A simulation study on how the resource competition and anti-predator cooperation impact the motile-phytoplankton groups’ formation under predation stress

The phytoplankton's spatial aggregation is a very important phenomenon that can give responses to many questions such as the passage from the unicellularity to the multicellularity. In this work, we are interested by predator-induced aggregations in motile phytoplankton. Our aim is to bring, through a simulation study, some explanations on how these groups form and analyze the simultaneous effect of both resource competition and anti-predation cooperation on the groups’ formation process. For this purpose, we developed a 3D individual-based model (IBM) that takes into account small-scale biological processes for the phytoplankton cells that are: (1) motion, described by a stochastic differential equation in which the drift term is density-dependent to take into account the attraction mechanism between cells due to their chemosensory abilities and the dispersal term representing the diffusion of cells in water, (2) a density-dependent birth–death process to describe the demographical process in phytoplankton cells. In the latter, division and death rates were considered density-dependent to include a local competition for resources that slows up the cell's division and a local cooperation in phytoplankton that reduces the cell's predation death. We implemented the IBM and considered several scenarios that combine three different levels of resource competition with three different intensities of cooperation. The different scenarios were tested using real parameter values for phytoplankton.