Although eutrophication induced by anthropogenic nutrient enrichment is a driver of shifts in community composition and eventually a threat to marine biodiversity, the causes and consequences on ecosystem functioning remain greatly unknown. In this study, by applying a trait-based approach and measuring niche breadth of diatoms and copepods, the drivers and underlying mechanisms of the seasonal species succession of these ecological communities in a coastal system dominated in spring by Phaeocystis blooms were explored. It is suggested that the seasonal succession of diatoms and copepods is the result of several trade-offs among functional traits that are controlled by the seasonal abiotic and biotic pressure encountered by the plankton communities. The results of this study highlight that a trade-off between competition and predator, i.e., weak competitors are better protected against predation, plays an important role in promoting plankton species richness and triggers the Phaeocystis bloom. As often observed in eutrophicated ecosystems, only the biotic homogenization of the copepod community and the shift in the diet of copepods toward Phaeocystis detrital materials have been detected during the Phaeocystis bloom. The diatom and copepod communities respond synchronously to fluctuating resources and biotic conditions by successively selecting species with specific traits. This study confirms the key role of competition and predation in controlling annual plankton succession.

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While ecological deterministic processes are conducive to modeling, stochastic ones are far less predictable. Understanding the overall assembly processes of phytoplankton is critical in tracking and predicting future changes.

Temporal dynamics of Syndiniales Group II were investigated combining 18S rDNA amplicon sequencing and direct microscopy counts (fluorescence in situ hybridization-tyramide signal amplification [FISH-TSA]) during 5 years. The study was undertaken in meso-eutrophic coastal ecosystem, dominated by diatoms, the haptophyte Phaeocystis globosa and exhibiting relatively low dinoflagellate abundance (max. 18.6 Â 10 3 cells L À1 ). Consistent temporal patterns of Syndiniales Group II were observed over consecutive years highlighting the existence of local populations. According to sequencing data, Syndiniales Group II showed increasing abundance and richness in summer and autumn. Dinospores counted by microscopy, were present at low abundances and were punctuated by transient peaks. In summer dinospore highest abundance (559 Â 10 3 L À1 ) and prevalence (38.5%) coincided with the peak abundance of the dinoflagellate Prorocentrum minimum (13 Â 10 3 L À1 ) while in autumn Syndiniales Group II likely had more diversified hosts. Although, several peaks of dinospore and read abundances coincided, there was no consistent relation between them. Ecological assembly processes at a seasonal scale revealed that stochastic processes were the main drivers (80%) of the Group II community assembly, though deterministic processes were noticeable (20%) in June and July. This latter observation may reflect the specific Syndiniales-dinoflagellate interactions in summer.Urania Christaki and Dimitra-Ioli Skouroliakou contributed equally to this study.

The effect of environmental change in structuring the phytoplankton communities of the coastal waters of the Eastern English Channel was investigated by applying a trait-based approach on two decades (1996-2019) of monitoring on diatoms and Phaeocystis. We show that phytoplankton species richness in an unbalanced nutrient supply context was influenced by wind-driven processes, ecological specialization for dissolved inorganic phosphorous, temporal niche differentiation, and a competition-defense and/or a growth-defense trade-off, a coexistence mechanism where weak competitors (i.e., slower growing) are better protected against predation. Under the influence of both environmental perturbations (e.g., wind-driven processes, freshwater influence, unbalanced nutrient levels) and biotic interactions (e.g., competition, predation, facilitation), phytoplankton species exhibited specific survival strategies such as investment on growth, adaptation and tolerance of species to environmental stresses, silicification and resource specialization. These strategies have led to more speciose communities, higher productivity, functional redundancy and stability in the last decade. Our results revealed that the unbalanced nutrient reduction facilitated Phaeocystis blooms and that anthropogenic climate warming and nitrate reduction may threaten the diatom communities of the eastern English Channel in a near future. Our results provide strong support for biogeographical historical and niche-based processes in structuring the phytoplankton community in this temperate region. The variety of species responses that we characterized in this region may help to better understand future changes in pelagic ecosystems, and can serve as a basis to consider functional approaches for future ecosystem management.