ABSTRACT. During an ice-tank experiment, samples were taken to study the processes of acquisition and alteration of the gas properties in young first-year sea ice during a complete growth^warming^cooling cycle. The goal was to obtain reference levels for total gas content and concentrations of atmospheric gases (O 2 , N 2 , CO 2 ) in the absence of significant biological activity. The range of total gas-content values obtained (3.5^18 mL STP kg^1) was similar to previous measurements or estimates. However, major differences occurred between current and quiet basins, showing the role of the water dynamics at the ice^water interface in controlling bubble nucleation processes. Extremely high CO 2 concentrations were observed in all the experiments (up to 57% in volume parts). It is argued that these could have resulted from two unexpected biases in the experimental settings. Concentrations in bubbles nucleated at the interface are controlled by diffusion both from the ice^water interface towards the wellmixed reservoir and between the interface water and the bubble itself.This double kinetic effect results in a transition of the gas composition in the bubbles from values close to solubility in sea water toward values close to atmospheric, as the ice cover builds up. FRAMEWORKThe impact of sea ice on the ocean^atmosphere^biosphere interactions in the polar regions is well recognized. Annual growth of a continuous sea-ice cover is considered to impede gaseous exchanges with the atmosphere so efficiently that global coupled models either assume a yes/no scenario, respectively, for absence/presence of sea ice (England and others, 1994; Dixon and others, 1996), or, more recently (Goosse and others, 1999), reduce winter exchanges to thè`l ead'' areas (open-water fraction, generally <10%, of the total area covered with sea ice in mid-winter). However, since for temperatures higher than roughly^5³C a marked transition is assumed to occur toward increased permeability for inclusions in sea ice (Weeks and Ackley, 1986; Golden and others, 1998), this should allow, in both autumn and spring, gas transfer between the ocean and the atmosphere across the sea-ice cover mainly via the brine-channel network. On the other hand, recent studies of the biogeochemistry of the sea-ice habitat others,1995,1996) have shown that under favourable circumstances such as summer algal blooms, biological activity can profoundly affect the brine geochemistry, including the gaseous components. This has important implications for pathways of algal carbon acquisition (and therefore ice-algal species succession during summer blooms), carbon isotope fractionation and species distribution in the open-water phytoplankton. These two examples point to the need for a better understanding of the behaviour of the gaseous components of sea ice, both as gas bubbles and dissolved in brines.Datasets on total gas content and gas composition of sea ice are scarce. To our knowledge, only two attempts, both in the mid-1960s, have been made to measure gas propertie...

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The haptophyte Phaeocystis antarctica Karsten is a dominant species within the seasonal bloom in the Ross Sea. One of the unique characteristics of this form is that carbon is partitioned between the cells and the colonial matrix, a relationship that is poorly documented for this region. We combined particulate organic carbon measurements and microscopic analysis of P. antarctica‐dominated samples to assess the contribution of single cells, colony‐associated cells, and mucilage to the carbon concentrations of waters with P. antarctica. Two cruises to the Ross Sea were completed, one in austral spring 1994 and one in summer 1995–1996. In 1994 the bloom was dominated by colonial P. antarctica that contributed up to 96% of the total autotrophic carbon, whereas in 1995–1996 a mixture of P. antarctica and diatoms occurred. P. antarctica colony volume (V ) was related to colonial cell number (NC) by the relationship V = 417 ×NC1.67. Total colony carbon (CCOL) was calculated as the sum of cell carbon (CCC) and mucus‐related carbon (CM). We found the contribution of mucus carbon to be 213 ng C mm−3 of colony volume. For P. antarctica‐dominated assemblages sampled at the peak of the bloom, CM represented a minor fraction (14 ± 4%) of colony carbon, and during early summer conditions CM was at most 33% of CCOL. This organism plays a cardinal role in the carbon cycle of many regions. These results constrain the partitioning of carbon between cellular material and the colony matrix, information that is necessary to accurately describe the biogeochemical cycles influenced by this species.