Towards a model of cyanobacteria life cycle-effects of growing and resting stages on bloom formation of N2-fixing species

Cyanobacteria blooms are a common phenomenon in aquatic environments but although considerable effort has been devoted to study various aspects of bloom formation, the processes involved are still not fully understood. Most of the factors that have been investigated can be categorised as external (e.g. N/P-ratio, temperature), whereas internal factors on the generation of cyanobacteria blooms through their distinctive life cycle have not yet been sufficiently considered. To fill this gap and to investigate the dynamics of cyanobacteria life cycles, a numerical model has been developed. The model assumes that the life cycle is governed by the internal energy and nitrogen quotas of the cells, and discriminates four different stages: vegetative cells, vegetative cells with heterocysts, akinetes and recruiting cells (including germinates). The seasonal succession of life stages is simulated in a one-dimensional framework, and a typical bloom is successfully simulated with a set of plausible parameters. Observed interannual variations in the relative proportions of different life cycle stages can be explained as the direct result of life cycle dynamics. The results show that life cycle simulations are feasible and can be used to test hypotheses and to determine sensitivities regarding the role of cyanobacteria life cycles in marine and limnic environments. Our study indicates that prediction of cyanobacteria blooms has to be based on a detailed knowledge of all stages of the life cycle.