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. 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. Furthermore, we found that polar pelagic and ice-associated taxa, such as Fragilariopsis cylindrus and Melosira arctica, were more common in Atlantic conditions, while temperate taxa, such as Odontella aurita and Proboscia alata, were less abundant. This suggests that sea ice melt may act as a barrier to the northward expansion of temperate phytoplankton, preventing their dominance in regions still strongly influenced by polar conditions. Our findings highlight the complex interactions between sea ice melt, phytoplankton dynamics, and biodiversity in the Arctic.

Beyond blooms: A novel time series analysis framework predicts seasonal keystone species and sheds light on Arctic pelagic ecosystem stability

“…Names are based on light availability (as defined in Oldenburg et al (2024)), categorized in transition areas between dark and light (T), high light (H) and low light (L) phases based on PAR parameter (Figure 4) and the season spring (S), summer(S), autumn (A) and winter (W).…”

“…In our study, we found distinct taxonomic compositions within various clusters. Photosynthetic organisms like Ochrophyta and Haptophyta dominate the light phases (Oldenburg et al, 2024). In late spring (cluster 08TS) phototrophs make up more than 75% of ASVs, while during summer (clusters 08TS and 09HS) and early autumn (10HS) they still comprise over 25% of all ASVs.…”

“…Samples were collected with Remote Access Samplers (RAS; McLane) deployed in conjunction with oceanographic sensors over four annual cycles (01.08.2016cycles (01.08. to 16.09.2020) at the F4 mooring (79.0118N 6.9648E) of LTER HAUSGARTEN and FRAM in the Fram Strait (Soltwedel et al, 2005;Oldenburg et al, 2024). Each RAS contains 48 sterile bags, each collecting water samples of 500 mL at programmed sampling intervals .…”