Physical and biological properties of Antarctic winter sea ice: Ross Sea vs Weddell Sea

Physical and biogeochemical properties of Antarctic winter pack ice are typically under-sampled. Here, we present the results of physical and biological investigations on early-winter pack ice in the Ross Sea (April–June 2017). Ice textures, temperature, bulk salinity, brine volume, brine salinity and Rayleigh number help us characterize the physical environment in which biological activity will be able to develop in early winter, with contrasted behavior between polynyas, the marginal ice zone (MIZ) and the central Ross Sea. As expected, the very dynamic Terra Nova Bay polynya and the MIZ are dominated by granular ice, while columnar ice takes over the central Ross Sea, with decreasing proportions as one progresses north from the Ross Sea polynya. Snow is present in negligible amounts (a few centimeters) almost everywhere, except in the northern section of the central Ross Sea (10–20 cm), which has a longer and more spatially extended growth history, as shown by satellite imagery. The latter also clearly shows that the southern section has a much shorter growth history, due to a very late ice-growth onset, a remarkable feature of year 2017. The result is a globally low chl-a standing stock, as compared to previous Ross Sea winter cruises (1995–98). The Terra Nova Bay polynya, the MIZ and the southern central Ross Sea show low internal chl-a (≤1 μg L–1). A bottom community, however, develops with time and ice growth, showing higher chl-a concentrations, both from the outer skirt of the Terra Nova Bay polynya northward, and from the MIZ southwards. The highest chl-a levels (>30 μg L–1) are found in rafted coastal floes within the transition zone to the central Ross Sea. Finally, the northern central Ross Sea floes, with a longer growth history and thicker ice cover, show an increase in the internal community chl-a, potentially triggered by brine-tube development. A recent update on the biogeochemical impact of snow cover and cyclonic intrusions on the winter pack ice in the Weddell Sea has shown that winter 2013 (June–August) conditions were favorable to high ice permeability and cyclic events of brine movements within the sea-ice cover (via well-developed brine tubes), favoring relatively high internal chlorophyll-a (chl-a) concentrations. We will discuss if this contrast with the Ross Sea conditions in 2017 is solely due to the time difference in the season, or to contrasted initial conditions of sea-ice growth.