Meltwater ponds are oases of life on the Antarctic continent in which the freeze-thaw cycle and availability of liquid water are critical to biological structure and productivity. For logistical regions, almost all previous study of these ponds has been during the summer period of maximum thaw, but we suspected that extreme conditions experienced during winter freezing may be just as important in controlling life. The aim of this IPY project was therefore to document the physical and chemical changes occuring in meltwater ponds during freezing, and determine its effects on both planktonic and benthic microbial metabolism. From the end of January 2008 to early April observations were made of ice formation, water column attributes, and metabolism in four ponds on the McMurdo Ice shelf. 

Results/Conclusions

The ponds transitioned from ice-free to having >80 cm of ice with ice growth close to linear at 1.5 cm d^-1, reflecting a stable temperature-depth gradient within the ice cover due to a general decline in air temperature as ice thickened.  As ice formed, salts and gases were excluded from the ice, resulting in the conductivity of the remaining liquid water in the smallest of the ponds rising from 3-5 mS cm^-1 to over 60 mS cm^-1.  Due to the rapid growth in ice, gas bubbles trapped beneath the ice were then incorporated into the ice matrix and resulted in a high ice albedo and under-ice declined faster than incident irradiance, falling below 1 W m^-2 by early April.  Up to 10% of the volume of the deeper ponds was still liquid in April, but nearly 40% of the pond floor supporting benthic microbial mats remained unfrozen.  Light attenuation meant that the benthic mats ceased photosynthesizing in late February, whereas for phytoplankton photoinhibition was relieved and photosynthesis continued until late March. The period of freezing was at least as long as the period of open water.  We suggest that the stress that freezing imposes may be as important to structuring biotic communities in these ponds as favorable conditions that occur during the summer.