Long-term trends in methane flux from the sediments of Onondaga Lake, NY: Sediment diagenesis and impacts on dissolved oxygen resources

Long-term trends in diffusive and ebullitive fluxes of methane (CH 4 ) from the sediments of hypereutrophic Onondaga Lake, New York, are documented for the 1989 to 2004 interval, following an abrupt reduction in deposition of particulate organic carbon. Assessments are based on June to October monitoring of dissolved CH 4 in the water column and ebullitive gas collected above the sediments. Reductions in overall CH 4 flux were first observed in ebullition, which decreased abruptly from 6 mmol m -2 d -1 before 1992 to 1 mmol m -2 d -1 thereafter. The contribution of ebullition to total CH 4 flux decreased from 35 to 9 % over the same period. Substantial reductions in dissolved CH 4 flux were first observed approximately two years after the major reductions in ebullition. Sediment release of dissolved CH 4 decreased 38 %, from an average rate of 11.7 mmol m -2 d -1 for the 1989 to 1991 interval, to 7.3 mmol m -2 d -1 for the 2002 to 2004 interval. The decrease in CH 4 flux 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 CH4 flux is consistent with previously documented decreases in primary production, organic carbon deposition, and the areal hypolimnetic oxygen deficit. The time course of the decrease in CH 4 flux reflects sediment diagenesis processes. Oxidation of large (3.3 - 8.2 x 10 6 mol) hypolimnetic accumulations of CH 4 during the fall mixing period contributed significantly to oxygen depletion observed annually in the lake's upper waters. Decreases in CH 4 flux from the sediments resulted in smaller hypolimnetic CH 4 pools in late summer and contributed to improved dissolved oxygen resources in fall.