Phyto-plankton Blooms, Vortices & Ocean Water

 

Turbulence and coherent circulation structures, such as submesoscale and mesoscale eddies, convective plumes and Langmuir cells, play a critical role in shaping phytoplankton spatial distribution and population dynamics. We use a framework of advection–reaction–diffusion equations to investigate the effects of turbulent transport on the phytoplankton population growth and its spatial structure in a vertical two-dimensional vortex flow field. In particular, we focus on how turbulent flow velocities and sinking influence phytoplankton growth and biomass aggregation. Our results indicate that conditions in mixing and growth of phytoplankton can drive different vertical spatial structures in the mixed layer, with the depth of the mixed layer being a critical factor to allow coexistence of populations with different sinking speed. With increasing mixed layer depth, positive growth for sinking phytoplankton can be maintained with increasing turbulent flow velocities, allowing the apparently counter-intuitive persistence of fast sinking phytoplankton populations in highly turbulent and deep mixed layers. These dynamics demonstrate the role of considering advective transport within a turbulent vortex and can help to explain observed phytoplankton biomass during winter in the North Atlantic, where the overturn of deep convection has been suggested to play a critical role in phytoplankton survival.

1. Introduction

Phytoplankton play a central role in the global carbon cycle as it absorbs dissolved inorganic carbon during photosynthesis, some of which is exported to the ocean’s interior [1,2]. Ocean dynamics can drive primary production and carbon export at local and global scales [3,4], by controlling biotic and abiotic factors such as light and nutrient limitation, grazing [5] and viruses [6]. Being ultimately driven by light, primary production depends on the net phytoplankton growth between the sea surface and the bottom of the surface mixed layer, the mixed layer depth (MLD). In a seminal paper, Huisman et al. [7] investigated the relation between turbulence diffusion, MLD and phytoplankton growth within a homogeneous mixed surface layer and found that growth conditions are not sufficiently described by the classical critical depth [8], defined as the depth where the vertically integrated production balances the net losses within an actively mixed layer. Using a general water column model, they showed that sinking phytoplankton cells manage to persist at intermediate levels of turbulence irrespective of the critical depth [7,9]. Turbulent mixing in the ocean does affect the vertical displacement of phytoplankton cells and, depending on the sinking or buoyancy properties of these cells, different aggregation layers can be found in the water column [10,11].

At high turbulent levels above a critical threshold [7], the increase in light limitation due to a lower average position of the cells in the water column creates unfavourable conditions for growth. On the other hand, low turbulence levels may not be able to sustain sinking cells, thus limiting growth and survival [12]. This relation depends on turbulent mixing and cell sinking rates [13], the latter of which can vary with cell size, species and physiological state [14,15]. Consequentially, the theoretical basis for these critical threshold values of turbulence has been developed in a one-dimensional framework which includes homogeneous vertical eddy diffusion and cell sinking as mechanisms for particle re-suspension and persistence in the water column [7,12]. Eddy diffusion is a common way of including diffusive and advective processes in a single parameter; however, in the ocean both diffusion and advection by themselves can contribute to the mixing of suspended matter, regulating phytoplankton growth and distribution [16]. Indeed, three-dimensional eddy-like structures of various shapes, sizes and intensities are ubiquitous features in the ocean that set the underlying physics driving spatial heterogeneity and the dispersion of marine plankton [17], hence regulating growth and survival. Several physical mechanisms can shape these structures and provide different properties in terms of sinking and dispersion of the suspended matter. For example, shear forces originating from horizontal density gradients can generate mesoscale (10–200 km) and submesoscale (0.1–10 km) eddies [17,18], which typically provide nutrients to the euphotic zone and regulate particle sinking [1921]. Baroclinic instability in the inner part of the ocean can drive the formation of convective plumes (<1 km) with localized areas of strong downward currents (downwelling) and larger areas of upward currents (upwelling) [22]. On smaller scales, strong and persistent wind conditions can force Langmuir circulation (0.1–200 m) hence generating vortex structures in the vertical with the distinctive long horizontal bands of inert material aligned with wind [23,24].

Although different in their physical nature, all these processes share a similar spatial structure with alternating zones of upwelling and downwelling and zero net transport within a vortex cell [17,22,24]. While these structures are typically three-dimensional, two-dimensional sections can be used to study these systems, capturing relevant advective and diffusive properties of suspended matter [17,18,2527].

In the present paper, we analysis the effects of turbulent transport in regulating phytoplankton distribution and growth using a novel two-dimensional framework. We use this approach to demonstrate growth and survival of phytoplankton in a turbulent vortex flows at scales commonly found in aquatic ecosystems. We compare our results to previous hypothesis of phytoplankton survival in light-limited environments [7] and apply the model to study the role of deep convection in promoting phytoplankton growth and persistence in winter in the North Atlantic. In these conditions convective overturn has been suggested to maintain phytoplankton cell with sinking rates of several metres per day (electronic supplementary material, appendix C, table C1) within the deep mixed layer and thus to play an important role in sustaining primary production [28,29].

3. Discussion

Turbulent structures in the ocean play an important role in driving population dynamics of phytoplankton and their spatial structures [3]. They do so not only through regulating nutrient availability [19] but also by directly impacting on their spatial distribution [16] and hence their access to light. Our results confirm that the interplay between growth, sinking and turbulent transport can drive phytoplankton blooms and affect phytoplankton spatial distribution within the mixed layer. Moreover, explicit consideration of the turbulent vortex flow has a significant effect on the net phytoplankton growth and survival. We apply a numerical technique based on the analyses of a transition matrix that is commonly used in demographic studies in ecology [34]. We extended the method to analyse spatial dynamics on a two-dimensional grid and applied it to study the spatially structuring effects of vortices of turbulent flow. In addition to the net population growth rate, the method provides quantitative information on biomass accumulation (figure 2) and growth potential (figure 3) of phytoplankton in the spatial domain, using the right and left eigenvectors, respectively.

Using this method, this study demonstrates the spatially structuring effect of turbulent vortex flow on phytoplankton growth, and biomass accumulation can explain the existence of a viable phytoplankton stock with significant sinking rates commonly found in the North Atlantic convective systems during winter [28].

Footnotes

Electronic supplementary material is available online at https://dx.doi.org/10.6084/m9.figshare.c.3911878.

 

 

 

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