Abstract
In temperate marine ecosystems, the major part of the annual primary production is generated in spring during rapid phytoplankton biomass accumulation periods, called ‘blooms’, supporting the diversity and the functioning of these ecosystems. Several physical, chemical and biological mechanisms triggering the bloom initiation were evocated for these ecosystems. However, for shallow coastal zones, under the influence of complex environmental forcing factors, mechanisms triggering blooms are not well known. The objective of the present thesis was to identify and classify the forcing factors contributing to the bloom initiation in these zones, especially the role of physical and chemical forcing factors and biological interactions in the microbial network, but also to understand the consequences of the temperature elevation on this functioning in the global warming context.In this frame, a monitoring with a dual approach was carried out in Thau lagoon: a high frequency (15 min) in situ monitoring of hydrological , meteorological and biological parameters; and a weekly monitoring of the abundance of the microbial community (virus, bacteria, phytoplankton, heterotrophic flagellates and ciliates), and its diversity, with a particular look at phytoplankton. These monitoring were carried out from winter to spring in two consecutive years, 2015 and 2016. Besides these monitoring, an in situ mesocosm experiment was carried out during the 2018 spring to simulate the temperature elevation according to the global warming scenario, in the presence and the absence of mesozooplankton. The objective of this experiment was to identify the direct effect of warming and the indirect effect of the zooplankton on the phytoplankton dynamic, the pigment composition and succession, during the pre-bloom, bloom and post-bloom periods. A correlation network analysis between 110 various groups/taxa/species highlighted the major interactions characterizing the microbial interaction network during the bloom and the non-bloom periods and the differences between these two years. During the bloom periods, intraguild phytoplankton competition and mutualism between phytoplankton and heterotrophic bacteria dominated the microbial food web. This suggested an energy transfer based on both bacterial and phytoplanktonic biomass, through the microzooplankton predation. During the non-bloom periods, interaction between ciliates and heterotrophic bacteria (bacterivory) dominated, suggesting an energy transfer mainly based on bacterial biomass. Besides, the high frequency monitoring highlighted the predominant role of the water temperature increase, especially during the early spring, in the initiation of the phytoplankton blooms. The combination between the phytoplankton metabolism stimulated by the temperature increase and the low grazing pressure triggered the phytoplankton biomass accumulation starting the blooms. Furthermore, 2016 year, with the warmer winter recorded in France (Meteofrance), was characterized by a weaker phytoplankton biomass accumulation during the early spring, a dominance of the small phytoplankton at the expanse of diatoms, and a dominance of interactions between small size microorganisms. The mesocosm experiment confirmed the role of the temperature elevation on the bloom amplitude reduction (diminution of 50% of the chlorophyll a concentration) and the promotion of small phytoplankton such as small green algae and dinoflagellates, at the expanse of diatoms. This amplitude and composition modification of phytoplankton blooms was mainly due to the indirect effect of the zooplankton grazing increase under warming. Furthermore, the results underlined that it was microzooplankton which mainly controlled the phytoplankton dynamic and biomass and the mesozooplankton was mainly accomplished the role of the secondary consumer in this system.