Sea-ice retreat may decrease carbon export and vertical microbial connectivity in the Eurasian Arctic basins

Abstract

Arctic Ocean sea-ice cover is shrinking due to warming. Long-term sediment trap data show higher export efficiency of particulate organic carbon in regions with seasonal sea-ice compared to regions without sea-ice. To investigate this sea-ice enhanced export, we compared how different phytoplankton communities in seasonally ice-free and ice-covered regions of the Fram Strait affect carbon export and vertical dispersal of microbes. In situ collected aggregates, combined with microbial source tracking revealed that larger aggregates from sea-ice and under-ice diatom blooms were responsible for higher export efficiency and vertical microbial connectivity. During early summer, Phaeocystis aggregates dominated the ice-free regions and exported two-fold less carbon than diatom aggregates in ice-covered regions, and also less surface-born microbial clades to the deep-sea. This suggests that continuous ice-loss will further decrease pelagic-benthic coupling, impacting the quantity and quality of food input due to formation of slow-settling aggregates, with potential repercussions for Arctic deep-sea ecosystems.

Spatial distribution of Arctic bacterioplankton abundance is linked to distinct water masses and summertime phytoplankton bloom dynamics (Fram Strait, 79°N)

“…Overall, across all three regions, coverage of the Archaea-specific probe was significantly higher at depth (ANOVA; F 3 =34.31, p<0.01), reaching 13-17% of DAPI-stained cells (Table S3). This trend implies an increase in relative abundance of Archaea with depth (Müller et al, 2018a;Fadeev et al, 2020). Taken together, our findings confirm previously observed higher abundances of Archaea in bacterioplankton communities of ice-covered waters (Wilson et al, 2017;Müller et al, 2018b;Fadeev et al, 2020), and correspond to the globally observed trend of an increasing archaeal importance at depth (Herndl et al, 2005;Teira et al, 2006;Galand et al, 2009b).…”

“…The consumed silica (ΔSiO ), used by diatoms, did not show a significant correlation to ΔPO 4 and ΔNO 3 . This further supports the impact of different phytoplankton populations across the Strait (i.e., diatoms vs. Phaeocystis; Fadeev et al, 2020). Phytoplankton bloom-associated environmental parameters (chlorophyll a concentration and the consumed inorganic nutrients) revealed weaker relationships with cell abundances of different taxonomic groups (Table S2).…”

“…Jointly, these classes comprised up to 50% of the analyzed bacterioplankton communities (Table 1). Other taxonomic groups, which were previously associated with the Arctic water masses and winter communities in the Fram Strait (e.g., the class Deltaproteobacteria and the Thaumarchaeota) (Wilson et al, 2017;Fadeev et al, 2018Fadeev et al, , 2020Müller et al, 2018b), showed higher cell abundances in the ice-covered EG stations, as compared to the ice-free HG and ice-margin N stations (Table 1). Hence, the spatial variability in cell abundances of different taxonomic groups was apparently associated with different stages of the phytoplankton bloom and different water masses.…”

“…Based on the known hydrography of the Strait (Rudels et al, 2012), and observed sea-ice conditions, the sampled stations were grouped into three distinct regions (Figure 1): the eastern ice-free region associated with the WSC (Beszczynska-Möller et al, 2012), the western ice-covered region associated with the EGC (de Steur et al, 2009), and the partially ice-covered region in the north-east that is associated with the highly productive ice-margin zone (further addressed as North-"N") (Hebbeln and Wefer, 1991;Perrette et al, 2011). At the time of sampling in June-July 2016, microscopy counts of phytoplankton communities and chlorophyll a concentration, showed a late phytoplankton bloom across the Strait (Table S1; described in detail by Fadeev et al 2020). The phytoplankton communities of the ice-covered EG and the ice-margin N stations were dominated by diatoms, in contrast to the ice-free HG stations that were dominated by Phaeocystis spp.…”

“…Previous observations in the Fram Strait, acquired using high-throughput sequencing of the 16S rRNA gene, revealed a strong influence of the summerly phytoplankton bloom conditions on the bacterioplankton communities (Wilson et al, 2017;Müller et al, 2018b), differing between the ice-covered and ice-free regions of the Strait (Fadeev et al, 2018). During our sampling period, distinct phytoplankton bloom communities in surface waters across the Strait were observed, with a Phaeocystis-dominated bloom in the ice-free HG stations, a diatom-dominated bloom in the ice-covered EG stations, and mixed diatoms and Phaeocystis populations bloom in the ice-margin N stations (Fadeev et al, 2020). Along with this, we observed significantly higher cell abundances in the surface water total bacterioplankton communities of the HG and N stations (13-21×10 5 cells mL -1 ), as compared to the EG stations (0.5-2×10 5 cells mL -1 ; Kruskal-Wallis test; Chi square=81.85, df=2, p-value<0.01).…”