ABSTRACT. Environmental properties of the lakes in the McMurdo Dry Valleys are largely controlled by the presence of perennial ice covers (2.8-6 m thick). Ice cover properties are, in turn, regulated by the extreme seasonality of Antarctica and local climate. An ice cover eliminates wind-generated currents, restricts the exchange of gases between the water column and the atmosphere, reduces light penetration, and limits sediment deposition. Ice cover properties, particularly ice thickness, have changed significantly since measurements first began in the 1960s. For example, the ice sheet on Lake Hoare in Taylor Valley thinned from 5.5 to 3.5 m between 1978 and 1983. Two hypotheses are presented to explain the ice thinning at Lake Hoare: (1)a "regional" hypothesis which posits that the thinning is due to a change in some regional climatic parameter (e.g., mean annual air temperature) which has altered the ice cover energy balance and (2)a "transitional" hypothesis that proposes that the thinning is a result of a change in the mechanisms of mass loss from the surface of the ice cover. As an ice cover thins, it becomes structurally weaker and develops cracks that enhance the movement of its sediment burden into the lake. A thinner ice cover that has less sediment will result in increased light penetration into the water column. A lake's productivity should then increase, resulting in greater deposition of organic matter to the benthic microbial mats. The decomposition of this organic matter will consume O₂ and, if the abiological sources of O₂ are reduced (due to loss of O₂ to the atmosphere through a more permeable ice cover), the deep oxygenation of sediments currently observed in Lake Hoare may cease to occur.