Physical processes and hypoxia in the central basin of Lake Erie.

The circulation and exchange processes during summer stratification were examined using time-series data of horizontal velocity, temperature, and dissolved oxygen profiles during 2004 and 2005 in the mid-central basin of Lake Erie. The current and temperature spectra showed a prominent peak at around 18 h, indicating the presence of clockwise rotating inertial waves. The mean bottom currents were strong (>0.1 m s^-1) and flowed in opposite direction to winds because of the surface pressure gradient due to wind set-up. The general range of horizontal exchange coefficients in the central basin is 0.2-1.2 m² s^-1. Vertical exchange coefficients varied from 1 x 10^-5 to 1 x 10^-4 m² s^-1. The high values usually occurred in the surface layer because of surface winds. The source of turbulent energy is current shear due to near-inertial oscillations in and above the thermocline and shearing stress due to the effect of mean currents and wave-induced motions during energetic wind events at the lake bottom. During strong wind episodes significant wave-induced currents were observed close to the bottom. The short-term oxygen depletion rates varied considerably between +0.87 mg L^-1 d^-1 and -1.16 mg L^-1 d^-1 in 2004 mainly because of physical processes in the central basin. When the hypolimnion depth is sufficiently thick (4 m), shortterm changes in dissolved oxygen concentrations were partly due to vertical mixing and partly due to horizontal transport and mixing. [ABSTRACT FROM AUTHOR]