In situ community dynamics influences the temperature- and light- dependent succession of seasonal phytoplankton

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