Hypolimnetic aeration in Lake Tegel, Berlin

Hypolimnetic aeration has been part of a successful restoration strategy for the shallow urban lake, Lake Tegel, in Berlin since 1980. From this time to the present, extensive limnological data on the lake's water quality have been collected which have been used to validate a one-dimensional water quality model. Three cases are discussed: full operation of a network of 15 aerators, intermittent operation as water quality improved and no aeration as is the present situation. The effect of the hypolimnetic aerator was modelled once as a stirrer and also as a bubbler. While both are designed to impart mechanical energy to the water column to promote mixing by means of a column of bubbles, stirrers are meant to restrict the mixing between the epilimnion and hypolimnion by collecting the bubble streams before they penetrate the upper layer. On the other hand, for bubblers the bubble column continues unimpeded to the surface. We conclude that the design of the hypolimnetic aerators employed in Lake Tegel behave more closely to a bubble plume and thus there is considerable mixing between the hypolimnion and epilimnion. (C) 1997 Elsevier Science Ltd.