Distribution of dissolved organic carbon linked to bacterial community composition during the summer melting season in Arctic fjords

Melting glaciers play a substantial role in the Arctic hydrography of Svalbard and the associated bacterial processes. In this study, we surveyed the spatial variability in dissolved organic carbon (DOC), fluorescent dissolved organic matter (FDOM), and bacterial biogeography (diversity and community composition) using 16S rRNA gene metabarcoding at four fjords in Svalbard in August 2019. The environmental heterogeneity of Arctic fjord seawater was represented by the water mass balance parameters of surface water (SW), subsurface water (SSW), and intermediate water (IW). The DOC concentrations in the SW (avg. 67 ± 7 μM), IW (avg. 68 ± 8 μM), and SSW (avg. 68 ± 7 μM) of the surveyed Arctic fjords did not differ significantly ( P > 0.01). We found that the DOC concentration depended on the microbial community because of the strong association between DOC and marine humic-like FDOM ( R 2 = 0.48 for SW and R 2 = 0.45 for SSW), along with the humification, biological, and fluorescence indices. Metabarcoding results revealed that six heterotrophic bacteria ( Alphaproteobacteria , Gammaproteobacteria , Bacteroidia , Verrucomicrobiae , Actinobacteria , and Acidimicrobiia ) were dominant, and their population dynamics represented the water mass balance. Furthermore, our statistical results indicated that Actinobacteria may significantly affect the DOC distribution during the glacier melt season. Our results confirm that the water mass balance shapes the bacterial community composition and suggest that DOC derived from microbial activities warrants significant attention considering the effects of climate change during the glacier melt season in the Arctic fjords around Svalbard.