Abstract
The Arctic is experiencing dramatic changes including increases in precipitation, glacial melt, and permafrost thaw, resulting in increasing freshwater runoff to coastal waters. During the melt season, terrestrial runoff delivers carbon- and nutrient-rich freshwater to Arctic coastal waters, with unknown consequences for the microbial communities that play a key role in determining the cycling and fate of terrestrial matter at the land-ocean interface. To determine the impacts of runoff on coastal microbial (bacteria and archaea) communities, we investigated changes in pelagic microbial community structure between the early (June) and late (August) melt season in 2018 in the Isfjorden system (Svalbard). Amplicon sequences of the 16S rRNA gene were generated from water column, river and sediment samples collected in Isfjorden along fjord transects from shallow river estuaries and glacier fronts to the outer fjord. Community shifts were investigated in relation to environmental gradients, and compared to river and marine sediment microbial communities. We identified strong temporal and spatial reorganizations in the structure and composition of microbial communities during the summer months in relation to environmental conditions. Microbial diversity patterns highlighted a reorganization from rich communities in June toward more even and less rich communities in August. In June, waters enriched in dissolved organic carbon (DOC) provided a niche for copiotrophic taxa including Sulfitobacter and Octadecabacter. In August, lower DOC concentrations and Atlantic water inflow coincided with a shift toward more cosmopolitan taxa usually associated with summer stratified periods (e.g., SAR11 Clade Ia), and prevalent oligotrophic marine clades (OM60, SAR92). Higher riverine inputs of dissolved inorganic nutrients and suspended particulate matter also contributed to spatial reorganizations of communities in August. Sentinel taxa of this late summer fjord environment included taxa from the class Verrucomicrobiae (Roseibacillus, Luteolibacter), potentially indicative of a higher fraction of particle-attached bacteria. This study highlights the ecological relevance of terrestrial runoff for Arctic coastal microbial communities and how its impacts on biogeochemical conditions may make these communities susceptible to climate change.
Highlights
Arctic regions are warming two times faster than lower latitudes (Serreze and Barry, 2011; Osborne et al, 2018)
We identified a total of 35,280 Operational Taxonomic Units (OTUs) across all samples
Richness was highest in June (p < 0.005 Chao1 and Abundance-based Coverage Estimator (ACE)), whereas evenness was highest in August (p < 0.001 Pielou’s) (Figures 2A,B and Supplementary Figure 2), resulting in higher Shannon’s and inverse Simpson’s diversity in August (Figure 2C and Supplementary Figure 2)
Summary
Arctic regions are warming two times faster than lower latitudes (Serreze and Barry, 2011; Osborne et al, 2018). Seasonal changes (e.g., in light and nutrient availability, temperature, and salinity) strongly affect microbial community function and composition (Gilbert et al, 2012; Cram et al, 2015). In coastal areas, these variables are altered by terrestrial runoff from Arctic thawing catchments (McGovern et al, 2020). In the Arctic, climate change is amplifying seasonal changes in temperature (Serreze and Barry, 2011) and terrestrial runoff (Holmes et al, 2012), and will likely affect coastal microbial communities (Kirchman et al, 2009; Vincent, 2010; Cavicchioli et al, 2019). Understanding how these communities respond to environmental variables through the melt season is needed to predict how climateinfluenced changes might affect microbial communities in Arctic coastal waters
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