Vertical zonation of bacterial assemblages attributed to physical stratification during the summer relaxation of the coastal upwelling off Galicia (NW Spain)

• Published in: Estuarine, coastal and shelf science Vol. 245; p. 106791
• Main Authors: Montes, Tania, Guerrero-Feijóo, Elisa, Moreira-Coello, Víctor, Bode, Antonio, Ruiz-Villarreal, Manuel, Mouriño-Carballido, Beatriz, Varela, Marta M.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 30.10.2020
• Subjects: 454 pyrosequencing; Bacteria; CARD-FISH; Microplankton; NW Spain; Summer upwelling relaxation; Upwelling pulse; Microplankton; Summer upwelling relaxation; NW Spain; Bacteria; 454 pyrosequencing; Upwelling pulse; CARD-FISH
• ISSN: 0272-7714; 1096-0015
• DOI: 10.1016/j.ecss.2020.106791

We combined flow cytometry, CARD-FISH, and 16S rRNA gene tag pyrosequencing to investigate bacterioplankton dynamics along a transect in shelf waters off A Coruña (Galicia, NW Spain). Over five days (16-20th July 2012) we sampled during the relaxation of a summer upwelling pulse, providing an opportunity to examine the impact of pulses of cold nutrient-rich water into coastal microbial communities. The hydrographic conditions, characterized by intense density stratification of surface waters and the presence of a deep chlorophyll maximum (DCM) at 20–30 m, were relatively maintained over the sampling period. Indeed, bacterial abundance and composition displayed low day to day variation. Alpha diversity analysis suggested that species richness and diversity increased from coastal to shelf stations and from the surface down to the coastal DCM, which could be caused by the mixing of upwelled bacteria with the coastal surface waters. SAR11, SAR86, and Roseobacter were the most abundant bacteria detected in the samples by using CARD-FISH. The assemblages observed by pyrosequencing displayed a strong vertical zonation along the transect. Rhodobacteraceae (under class Alphaproteobacteria) and Bacteriodetes dominated the surface waters and decreased during the upwelling pulse, while SAR 86 (under class Gammaproteobacteria), Actinobacteria and SAR11 clade increased their relative abundance at the coastal DCM with upwelling relaxation, particularly at the shelf stations. Bacterial assemblages from surface waters were associated with higher temperature and light conditions, while coastal DCM assemblages were rather associated to salinity, inorganic nutrients and a diatom-bloom leading to high chlorophyll-a. Our findings suggest that the vertical variability in environmental conditions induced by the intense density stratification, the exportation of warmer and less saline surface water from the rias to the adjacent shelf, and the fertilizing effect of recently upwelled water at the deeper layer, determined the composition of distinct bacterial assemblages at the subsurface and DCM layers. •No previous studies have been found in the literature showing how bacterial community composition evolves during an episode of relaxation of an upwelling, by characterizing bacterial assemblages (using different diversity measurement techniques) along with hydrography, nutrient concentration and microbial abundance in a section off A Coruña (from the ría to the adjacent shelf).•The three different approaches (flow cytometry, CARD-FISH and Illumina MiSeq) provided complementary information on the abundance, diversity, community composition and distribution patterns of bacterial communities showing up robustness and solidity to our results and.•Different diversity techniques show how bacterial community composition evolve during an episode of relaxation of an upwelling.