Résumé: Microbial food web organisms are at the base of the functioning of pelagic ecosystems and support the whole marine food web. They are very reactive to environmental changes and their interactions are modified in response to different productive periods such as phytoplankton bloom and non-bloom as well as contrasted climatic years. To study ecological associations, identify potential interactions between microorganisms and study the structure of the microbial food web in coastal waters, a weekly monitoring was carried out in the Thau Lagoon on the French Mediterranean coast. The monitoring lasted from winter to late spring during two contrasting climatic years, a typical Mediterranean (2015) and a year with an extreme warm winter (2016). Correlation networks comprising 110 groups/taxa/species were constructed to characterize potential possible interactions between the microorganisms during bloom and non-bloom periods. Complex correlation networks during the bloom and dominated by negative intraguild correlations and positive correlations of phytoplankton with bacteria. Such pattern can be interpreted as a dominance of competition and mutualism. In contrast, correlation networks during the non-bloom period were less complex and mostly dominated by tintinnids associations with bacteria mostly referring to potential feeding on bacteria, which suggests a shift of biomass transfer from phytoplankton-dominated food webs during bloom to more bacterioplankton-based food webs during non-bloom. Inter-annual climatic conditions significantly modified the structure of microbial food webs. The warmer year favored relationships among smaller group/taxa/species at the expense of large phytoplankton and ciliates, possibly due to an intensification of the trophic cascade with a potential shift in energy circulation through microbial food web. Our study compares a typical Mediterranean spring with another mimicking the prospected intensification of global warming; if such consideration holds true, the dominance of future coastal marine ecosystems will be shifted from the highly productive herbivorous food web to the less productive microbial food web.

Résumé: Sardine populations worldwide can fluctuate drastically over short time periods, in terms of both biomass and biological characteristics. Fluctuations might be amplified by pathogens, but such hypotheses have never been considered in the absence of clear macroscopic symptoms. In the Gulf of Lions (NW Mediterranean), an enduring severe decrease in sardine (Sardina pilchardus) size, condition and age has been observed since 2008, resulting in a strong decline in landings. This situation might have been caused or aggravated by diseases, especially as other drivers such as fisheries are not expected to be important. Therefore, we developed and performed a general veterinary study, aimed at detecting a wide range of potential pathogens, including parasites, viruses and bacteria. We explored which infectious agents are most likely to produce a causal relationship with sardine health, important information for future infection experiments. Among about 1300 sardines sampled during June 2014-July 2015, microscopic parasites (often trematodes and coccidians) and bacteria Tenacibaculum and Vibrio spp. were found. However, no clear damage to tissue was observed and there was generally no link between the agents' presence and host size or condition, so that no strong indications of pathogenicity were present. Nonetheless, 54 % of the sardines analysed in 2015 had elevated quantities of melano-macrophage centres (macrophage aggregates), indicating stress on the fish that might potentially be related to starvation and/or pollution.