Stephen F. Ackley

The Growth, Structure, and Properties of Sea Ice

On the geophysical scale sea ice is a thin, fragile, dynamic, solid layer that forms under the thermodynamic conditions that occur near the Poles. There it serves as a boundary between two much larger fluid bodies — the ocean and the atmosphere. Typical scales of interest would be 103 to 106 m. In the present paper we take a more detailed view, focusing on the ice itself at scales ranging between 100 and 10−3 m, with an occasional glimpse at a scale of 10−10 m. It is our purpose to help the reader understand the internal structure of sea ice, how this structure develops, and how it affects the bulk properties of the ice. Although this is a subject that has received little attention in comparison to similar studies of metals and ceramics, it is, in our view, very important, as many details in the behavior of sea ice are structurally controlled. The variations in structure are, in turn, determined by the environmental conditions under which the ice has formed.

Ecology of sea ice biota - 2. Global significance

SummaryThe sea ice does not only determine the ecology of ice biota, but it also influences the pelagic systems under the ice cover and at ice edges. In this paper, new estimates of Arctic and Antarctic production of biogenic carbon are derived, and differences as well as similarities between the two oceans are examined. In ice-covered seas, high algal concentrations (blooms) occur in association with several types of conditions. Blooms often lead to high sedimentation of intact cells and faecal pellets. In addition to ice-related blooms, there is progressive accumulation of organic matter in Arctic multi-year ice, whose fate may potentially be similar to that of blooms. A fraction of the carbon fixed by microalgae that grow in sea ice or in relation to it is exported out of the production zone. This includes particulate material sinking out of the euphotic zone, and also material passed on to the food web. Pathways through which ice algal production does reach various components of the pelagic and benthic food webs, and through them such top predators as marine mammals and birds, are discussed. Concerning global climate change and biogeochemical fluxes of carbon, not all export pathways from the euphotic zone result in the sequestration of carbon for periods of hundreds of years or more. This is because various processes, that take place in both the ice and the water column, contribute to mineralize organic carbon into CO2 before it becomes sequestered. Processes that favour the production and accumulation of biogenic carbon as well as its export to deep waters and sequestration are discussed, together with those that influence mineralization in the upper ice-covered ocean.

Ecology of sea ice biota

SummaryPolar regions are covered by extensive sea ice that is inhabited by a variety of plants and animals. The environments where the organisms live vary depending on the structure and age of the ice. Many terms have been used to describe the habitats and the organisms. We here characterize the habitats and communities and suggest some standard terms for them. We also suggest routine sampling methods and reporting units for measurements of biological and chemical variables.

Ecology of sea ice biota - 1. Habitat, terminology, and methodology

SummaryPolar regions are covered by extensive sea ice that is inhabited by a variety of plants and animals. The environments where the organisms live vary depending on the structure and age of the ice. Many terms have been used to describe the habitats and the organisms. We here characterize the habitats and communities and suggest some standard terms for them. We also suggest routine sampling methods and reporting units for measurements of biological and chemical variables.

Physical controls on the development and characteristics of Antarctic sea ice biological communities— a review and synthesis

Abstract Ice structures found in Antarctic sea ice and related morphological processes are summarized, including: frazil ice growth; the flooded snow layer; pressure ridge induced flooding; thermally driven brine drainage; and platelet-ice formation. The associated colonization, physiological adaptation, and growth of sea ice biota within these structures, to the levels presently identifiable, are also reviewed. A strong interaction exists between the physical processes that form, evolve and deteriorate sea ice, and the biological communities located within sea ice. Variability of ice structure and associated biological communities over small spatial scales necessitated analysis of the biological component in combination with physical and chemical properties of the sea ice. The ice microstructure provides indications of the growth and evolution of the ice properties and initially defines how ice biota colonize the ice. The light, temperature, space and nutrient fields within which ice biota subsequently adapt and grow, are the other key determinants of the biology. While the ice microstructure shapes the localized biological response, relatively large regions of pack ice have characteristic microstructures. Regional patterns of biomass and biological productivity within the Antarctic sea ice zone may therefore be predictable as a result of these physical-biological associations. Examples from the drifting pack ice and fast ice zones of the Weddell and Ross Seas are given.

Thickness distribution of Antarctic sea ice

[1] Ship-based observations are used to describe regional and seasonal changes in the thickness distribution and characteristics of sea ice and snow cover thickness around Antarctica. The data set comprises 23,373 observations collected over more than 2 decades of activity and has been compiled as part of the Scientific Committee on Antarctic Research (SCAR) Antarctic Sea Ice Processes and Climate (ASPeCt) program. The results show the seasonal progression of the ice thickness distribution for six regions around the continent together with statistics on the mean thickness, surface ridging, snow cover, and local variability for each region and season. A simple ridge model is used to calculate the total ice thickness from the observations of level ice and surface topography, to provide a best estimate of the total ice mass, including the ridged component. The long-term mean and standard deviation of total sea ice thickness (including ridges) is reported as 0.87 ± 0.91 m, which is 40% greater than the mean level ice thickness of 0.62 m. Analysis of the structure function along north/south and east/west transects revealed lag distances over which sea ice thickness decorrelates to be of the order of 100–300 km, which we use as a basis for presenting near-continuous maps of sea ice and snow cover thickness plotted on a 2.5 � 5.0 grid.

Sea Ice Microbial Communities in AntarcticaThese communities may provide an important food resource in deep-water pelagic systems

ccounts of early naturalists first suggested the richness of Antarctic waters. Along with observations of abundant stocks of great whales, seals, and seabirds came reports of sea ice floes stained and discolored by algae. Although sea ice microbial communities (SIMCOs) have been observed and studied for several decades, fundamental questions about their role remain. Do these communities represent a significant food source for benthic or pelagic food webs in ice-covered oceans? If so, what factors contribute to their productivity? Studies of SIMCOs in the landfast ice at McMurdo Sound and in the pack ice region of the Weddell Sea are part of an ongoing effort to understand the role and importance of sea ice communities in the Antarctic marine ecosystem and the physical and chemical attributes of their habitat.

Autumn Bloom of Antarctic Pack-Ice Algae

An autumn bloom of sea-ice algae was observed from February to June of 1992 within the upper 0.4 meter of multiyear ice in the Western Weddell Sea, Antarctica. The bloom was reliant on the freezing of porous areas within the ice that initiated a vertical exchange of nutrient-depleted brine with nutrient-rich seawater. This replenishment of nutrients to the algal community allowed the net production of 1760 milligrams of carbon and 200 milligrams of nitrogen per square meter of ice. The location of this autumn bloom is unlike that of spring blooms previously observed in both polar regions.

On the Differences in Ablation Seasons of Arctic and Antarctic Sea Ice

Abstract Arctic sea ice is freckled with melt ponds during the ablation season; Antarctic sea ice has few, if any. On the basis of a simple surface beat budget, we investigate the meteorological conditions necessary for the onset of surface melting in an attempt to explain these observations. The low relative humidity associated with the relatively dry winds off the continent and an effective radiation parameter smaller than that characteristic of the Arctic are primarily responsible for the absence of melt features in the Antarctic. Together these require a surface-layer air temperature above 0°C before Antarctic sea ice can melt. A ratio of the bulk transfer coefficients CH/CE less than 1 also contributes to the dissimilarity in Arctic and Antarctic ablation seasons. The effects of wind speed and of the sea-ice roughness on the absolute values of CH and CE seem to moderate regional differences, but final assessment of this hypothesis awaits better data, especially from the Antarctic.

Standing crop of algae in the sea ice of the Weddell Sea region

Abstract Physical and biological measurements were made of sea ice cores taken fron 68° to 78° S in the Weddell Sea. Fluorescence measurements indicated an algal community that was strongly associated with salinity maxima within the ice. Maximum concentrations of chlorophyll a ranged from 0.31 to 4.54 mg m −3 . Comparisons with standing crops in the water column indicate that the standing crop within the ice can represent a minor but significant fraction of the total standing crop for the region. The sea ice algal community is apparently distinct from others that have been described for land-fast ice in McMurdo Sound, sea ice in the Arctic, and pack ice off East Antarctica. The highest concentrations of biological material are found in the bottom or top samples from those regions, whereas the Weddell Sea maxima are concentrated at intermediate depths (0.65 to 2.15 m) within the ice. A qualitative model indicating the relationship between thermally induced brine migration and subsequent algal growth is presented. The model indicates that the distribution of algae within the ice depends on the thermal and physical setting for Weddell Sea pack ice where brine drainage is initiated by spring and summer warming but is not carried through so completely as in other regions. It is concluded that the ecological significance of the release of ice algae is greater for the community in the Weddell Sea than elsewhere.