EXCHANGE OF WATER AND OXYGEN BETWEEN THE 2 BASINS OF AMISK LAKE

Amisk Lake was artificially oxygenated year-round by bubbling oxygen t hrough a diffuser installed at the deepest point of its north basin. T he hypolimnion of both basins became anoxic in summer before operation of the system whereas anoxia was never observed while it was fully op erational (July 1990 until October 1993). The system increased the ave rage hypolimnetic oxygen content of the south basin by an average of 8 3 tonnes (t) more than under natural conditions between 2 September an d 18 May. This considerable increase is approximately accounted for by the transport of dissolved oxygen (DO) through the connecting channel by currents driven by the bubble plume (approximate to 52 t) and by i nternal seiche pumping (approximate to 17 t): both mechanisms are cont ingent on the thermocline depth being less than that of the connecting channel. Internal seiche pumping was possible because the connecting channel is shorter than the maximum distance across which hypolimnetic fluid moved between flow reversals. Other factors that may have been important include changes in the replenishment of DO during turnover, oxygen transfer during winter through the ice-free patch created by th e bubble plume, and an increase in DO consumption.