Dissolved Oxygen Depletion in a Small Deep Lake with a Large Littoral Zone

Abstract

ABSTRACT Advection and its influence on water quality (density currents and the fate of transported materials, metalimnetic oxygen depletion, etc.) are well established for reservoirs and other lakes with well-defined inflows. This paper provides an example of the potential importance of these processes in small lakes with limited inflow. Viewing such lakes as one-dimensional systems may severely limit our understanding of their dynamics. Holland Lake, a small and deep mesotrophic lake (0.14 km2 surface area and 18.8 m maximum depth) has been considered for stocking with brown trout because it provides extensive cold-water during the summer. Unfortunately, an anoxic layer of about 1.5 to 3 m thickness develops in the upper metalimnion resulting in a negative heterograde dissolved oxygen profile, and eliminating any trout habitat during July and August. The findings of a field study suggest that the occurrence of the metalimnetic oxygen minimum is related to the presence of substantial macrophyte beds in the shallow bays, the particular morphometry of the lake (69% littoral zone), and a significant groundwater inflow and outflow. Density current intrusions that transport oxygen depleted water of high detritus content from the bottom of the shallow bays into the metalimnion of the deep basin in early summer are consistent with exceptionally high observed oxygen depletion rates. It is this horizontal convective transport of materials within the lake in addition to phytoplankton settling from the photic zone above, that strongly affects the dissolved oxygen deficiency in the upper metalimnion of the deep main basin. To create a brown trout habitat, metalimnetic aeration will be required.