Effects of oxygen transport on the areal hypolimnetic oxygen deficit

Abstract

Using a modified flux gradient technique, transmetalimnetic oxygen transport was evaluated in Lakes Mendota and Delavan, two eutrophic Wisconsin basins featuring moderate depth (17 < zmax < 25 m), good wind exposure, and classic clinograde oxygen profiles. The biweekly fluxes intensified between mid‐May and late July 1972, mainly on account of seasonal steepening of the metalimnetic O2 gradients. Fluxes also increased sharply during windy intervals characterized by accelerated transmetalimnetic heat fluxes. The fluxes across the lower epilimnetic boundary exceed those across the lower metalimnetic boundary; this differential is consistent with the metalimnion's role as an oxidation zone in these turbid lakes. The vertical fluxes are a large but variable (over time) fraction of the net areal oxygen deficit in both lakes, thus complicating statistical analysis of seasonal and intergenerational trends in lake productivity using the areal hypolimnetic oxygen deficit method. Even after correcting for the effects of O2 transport, the deficit in Lake Mendota exceeds that in Delavan following the onset of hypolimnetic anoxia in the shallower lake. This difference likely results, in part, from transmetalimnetic fluxes of reducing substances (CH4, NH3, etc.) into the epilimnion.