EVIDENCE OF DENSITY CURRENTS WITH THE POTENTIAL TO PROMOTE MEROMIXIS IN ICE-COVERED SALINE LAKES

Ice covers all but the most saline of the Vestfold Hills' lakes for about 8-12 months of each year and precludes wind-induced turbulent mixing over the winter, when strong winds are most frequent. Nevertheless, the progressive growth of ice volume through the austral winter and spring months generates haline convection capable of mixing the water column to a depth at least as great as that achieved by wind-induced turbulence in the summer ice-free period. Further, cold and very saline brines may form at the shallow periphery of a lake and flow downslope, penetrating to the lake centre below the convectively mixed layer. Detailed temperature profiles of hypersaline Organic Lake provide the first evidence for these density currents in saline lakes of the Vestfold Hills. These data indicate a dynamic responsiveness to periods of relatively cold weather, and that the resulting density currents may be sufficiently small in volume to have little effect on the anoxia of bottom layers in these meromictic lakes. Alternating periods of negative and positive water balance may also be significant in the formation and destruction of meromixis in these saline lakes, which lack outflow streams. If a lake has been through a period of negative water balance, becoming relatively more saline, and then begins to be diluted during a subsequent period of positive water balance, the winter haline convection will penetrate to progressively shallower depths and deeper layers may stagnate, marking the onset of meromixis. An increase in the winter level of Organic Lake over ten years indicates the speed with which the Vestfold Hills' lakes may experience significant change in salinity despite the generally small catchments of these lakes.