Entrainment by bottom currents in Lake Erie

Previous studies of the central basin of Lake Erie have indicated, on the basis lakewide budgets of heat and dissolved oxygen, that the thickness of the hypolimnion and the interaction of the hypolimnion with the overlying fluid are important factors governing the dissolved oxygen concentration in the near‐bottom water. Data collected during an intensive field program in 1979 contain an example of an event during which both the thickness and temperature of the hypolimnion increase due to an erosion of the thermocline from below. This thickening of the hypolimnion requires two conditions. First, the thermocline region, or metalimnion, must be thick enough so that the effects of the energetic surface mixing processes are confined to the upper portion of the thermocline. And second, the currents in the thin hypolimnion must be strong enough to entrain overlying metalimnion water down into the hypolimnion. An analysis of the current meter data suggests the source of turbulent energy driving the mixing is shearing stresses at the bottom. During the entrainment event, the contribution to bottom dissolved oxygen supply is about 10% of the daily demand.