Carbon Export in the Seasonal Sea Ice Zone North of Svalbard From Winter to Late Summer

Christine Dybwad,Philipp Assmy,Anna Nikolopoulos,Ilka Peeken,Marit Reigstad,Józef M Wiktor,Lasse M Olsen,Thomas Krumpen,Achim Randelhoff,Agnieszka Tatarek

doi:10.3389/fmars.2020.525800

Abstract

Phytoplankton blooms in the Arctic Ocean's seasonal sea ice zone are expected to start earlier and occur further north with retreating and thinning sea ice cover. The current study is the first compilation of phytoplankton bloom development and fate in the seasonally variable sea ice zone north of Svalbard from winter to late summer, using short-term sediment trap deployments. Clear seasonal patterns were discovered, with low winter and pre-bloom phytoplankton standing stocks and export fluxes, a short and intense productive season in May and June, and low Chl a standing stocks but moderate carbon export fluxes in the autumn post-bloom conditions. We observed intense phytoplankton blooms with Chl a standing stocks of &gt;350 mg m−2 below consolidated sea ice cover, dominated by the prymnesiophyte Phaeocystis pouchetii. The largest vertical organic carbon export fluxes to 100 m, of up to 513 mg C m−2 day−1, were recorded at stations dominated by diatoms, while those dominated by P. pouchetii recorded carbon export fluxes up to 310 mg C m−2 day−1. Fecal pellets from krill and copepods contributed a substantial fraction to carbon export in certain areas, especially where blooms of P. pouchetii dominated and Atlantic water advection was prominent. The interplay between the taxonomic composition of protist assemblages, large grazers, distance to open water, and Atlantic water advection was found to be crucial in determining the fate of the blooms and the magnitude of organic carbon exported out of the surface water column. Previously, the marginal ice zone was considered the most productive region in the area, but our study reveals intense blooms and high export events in ice-covered waters. This is the first comprehensive study on carbon export fluxes for under-ice phytoplankton blooms, a phenomenon suggested to have increased in importance under the new Arctic sea ice regime.

Highlights

Vertical carbon export plays a key role in the biological carbon pump by contributing to the sequestration of CO2 from the surface to the deep ocean, as well as providing a food source for mesopelagic, deep-sea and benthic ecosystems (Ducklow et al, 2001; Boyd and Trull, 2007)
Vertical carbon export was studied during three field campaigns in the sea ice zone (SIZ) north of Svalbard: CarbonBridge (Bridging productivity regimes in the Arctic Ocean, with R/V Helmer Hanssen), N-ICE2015 (Norwegian young sea ICE Cruise, with R/V Lance), and TRANSSIZ (Transitions in the Arctic Seasonal Sea Ice Zone, PS92, ARK XXIX/1, with R/V Polarstern)
The ice cover in the visited areas ranged from 0 to 100% in sea ice cover and was influenced to various degrees by Atlantic water transported by the West Spitsbergen Current (WSC) and by overlaying Arctic surface waters (ArSW) advected in the opposite direction (Rudels et al, 2000; Fer et al, 2017; Meyer et al, 2017; Crews et al, 2018; Menze et al, 2019) (Figure 1)

Summary

Introduction

Vertical carbon export plays a key role in the biological carbon pump by contributing to the sequestration of CO2 from the surface to the deep ocean, as well as providing a food source for mesopelagic, deep-sea and benthic ecosystems (Ducklow et al, 2001; Boyd and Trull, 2007). The proportion of multiyear ice (MYI) has declined by 90% since 1979 (Stroeve and Notz, 2018; IPCC report, 2019), and the Arctic Ocean is expected to be ice free in summer by the end of the century or earlier (Massonnet et al, 2012; Wang and Overland, 2012). As a result of the shorter, more transparent, and dynamic sea ice cover, large under-ice phytoplankton blooms in the Arctic have been reported (Fortier et al, 2002; Arrigo et al, 2012; Mundy et al, 2014; Assmy et al, 2017; Johnsen et al, 2018; Ardyna and Arrigo, 2020; Ardyna et al, 2020). Recent studies have described the phytoplankton dynamics and environmental drivers of these under-ice blooms (Ardyna et al, 2020), but the fate of the carbon produced by these blooms is yet to be explored (Ardyna and Arrigo, 2020)

Methods

Results

Discussion

Conclusion