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24 pages, 2 figures, 1 table, supporting Information https://doi.org/10.1111/brv.12679, Important findings from the second decade of the 21st century on the impact of environmental change on biological processes in the Antarctic were synthesised by 26 international experts. Ten key messages emerged that have stakeholder¿relevance and/or a high impact for the scientific community. They address (i) altered biogeochemical cycles, (ii) ocean acidification, (iii) climate change hotspots, (iv) unexpected dynamism in seabed¿dwelling populations, (v) spatial range shifts, (vi) adaptation and thermal resilience, (vii) sea ice related biological fluctuations, (viii) pollution, (ix) endangered terrestrial endemism and (x) the discovery of unknown habitats. Most Antarctic biotas are exposed to multiple stresses and considered vulnerable to environmental change due to narrow tolerance ranges, rapid change, projected circumpolar impacts, low potential for timely genetic adaptation, and migration barriers. Important ecosystem functions, such as primary production and energy transfer between trophic levels, have already changed, and biodiversity patterns have shifted. A confidence assessment of the degree of `scientific understanding¿ revealed an intermediate level for most of the more detailed sub¿messages, indicating that process¿oriented research has been successful in the past decade. Additional efforts are necessary, however, to achieve the level of robustness in scientific knowledge that is required to inform protection measures of the unique Antarctic terrestrial and marine ecosystems, and their contributions to global biodiversity and ecosystem services, With the funding support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S), of the Spanish Research Agency (AEI)

Ice-associated microalgae make a significant seasonal contribution to primary production and biogeochemical cycling in polar regions. However, the distribution of algal cells is driven by strong physicochemical gradients which lead to a degree of microspatial variability in the microbial biomass that is significant, but difficult to quantify. We address this methodological gap by employing a field-deployable hyperspectral scanning and photogrammetric approach to study sea-ice cores. The optical set-up facilitated unsupervised mapping of the vertical and horizontal distribution of phototrophic biomass in sea-ice cores at mm-scale resolution (using chlorophyll a [Chl a] as proxy), and enabled the development of novel spectral indices to be tested against extracted Chl a (R2 ≤ 0.84). The modelled bio-optical relationships were applied to hyperspectral imagery captured both in situ (using an under-ice sliding platform) and ex situ (on the extracted cores) to quantitatively map Chl a in mg m−2 at high-resolution (≤ 2.4 mm). The optical quantification of Chl a on a per-pixel basis represents a step-change in characterising microspatial variation in the distribution of ice-associated algae. This study highlights the need to increase the resolution at which we monitor under-ice biophysical systems, and the emerging capability of hyperspectral imaging technologies to deliver on this research goal.

Sea ice, which forms in polar and nonpolar areas, transmits light to ice-associated (sympagic) algal communities. To noninvasively study the distribution of sea-ice algae, empirical relations to estimate its biomass from under-ice hyperspectral irradiance have been developed in the Arctic and Antarctica but lack for nonpolar regions. This study examines relationships between normalised difference indices (NDI) calculated from hyperspectral transmittance and sympagic algal biomass in the nonpolar Saroma-ko Lagoon. We analysed physico-biogeochemical properties of snow and land-fast sea ice supporting 27 paired bio-optical measurements along three transects covering an area of over 250 m × 250 m in February 2019. Snow depth (0.08 ± 0.01 m) and ice-bottom brine volume fraction (0.21 ± 0.02) showed low (0.06) and high (0.58) correlations with sea-ice core bottom section chlorophylla(Chl.a), respectively. Spatial analyses unveiled the patch size of sea-ice Chl.ato be ~65 m, which is in the same range reported from previous studies. A selected NDI (669, 596 nm) explained 63% of algal biomass variability. This reflects the bio-optical properties and environmental conditions of the lagoon that favour the wavelength pair in the orange/red part of the spectrum and suggests the necessity of a specific bio-optical relationship for Saroma-ko Lagoon.

The Arctic sea-ice-scape is rapidly transforming. Increasing light penetration will initiate earlier seasonal primary production. This earlier growing season may be accompanied by an increase in ice algae and phytoplankton biomass, augmenting the emission of dimethylsulfide and capture of carbon dioxide. Secondary production may also increase on the shelves, although the loss of sea ice exacerbates the demise of sea-ice fauna, endemic fish and megafauna. Sea-ice loss may also deliver more methane to the atmosphere, but warmer ice may release fewer halogens, resulting in fewer ozone depletion events. The net changes in carbon drawdown are still highly uncertain. Despite large uncertainties in these assessments, we expect disruptive changes that warrant intensified long-term observations and modelling efforts., info:eu-repo/semantics/published

Antarctic krill is a key species of important Southern Ocean food webs, yet how changes in ocean temperature and primary production may impact their habitat quality remains poorly understood. We provide a circumpolar assessment of the robustness of krill growth habitat to climate change by coupling an empirical krill growth model with projections from a weighted subset of IPCC Earth system models. We find that 85% of the study area experienced only a moderate change in relative gross growth potential (± 20%) by 2100. However, a temporal shift in seasonal timings of habitat quality may cause disjunctions between krill’s biological timings and the future environment. Regions likely to experience habitat quality decline or retreat are concentrated near the northern limits of krill distribution and in the Amundsen–Bellingshausen seas region during autumn, meaning habitat will likely shift to higher latitudes in these areas.

Antarctic sea ice is an important temporal reservoir of iron which can boost primary production in the marginal ice zone during the seasonal melt. While studies have reported that Antarctic fast ice bears high concentrations of iron due to the proximity to coastal sources, less clear is the biogeochemical changes this iron pool undergoes during late spring. Here we describe a 3-weeks? time series of physical and biogeochemical data, including iron, from first-year coastal fast ice sampled near Davis Station (Prydz Bay, East Antarctica) during late austral spring 2015. Our study shows that dissolved and particulate iron concentrations in sea ice were up to two orders of magnitude higher than in under-ice seawater. Furthermore, our results indicate a significant contribution of lithogenic iron from the Vestfold Hills (as deduced from the comparison with crustal element ratios) to the particulate iron pool in fast ice after a blizzard event halfway through the time series. Windblown dust represented approximately 75% of the particulate iron found in the ice and is a potential candidate for keeping concentrations of soluble iron stable during our observations. These results suggest that iron entrapped during ice formation, likely from sediments, as well as local input of coastal dust, support primary productivity in Davis fast ice. As ice-free land areas are likely to expand over the course of the century, this work highlights the need to quantify iron inputs from continental Antarctic dust and its bioavailability for ice algae and phytoplankton. Plain summary Oceanic single-celled algae are the base of the ocean food web and play an important role in the Earth climate. In the Southern Ocean, the growth of these microorganisms is limited by the naturally low concentration of iron in the seawater. Microalgae benefits from the presence of the Antarctic sea ice since iron there are many times concentrated relative to the seawater. Less clear though, is the contribution of the potential sources of iron to the sea ice. We collected and analyzed sea ice cores for a series of parameters, including iron, from first-year coastal sea ice sampled near Davis Station (Prydz Bay, East Antarctica) during late austral spring 2015. Our results suggest that iron entrapped during ice formation, likely from seafloor sediments, as well as dust blown by winds from the neighboring Vestfold Hills, are the main sources of iron to Davis coastal sea ice. Since we can expect the expansion of ice-free areas and exposed grounds over the course of this century, our results highlight the need to quantify the amount of iron coming from continental Antarctic dust and to access if sea algae can access this form of iron for their basic physiological needs.

Climate change alters the extent and structure of sea-ice environments, which affects how they function as a habitat for polar species. Identifying sea-ice characteristics that serve as indicators of habitat quality will be crucial to the monitoring and management of climate change impacts. In the Southern Ocean, Antarctic krill is a key prey species and fishery target. Krill larvae depend upon sea-ice habitats to survive the winter and recruit to the population in spring. Existing observations of sea-ice characteristics lack sufficient spatiotemporal coverage to quantify which ones contribute to favourable overwintering habitat, leading to uncertainties in how current and future changes in sea ice affect krill populations. Here, we derive regional-scale indices of annual krill recruitment spanning 35 years across the southwest Atlantic. To develop meaningful indicators of sea-ice habitat, we selected variables from a high-resolution sea-ice model that are hypothesized as relevant for larval habitat use. The resulting correlations between recruitment and sea-ice indicators vary by region and show remote connections to sea ice that correspond with established theories of larval transport. Through an improved representation of sea-ice habitat quality, as compared with using more traditional satellite-derived variables such as sea-ice extent and duration, we highlight plausible regions of overwintering habitat. Our findings improve current understanding of how krill are likely responding to changing sea ice and support emerging views that larval habitat use is complex. Furthermore, regional variation in larval dependence on sea ice may provide pockets of resilience to change for the broader krill population.

Historical sea ice core chlorophyll-a (Chla) data are used to describe the seasonal, regional, and vertical distribution of ice algal biomass in Antarctic landfast sea ice. The analyses are based on the Antarctic Fast Ice Algae Chlorophyll-a data set, a compilation of currently available sea ice Chla data from landfast sea ice cores collected at circum-Antarctic nearshore locations between 1970 and 2015. Ice cores were typically sampled from thermodynamically grown first-year ice and have thin snow depths (mean = 0.052 ± 0.097 m). The data set comprises 888 ice cores, including 404 full vertical profile cores. Integrated ice algal Chla biomass (range, SCOPUS: ar.j, info:eu-repo/semantics/published

Publisher Copyright: © 2021 The Author(s). A rigorous synthesis of the sea-ice ecosystem and linked ecosystem services highlights that the sea-ice ecosystem supports all 4 ecosystem service categories, that sea-ice ecosystems meet the criteria for ecologically or biologically significant marine areas, that global emissions driving climate change are directly linked to the demise of sea-ice ecosystems and its ecosystem services, and that the sea-ice ecosystem deserves specific attention in the evaluation of marine protected area planning. The synthesis outlines (1) supporting services, provided in form of habitat, including feeding grounds and nurseries for microbes, meiofauna, fish, birds and mammals (particularly the key species Arctic cod, Boreogadus saida, and Antarctic krill, Euphausia superba, which are tightly linked to the sea-ice ecosystem and transfer carbon from sea-ice primary producers to higher trophic level fish, mammal species and humans); (2) provisioning services through harvesting and medicinal and genetic resources; (3) cultural services through Indigenous and local knowledge systems, cultural identity and spirituality, and via cultural activities, tourism and research; (4) (climate) regulating services through light regulation, the production of biogenic aerosols, halogen oxidation and the release or uptake of greenhouse gases, for example, carbon dioxide. The ongoing changes in the polar regions have strong impacts on sea-ice ecosystems and associated ecosystem services. While the response of sea-ice–associated primary production to environmental change is regionally variable, the effect on ice-associated mammals and birds is predominantly negative, subsequently impacting human harvesting and cultural services in both polar regions. Conservation can help protect some species and functions. However, the key mitigation measure that can slow the transition to a strictly seasonal ice cover in the Arctic Ocean, reduce the overall loss of sea-ice habitats from the ocean, and thus preserve the unique ecosystem services provided by sea ice and their contributions to human well-being is a reduction in carbon emissions.

Iron (Fe) has been shown to limit growth of marine phytoplankton in the Southern Ocean, regulating phytoplankton productivity and species composition, yet does not seem to limit primary productivity in Antarctic sea ice. Little is known, however, about the potential impact of other metals in controlling sea-ice algae growth. Here, we report on the distribution of dissolved and particulate cadmium (Cd), cobalt (Co), copper (Cu), manganese (Mn), nickel (Ni), and zinc (Zn) concentrations in sea-ice cores collected during 3 Antarctic expeditions off East Antarctica spanning the winter, spring, and summer seasons. Bulk sea ice was generally enriched in particulate metals but dissolved concentrations were similar to the underlying seawater. These results point toward an environment controlled by a subtle balance between thermodynamic and biological processes, where metal availability does not appear to limit sea-ice algal growth. Yet the high concentrations of dissolved Cu and Zn found in our sea-ice samples raise concern about their potential toxicity if unchelated by organic ligands. Finally, the particulate metal-to-phosphorus (P) ratios of Cu, Mn, Ni, and Zn calculated from our pack ice samples are higher than values previously reported for pelagic marine particles. However, these values were all consistently lower than the sea-ice Fe:P ratios calculated from the available literature, indicating a large accumulation of Fe relative to other metals in sea ice. We report for the first time a P-normalized sea-ice particulate metal abundance ranking of Fe >> Zn ≈ Ni ≈ Cu ≈ Mn > Co ≈ Cd. We encourage future sea-ice work to assess cellular metal quotas through existing and new approaches. Such work, together with a better understanding of the nature of ligand complexation to different metals in the sea-ice environment, would improve the evaluation of metal bioavailability, limitation, and potential toxicity to sea-ice algae.

Antarctic sea ice can incorporate high levels of iron (Fe) during its formation and has been suggested as an important source of this essential micronutrient to Southern Ocean surface waters during the melt season. Over the last decade, a limited number of studies have quantified the Fe pool in Antarctic sea ice, with a focus on late winter and spring. Here we study the distribution of operationally defined dissolved and particulate Fe from nine sites sampled between Wilkes Land and King George V Land during austral summer 2016/2017. Results point toward a net heterotrophic sea‐ice community, consistent with the observed nitrate limitation (−1) which can serve as a hotspot for bacterial activity. Finally, results indicate a potential relationship between glacial meltwater discharged from the Moscow University Ice Shelf and the occurrence of Fe‐rich (∼4.3 μM) platelet ice in its vicinity. As climate change is expected to result in enhanced Fe‐rich glacial discharge and changes in summer sea‐ice extent and quality, the processes influencing Fe distribution in sea ice that persists into summer need to be better constrained.

The durability of plastics in the marine environment has led to concerns regarding the pervasiveness of this debris in remote polar habitats. Microplastic (MP) enrichment in East Antarctic sea ice was measured in one ice core sampled from coastal land-fast sea ice. The core was processed and filtered material was analyzed using micro Fourier-Transform Infrared (μFTIR) spectroscopy. 96 MP particles were identified, averaging 11.71 particles L−1. The most common MP polymers (polyethylene, polypropylene, and polyamide) were consistent with those most frequently represented in the majority of marine MP studies. Sea-ice MP concentrations were positively related with chlorophyll a, suggesting living biomass could assist in incorporating MPs in sea ice. Our preliminary results indicate that sea ice has the potential to serve as a reservoir for MP debris in the Southern Ocean, which may have consequences for Southern Ocean food webs and biogeochemistry.

This chapter describes a period of low air temperatures, darkness or very little light, cold ice, minimum brine volumes, and strongly reduced ice permeability. The first section provides an overview of the physical and optical conditions of this winter ice (3.1). Both snowfall and snowdrift are frequent during winter and an overview of the effects of a thick snow cover on sea ice is provided (3.2). Increased colonization of the sea ice bottom by bacteria and ice algae is initiated in late winter around first light, and an example is given (3.3). Once these microorganisms have colonized the ice the important growth limitations of bacteria and algae are considered (3.4), along with how they are able to adapt to these extreme conditions, and their seasonal species succession (3.5). The purposes of EPS and the functioning of different pigments found in ice algae are described (3.6). This is combined with a description of low light conditions and related photosynthesis (3.7), and exemplified with a case study from Station North, NE Greenland (3.8).

This chapter is a description of the field and laboratory methods that are generally applied in sea ice ecological research. The first section describes sampling techniques and handling of sea ice samples (6.1). Calculations of brine and gas volumes are dealt with in (6.2), and how to measure and calculate optical parameters (6.3). Sampling, handling, and calculation of ice bacterial production are described (6.4), and methods for ice algae biomass spatial distributions are dealt with in (6.5). Measuring photosynthesis and ice algae primary production are described (6.6), and the determination of Chl a content is explained (6.7). The recently developed fluorescence imaging of ice algae distributions and derived photosynthetic parameters are described (6.8). This is followed by a description of the IP25, a substance synthesized by ice algae and applied for mapping sea ice and food web studies (6.9).

This chapter describes the period of development of new ice during autumn freeze-up, different ice types, and development of the brine channels (2.1). Optical properties of young and developing sea ice are described in terms of the transition from open water to bare sea ice, that later can be covered by snow (2.2). The incorporation and scavenging of ice algae, bacteria and meiofauna from the water column during ice growth and freeze-up are then described (2.3). Development of platelet ice, a prominent under-ice feature in Antarctica and recently observed in the Arctic, is described (2.4). The final section describes frost flowers and brine skim development, and the dynamics of the bacteria communities in the frost flowers and brine skim on top of newly formed sea ice (2.5).

This chapter describes the structure of the book and the diverse significance and importance of sea ice. The first section explains how and why the book is structured following the seasonal events governing the abiotic and biotic parameters in sea ice (1.1). The second section places sea ice into a physical and geopolitical perspective with the new and developing international borders in the Arctic Ocean (1.2). Sea ice ecosystems are then compared to other ecosystems (1.3), followed by a section describing the important and global ecological features of sea ice (1.4).

This chapter describes one of the most dynamic and ecological important seasons. The onset of ice melt initiates significant changes in the physical properties of the ice (4.1), and the related biotic processes in the spring/summer sea ice are addressed (4.2). Ice algal spring blooms are described and exemplified with two studies from contrasting Arctic sites (4.3). A Case Study 2 has a focus on effects and consequences for ice algae of increased irradiances at the ice bottom in terms of pigments, fatty acids, and MAAs (4.4). Ice algae bloom dynamics are investigated in a combined model and field study (4.5). Melt ponds termed “windows to the ocean” develop on the surface of the ice with increased light transmittance (4.6). Seeding of the ice or water column below with viable ice algae can initiate ice algae or pelagic blooms, and mechanisms are explored in model and field studies (4.7). Case study 3 show that species of meiofauna graze directly on the ice algae, and that the meiofauna establishes an important ecological component in the sea ice (4.8).

This chapter is a description of the role and importance of sea ice and sea ice biota on large scales and in relation to the effects of climate change. The decrease in summer sea ice extent and thickness are evident and described in (5.1). A Case Study 4 based on our observations in the Fram Strait and the Arctic Ocean illustrates some of the consequences and effects of increased inflow of warm Atlantic water (5.2). The question whether more light in an ice-free water column will increase pelagic primary production in the Arctic Ocean, is addressed with a model (5.3). Sea ice plays an important role in the exchange of CO2 between ocean and atmosphere and the sea ice CO2 pump is described in (5.4).

International audience; We use field observations from late spring and a one‐dimensional sea‐ice model to explore a high nutrient, high chlorophyll system in Antarctic land‐fast ice. Lack of variability in chlorophyll a concentration and organic carbon content over the 17‐day sampling period suggests a balance between macronutrient sources and biological uptake. Nitrate, nitrite, phosphate, and ammonium were measured at concentrations well above salinity‐predicted levels, indicating nutrient accumulation fueled by remineralization processes. However, silicic acid (DSi) was depleted relative to seawater and was potentially limiting. One‐dimensional physical‐biogeochemical sea‐ice model simulations at the observation site achieve extremely high algal growth and DSi uptake with a DSi half‐saturation constant used for pelagic diatoms (KSi = 3.9 μM) and are not sufficiently improved by tuning the DSi:carbon ratio or DSi remineralization rate. In contrast, diatom biomass in the bottom ice, which makes up 70% of the observed chlorophyll, is simulated using KSi an order of magnitude higher (50 μM), a value similar to that measured in a few Antarctic diatom cultures. Some sea‐ice diatoms may therefore experience limitation at relatively high ambient DSi concentrations compared to pelagic diatoms. Our study highlights the urgent need for observational data on sea‐ice algal affinity for DSi to further support this hypothesis. A lower algal growth rate increases model predictions of DSi in the upper sea ice to more accurate concentrations. The model currently does not account for the non‐diatom communities that dominate those layers, and thus, modeling diatom communities overpredicts DSi uptake in the upper ice.

Primary productivity in large areas of the Southern Ocean is limited by the availability of a key micronutrient—iron (Fe). Recently it has been suggested that marine animals could play an important role in recycling Fe through their diet and subsequent defecation, however there is no information on the relative bioavailability of faecal Fe for uptake. The bioavailability of Fe in seawater is controlled by a number of complex interactions. The physical separation between the dissolved ( 0.2 μm) and particulate (> 0.2 μm) fractions is one common measure used to determine element bioavailability. Here, the size fractionation of Fe from 3 whale faecal samples in 4 different size classes ( 0.2 μm, 0.2–10 μm, 10–60 μm and > 60 μm) was investigated, along with the leaching of particulate Fe over time. Although the total particulate fraction (> 0.2 μm, 5026–22,526 nmol L− 1) dominated the total Fe pool, the concentrations of dissolved Fe in whale faecal samples (186–754 nmol L− 1 ) were three order of magnitude higher than published Southern Ocean surface seawater concentrations. Furthermore, results from the leaching experiment suggest that Fe is continually leached from faecal particles over an initial 12-hour period, thus increasing the concentration of bioavailable Fe in surface seawater. Although the concentrations measured here are some of the highest reported in the literature, the true supply of Fe back to surface seawater will be controlled by processes such as organic complexation , scavenging and sinking by particles, remineralisation , and vertical transport, not measured in this study.

Antarctic pack ice serves as habitat for microalgae which contribute to Southern Ocean primary production and serve as important food source for pelagic herbivores. Ice algal biomass is highly patchy and remains severely undersampled by classical methods such as spatially restricted ice coring surveys. Here we provide an unprecedented view of ice algal biomass distribution, mapped (as chlorophyll a) in a 100 m by 100 m area of a Weddell Sea pack ice floe, using under-ice irradiance measurements taken with an instrumented remotely operated vehicle. We identified significant correlations (p

Snow on sea ice alters the properties of the underlying ice cover as well as associated physical and biological processes at the interfaces between atmosphere, sea ice, and ocean. The Antarctic snow cover persists during most of the year and contributes significantly to the sea-ice mass due to the widespread surface flooding and related snow-ice formation. Snow also enhances the sea-ice surface reflectivity of incoming shortwave radiation and determines therefore the amount of light being reflected, absorbed, and transmitted to the upper ocean. Here, we present results of a case study of spectral solar radiation measurements under Antarctic pack ice with an instrumented Remotely Operated Vehicle in the Weddell Sea in 2013. In order to identify the key variables controlling the spatial distribution of the under-ice light regime, we exploit under-ice optical measurements in combination with simultaneous characterization of surface properties, such as sea-ice thickness and snow depth. Our results reveal that the distribution of flooded and nonflooded sea-ice areas dominates the spatial scales of under-ice light variability for areas smaller than 100 m-by-100 m. However, the heterogeneous and highly metamorphous snow on Antarctic pack ice obscures a direct correlation between the under-ice light field and snow depth. Compared to the Arctic, light levels under Antarctic pack ice are extremely low during spring ( < 0.1%). This is mostly a result of the distinctly different dominant sea ice and snow properties with seasonal snow cover (including strong surface melt and summer melt ponds) in the Arctic and a year-round snow cover and widespread surface flooding in the Southern Ocean.

This study describes within-ice concentrations of dimethylsulfoniopropionate (DMSP), its degradation product dimethylsulphide (DMS), as well as nutrients and chlorophyll a, that were sampled during the Sea Ice Physics and Ecosystems eXperiment-2 (SIPEX-2) in 2012. DMSP is a methylated substrate produced in large amounts annually by ice-associated microalgae, while DMS plays a significant role in carbon and sulphur cycling in the Southern Ocean. In the East Antarctic study area between 115–125°E and 64–66°S, ice and slush cores, brine, under-ice seawater and zooplankton (Antarctic krill) samples were collected at 6 ice stations. The pack-ice was characterised by high snow loading which initiated flooding events and triggered nutrient supply to the sea-ice surface, while variation in ice conditions influenced sea-ice permeability. This ranged from impermeable surface and middle sections of the sea ice, to completely permeable ice cores at some stations. Chlorophyll a maxima shifted from the sea-ice surface horizon at the first station to the sea ice bottom layer at the last station. Highest DMSP concentrations were detected in brine samples at the sea-ice surface, reflecting a mismatch with respect to the distribution of chlorophyll a. Our data suggest enhanced DMSP production by sea-ice surface algal communities and its release into brine during freezing and melting, which in turn is coupled to flooding events early in the season. A time-cycle of DMS production by DMSP degradation and DMS efflux is evident at the sea ice-snow interface when slush is formed during melt. Seawater under the ice contained only low concentrations of DMSP and DMS, even when brine drainage was evident and the sea ice became permeable. We postulate that in situ grazing by zooplankton may act as sink for the DMSP produced early in the season.

Sea ice is an influential feature in Southern Ocean-Antarctic marine environments creating a 2-phase vertical ecosystem. The lack of information on how this system influences community structure during the winter-spring transition, however, is largely lacking. Zooplankton form the link that bridges these environments, with the meiofaunal and algal communities within sea ice directly influencing the epipelagic zooplankton community at the ice-water interface. A combination of methods including sea-ice coring, umbrella net sampling and Laser Optical Plankton Counter were used to describe the vertical structure of zooplankton and meiofaunal communities. The distribution of meiofauna and chlorophyll a both played important roles in structuring the zooplankton community within this dynamic region. Many dominant taxa, including Calanus propinquus and Oithona similis, directly responded to the high availability of algae present within the bottom strata of sea ice. The sea-ice associated species Stephos longipes represented a strong link between this 2-phase ecosystem. Observations of the vertical distribution of biomass obtained from the LOPC suggests that the responses of these species to the sea ice directly influences the vertical structure of zooplankton during the winter-spring transition.

This editorial introduces a suite of articles resulting from the second Sea Ice Physics and Ecosystems eXperiment(SIPEX-2) voyage by presenting some background information on the study areaandAntarcticsea-ice conditions,and summarising the key findings from the project.Using the Australian iceb reaker RV Aurora Australis, SIPEX-2 was conducted in the area between 115–125°E and 62–66°S off East Antarctica during September to November 2012. This region had been sampled during two previous experiments,i.e. ARISE in 2003 (Massom etal.,2006a) and SIPEX in 2007(Worbyetal.,2011a). The 2012 voyage combined traditional and newly developed sampling methods with satellite and other data to measure sea-ice physical properties and pro- cesses on large scales,which provided context for bio geochemical and ecological case studies. Thes pecific goals of the SIPEX-2 project were to:(i)measure the spatial variability in sea-ice and snow-cover properties over small-to regional-length scales;(ii) improve understanding of sea-ice kinematic processes;and(iii) advance knowledge of the links between sea-ice physical characteristics,sea-ice biogeochemical cycling and ice-associated food-web dynamics.Our field-based activities were designed to inform modelling approaches and to improve our capability to assess impacts of predicted changes in Antarctic sea ice on Southern Ocean biogeochemical cycles and ecosystem function.

Understanding how sea ice serves zooplankton species during the food-limited season is crucial information to evaluate the potential responses of pelagic food webs to changes in sea-ice conditions in the Southern Ocean. Stable isotope analyses (13C/12C and 15N/14N) were used to compare the dietary preferences and trophic relationships of major zooplankton species under pack ice during two winter–spring transitions (2007 and 2012). During sampling, furcilia of Euphausia superba demonstrated dietary plasticity between years, herbivory when feeding on sea-ice biota, and with a more heterotrophic diet when feeding from both the sea ice and the water column. Carbon isotope signatures suggested that the pteropod Limacina helicina, small copepods Oithona spp., ostracods and amphipods relied heavily on sea-ice biota. Post larval E. superba and omnivorous krill Thysanoessa macrura consumed both water column and ice biota, but further investigations are needed to estimate the contribution from each source. Large copepods and chaetognaths overwintered on a water column-based diet. Our study suggests that warm and permeable sea ice is more likely to provide food for zooplankton species under the ice than the colder ice.

Antarctic pack ice comprises about 90% of the sea ice in the southern hemisphere and plays an important structuring role in Antarctic marine ecosystems, yet measurements of ice algal primary production and nitrogen uptake rates remain scarce. During the early austral spring of 2012, measurements for primary production rates and uptake of two nitrogen substrates (nitrate and ammonium) were conducted at 5 stations in the East Antarctic pack ice (63–66°S, 115–125°E). Carbon uptake was low (3.52 mg C m -2 d -1 ) but a trend of increased production was observed towards the end of the voyage suggesting pre-bloom conditions. Significant snow covers reaching, up to 0.8 m, induced strong light limitation. Two different regimes were observed in the ice with primarily nitrate based ‘new’ production (f-ratio: 0.80–0.95) at the bottom of the ice cover, due to nutrient-replete conditions at the ice–water interface, and common for pre-bloom conditions. In the sea-ice interior, POC:PN ratios (20–70) and higher POC:Chl a ratios suggested the presence of large amounts of detrital material trapped in the ice and here ammonium was the prevailing nitrogen substrate. This suggests that most primary production in the sea-ice interior was regenerated and supported by a microbial food web, recycling detritus.

Microbial communities have evolved mechanisms to allow them to survive within the challenging and changing pack ice environment. One such mechanism may be the exudation of photosynthetically-derived organic carbon into various extracellular pools. During the 2nd Sea Ice Physics and Ecosystems eXperiment (SIPEX-2), East Antarctic pack ice productivity and subsequent carbon allocation were quantified, together with physico-biogeochemical characteristics (29 September–28 October, 2012). Mean ice thickness ranged between 0.80 and 2.16 m, and typically exhibited a warm ice interior with weak temperature gradients. All stations, with one exception, were layered with granular (mean: 78%), columnar (mean: 15%), and mixed granular/columnar (mean: 4%) ice. Highest ice brine-volume fractions were at the ice–water interface, but all ice had high brine-volume fractions conducive for brine percolation (mean: 15%). Dissolved inorganic nutrient concentrations in the brine were scattered around theoretical dilution lines (TDLs), with some values of nitrate and nitrite, ammonium and silicic acid falling below TDLs, indicating nutrient depletion. Bulk ice dissolved organic carbon was low (mean: 64 µmol kg−1), but most samples showed enrichment in relation to TDLs. Microbial biomass (bacterial and algal) was low, and generally showed maxima in the sea-ice interior. Bottom ice algal communities were dominated by pennate diatom species (mean: 86% of total cell abundance). 14C-total primary productivity (14C-TPP) ranged from

Antarctic krill may play a significant role in the Southern Ocean iron cycle. However, understanding the control on iron budgets by Antarctic krill is hampered by the large range in the reported iron concentration of krill. The aim of this study was to investigate the causes of the large range of iron concentrations in krill reported in the literature (6–190 mg kg−1). Antarctic krill samples were collected from three research voyages to Pyrdz Bay, Antarctica, and analysed individually. Iron concentrations were measured using sector field inductively coupled plasma mass spectrometry in whole krill specimens and in the isolated stomach, digestive gland, muscle, body (whole krill excluding stomach and digestive gland), exoskeleton and faecal pellets. Iron concentrations in stomach (6–98 mg/kg), digestive gland (14–82 mg kg−1), and faecal pellet (683–1039 mg kg−1) were higher compared to muscle (4–7 mg kg−1), exoskeleton (6–15 mg kg−1), and body (4–18 mg kg−1) indicating that krill may ingest more iron than they require for physiological processes. Iron concentrations in whole krill from March 2012 (10 ± 3 mg kg−1) were significantly lower compared to February 2003 (19 ± 7 mg kg−1) and February 2015 (18 ± 12 mg kg−1). Overall, the iron concentrations in krill from this study were consistently at the lower end of the published range. We propose that the large range in reported whole iron concentrations of krill can be accounted for by a combination of seasonal and regional differences in sampling, reflecting differences in the quantity and quality of their diet.

Our current knowledge of broad-scale patterns of primary production in the Southern Ocean is derived from satellite ocean-colour estimates of chlorophyll a (Chl a) in the open ocean, typically in spring-summer. Here, we provide evidence that large-scale intra-ice phytoplankton surface aggregation occur off the coast of Antarctica during austral autumn, and that these "blooms" are largely undetected in satellite ocean-colour time series (which mask the ice-covered ocean). We present an analysis of (i) true-colour (visible) satellite imagery in combination with (ii) conventional ocean-colour data, and (iii) direct sampling from a research vessel, to identify and characterise a large-scale intra-ice algal occurrence off the coast of East Antarctica in early autumn (March) 2012. We also present evidence of these autumn "blooms" in other regions (for example, Princess Astrid Coast in 2012) and other years (for example, Terra Nova Bay in 2015) implying regular and widespread occurrence of these phenomena. The occurrence of such undetected algal accumulations implies that the magnitude of primary production in the Southern Ocean is currently underestimated.

This study presents the first laboratory-controlled sea-ice growth experiment conducted under trace metal clean conditions. The role played by organic matter, in the incorporation of iron (Fe) into sea ice was investigated by means of laboratory ice-growth experiments using a titanium cold-finger apparatus. Experiments were also conducted to understand the role of extracellular polymeric substances (EPS) in the enrichment of ammonium in sea ice. Sea ice was grown from several seawater solutions containing different quantities and qualities of particulate Fe (PFe), dissolved Fe (DFe) and organic matter. Sea ice and seawater were analyzed for particulate organic carbon and nitrogen, macro-nutrients, extracellular EPS, PFe and DFe, and particulate aluminium. The experiments showed that biogenic PFe is preferentially incorporated into sea ice compared to lithogenic PFe. Furthermore, sea ice grown from ultra-violet (UV) and non-UV treated seawaters exhibits contrasting incorporation rates of organic matter and Fe. Whereas the effects of UV-treatments were not always significant, we do find indications that the type or organic matter controls the enrichment of Fe in forming sea ice.. Specifically, we come to the conclusion that the incorporation of DFe is favored by the presence of organic ligands in the source solution.

Sea ice is one the largest biomes on earth, yet it is poorly described by biogeochemical and climate models. In this paper, published and unpublished data on sympagic (ice-associated) algal biodiversity and productivity have been compiled from more than 300 sea-ice cores and organized into a systematic framework. Significant patterns in microalgal community structure emerged from this framework. Autotrophic flagellates characterize surface communities, interior communities consist of mixed microalgal populations and pennate diatoms dominate bottom communities. There is overlap between landfast and pack-ice communities, which supports the hypothesis that sympagic microalgae originate from the pelagic environment. Distribution in the Arctic is sometimes quite different compared to the Antarctic. This difference may be related to the time of sampling or lack of dedicated studies. Seasonality has a significant impact on species distribution, with a potentially greater role for flagellates and centric diatoms in early spring. The role of sea-ice algae in seeding pelagic blooms remains uncertain. Photosynthesis in sea ice is mainly controlled by environmental factors on a small scale and therefore cannot be linked to specific ice types. Overall, sea-ice communities show a high capacity for photoacclimation but low maximum productivity compared to pelagic phytoplankton. Low carbon assimilation rates probably result from adaptation to extreme conditions of reduced light and temperature in winter. We hypothesize that in the near future, bottom communities will develop earlier in the season and develop more biomass over a shorter period of time as light penetration increases due to the thinning of sea ice. The Arctic is already witnessing changes. The shift forward in time of the algal bloom can result in a mismatch in trophic relations, but the biogeochemical consequences are still hard to predict. With this paper we provide a number of parameters required to improve the reliability of sea-ice biogeochemical models.

The presence of algal pigments in sea ice alters under-ice irradiance spectra, and the relationship between these variables can be used as a non-invasive means for estimating ice- associated algal biomass on ecologically relevant spatial and temporal scales. While the influence of snow cover and ice algal biomass on spectra transmitted through the snow-ice matrix has been examined for the Arctic, it has not been tested for Antarctic sea ice at regional scales. We used paired measurements of sea ice core chla concentrations and hyperspectral-transmitted under-ice irradiances from 59 sites sampled off East Antarctica and in the Weddell Sea to develop algorithms for estimating algal biomass in Antarctic pack ice. We compared 4 approaches that have been used in various bio-optical studies for marine systems: normalised difference indices, ratios of spectral irradiance, scaled band area and empirical orthogonal functions. The percentage of vari- ance explained by these models ranged from 38 to 79%, with the best-performing approach being normalised difference indices. Given the low concentrations of integrated chl a observed in our study compared with previous studies, our statistical models performed surprisingly well in explaining variability in these concentrations. Our findings provide a basis for future work to develop methods for non-invasive time series measurements and medium- to large-scale spatial mapping of Antarctic ice algal biomass using instrumented underwater vehicles.

Antarctic fast ice provides a habitat for diverse microbial communities, the biomass of which is mostly dominated by diatoms capable of growing to high standing stocks, particularly at the icewater interface. While it is known that ice algae exude organic carbon in ecologically significant quantities, the mechanisms behind its distribution and composition are not well under- stood. This study investigated extracellular organic carbon dynamics, microbial characteristics, and ice algal photophysiology during a bottom-ice algal bloom at McMurdo Sound, Antarctica. Over a 2 wk period (November to December 2011), ice within 15 cm from the icewater interface was collected and sliced into 9 discrete sections. Over the observational period, the total concen- trations of extracellular organic carbon components (dissolved organic carbon (DOC) and total carbohydrates (TCHO) — the sum of monosaccharides (CHOMono) and polysaccharides (CHOPoly)) increased, and were positively correlated with algal biomass. However, when normalised to chlorophyll a, the proportion of extracellular organic carbon components substantially decreased from initial measurements. Concentrations of DOC generally consisted of

Ice algae are a key component in polar marine food webs and have an active role in large-scale biogeochemical cycles. They remain extremely under-sampled due to the coarse nature of traditional point sampling methods compounded by the general logistical limitations of surveying in polar regions. This study provides a first assessment of hyperspectral imaging as an under-ice remote-sensing method to capture sea-ice algae biomass spatial variability at the ice/water interface. Ice-algal cultures were inoculated in a unique inverted sea-ice simulation tank at increasing concentrations over designated cylinder enclosures and sparsely across the ice/water interface. Hyperspectral images of the sea ice were acquired with a pushbroom sensor attaining 0.9 mm square pixel spatial resolution for three different spectral resolutions (1.7, 3.4, 6.7 nm). Image analysis revealed biomass distribution matching the inoculated chlorophyll a concentrations within each cylinder. While spectral resolutions >6 nm hindered biomass differentiation, 1.7 and 3.4 nm were able to resolve spatial variation in ice algal biomass implying a coherent sensor selection. The inverted ice tank provided a suitable sea-ice analogue platform for testing key parameters of the methodology. The results highlight the potential of hyperspectral imaging to capture sea-ice algal biomass variability at unprecedented scales in a non-invasive way.

A dominant Antarctic ecological paradigm suggests that winter sea ice is generally the main feeding ground for krill larvae. Observations from our winter cruise to the southwest Atlantic sector of the Southern Ocean contradict this view and present the first evidence that the pack-ice zone is a food-poor habitat for larval development. In contrast, the more open marginal ice zone provides a more favourable food environment for high larval krill growth rates. We found that complex under-ice habitats are, however, vital for larval krill when water column productivity is limited by light, by providing structures that offer protec- tion from predators and to collect organic material released from the ice. The larvae feed on this sparse ice-associated food during the day. After sunset, they migrate into the water below the ice (upper 20 m) and drift away from the ice areas where they have previously fed. Model analyses indicate that this behaviour increases both food uptake in a patchy food environment and the likelihood of overwinter transport to areas where feeding conditions are more favourable in spring.