Electron budgets for the hypolimnion of a recovering urban lake, 1989-2004: Response to changes in organic carbon deposition and availability of electron acceptors

Hypolimnetic electron budgets were developed for sulfate-rich, eutrophic Onondaga Lake, New York, for the summer stratification periods of 16 years (1989-2004) to describe seasonal, long-term, and interannual changes in hypolimnetic metabolism. A mass balance approach was applied to the hypolimnion to estimate the consumption of electron acceptors (e.g., O₂, NO ₃^-) and production of reduced by-products (e.g., H ₂S, CH₄) associated with the decomposition of organic matter. The stoichiometry of the associated redox reactions supported calculation of electron transfer in the hypolimnion and partitioning of the contributions of various metabolic pathways. In 12 of the 16 study years SO ₄^2- 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 to overall hypolimnetic metabolism 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. Changes in the importance of the different decomposition pathways were generally consistent with the thermodynamic constraints that regulate the spatial and temporal patterns of redox processes in aquatic sediments. On average, the discrepancy between observed dissolved inorganic carbon accumulation and that calculated from the four redox pathways was 14%; potential causes for this discrepancy include storage of reduced forms of particulate organic carbon and metabolic by-products in the sediment.