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

Abstract

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 z(c)(-2)<[1/(C-1 delta(E))(2)+1/(C-2 delta(S))(2)], where delta(E) (= u(*)/f) is the scale for the Ekman boundary layer, delta(S) (=u(*)(2)/(fQ(0))(1/2)) 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(C-2 delta(S)/C-1 delta(E))(2)<<1, using the mixing layer depth instead of the mixed layer depth, as z(c) > C-2 delta(S), but the critical turbulence hypothesis can be applied, as z(c) > C-1 delta(E), when(C-2 delta S/C-1 delta E)(2)>>1. A spring bloom is more likely to occur at higher atitudes, 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. Plain Language Summary Phytoplankton concentrations increase rapidly in early spring in the high-latitude ocean, which is known as a spring phytoplankton bloom. It is caused by the weakened vertical mixing after surface cooling in winter switches to surface heating, which makes plankton spend more time near the surface, where the sunlight for photosynthesis is plentiful. In order to understand this phenomenon and predict its occurrence, a new type of plankton model is developed, in which a large number of particles, representing a parcel of plankton, move around in turbulent flows of the ocean. The spring bloom is simulated by the plankton model. The simulations show that a spring bloom occurs when the depth of a seasonal thermocline is shallower than the critical depth determined by radiation penetration. It can also occur in the absence of surface heating, when wind is very weak. Furthermore, results show that spring blooms are more likely to occur at higher latitudes, even if the atmospheric condition is the same. A new criterion for the onset of the spring bloom is proposed. Various statistical analyses of plankton particles are carried out to illustrate how plankton move and grow at different latitudes under the influence of the diurnal variation.