Editorial: Rapid change in polar pelagic ecosystems: new challenges on causes and effects of climate variability

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Polar pelagic ecosystems are undergoing rapid change as climate variability intensifies and human pressures increase. These waters support productive and diverse communities whose dynamics are closely linked to sea ice, water-mass structure and extreme seasonal variation in light. The Arctic and Southern Ocean differ markedly in their physical oceanography, and these contrasts influence how each region responds to environmental change (Meredith et al., 2019). As climate-driven pressures increase, regional differences are becoming more pronounced, contributing to shifts in biodiversity, food-web structure and ecosystem functioning, with implications for global biogeochemical cycles (IPCC, 2022).Pelagic species are responding rapidly to changing environmental conditions and are increasingly recognised as indicators of ecosystem change. Altered sea-ice regimes, changing upper-ocean structure and ocean warming affect species distributions, migration patterns, dietary preferences and seasonal life cycles, with consequences that propagate through food webs (Atkinson et al., 2019). In the Arctic, declining multiyear ice, earlier melt onset and changing stratification are modifying habitat availability and the timing of key life-history events (Ardyna & Arrigo, 2020). In the Southern Ocean, changes in temperature and sea ice influence krill recruitment and distribution, altering predator foraging patterns and carbon transfer to the deep ocean (Kawaguchi et al., 2013). These climate pressures are compounded by ocean acidification and expanding fisheries, which further threaten food-web stability, carbon storage and habitat conservation (Cavan et al., 2019).The papers in this Research Topic address the effects of climate change on polar marine ecosystems, including six Arctic and two Antarctic studies. Together, the contributions span multiple trophic levels, from prokaryotes and protists to zooplankton, fish and marine predators. A common theme is the ecological impact of diminishing sea ice. In the Arctic, seaice loss has contributed to declines in species that depend on ice-associated habitats, including polar bears, walruses and seals (Le Moullec & Bender, 2022). In the Antarctic, reduced sea ice affects krill nursery habitat and may disrupt the synchrony between plankton production and herbivore development, with consequences for higher trophic levels (Fraser et al., 2023;Swadling et al., 2023).Arctic sea ice is highly heterogeneous, yet many habitat types remain poorly studied. Castellani et al. examined sea-ice ridges, one of the least sampled components of the Arctic system. Their study showed that variability measured on a single floe can be representative of larger-scale patterns, providing a framework for improving assessments of sea-ice habitats.Increasing glacial meltwater input is another major driver of ecological change. Lo Giudice et al. investigated how freshwater influx alters salinity, turbidity, nutrient availability and organic matter dynamics in Arctic fjords. Their results demonstrated strong effects on prokaryotic community composition while highlighting ongoing challenges in linking taxonomic shifts to ecosystem processes and long-term change. Dabrowska et al. examined protistan plankton communities along a terrestrial-marine gradient in Isfjorden, Svalbard, during the exceptionally warm year of 2018. Their results suggest that continued warming may favour small opportunistic flagellates over diatom-dominated communities, with implications for food-web dynamics and carbon cycling. Freshwater sources vary in their ecological effects. Sumiyoshi et al. compared the influence of river runoff and sea-ice meltwater on microplankton communities in the western Arctic Ocean. Phytoplankton responses differed among regions, indicating that biological outcomes depend on both freshwater source and local environmental conditions.Nicolai et al. investigated the temporal dynamics of two appendicularian species in a high-Arctic fjord influenced intermittently by Atlantic Water intrusions. Their findings improve understanding of appendicularian population ecology under changing oceanographic conditions and support their potential use as sentinel species of environmental change. Heide-Jørgensen et al. examined ecosystem responses to warming along the East Greenland coast, where sea-surface temperatures shifted markedly during the late 1990s. They documented a redistribution of capelin towards East Greenland and associated increases in the use of coastal waters by boreal baleen whale species.The two Southern Ocean studies highlighted the importance of environmental heterogeneity in shaping pelagic communities. Granata et al. showed that phytoplankton and zooplankton distributions in the western Ross Sea are structured by sea-ice retreat, hydrography and regional environmental gradients. The study identified a mosaic of ecological subsystems whose characteristics were influenced by temperature, salinity and fluorescence. Minutoli et al. provided a baseline assessment of pelagic amphipod biodiversity across the South Pacific sector and western Ross Sea. This work establishes an important reference point for detecting future ecological change.Although rapid changes in polar pelagic ecosystems are widely documented, the mechanisms linking physical climate change to biological responses remain incompletely understood (Henley et al., 2020). Large-scale climate modes influence sea ice and nutrient supply, yet their effects on plankton phenology, species interactions and key survival traits are still poorly represented in predictive models (Atkinson et al., 2019;Freer et al., 2021;Thompson et al., 2022).Emerging technologies are helping to address these gaps. Autonomous observing platforms now provide access to remote regions and seasons that were previously difficult to sample. Nevertheless, winter and under-ice observations remain limited, restricting our understanding of fine-scale behaviour and physiological adaptation (Riser et al., 2021). Molecular approaches, including environmental DNA, and advances in machine learning are improving predictions of species distributions and climate responses. Integrating these observations with physical and biogeochemical models remains a major challenge (Grillo et al., 2022). Progress in forecasting ecosystem change, refining carbon-cycle projections and supporting effective conservation will depend on sustained observations and stronger integration across disciplines.

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