A modelling study of the role of resting cysts in Alexandrium minutum population dynamics

Abstract

We implemented a numerical model to simulate the population dynamics of Alexandriumm minutum in a Mediterranean semi-enclosed water body (Arenys de Mar Harbor, NW Mediterranean). The effect of encystment and excystment rates, and length of the cyst dormancy period on intensity and duration of blooms of vegetative cells was tested by means of a series of scenarios, constrained when possible by experimentally-measured parameters. Excystment fluxes could substantially increase the population size of vegetative cells in initial phases of the bloom and when net growth rates of the vegetative population were low or negative, but had negligible effect during periods of exponential growth. Encystment rates not exceeding the maximum values determined in the study area (0.01 d-1) contributed to reduce the peak concentration of vegetative cells but did not have a dominant effect on bloom termination. Increasing the dormancy times of the resting cysts in the sediment resulted in greater losses of sediment cysts and reduced excystment fluxes. Our simulations highlight the importance, regarding A. minutum population dynamics, of considering the magnitude and variability not only of growth and life-cycle transition rates, but also of loss rates of both vegetative cells and cysts