Seasonal temporal dynamics of marine protists communities in tidally mixed coastal waters

Major seasonal community reorganizations and associated biomass variations are landmarksof plankton ecology. However, the processes determining marine species and communityturnover rates have not been fully elucidated so far. Here, we analyse patterns of planktonicprotist community succession in temperate latitudes, based on quantitative taxonomic datafrom both microscopy counts and ribosomal DNA metabarcoding from plankton samplescollected biweekly over 8 years (2009-2016) at the SOMLIT-Astan station (Roscoff, WesternEnglish Channel). Considering the temporal structure of community dynamics (creatingtemporal correlation), we elucidated the recurrent seasonal pattern of the dominant speciesand OTUs (rDNA-derived taxa) that drive annual plankton successions. The use ofmorphological and molecular analyses in combination allowed us to assess absolute speciesabundance while improving taxonomic resolution, and revealed a greater diversity. Overall,our results underpinned a protist community characterised by a seasonal structure, which issupported by the dominant OTUs. We detected that some were partly benthic as a result ofthe intense tidal mixing typical of the French coasts in the English Channel. While theoccurrence of these microorganisms is driven by the physical and biogeochemical conditionsof the environment, internal community processes, such as the complex network of bioticinteractions, also play a key role in shaping protist communities