Sink or swim? Vertical movement and nutrient storage in phytoplankton

A simulation model of vertically migrating phytoplankton is presented, using a Lagrangian, individual-based computational approach. Algal cells acquire and store nutrient at the bottom of the habitat, using stored nutrient to grow while in shallower waters. Stored nutrient also governs movement: cells sink when their nutrient quota falls below a threshold; otherwise they rise (or at least sink more slowly). Although the bottom of the habitat provides the growth-limiting nutrient, it also entails a risk of mortality. For a parameter set representing phosphorus-limited algae with a fixed nutrient storage capacity, neither continual sinking nor continual rising are optimal strategies. Instead, an adaptive dynamics approach suggests there is an optimal movement strategy in which cells rise when their storage capacity is partially filled, and otherwise sink. When the movement strategy is fixed in such a way and storage capacity is free to evolve, storage capacity approaches an optimal value several times higher than the minimal quota permitting population growth. Vertical movement and nutrient storage affect the vertical distribution of total nutrient. When cells always sink, total nutrient declines exponentially from the nutrient source at the bottom to a surface minimum. When cells always rise, there is a peak of total nutrient at the bottom, and another at the surface, with a minimum between. When cells move optimally, the vertical distribution of total nutrient can be close to uniform, or have a peak at mid-depth.