Small eukaryotic plankton composition and dynamics in the ría de Vigo (NW Spain) during the upwelling season

2nd Meeting of the Iberian Ecological Society (SIBECOL), XXI conference of the Iberian Association of Limnology (AIL) and 21st National Congress of the Portuguese Ecological Society (SPECO), 3-8 July 2022, AveiroBiodiversity and dynamics of small plankton from the NW Iberian coast are poorly described. In this study, we applied high-throughput sequencing targeting the V9 region of 18S rDNA to identify the main taxonomic groups of pico- (0.2-2 μm) and nanoeukaryotes (2-20 μm) in the Ría de Vigo during the upwelling season. Sampling took place in June 2017, every 2/3 days over 3 weeks. On each sampling day, physical, chemical and biological data were also collected to characterize environmental conditions. All metrics indicated greater diversity of nanoeukaryotes compared with picoeukaryotes. The phylogenetic diversity of picoeukaryotes mainly involved sequences of SAR supergroup (46%) and Archaeplastida (19%). Within SAR supergroup, Stramenopiles accumulated the higher abundance (33%). Likewise, SAR supergroup also accounted for the largest share of nanoeukaryotes (89%), but in this case was strongly represented by Alveolates (46%). Furthermore, taxonomic composition of the small eukaryotes changed over time following the upwellingrelaxation-upwelling-downwelling-upwelling cycle observed. Within picoeukaryotes, an abrupt change in taxonomic composition occurred. Initially, SAR supergroup dominated (>60%) associated with upwelling, but reached a low point during downwelling. Conversely, an unknown category of picoeukaryotes marked a peak (70%) precisely during downwelling, after having gradually strengthened during relaxation. At higher taxonomic resolution, it was noted that upwelling favoured Ochrophyta, whereas MAST were promoted by relaxation. Instead, downwelling caused a decrease on both. On the other hand, the relative abundance of SAR supergroup within nanoeukaryotic community hardly changed over time, unlike some of its components, that exhibited large temporal variability. In particular, Dinoflagellata and Ochrophyta showed the opposite trend during the study period, so that the former was more abundant during relaxation and downwelling while the later remarkably increased just before and after downwelling. Interestingly, a close examination of minority taxa (