Microalgal assemblages associated with the winter Antarctic pack ice sheet were examined for photosynthesis-irradiance characteristics in relation to light availability. Pack ice, the dominant form of sea ice in the Southern Ocean, and the underlying water column were sampled from a site, approximately 150 nautical miles north of the French base - Dumont d'Urville, during Voyage 1 of RSV Aurora ... Australis for the 1995-1996 season. Microalgal cell concentrations in first-year pack ice and the underlying water column were extremely low (~100 to 1000 cells per litre) in comparison to the multi-year site, where the ice was stained brown due to high concentrations of microalgae dominated by pennate and centric diatoms. P-I analyses could not determine whether Antarctic winter pack ice microalgae or phytoplankton were actively photosynthesising in situ during the study period, because analytical temperatures differed from those in situ. However, the relatively high vertebrate and invertebrate activity noted in the vicinity of brown pack ice indicated that established microalgal assemblages associated with multi-year ice were supporting secondary production during the winter months. Due to a mishap during pigment extraction, it was considered inappropriate to use the Chl a data to standardise the parameters describing photosynthesis (P max, alpha). Relative Chl a levels indicated that biomass levels were higher in the pack ice compared to the underlying water column, and within the first-year ice sheet the biomass was generally concentrated in the bottom layers. As expected, the maximum photosynthetic capacities (P max) reflected the trend noted in Chl a distribution in both the first-year ice sheet and the water column. The relatively low Ik values observed in first-year pack ice indicate extreme shade adaptation. In comparison, under-ice phytoplankton generally displayed higher Ik. Photoinhibition was noted in all first-year ice samples displaying photosynthetic activity and most phytoplankton assemblages. The snow-free, multi-year ice at Brown Sugar displayed the highest P max and biomass values. High levels of bacterial dark carbon uptake were noted at three of the five first-year ice stations. Dark uptake was not observed in any of the water column samples. The high dark uptake noted in the ice samples is thought to occur as a result of bacterial proliferation during the ice thawing process. The study was also able to highlight some of the problems involved with the analysis of sea ice microalgae, using a technique that was designed to assess the P-I characteristics of phytoplanktonic assemblages. One major problem that is proposed is the modification of P-I parameters during extended ice core thawing periods (up to 4 days).
The primary objective for the study was: To determine whether microalgae associated with the pack ice north of Dumont d'Urville were making a significant contribution to the energy flow of the Southern Ocean by supporting the associated secondary production during the winter months.
To meet the primary objective, a number of secondary objectives were also set. These were:
1) To determine whether Antarctic pack ice microalgae were photosynthetically active during the winter months;
2) To determine the photosynthesis-irradiance (P-I) characteristics of Antarctic winter pack ice microalgae in relation to ambient light intensity;
3) To compare the P-I characteristics of microalgae associated with a range of winter pack ice microhabitats sampled vertically from ice cores;
4) To compare the results obtained in 2) with those P-I characteristics obtained for Antarctic phytoplankton; and
5) To determine the suitability of the 14C technique, used in ocean productivity studies, for sea ice productivity studies.
Enroute to the sea ice, the Aurora Australis cruised SSW until intersecting 139 degrees east and then due south along the World Ocean Circulation Experiment (WOCE) SR3 transect. The study sites were located about 150 nautical miles within the ice edge (between 64 and 66 south and 138 and 141 east).
The fields in these datasets are:
Ice Floe Age
Average Ambient Surface Irradiance
Average Under-Ice Irradiance
Percentage of Surface Irradiance
Daily Average Surface Air Temperature
Average Ice Surface Temperature
Range of Ice Surface Temperatures
Approximate Ice &Interior& Temperatures
Mixed Layer Depth
Temperature Below Mixed Depth
Dark Carbon Uptake
Percentage of Total Primary Production