Mechanisms governing phytoplankton blooms
In temperate climate zones, build up of biomass by phytoplankton often take the shape of abrupt, short-lived `blooms'. The reasons for the so-called exponential growth phase are relatively well understood; It is typically governed by the establishment of thermal stratification, hence, improvement of average light conditions experienced by the organisms, and abundantly available dissolved inorganic nutrients. The ending of a phytoplankton bloom is also often abrupt, and there are various interdependent mechanisms which may contribute to a rapid collapse, such as a cease of growth due to the depletion of nutrients, consumption of the poorly defended, smaller phytoplankton species by the fast growing microzooplankton groups such as ciliates, increased lysis rates due to viral mortality, and entrapment of phytoplankton cells in flocs through aggregation processes.
We use numerical models to better understand the role of various processes involved in the regulation of phytoplankton blooms, that are, for instance, observed in mesocosm experiments. After mathematically describing all the processes involved, calibrating the free parameters of the model, and obtaining a realistic simulation, such models allow quantifying and analyzing the fluxes between various model components, which can help gaining insight into the relevance of various mechanisms during different phases of the bloom.