| A comprehensive retrospective analysis of the hypolimnetic metabolism of dimictic, sulfate-rich, culturally eutrophic Onondaga Lake (Syracuse, NY) is presented. Changes in hypolimnetic metabolism are described for the 1978 to 2005 interval, based upon an extensive long-term data set that includes temporally and vertically detailed water column measurements of key drivers and indicators of aerobic and anaerobic decomposition processes. This 28 year interval is particularly interesting because it includes major changes in important drivers, including abrupt decreases in primary production and organic carbon deposition to the hypolimnion and substantial variations in the availability of electron acceptors to support decomposition in the hypolimnion. A hypolimnetic mass balance approach is employed for the summer stratification interval to investigate the impacts of these changes on depletion of dissolved oxygen (DO) and nitrate (NO3-) and accumulation of total sulfide (SigmaS2-), methane (CH4), and dissolved inorganic carbon (DIC).; After 1987 the areal hypolimnetic oxygen deficit (AHOD) and CH4 flux from the sediments decreased gradually by 49% and 46%, respectively. The magnitude of these decreases compares reasonably well with concomitant decreases in both primary production in the trophogenic zone (32-39%) and organic carbon deposition to the tropholytic zone (37%). The trajectories of the decreases in AHOD and CH4 flux are consistent with localization of oxygen depletion and methanogenesis within the lake sediments, reflecting the progression of sediment diagenesis. In contrast, epilimnetic DO concentrations responded rapidly to the abrupt change in lake metabolism. Hypolimnetic electron budgets developed for the 1989-2004 interval revealed that SO4 2- reduction was the most important decomposition pathway, accounting for an average of 44% of the electron transfer in the hypolimnion. Aerobic decomposition (28%), methanogenesis (19%), and denitrification (9%) were also quantitatively important mineralization processes. The relative importance of the various decomposition pathways varied widely in response to an abrupt decrease in organic carbon deposition in 1987, incomplete spring turnover in 1993, and improved nitrification treatment at a contributing wastewater treatment facility in 2004. The implications of these findings are discussed within the context of ongoing rehabilitation programs designed to address the issues of cultural eutrophication and mercury contamination.; Keywords. decomposition, denitrification, diagenesis, dissolved oxygen, eutrophication, lake rehabilitation, methanogenesis, organic matter, redox, sediment flux, sulfate reduction... |
