Titre de la thèse « Exploring temporal patterns of marine plankton communities in the Western English Channel through the analyses of morphological and metabarcoding time-series » . La soutenance se déroulera en anglais.
Le jury est composé de:
Dr. Marina Montresor Stazione Zoologica Anthon Dorne, Naples, Italy Rapportrice
Dr. Ramiro Logares Institut de Ciències del Mar, CSIC, Barcelona, Spain Rapporteur
Pr. John Dolan Observatorie Océanologique de Villefranche-sur-Mer, France Examinateur
Pr. Lars Stemmann Institut de la Mer de Villefranche (IMEV), France Examinateur
Pr. Alexandra Worden GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany Examinatrice
Pr. Eric Thiebaut Station Biologique de Roscoff, Sorbonne Université, CNRS, France Examinateur
Dr. Nicolas Henry Station Biologique de Roscoff, Sorbonne Université, CNRS, France Co-Directeur de Thèse
Dr. Nathalie Simon Station Biologique de Roscoff, Sorbonne Université, CNRS, France Directrice de Thèse
Marine plankton, which incorporates all organisms that drift with currents, is responsible for about half of global primary production, plays a major role in global biogeochemical cycles and facilitates climate regulation. Understanding the dynamics of marine plankton is thus crucial in improving our capacity to accurately predict the environmental consequences of global changes. Time-series are witnesses of long-term ecological change and also provide a great opportunity to understand how the environment shapes planktonic communities to be able to predict their composition in the future.
The main goals of this thesis are to describe the temporal patterns of planktonic protist communities in coastal tidally-mixed waters and unveil their main drivers. The work presented in this manuscript is based on the analysis of microscopy counts and DNA metabarcoding (rDNA 18S V4) data from plankton samples collected biweekly over 8 years (2009-2016) at the Somlit-Astan station (Roscoff, Western English Channel). Morphological data allowed us to assess absolute abundance of eukaryotic phytoplankton and metabarcoding data revealed a great diversity of both phototrophic and heterotrophic microeukaryotes.
First we elucidated the recurrent seasonal pattern of the dominant species and OTUs (rDNA-derived taxa) that drive annual plankton successions at the Somlit-Astan station and identified diatoms as the dominant group of phytoplankton. Then, we partitioned the planktonic taxa into two groups (core and transient) using a new metric estimating the temporal regularity of occurrence of organisms over the years (perplexity index), and we explored how these two groups interact with each other and with the environment. Core taxa persist through time and are very abundant whereas transient taxa occur intermittently. Finally, we compared the Somlit-Astan time-series with another one in the Gulf of Naples (Lter-Marechiara) focusing on diatoms. We highlighted six unique sequences abundant at both sites (maximum abundance > 10%) despite the difference of temperature.
This thesis supports evidence that microbial planktonic communities are shaped by time, and more specifically they are responding to the temporal structure of the environmental factors on one hand, and the self-organization processes of the community assembly (i.e. interactions), on the other hand. Those are major forces shaping annual plankton successions and the notable resilience of core species. Transient species instead are mainly driven by stochastic events.
Key words: plankton, community ecology, temporal pattern, metabarcoding, ecological succession, core and transient species, environmental forcing, biotic interactions.