Report 18 -- Q5

Do ice and ice biota influence the system?

Each autumn and winter about 16 million km² of the Southern Ocean's surface (about 40% of the waters south of the APFZ and 4% of the global ocean) freezes (e.g., Zwally et al., 1983; Comiso 1991). Each spring and summer, this ice melts . This seasonal advance and retreat of ice in the Southern Ocean is probably the most profound seasonal cycle that occurs in any ocean. An additional about 4 million km² of the Southern Ocean is permanently ice covered, primarily in the western Weddell Sea (ibid). Sea ice has been shown or hypothesized to have direct effects on ocean/atmosphere gas and momentum exchange, light penetration and deep water formation (e.g., Gow and Tucker, 1990). In addition to the direct effects of ice on physical and chemical exchange, the seasonal formation and melting of ice have been shown to enhance upper water-column stability, diminish mixed layer depths, and increase primary productivity in spring and summer (Smith and Sakshaug, 1990), with associated increases in the abundances of heterotrophic organisms ranging in size and function from bacteria to seals and seabirds (e.g., Ainley and DeMaster, 1990). Melting sea ice has also been hypothesized to be a quantitatively significant source of iron to surface waters, resulting in enhanced ice-edge productivity (Martin et al., 1990). Any field study of biogeochemical processes in the Southern Ocean would be enormously deficient if it failed to take these effects of sea ice into account, as would any model of the Southern Ocean system.

Sea ice is also itself a habitat for biota. Dense algal layers can form within the ice and at its under surface (e.g., Garrison et al., 1986), and those algal layers provide the apparent food source for dense and diverse communities of microheterotrophs that inhabit interstitial waters within the ice (Garrison, 1991). Direct observations from ROVs have shown that ice-algal layers at and near the under surface of pack ice can be a major winter feeding ground for first-year krill which, unlike adult krill, must feed through the winter (ibid.). Being trapped within the forming sea ice and remaining viable within the ice appears to be an important overwintering strategy for many Antarctic phytoplankton species, several of which are dominant within the ice-edge blooms that develop in the water column as the ice retreats in spring and summer (Wilson et al., 1986). There are thus very significant interactions between the biota of the sea ice and the water column, especially in areas where ice cover is seasonal.

Seasonal pack ice does not advance as far north as the APFZ, but it completely covers the Ross Sea in winter. Thus the effects of ice do not have be considered in field studies or models of the APFZ or other northern parts of the Southern Ocean. However it will be crucial to understanding the Ross Sea and other seasonally ice-covered areas, from both a scientific and a logistical perspective. For example, the presence of sea ice greatly affects the flux of CO₂ between the ocean and atmosphere (Hibler, 1992). In addition to those effects of ice cover that warrant scientific study, the seasonal ice cover severely restricts the ability of ships to get into the Ross Sea in winter and to recover moored instruments in any area of significant ice cover, even in summer.

Implications for field measurements: Biogeochemical studies in the Ross Sea should commence as soon after the first appearance of sunlight and open water as is logistically possible, and should continue until advancing ice makes it necessary for the ship to leave. The first light appears in September, the first open water is generally observed in mid-October, and ice is usually advancing rapidly by late March. Key data sets that can be obtained from moorings while no ship is in the area (e.g., currents, transmissometry, biogenic particle flux) should continue through the winter. Studies of primary productivity, grazing, nutrient removal and regeneration, biogenic particle flux, CO₂ exchange etc. should all take into account both the intense, diatom-dominated phytoplankton blooms that form in the ice-edge zone of the Ross Sea in summer and the Phaeocystis-dominated blooms that are now known to develop very rapidly after the first appearance of light and open water in spring.

Implications for modeling: Physical and biogeochemical models of the Ross Sea and other seasonally ice-covered areas should have explicit submodels dealing with the sea-ice system, and should account in a realistic way for each of the known or hypothesized physical, chemical and biological interactions between sea ice (including the seasonal formation/melt cycle) and the water column. Because of the great apparent importance of mesoscale phytoplankton blooms in the export of biogenic material to the deep water column and the sea floor in the Southern Ocean (discussed below), these models should pay particular attention to the mechanisms by which blooms develop near the receding ice edge in spring and summer, and to the biogeochemical consequences of enhanced primary productivity near the ice edge.