A Lagrangian plankton model (LPM) is developed, in which the motion of a large number of Lagrangian particles, representing a plankton community, 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 generation is investigated. Mixing by convective eddies during the night helps to maintain the uniform concentration of phytoplankton within the mixed layer, even if the daily mean surface heat flux is positive in spring. Accordingly, the spring bloom can be predicted by the critical depth hypothesis, if the mixing layer is used instead of the mixed layer. The shoaling of the mixing layer occurs immediately after the start of surface heating, but the shoaling of the mixed layer is delayed. A new criterion for the spring bloom is proposed, which predicts that spring blooms are more likely to occur at higher latitudes, even if the atmospheric forcing is the same. Furthermore, various statistics of Lagrangian particles, such as the vertical migration of plankton, the residence time of plankton within the euphotic zone, and the growth of plankton are investigated by taking advantage of the LPM.