In situ hypolimnetic oxygen depletion measurements were conducted during four summer cruises in 1979 at two central basin stations in Lake Erie to evaluate the relative contribution of the sediments to the oxygen demand. Sediment oxygen demand (SOD) rates were determined by measuring the rate of oxygen decrease within a triangular benthic chamber; water column oxygen demand (WOD) rates were determined using 24-hour light and dark bottles placed in situ. Results indicated that the SOD contribution to the hypolimnetic oxygen depletion rate decreased throughout the summer from about 81% to only 30% with an initially high rate due to spring algal biomass sedimentation and lower rates in late summer due to depressed oxygen levels. The WOD rate contribution increased from 19% to 70% throughout the stratified period due to the decomposition of settling algal cells. Comparing the overall volumetric summer in situ rates (0.126 mg O2/L/day) with cruise-interval depletion rates (0.365 mg O2/L/day), the in situ rates were about 300% higher. This is attributed to unaccounted oxygen sources to the hypolimnion and because the in situ rates measure the gross WOD and SOD rather than measuring the net effects they exert.