Abstract : To assess the importance of these two parameters experimentally, microcosms were
14 conducted on seven picoplankton communities (<3 μm) sampled in December, March, June and
15 September 2015 and 2016 in the North Western Mediterranean Sea. Each community was
16 exposed to 4 realistic seasonal conditions (December, March, June and September).
17 Metabarcoding was used to investigate the eukaryotic diversity in the 56 microcosms
18 experiments in parallel to high-frequency monitoring of environmental diversity in the sea. The
19 three major lineages identified were the Stramenopiles, Alveolata and Archaeplastida. Overall,
20 the five-day incubations were not sufficient to reshape the initial microbial communities
21 completely. The microcosm outcome was strongly influenced by the dynamics of phytoplankton
22 starting communities. In pre-bloom conditions, phytoplanktonic species were the most sensitive
23 to temperature and light conditions. During a bloom, species belonging to diatoms or
24 Chlorodendrophyceae usually did not respond to light and temperature in microcosms and
25 continued to bloom independently of the applied seasonal condition. Together, these results
26 suggest that light and temperature seasonal conditions play a crucial role in regulating
27 phytoplankton dynamics in pre-bloom conditions and biotic interactions may be preponderant in
28 bloom and post-bloom conditions