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 m(2) s(-1). Vertical exchange coefficients varied from 1 3 1025 to 1 x 10(-4) m(2) 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), short-term changes in dissolved oxygen concentrations were partly due to vertical mixing and partly due to horizontal transport and mixing.