Phytoplankton diversity and zooplankton diet across Fram Strait: Spatial patterns with implications for the future Arctic Ocean

Abstract

The rapid transformations in the European sector of the Arctic Ocean, characterized by a pronounced increase in Atlantic water inflow and a decline in sea ice, are reshaping the trophic structures within this ecosystem. This study investigates the effects of such environmental changes on the planktonic trophodynamics in Fram Strait, focusing on the comparison between Arctic and Atlantic influenced regions. Utilizing a combination of fatty acid and stable isotope biomarker analyses, we analyze the dietary composition and trophic levels of key zooplankton species, including Arctic and boreal-Atlantic sister species of Calanus, Paraeuchaeta and Themisto. The findings reveal significant differences in the dietary structures between the different areas in Fram Strait. In the Arctic region, the diet of zooplankton species is primarily influenced by diatoms, as indicated by the dominance of diatom-specific fatty acid trophic markers. Conversely, the Atlantic region exhibits a different dietary pattern, pointing to a reduced influence of diatoms and increased importance of (dino-)flagellates for Calanus spp., as well as a higher degree of carnivory in higher trophic levels. Moreover, the comparison of sister species unveils a distinct overlap in dietary niches between congeners of Calanus and Paraeuchaeta, highlighting the increasing potential for competitive interactions as Atlantification progresses. These results underscore the complex relationships within Arctic food webs and the profound impact of environmental changes on trophodynamics. They also emphasize the importance of considering physiological adaptability, life history traits and ecological interactions in predicting the future of Arctic marine ecosystems amidst ongoing climate change.