Sea-ice melt determines seasonal phytoplankton dynamics and delimits the habitat of temperate Atlantic taxa as the Arctic Ocean atlantifies

Abstract

The Arctic Ocean is one of the regions where anthropogenic environmental change is progressing most rapidly and drastically. The impact of rising temperatures and decreasing sea ice on Arctic marine microbial communities is yet not well understood. Microbes form the basis of food webs in the Arctic Ocean, providing energy for larger organisms. Previous studies have shown that Atlantic taxa associated with low light are robust to more polar conditions. In this study, we compared to which extent sea ice melt influences light-associated phytoplankton dynamics and biodiversity over two years at two mooring locations in the Fram Strait. One mooring is deployed in pure Atlantic water, and the second in the intermittently ice-covered Marginal Ice Zone. Time-series analysis of amplicon sequence variants abundance over a two-year period, allowed us to identify communities of co-occurring taxa that exhibit similar patterns throughout the annual cycle. We then examined how alterations in environmental conditions affect the prevalence of species. During high abundance periods of diatoms, polar phytoplankton populations dominated, while temperate taxa were weakly represented. Generally, polar pelagic and ice-associated taxa (such asFragilariopsis cylindrusorMelosira arctica) were more prevalent in Atlantic conditions whereas temperate taxa (such asOdontella auritaorProboscia alata) have limited potential to persist in colder ice-impacted waters. In contrast to previous assumptions, we think that sea-ice melt acts as a barrier to the horizontal extent of temperate diatoms by preventing their succession at places strongly influenced by polar conditions such as the melting sea ice.

The Arctic summer microbiome across Fram Strait: Depth, longitude, and substrate concentrations structure microbial diversity in the euphotic zone

“…Although depth and longitude were the strongest drivers of microbial composition (Figure 3), the influence of Julian day (i.e., the time of sampling) was pronounced for samples from surface and chl‐max depths (Figure 6A). These patterns likely corresponded to varying mixed layer depth and productivity between June and September (Oldenburg et al, 2023; Wietz et al, 2021). In parallel, substrate regimes shift to more refractory compounds once phytoplankton blooms collapse (von Jackowski et al, 2022).…”

“…Future ocean scenarios predict substantial ecosystem shifts in the Arctic, supported by a changing microbiome structure at higher temperatures (Ahme et al, 2023). One important aspect is the impact of northward expanding Atlantic waters, termed Atlantification, on microbial diversity and the biological carbon pump (Carter‐Gates et al, 2020; Oldenburg et al, 2023; Oziel et al, 2020).…”

Seasonal recurrence and modular assembly of an Arctic pelagic marine microbiome

“…The temporal analysis of ASVs and gene clusters as well as the construction of co-occurrence networks was performed using the same workflow as that presented recently by authors of this paper 60 . In brief, for each ASV and gene cluster, we calculated a time-series signal using the signal using the segmenTier / segmenTools (https://github.com/raim/segmenTools) packages https://www.nature.com/articles/s41598-017-12401-8) using relative (ASV) and normalized (gene cluster) abundance data.…”

“…The temporal analysis of ASVs and gene clusters as well as the construction of co-occurrence networks was performed using the same workflow as that presented recently by authors of this paper 60 . In brief, for each ASV and gene cluster, we calculated a time-series signal using the signal using the…”