Understanding how the collective behaviour of phytoflagellates is affected by light attenuation and diel vertical migration using individual-based modelling.

In this article, we investigate the question of the impact of the diel vertical migration (DVM) and the light attenuation by the cells on both spatial pattern and population dynamics of phytoflagellates. For these purposes, we performed a simulation study by using a spatially explicit individual-based model (IBM). The designed IBM includes 2 global mechanisms: cellular motion which is modelled through a stochastic differential equation and demographic process that is density and light intensity dependent. We showed that under no-DVM, for competitive environment, increasing the light absorption helps the creation of a strong oscillatory behaviour, that breeds aggregation-break up rhythm, which is beneficial. However, when we pass to DVM regime, it will be more advantageous for the cells to reduce their light absorption. In regards of DVM effect, we showed that it has other benefits, apart from the acquisition of resources, such as enhancing the attraction mechanism that promotes cooperation and also helping the creation of spatial voids that permit the penetration of the light. Also, we showed that the DVM reduces the predation rate, so we prove quantitatively that the DVM reduces, in general, the grazing losses. Also, we found that the DVM strategy depends on the impact of the competition and cooperation between the cells on the division and death rates. So, probably for the motile species, the local competition is not high in order to let the cells gain advantages from the DVM regarding their survivability that is explained by an annual cyclic behaviour under seasonal environment.

A 3D Individual-Based Model to Study Effects of Chemotaxis, Competition and Diffusion on the Motile-Phytoplankton Aggregation.

In this paper, we develop a 3D-individual-based model (IBM) to understand effect of various small-scale mechanisms in phytoplankton cells, on the cellular aggregation process. These mechanisms are: spatial interactions between cells due to their chemosensory abilities (chemotaxis), a molecular diffusion and a demographical process. The latter is considered as a branching process with a density-dependent death rate to take into account the local competition on resources. We implement the IBM and simulate various scenarios under real parameter values for phytoplankton cells. To quantify the effects of the different processes quoted above on the spatial and temporal distribution of phytoplankton, we used two spatial statistics: the Clark-Evans index and the group belonging percentage. Our simulation study highlights the role of the branching process with a weak-to-medium competition in reinforcing the aggregating structure that forms from attraction mechanisms (under suitable conditions for diffusion and attraction forces), and shows by contrast that aggregations cannot form when competition is high.

On phytoplankton aggregation: a view from an IBM approach.

In this paper, we build up an individual-based model (IBM) that describes the aggregative behavior in phytoplankton. The processes in play at the individual level (an individual=a phytoplankton cell) are: a random dispersal, a displacement due to the net effect of cells present in a suitable neighborhood (spatial interactions) and a branching (cell division and death). The IBM model provides a virtual world where phytoplankton cells appear to form clusters. Using this model, we explore the spatial structure of phytoplankton and present some numerical simulations that help the understanding of the aggregation phenomenon.