Interannual differences in sea ice regime in the north-western Barents Sea cause major changes in summer pelagic production and export mechanisms

Abstract

The Barents Sea is a highly dynamic and productive marine ecosystem and a hotspot of global warming. Variability in sea ice extent is a common feature in the Barents Sea with substantial movements of the sea ice edge on short-term, seasonal to interannual time scales. Historically the northern Barents Sea (north of 75◦N) has been ice-covered in winter, but recently it has become the area with most winter ice loss in the Arctic, and year-round ice-free conditions are predicted for the second half of the 21st century. These environmental changes have significant implications for the marine ecosystem. In this study we used contrasting sea ice regimes in August 2018 and August 2019 to explore the response of phytoplankton and bacterial production, microbial abundance, and vertical carbon flux in the north-western Barents Sea (between 76◦N and 83◦N) to the variability of sea ice. While the study area was ice-free in August 2018, extensive areas north of 79◦N were ice-covered in 2019. When the northern parts of the transect were still ice covered, diatoms and other larger phytoplankton were dominant and highest abundances were observed following the receding ice edge. In contrast, under ice-free conditions in 2018, the pelagic ecosystem resembled a post-bloom stage of the seasonal succession with higher abundance of small phytoplankton and heterotrophic protists and low vertical flux throughout the water column. While phytoplankton biomass, bacterial production and downward vertical flux of particulate organic carbon in the upper 60 m were on average higher in 2019, primary production and carbon export below the euphotic layer were comparable between both years. However, overall highest primary production, bacterial production and abundance of both photosynthetic and heterotrophic microorganisms were observed in surface waters (upper 30 m) in 2019, connected to the retreating ice edge, where also vertical particle flux was higher and characterized by a strong attenuation curve. The results clearly demonstrate that differences in ice cover affect the phenology of pelagic primary production and associated biological processes in the Barents Sea.

Barentshavet er et svært dynamisk og produktivt marint økosystem, og er nå en global hotspot i den raske oppvarming av havområdene i Arktis. Iskanten i Barentshavet kan variere over kortsiktige, sesongmessige og interannuelle tidsrammer. Historisk sett har den nordlige delen av Barentshavet (nord for 75°N) vært dekket av havis om vinteren, men i den senere tid har området hatt et tap av havis om vinteren som er unikt i Arktis, og det er estimert at området vil være isfritt gjennom hele året i andre halvdel av det 21. århundre. Denne miljøendringen har betydelige implikasjoner for det marine økosystemet. I denne studien undersøker vi endringer i planteplankton og bakterieproduksjon og tettheten, og vertikalt karbonfluks i det nordvestlige Barentshavet (mellom 76°N og 83°N) under ulike havisforhold i august 2018 og august 2019. Mens hele studieområdet var isfritt i august 2018, var områdene nord for 79°N dekket av havis i august 2019. Fordi de nordlige delene var isdekket i 2019, dominerte diatoméer og andre større planteplankton, og de høyeste tettheter, vertikal fluks og produksjon ble observert langs iskanten. Derimot dominerte små arter av fytoplankton og heterotrofe protister, og den vertikale fluksen var lav, under de isfrie forholden i 2018. Våre resultater bekrefter at forskjeller i havisdekket påvirker i betydelig grad pelagisk primærproduksjon og tilknyttede biologiske prosesser i Barentshavet.