Long-term changes in the areal hypolimnetic oxygen deficit (AHOD) of Onondaga Lake: Evidence of sediment feedback

Long-term trends in the rate of depletion of hypolimnetic dissolved oxygen (DO) are documented for ionically enriched hypereutrophic Onondaga Lake, New York, for the 1978–2002 interval. Depletion rates, represented as areal hypolimnetic oxygen deficits (AHOD, g m22 d21), are calculated on the basis of weekly DO profiles of 1-m resolution and estimates of coincident inputs of DO from overlying layers driven by vertical mixing. Vertical mixing inputs of DO are important in this system, representing from 15% to 37% (mean 25%) of AHOD. Interannual variations in hypolimnetic temperatures have comparatively minor (66%) effects on AHOD. AHOD decreased 49%, from an average of 2.12 g m22 d21 for the 1978–1986 interval, to an average of 1.08 g m22 d21 for the 1997– 2002 interval. This decrease was driven by an abrupt decrease in the deposition of particulate organic carbon into the hypolimnion starting in 1987. The magnitude of the decrease in AHOD closes reasonably well with decreases in both primary production in the trophogenic zone and organic carbon deposition to the tropholytic zone. The time course of the decrease in AHOD is consistent with localization of oxygen-demanding processes within the lake sediments, reflecting the progression of sediment diagenesis. Dissolved oxygen (DO) is a fundamental resource of lakes that has profound effects on lake chemistry and biology (Wetzel 2001). Most hypolimnia are isolated from the important oxygen source of photosynthesis and potential inputs of DO from atmospheric exchange. Oxygen consumption in hypolimnia and sediments reflects decomposition of settling and deposited particulate organic matter that is formed mostly through primary production in the overlying trophogenic zone (Hutchinson 1957). The rate of hypolimnetic oxygen depletion has long been recognized as an integrator of lake metabolism. The evolution of these concepts and various calculation protocols to quantify the rate of DO depletion as an index, or even an indirect measure (Hutchinson 1938; Mortimer 1941), of primary production has been reviewed by Wetzel (2001). The rate of loss of the mass of DO, normalized for the surface area of the hypolimnion, described as the areal hypolimnetic oxygen deficit (AHOD, g m22 d21), is recognized as a quantitative representation of this oxygen depletion (Wetzel and Likens 2000). Alternatively, the oxygen depletion rate can be normalized for hypolimnetic volume and represented as a volumetric hypolimnetic oxygen deficit (VHOD, g m23 d21; Burns 1995). The AHOD representation has been widely used as an index of the productivity of stratifying lakes, particularly for 1 Corresponding author ([email protected]).