Algal Hot Spots in a Changing Arctic Ocean: Sea-Ice Ridges and the Snow-Ice Interface

Mar Fernández-Méndez,Åse Ervik,A Malin Johansson,Jens K Ehn,Hanna M Kauko,Lasse M Olsen,Pedro Duarte,Amelie Meyer,Christopher J Mundy,Philipp Assmy,Brian K Sorrell,Anette Wold,Penelope M Wagner,Ioanna Merkouriadi,Anja Rösel,Haakon Hop

doi:10.3389/fmars.2018.00075

Abstract

During the N-ICE2015 drift expedition north-west of Svalbard, we observed the establishment and development of algal communities in first-year ice (FYI) ridges and at the snow-ice interface. Despite some indications of being hot spots for biological activity, ridges are under-studied largely because they are complex structures that are difficult to sample. Snow infiltration communities can grow at the snow-ice interface when flooded. They have been commonly observed in the Antarctic, but rarely in the Arctic, where flooding is less common mainly due to a lower snow-to-ice thickness ratio. Combining biomass measurements and algal community analysis with under-ice irradiance and current measurements as well as light modeling, we comprehensively describe these two algal habitats in an Arctic pack ice environment. High biomass accumulation in ridges was facilitated by complex surfaces for algal deposition and attachment, increased light availability, and protection against strong under-ice currents. Notably, specific locations within the ridges were found to host distinct ice algal communities. The pennate diatoms Nitzschia frigida and Navicula species dominated the underside and inclined walls of submerged ice blocks, while the centric diatom Shionodiscus bioculatus dominated the top surfaces of the submerged ice blocks. Higher light levels than those in and below the sea ice, low mesozooplankton grazing, and physical concentration likely contributed to the high algal biomass at the snow-ice interface. These snow infiltration communities were dominated by Phaeocystis pouchetii and chain-forming pelagic diatoms (Fragilariopsis oceanica and Chaetoceros gelidus). Ridges are likely to form more frequently in a thinner and more dynamic ice pack, while the predicted increase in Arctic precipitation in some regions in combination with the thinning Arctic icescape might lead to larger areas of sea ice with negative freeboard and subsequent flooding during the melt season. Therefore, these two habitats are likely to become increasingly important in the new Arctic with implications for carbon export and transfer in the ice-associated ecosystem.

Highlights

Current changes in sea-ice conditions have consequences for algal biomass and growth, with bottom-up cascading effects on the Arctic marine food web (Wassmann et al, 2011)
When comparing the snow-ice interface community with the ridge communities, we found that ridge communities had significantly higher light saturation level (ANCOVA test for homogeneity of regression curves; Sokal and Rohlf, 2012), implying that these communities were acclimated to higher light intensities than the snow infiltration community (Figure 9)
Algal accumulations in complex structures such as ridges and in hidden layers at the snow-ice interface are understudied in the Arctic Ocean and have not been accounted for in seaice algal biomass estimates

Summary

Introduction

Current changes in sea-ice conditions have consequences for algal biomass and growth, with bottom-up cascading effects on the Arctic marine food web (Wassmann et al, 2011). The significant decline in sea-ice extent and thickness during the last 30 years has caused an increase in the light available for phytoplankton (Arrigo and van Dijken, 2011; Bélanger et al, 2013) and, an increase in phytoplankton net annual primary production (Arrigo and van Dijken, 2015). Ice algal areal production is probably decreasing on a pan-Arctic scale due to the loss of sea-ice habitat (Dupont, 2012). Biomass standing stocks are low in young ice compared to the disappearing older ice, probably limited by recruitment, adding to the reduction in sea-ice algal areal production (Lange et al, 2017a; Olsen et al, 2017). The trend toward earlier ice melt and later ice formation may cause a mismatch in the timing between primary and secondary producers, diminishing the amount of carbon and energy transferred up the food chain (Søreide et al, 2010; Leu et al, 2011; Ji et al, 2013)

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