The Route to Spring Phytoplankton Blooms Simulated by a Lagrangian Plankton Model

A Lagrangian plankton model (LPM) is developed, in which the motion of a large number of Lagrangian particles, representing a parcel of plankton, is calculated under the turbulence field simulated by large-eddy simulation. A spring phytoplankton bloom is realized using the LPM, and the mechanism for its 1 generation is investigated. The criterion based on these results is proposed as (Formula presented.), where δ _E (= u _∗/f) is the scale for the Ekman boundary layer, δ _S (= (Formula presented.)) is the scale for the depth of a seasonal thermocline, u _∗ is the wind stress, Q ₀ is the surface buoyancy flux, f is the Coriolis parameter, and C ₁ and C ₂ are constants. The critical depth hypothesis can be applied for the onset of a spring bloom, when (Formula presented.), using the mixing layer depth instead of the mixed layer depth, as z _c> C ₂ δ _S, but the critical turbulence hypothesis can be applied, as z _c> C ₁ δ _E, when (Formula presented.). A spring bloom is more likely to occur at higher latitudes, even if the atmospheric forcing is the same. The diurnal variation of Q ₀ tends to increase the strength of the spring bloom at small u _∗. Furthermore, various statistics of Lagrangian particles, such as the mixing length of plankton, the residence time of plankton within the euphotic zone, and the growth of plankton clarify the movement and growth of plankton cells.