Editorial: Current and Future Threats to Marine Zooplankton in Changing Polar Oceans and Their Potential for Adaption and Coping

Doreen Kohlbach^1*Jean-Pierre Desforges²Nadjejda Espinel Velasco³

• ¹Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI), Bremerhaven, Germany
• ²University of Winnipeg, Winnipeg, Canada
• ³University of Gothenburg, Gothenburg, Sweden

Marine zooplankton are continuously and increasingly exposed to the impacts of anthropogenic climate change, which poses both immediate and long-term threats to e.g., food-source availability, distribution, reproductive success and ultimately survival (Le Moullec & Bender, 2022;Bargagli & Rota, 2024). Stressors such as rising seawater temperatures, ocean acidification, and shifts in sea-ice dynamics are altering the environmental conditions that zooplankton are adapted to (Espinel-Velasco et al., 2023). Warmer waters and longer open-water seasons can affect the timing and abundance of ice algal and phytoplankton blooms, disrupting the food supply for zooplankton, whose lifecycles are oftentimes perfectly embedded between sympagic (ice-associated) and pelagic primary production peaks (Søreide et al., 2010;Renaud et al., 2024). Many species depend on ice-associated algae as a food source in both hemispheres (Kohlbach et al., 2016(Kohlbach et al., , 2018)), a dietary resource that will likely have a more spatio-temporally restricted availability in the sea ice-retreating decades to come. These environmental changes are likely to cause a shift in zooplankton distribution, with some species migrating northwards seeking cooler waters while others may face habitat loss (Kuletz et al., 2024). While some zooplankton may exhibit short-term resilience through adaptive behaviors, such as altered migration patterns or shifts in reproductive timing, their ability to cope with these rapid environmental changes might be limited in the long term and our understanding of potential coping mechanisms is still in its infancy. Long-term survival will depend on the pace of change and the ability of zooplankton populations to evolve and adapt genetically or successfully conquer and inhabit regions closer to their natural living conditions. If these environmental stresses continue to accelerate, as it is expected, the overall structure and productivity of polar marine ecosystems could be severely impacted (Johnston et al., 2022). Additionally, and partly facilitated by changing environmental settings, the increasing presence of anthropogenic pollutants, such as persistent organohalogenated chemicals, heavy metals and microplastics (Xie et al., 2022), can have adverse effects on marine animals' behavioral patterns and overall body condition (Dietz et al., 2019;Bargagli & Rota, 2022;Borgå et al., 2022).In this research topic, we aimed to identify and examine the role of marine zooplankton in polar food webs and how the status quo is being altered by climate and environmental change. We collected five original research papers exploring innovative tools to study polar zooplankton dynamics under rapid change. Two papers apply state-of-the-art molecular techniques such as DNA metabarcoding and RNA sequencing to understand diet and stress responses, one paper uses field data to explore timing of reproductive development in Antarctic krill, one paper models zooplankton community dynamics under climate-change scenarios, and the last paper reviews our current understanding of metal sources and fate in zooplankton of Arctic marine food webs.By applying state-of-the-art biomolecular techniques, the diet of Arctic amphipods during the phytoplankton-poor polar night has been studied by Dischereit et al. (2024). Metabarcoding revealed a surprisingly diverse dietary composition including gelatinous zooplankton and fish. These findings provide novel and exciting insights since jellyfish were typically considered a trophic dead-end with little nutritional value. In Martinez-Alarcón et al. ( 2024), RNA sequencing was used in an experimental approach to study the impact of different temperature treatments on Themisto spp., key amphipods in Arctic food webs. Results showed that thermal stress impacts gene expression of species differently, suggesting species-specific regulatory capacities to temperature increase. Their findings are important to our understanding of the potential to cope with rising seawater temperatures and likelihoods of range shifts for marine amphipods in a warming Arctic. In Steinke et al. (2024), the impacts of climate and environmental variability on the timing of reproductive development in female Antarctic krill around the Antarctic Peninsula is studied. Pronounced variability in maturation status among multi-annual measurements was observed and Southern Annular Mode and the Multivariate El Niño Index were identified to be the most prominent drivers of this variability. Results of this work are especially useful for management of krill fisheries and species reliant on krill as part of their diet. Hibino et al. ( 2025) used a modeling approach to infer the effect of sea-ice cover on the zooplankton distribution in the Pacific Arctic Ocean. Under significant warming scenarios, generalized dissimilarity models (GDM) projected that zooplankton communities in high latitudes will be more affected by altered timings of sea-ice melt seasons and consequently changes in annual primary production than those on the southern shelf (northern Bering Sea to southern Chukchi Sea). The authors provide evidence that Arctic warming is not likely to uniformly yield high primary productivity. In a Mini-Review by Halsband et al. (2024), sources and transfer of metals within Arctic marine ecosystems are elucidated with a focus on bioavailability of different compounds and their potential for toxicity in marine copepods. Updated information on zooplankton responses to metal exposure in combination with other stressors, including increased temperature and lower pH, is provided and a number of research gaps are identified. Suggestions for future research foci, such as improved understanding on intra-and interspecific differences in metal toxicity, are discussed.Overall, the five papers in this research topic explore leading issues in polar marine ecosystems, such as rising ocean temperatures, shifts in sea-ice dynamics, and climate change influences on pollution exposure and effects. Understanding these ecological dynamics is critical to assessing the vulnerability vs. resilience of marine zooplankton populations to ongoing environmental stress of varying sources. Investigating the capacity of zooplankton to cope with such changesthrough physiological adaptation, behavioral and trophic plasticity, or shifts in distribution-is critical to evaluate whether these responses are sufficient to maintain their ecological roles in rapidly changing polar ecosystems.