Changing Arctic zooplankton: aspects of physiology, food web structures and connectivity

Abstract

The Arctic Ocean and adjacent ice-covered seas are the regions that are most rapidly affected by climate change. Air temperatures are rising four times faster in the Arctic than the global average, leading to a rapid and substantial loss in sea-ice volume. Concurrently, the inflows of warm, saline Atlantic water into the Arctic Basin through Fram Strait and the Barents Sea are increasing, a phenomenon referred to as Atlantification. This Atlantification is not only driving physical changes of the Arctic environment, but also facilitates a northward range shift of boreal taxa. Consequently, polar zooplankton species are confronted with increasing temperatures and changing food web structures whilst at the same time face increasing competition by boreal-Atlantic congeners. This thesis provides a comprehensive approach investigating the different factors of climate change on Arctic zooplankton species, focusing on calanoid copepods Calanus hyperboreus, Calanus glacialis, Paraeuchata glacialis and the hyperiid amphipod Themisto libellula, as key representatives. Boreal-Atlantic congeners Calanus finmarchicus, Paraeuchaeta norvegica and Themisto abyssorum are considered as expatriates, which are extending their distribution ranges into the Arctic in the course of global warming. For the assessment of physiological responses of Arctic and boreal zooplankton species to rising temperatures, their respiration rates were measured from 0 to 10°C. A key finding was the resilience of Arctic species to temperature increases, as evidenced by their wide thermal tolerance and lack of metabolic stress response (low Q10 ratios). On the other hand, boreal species exhibited a more pronounced and rapid increase in respiration rates with rising temperatures, suggesting enhanced metabolic activity and overall performance under warmer conditions. Consequently, the temperature threshold at which boreal species outperform their Arctic congeners is likely to be a key determinant of zooplankton dynamics in a warming Arctic, rather than the absolute physiological limits of the species. Analyses of food webs, utilizing both fatty acid and stable isotope biomarkers, across Arctic and Atlantic-influenced regions in Fram Strait were conducted to explore changes in trophic structures associated with the sea ice decline and increased Atlantification. The results emphasized the importance of (sea ice) diatoms in the Arctic ice-covered regions and shows a shift towards a more flagellate-based food web, with a higher degree of omnivory, in the Atlantic regime. The ability of Arctic Calanus species to rely on alternative food sources other than (sea ice) diatoms highlighted their dietary flexibility, which may become increasingly important with the predicted increase in flagellate production in the future Arctic Ocean. The high relevance of Calanus fatty acid trophic markers in higher trophic levels in the Atlantic regime was likely a reflection of high abundances of C. finmarchicus in this region, showing its importance to the diet of carnivorous zooplankton as Atlantification progresses. Among Arctic and boreal congeners, an increased dietary overlap was observed between C. glacialis and C. finmarchicus as well as between P. glacialis and P. norvegica in areas of co-occurrence. The evaluation of zooplankton data in context with the physical-oceanographic observations of a submesoscale filament emphasized the significance of such dynamics in shaping the pelagic environment. Strong horizontal and vertical velocities associated with these features play a major role in structuring the pelagic ecosystem, facilitating Atlantification processes and influencing species allocation and biological connectivity. Additionally, proteomic fingerprinting was demonstrated as a rapid and accurate methodology for identifying climate-relevant but morphologically similar indicator species C. glacialis and C. finmarchicus to species and even developmental stages level. This technique has thus the potential to significantly enhance species identification in long-term monitoring studies, which is vital for deepening our understanding of ecosystem responses to climate change. In conclusion, this thesis demonstrates that Arctic zooplankton exhibit a considerable resilience and adaptability to environmental changes, including elevated temperatures and alterations in the food web structures. However, it also emphasizes the challenges posed by the intrusion of boreal species, which, under more boreal-like conditions, may outcompete polar Arctic species. The findings underline that the future of Arctic species in a warming ocean depends not only on their physiological tolerance and ecological adaptability but also on the competitive interactions with boreal congeners.