Manganese flux associated with dissolved and suspended manganese forms in Lake Fukami-ike

Abstract

Redissolved, particulate and sedimentary managanese were studied in the anoxic hypolimnion of the small monomictic Lake Fukami-ike in central Japan. The amounts of particulate and dissolved manganese in the water column were observed. The release of dissolved managanese starts gradually beginning in March. DMn reaches especially high levels of 93 mgMn m −2 d −1 from the end of May to the end of June calculated from the gradient of a regression formula for the daily change in DMn. The rate of accumulated PMn is 0.64 mgMn m −2 d −1 from the end of July calculated from the gradient of a regression formula for the daily change in PMn. An accumulation of PMn and an associated decrease in dissolved organic carbon (DOC) were observed at the transition zone of oxic and anoxic layers. These facts seem to suggest that the DOC decrease involves the consumption of DOC as an electron donor by bacterial catalysis for the reduction of MnO 2. The upward and downward DOC fluxes, 0.320 mmolC m −2 d −1, were calculated by the mean gradient values of the upward and downward DOC concentrations. The mean DOC flux was calculated by both the mean gradient DOC and the vertical diffusion coefficient (0.0304 m −2 d −1). Manganese-reducing bacteria could contribute as much as 24% to the DOC decrease in the oxic and anoxic layers, considering bacterial assimilation efficiency (64%). Vertical diffusion Mn flux from the interstitial water to the hypolimnion is then assumed to be 2 mgMn m −2 d −1. Mn for the vertical eddy diffusion flux, the vertical diffusion flux from interstitial water of the bottom sediment, the sinking flux and microbiological Mn flux were obtained, and the biogeochemical managanese cycle in Lake Fukami-ike was calculated. DMn flux in the oxic and anoxic layers by manganese-reducing bacteria is 0.004 gMn m −2 d −1, a value corresponding to 10% of DMn flux.