A theoretical investigation of the diatom cell size reduction–restitution cycle

• We have developed an individual based model that describes the diatom life cycle.
• The size reduction–restitution cycle (SRRC) shows the observed saw tooth patterns.
• The size reduction–restitution cycle causes interannual variability.
• Inter- and intraspecific competition can explain observed size distribution patterns.
• Auxospores are rare and occur in the surface spring bloom and subsurface in summer.

An individual based model for the life cycle of diatoms (DiaLCM) has been developed to investigate the combined effects of the cell cycle and the cell size reduction–restitution cycle (SRRC) of this important marine phytoplankton group. Growth, grazing mortality and sinking are considered as size-dependent processes. The model is tested against laboratory results and reproduces the typical saw tooth pattern of mean population size as the result of the SRRC. The development of cell-size distribution patterns, their dependence on the environment and their influence on the ecosystem is investigated in a one-dimensional water column setting, where the diatoms face competition by a bulk phytoplankton species. Using a perpetual-year forcing, we find that the SRRC introduces interannual variability with periods of diatoms dominance interrupted by dominance of other phytoplankton. We find that inter- and intraspecific competition explains several of the observed characteristics in size frequency distributions, in particular the relatively narrow cell size spectra. In agreement with observations modeled auxosporulation events are rare with low numbers of auxospores.