Abstract
The Arctic is impacted by climate warming faster than any other oceanic region on Earth. Assessing the baseline of microbial communities in this rapidly changing ecosystem is vital for understanding the implications of ocean warming and sea ice retreat on ecosystem functioning. Using CARD-FISH and semi-automated counting, we quantified 14 ecologically relevant taxonomic groups of bacterioplankton (Bacteria and Archaea) from surface (0–30 m) down to deep waters (2,500 m) in summer ice-covered and ice-free regions of the Fram Strait, the main gateway for Atlantic inflow into the Arctic Ocean. Cell abundances of the bacterioplankton communities in surface waters varied from 105 cells mL–1 in ice-covered regions to 106 cells mL–1 in the ice-free regions. Observations suggest that these were overall driven by variations in phytoplankton bloom conditions across the Strait. The bacterial groups Bacteroidetes and Gammaproteobacteria showed several-fold higher cell abundances under late phytoplankton bloom conditions of the ice-free regions. Other taxonomic groups, such as the Rhodobacteraceae, revealed a distinct association of cell abundances with the surface Atlantic waters. With increasing depth (>500 m), the total cell abundances of the bacterioplankton communities decreased by up to two orders of magnitude, while largely unknown taxonomic groups (e.g., SAR324 and SAR202 clades) maintained constant cell abundances throughout the entire water column (ca. 103 cells mL–1). This suggests that these enigmatic groups may occupy a specific ecological niche in the entire water column. Our results provide the first quantitative spatial variations assessment of bacterioplankton in the summer ice-covered and ice-free Arctic water column, and suggest that further shift toward ice-free Arctic summers with longer phytoplankton blooms can lead to major changes in the associated standing stock of the bacterioplankton communities.
Highlights
Atmospheric and oceanic warming has a substantial impact on the Arctic Ocean already today (Dobricic et al, 2016; Sun et al, 2016; Dai et al, 2019)
Phytoplankton blooms in surface waters generally lead to an increased cell abundance of heterotrophic bacteria that are specialized in the degradation of organic matter from algal exudates and phytodetritus (Buchan et al, 2014; Teeling et al, 2016)
We found that in surface waters some taxonomic groups were associated with the distinct water masses of the Strait (e.g., Rhodobacteraceae with the Atlantic waters)
Summary
Atmospheric and oceanic warming has a substantial impact on the Arctic Ocean already today (Dobricic et al, 2016; Sun et al, 2016; Dai et al, 2019). The Fram Strait is the main deep-water gateway between the Atlantic and the Arctic Ocean It hosts two distinct hydrographic regimes; the West Spitsbergen Current (WSC) that carries relatively warm and saline Atlantic water northward along the Svalbard shelf (Beszczynska-Möller et al, 2012; von Appen et al, 2015), and the East Greenland Current (EGC) that transports cold polar water and sea ice southwards from the Arctic Ocean along the ice-covered Greenland shelf (de Steur et al, 2009; Wekerle et al, 2017). Sea ice conditions have a strong impact on the seasonal ecological dynamics in Fram Strait and the whole Arctic Ocean (Wassmann and Reigstad, 2011), affecting light availability and stratification in the water column. Previous summer observations in the Fram Strait already suggested that total cell abundances and productivity of bacterioplankton communities in surface waters are driven by environmental parameters associated with phytoplankton bloom dynamics (Fadeev et al, 2018), such as the availability and composition of organic matter (Piontek et al, 2015; Engel et al, 2019), with differences between ice-covered and ice-free regions (Piontek et al, 2014; Fadeev et al, 2018)
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