EVOLUTION OF TEMPERATURE AND SALT STRUCTURE OF LAKE-BONNEY, A CHEMICALLY STRATIFIED ANTARCTIC LAKE

A resurgence of interest in the ecology of perennially ice-covered lak es in the McMurdo dry valleys has necessitated a review of our knowled ge of the physical and chemical properties of these unusual lakes. Sal inities in the ice-covered lakes cover a range from freshwater to hype rsaline brines. Recent measurements of salt composition and concentrat ions in Lake Bonney reveal little change below the chemocline since ex tensive measurements made in 1960-1961, although lake level has risen by approximately 5 m since that time. The rise in lake level has resul ted in a thickening of the freshwater layer above the chemocline. Temp erature structure has adjusted to the effects of increased lake level on heat transfer processes such as transmission and absorption of sola r radiation in the water column. Questions about how water-column stab ility affects biology in Lake Bonney have motivated the formulation of a method to compute density from in situ measurements of temperature, conductivity and pressure. Owing to high salt concentration and uniqu e ion ratios, we modified the UNESCO Equation of State for seawater to predict density at salinities greater than 42. The modifications merg e smoothly with the UNESCO equations at a salinity of 42. At salinitie s below 42 the UNESCO equations give excellent predictions of density.