drivers
6.3.4 What is the net effect of summer stratification?
6.4 Shedding light on hidden future drivers
6.5 Summary and Conclusions
Conclusions and Perspectives
