Ocean fertilization experiments may initiate a large scale phytoplankton bloom

Abstract

[1] Oceanic plankton plays an important role in the marine food chain and through its significant contribution to the global carbon cycle can also influence the climate. Plankton bloom is a sudden rapid increase of the population. It occurs naturally in the North Atlantic as a result of seasonal changes. Ocean fertilization experiments have shown that supply of iron, an important trace element, can trigger a phytoplankton bloom in oceanic regions with low natural phytoplankton density. Here we use a simple mathematical model of the combined effects of stirring by ocean eddies and plankton evolution to consider the impact of a transient local perturbation, e.g., in the form of iron enrichment as in recent 'ocean fertilization' experiments. The model not only explains aspects of the bloom observed in such experiments but predicts the unexpected outcome of a large scale bloom that in its extent could be comparable to the spring bloom in the North Atlantic. [Truscott and Brindley, 1994a] suggested that plankton blooms might be explained by the excitability of the dynamical system describing biological interactions in the plankton ecosystem. The characteristic feature of excitable systems [Meron, 1992] is that perturbations exceeding a certain threshold can induce a temporary large deviation from the equilibrium state. The plankton bloom produced artificially by iron fertilization experiments has been shown to be consistent with excitable system behaviour [Pitchford and Brindley, 1999]. However, previous work has considered only the spatially homogeneous situation, and need not be relevant to the iron fertilization experiments, in which localised perturbations produce a localised bloom that is subsequently strongly affected by stirring by mesoscale ocean eddies.[3] The role of stirring has been recognised and investigated in the context of the SOIREE experiment [Boyd et al., 2000;Abraham et al., 2000] and the spatial characteristics of the bloom filament was found to be similar to that seen in tracer release experiments [Sundermeyer and Price, 1998], in having a roughly constant width and exponentially increasing length. However, the biological evolution in this analysis was modelled by a simple linear growth rate, which cannot be adequate for predicting the long term behavior of the bloom when nonlinearities, e.g. interactions with other species, become important.[4] Here we consider a, perhaps minimal, model that unifies the two approaches described above by taking into account spatial distribution, stirring and non-linear interactions between biological components. The model equations arewhere P, Z represent the phytoplankton and zooplankton concentration (in nitrogen currency) and F is the deviation of the iron concentration from the original homogeneous background. All three components are advected by a two-dimensional flow, v(r, t), obtained from a so-called seeded eddy model [Abraham, 1998], and diffusive terms represent the effect of small-scale mixing (k = 4 m 2 s À1 ). The biological terms on the right-han...

Plankton blooms induced by turbulent flows

“…The important message in this work is that with a minimal model and using experimentally determined values for the parameters, we obtain a natural period of just less than one year, which may be further synchronized by the external forcing. This self-initiating, periodic patch-forming mechanism differs from other related work on advection-enhanced blooms (Biktashev et al 1998;Neufeld et al 2002) as it requires no initial perturbation and is self-sustaining.…”

“…After eliminating the tildes this gives us the following non-dimensional scheme: We choose L, the simulation box size, to be 50 km (where l 0 is related to L in the sense that l 0 determines the scale of the eddies, and in this case we choose L < 10l 0 ; see figure 1) and the characteristic velocity to be u 0 = 0.1 m s 21 , similar to that used in both Abraham (1998) and Neufeld et al (2002). From Truscott & Brindley (1994), we take b = 0.162, x = 0.053, s = 0.130 and « = 0.05 (hence, the zooplankton population is the slow/recovery variable).…”

“…Furthermore, the breaking of excitation waves by shear flow has been proposed (Biktashev et al 1998) as a possible mechanism for plankton patchiness. Similarly, a chaotically advected excitable system, together with a driving influx of nutrient (in this case iron), has been investigated as an alternative mechanism (Neufeld et al 2002). In both cases, the system needs some non-vanishing perturbation to seed the excitation.…”

Synthetizing Hydrodynamic Turbulence from Noise: Formalism and Applications to Plankton Dynamics

“…However, with a minimal model and using experimentally determined values for the parameters, we obtain a natural period of almost one year, which may be further synchronized by the forcing. This self-initiating, periodic patch forming mechanism differs from other related work on advection enhanced blooms [17] as it requires no initial perturbation and is self-sustaining.…”

Sufficient noise and turbulence can induce phytoplankton patchiness

“…The simulation images created by reaction-advection-diffusion systems seem to show a closer similarity to real patchiness patterns than those by reaction-diffusion systems [3][4][5][6]. The studies by reaction-advection-diffusion systems usually adopt the seeded-eddy model to represent two-dimensional turbulent flows, which is developed by Dyke and Robertson [7].…”