Design of an Aeration System to Enhance Trout Habitat in Holland Lake, MN

Holland Lake, a small but deep mesotrophic lake in the Twin Cities Metropolitan Area, has been considered by the Minnesota Department of Natural Resources, Division of Fisheries, for stocking with brown trout. Holland Lake, with a surface area of 0.14 km2 (35 acres) and a maximum depth of about 18.8 m (61 ft) consists of two shallow bays covered with rooted macrophytes and a deep main basin. The deep basin is thennally suitable for brown trout. However, due to a high oxygen depletion rate in summer, the lake becomes anoxic below the surface mixed layer from late June to early July. The rate of oxygen depletion below the surface mixed layer, based on field measurements, was estimated to be about 0.47 mg/day·l. Field studies conducted in the summers of 1999 and 2000 indicated that only horizontal advection processes could explain the observed high dissolved oxygen (DO) depletion rates. Density currents transport low DO water with high BOD into the deep basin metalimnion. These currents from the shallow bays were attributed to the temperature regimes of the shallow bays and groundwater flow through the lake. To improve brown trout habitat in Holland Lake, an aeration system has been designed based on the observed sumnler conditions. The aeration system comprises two bubble curtains along the border of the shallow bays to enhance mixing in the shallow bays and one metalimnetic aerator in the deep basin. The bubble curtains deepen the surface mixed layer down to 4 m, and prevent the fomlation of density currents from the shallow bays into the deep basin.