Physical processes and hypoxia in the central basin of Lake Erie

Abstract

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 × 10-5 to 1 × 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), short-term changes in dissolved oxygen concentrations were partly due to vertical mixing and partly due to horizontal transport and mixing. © 2008, by the American Society of Limnology and Oceanography, Inc.