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

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.

Culturable diversity of Arctic phytoplankton during pack ice melting

“…Isolates with identical 18S rRNA may present cryptic diversity based on ITS divergence (Luddington et al, 2016). S. bioculatus is reported as dominating the top portion of submerged sea-ice ridges (Fernández-méndez et al, 2018).…”

An Under-Ice Hyperspectral and RGB Imaging System to Capture Fine-Scale Biophysical Properties of Sea Ice

“…Low-cost imagery sensors-such as RGB, near-infrared (NIR) or multispectral-have also served well in multiple close-range remote sensing applications to retrieve qualitative and quantitative information from biological targets [92][93][94]. For the under-ice environment, RGB imagery has been used to qualitatively assess the spatial distribution of algae [9,95,96]. Therefore, RGB or multispectral imagery should be considered from a cost-benefit analysis perspective based on desired research aims and available resources.…”

“…Recent studies have further observed and inquired about the role of under-ice topography and underlying currents on algal biomass distribution at multiple spatial scales [11,96,111]. Hydrodynamic shadows can foster the accumulation of diatoms, algal aggregates, and may also provide shelter for under-ice fauna [9,112,113]. The RGB imagery not only can provide under-ice roughness but it could also serve to gain further insight into grazing dynamics by sympagic fauna (Figure 7).…”

“…The spatio-temporal variability of ice biological properties is determined by some of the sea ice physical properties such as snow depth and ice thickness, governing light availability for the organisms. In addition, ice algal biomass has been linked to sea-ice structure, under-ice roughness, and their complex interplay with the biogeochemical properties of the water column controlled by currents and boundary layer exchange processes [9][10][11][12][13].…”

Quantifying the Potential for Snow‐Ice Formation in the Arctic Ocean

“…Snow ice forms when seawater floods and refreezes at the snow/ice interface due to a heavy snow load that submerges the ice surface below sea level. Snow ice is therefore a mixture of snow and seawater, and it can provide a habitat for snow infiltration communities, with implications on carbon export and transfer (Fernández-Méndez et al, 2018).…”