Role of phytoplankton cell size on the competition for nutrients and light in incompletely mixed systems.

We investigate the effects of algal cell size on the competition for nutrients and light in an incompletely mixed water column, employing a spatially explicit variable internal stores approach and previously published allometric scaling relationships for modeling phytoplankton growth. We analyze the interplay between the size-dependent vertical assimilation and uptake profiles and the role of environmental settings such as mixing intensity, nutrient loading and background turbidity for the outcome of competition. Our results suggest that a potentially beneficial factor for resource competition in spatially heterogeneous systems is a low ratio of subsistence nutrient quota to the maximal quota, q(min)/q(max), which is a decreasing function of cell size according to allometric relationships. Environmental parameters such as mixing intensity and nutrient availability are shown to modulate the relevance of the q(min)/q(max) ratio for the competitive outcome and thereby have non-monotonic impacts on the algal size selection. The outcome of competition further depends on the temporal and spatial variability of mixing. In particular, the presence of a metalimnion with low diffusivity and periodic perturbation of the depth of the metalimnion strongly influences the relative success of differently sized algae. This suggests that the anticipated reduction in wind induced mixing events due to climate warming will have context-dependent consequences for algal size selection.