Comparative analysis of prokaryotic communities, hydrography, and biogeochemistry in Atlantic vs non-Atlantic influenced Svalbard fjords

BACKGROUND: Fjords in Svalbard are undergoing significant changes due to climate warming. Those along the west coast of Spitsbergen are particularly affected by the increasing influence of “warm” Atlantic Water (AW), a process known as Atlantification. We compared Kongsfjorden, a relatively “warm” fjord on the west coast, with Rijpfjorden, a typical cold Arctic fjord on the north coast of Nordaustlandet, combining physical and biogeochemical data with 16S rRNA gene amplicon and shotgun metagenomic sequencing. We hypothesize that differences in fjords’ water masses and prokaryotic communities provide insight into the effects of Atlantification as it expands eastwards along the shelf north of Svalbard. RESULTS: We found that warm AW dominated in Kongsfjorden, whereas Rijpfjorden was dominated by cold Arctic Water and Winter Cooled Water. Our results suggest that the Atlantic-influenced Kongsfjorden is a nutrient sink, whereas Rijpfjorden showed similar behavior only in 2016, a particularly warm year, otherwise no clear sink/source role could be identified. Analysis of 16S rRNA gene sequences revealed that Proteobacteria had higher relative abundances in Kongsfjorden while Bacteroidota dominated in Rijpfjorden. Ammonium and nitrite-oxidizing prokaryotes were most prevalent in deeper water masses of both fjords. The archaeal taxa of the ammonia-oxidizing community, mainly Nitrosopumilus and Nitrosopelagicus, were consistently more dominant than ammonium and nitrite-oxidizing bacteria. Denitrification and nitrogen fixation genes differed between the fjords, with Kongsfjorden having a higher coverage of diazotroph genes. CONCLUSIONS: Kongsfjorden and Rijpfjorden displayed distinct hydrographic conditions, with Kongsfjorden being under a stronger influence of Atlantification. Our results suggest that warmer water masses are linked to higher nutrient uptake. The clear association between microbial communities and water masses offers insight into changes driven by Atlantification.