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Phytoplankton dynamics in systems dominated by variable forcing

Final Activity Report Summary - DYNALGAE (Phytoplankton dynamics in systems dominated by variable forcing)

The dynamics of phytoplankton is complex as it is dependent on a variety of interacting physical, chemical and biological factors in a constantly changing environment. This complexity therefore requires the combined approaches of numerical, experimental and observational methods.

The present study aimed at an improved understanding of phytoplankton dynamics under the influence of rapidly varying physical and chemical environment. Its innovation lies in quantifying the role of adaptive strategies of specific algal species in long term competition and dominance of species under a realistic fluctuating environment, in particular cyanobacteria.

We have developed a new technique for simultaneous measurements of the distributions of phytoplankton, and physical and chemical parameters, using a purpose built system of automated profiling device in collaboration with the Physics Department of the University of Rennes. We also deployed a fine scale prototype microstructure probe to characterise the turbulence environment to allow the determination of the intensity of mixing conditions encountered by the different phytoplankton species. Finally, we used an amplitude modulated (PAM) fluorometer to evaluate the in situ physiological status of the separate algal groups. All three types of field instruments were necessary to determine the short term response to a highly fluctuating environment.

In laboratory controlled conditions, we showed for the first time that photoacclimation, a physiological process in response to changes in light intensity, occurs in freshwater phytoplankton species and that its role in the outcome of competition is crucial in fluctuating light environments. We also showed that when nutrients are provided as pulsed inputs, which are often observed in stratified environments, the outcome of competition depends on the specific capacities for nutrient uptake and reserve of the species present as well as initial species concentrations. These results differ from those obtained under the generally studied case of constant nutrient supply. Finally, we showed that the level of turbulence intensity affects the length of cells in a way which depends on their physiological conditions.

All of the above results suggest that short term physiological adaptation in fluctuating environments will have a strong impact on the result of competition and needs to be included in model predictions. We have developed competition models in which specific processes of adaptive strategies of phytoplankton (i.e. photoacclimation, nutrient uptake) were included.