Seasonal variation in sulfate reduction and methanogenesis in peaty sediments of eutrophic Lake Loosdrecht,The Netherlands

During one year, concentration profiles of sulfate and methane were measured in sediment cores of eutrophic Lake Loosdrecht. Sulfate concentrations decreased exponentially with depth towards a constant threshold value of 7.6 ± 6.1 μM. Concentration profiles were used to calculate fluxes of sulfate and methane and to estimate the anaerobic mineralization rate. Anaerobic mineralization was highest in autumn which was probably due to an increased sedimentation of easily degradable organic carbon. At high rates (>600 μ mol organic carbon .m⁻².h⁻¹), sulfate reduction appeared to be limited by sulfate and methanogenesis accounted for over 80% of the anaerobic mineralization. At low anaerobic mineralization rates, measured in winter and spring, sulfate reduction was predominant. There was little methanogenesis below 5 cm depth in the sediment which indicated a rapid decrease of degradable organic matter with depth. There was a remarkable difference, especially in winter, between methane fluxes which were measured in batch experiments and those calculated from the concentration profiles in the sediment. These differences may be due to methane diffusing upward from deep layers.