Abstract
Upwelling areas are shaped by enhanced primary production in surface waters, accompanied by a well-investigated planktonic succession. Although bacteria play an important role in biogeochemical cycles of upwelling systems, little is known about bacterial community composition and its development during upwelling events. The aim of this study was to investigate the succession of bacterial assemblages in aging upwelled water of the Benguela upwelling from coastal to offshore sites. Water from the upper mixed layer at 12 stations was sampled along two transects from the origin of the upwelling to a distance of 220 km. 16S rRNA gene amplicon sequencing was then used in a bacterial diversity analysis and major bacterial taxa were quantified by catalyzed reporter deposition-fluorescence in situ hybridization. Additionally, bacterial cell numbers and bacterial production were assessed. Community statistical analysis revealed a reproducible zonation along the two transects, with four clusters of significantly different microbial assemblages. Clustering was mainly driven by phytoplankton composition and abundance. Similar to the temporal succession that occurs during phytoplankton blooms in temperate coastal waters, operational taxonomic units (OTUs) affiliated with Bacteroidetes and Gammaproteobacteria were dominant during algal blooming whereas “Pelagibacterales” were highly abundant in regions with low algal abundance. The most dominant heterotrophic OTU (9% of all reads) was affiliated with “Pelagibacterales” and showed a strong negative correlation with phytoplankton. By contrast, the second most abundant heterotrophic OTU (6% of all reads) was affiliated with the phylum Verrucomicrobia and correlated positively with phytoplankton. Together with the close relation of bacterial production and phytoplankton abundance, our results showed that bacterial community dynamics is strongly driven by the development and composition of the phytoplankton community.
Highlights
Ocean margins and upwelling systems in particular are sites of enhanced primary production and organic matter export
The analysis showed a zonation along the transects and a cluster analysis distinguished four bacterial community clusters defined at the operational taxonomic units (OTUs) level (Supplementary Figure S3)
During our study reproducible changes in Prokaryotic abundance (PA), bacterial protein production (BPP), and BCC were determined in the two sampled transects
Summary
Ocean margins and upwelling systems in particular are sites of enhanced primary production and organic matter export. The Benguela upwelling, one of the most intensive upwelling systems worldwide (Nelson and Hutchings, 1983), is characterized by constant upwelling of nutrient rich water from deeper layers with maximum upwelling between. Many studies on the Benguela upwelling have focused on the succession of phytoplankton and zooplankton. Their results have shown that the upwelling of nutrients leads to a characteristic phytoplankton succession in which the early bloom is dominated by diatoms or dinoflagellates (e.g., Barlow, 1982; Pitcher et al, 1998) that, after nutrient depletion, are replaced by other phytoplankton groups (Brown and Hutchings, 1987). There is an increase in nano- and mesozooplankton biomass and in the grazing impact of flagellates and copepods (Painting, 1993a)
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