Environmental conditions during freezing, and response of microbial mats in ponds of the McMurdo Ice Shelf, Antarctica

Environmental conditions, both external to and within three shallow ponds o f the McMurdo Ice Shelf, were measured over an annual cycle between January 1997 and January 1998. We combined this with a study of the response of th e benthic microbial mat communities to the transition from summer condition s to winter freezing. Over the study period air temperature was above 0 deg rees C for a few days during summer. at this time pond temperatures were hi gher than air temperatures, with evidence of thermo-haline stratification. The shallow areas of ponds froze between late February and early March, wit h bottom waters in the deepest pond remaining unfrozen until early June. Mi nimum winter air temperatures were below -40 degrees C. There was little ev idence of freezing point depression due to freeze-concentration of solutes, except at the very bottom of ponds. In the most conductive pond investigat ed, the temperature of basal freezing was -4 degrees C and conductivity did not exceed approximately 60 mS cm(-1) immediately prior to freezing. Micro bial mats remained photosynthetically active up to conductivities between 4 0 and 80 mS cm(-1), and were able to acclimate to lowered irradiance associ ated with ice formation. Although photosynthesis and respiration were reduc ed by 11% and 40% respectively at temperatures of -2 degrees C compared to 1 degrees C, there was no difference in light harvesting efficiency. Result s from this study suggest that light limitation of photosynthesis, or freez ing, determine the growth season for the microbial communities, depending o n depth.