A majority of the research on the MCM LTER occurs during the austral spring and summer (October-January), a period of continuous sunlight, when field support is readily available. Through additional logistical efforts, we were able to collect the first data on the MCM lakes during the transition from summer to winter (October-April). These data allowed us to examine ecosystem responses as photosynthetic input of new carbon stopped. Protein biosynthesis (leucine incorporation) increased in the east lobe of Lake Bonney during March and April (p<0.05). There was no change in protein synthesis in Lake Fryxell. No significant change (p<0.05) in DNA replication occurred during the transition to complete darkness, inferring that the bacteria do not rely directly on fresh phytoplankton primary production as their source of organic carbon. A high rate of dark ¹⁴C-labeled bicarbonate incorporation (0.01 to 0.09 µg C l^-1 d^-1) in Lake Fryxell implies that chemoautotrophic primary production is important in sustaining the ecosystem of this lake during the winter. A decoupling of bacterial protein biosynthesis and DNA replication also occurred during the transition to winter. Protein biosynthesis increased relative to DNA replication as darkness set in, which may signify decreasing rates of cell division. Such a shift in cellular function, in combination with continued bacterial production, indicates that bacterioplankton in the MCM lakes remain active during the polar night, but may direct more energy towards survival than reproduction.