Akihiko Yagi

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.

Prediction of monthly mean chlorophylla concentration in Mikawa Bay using GMDH

Data on organic and inorganic nitrogen and phosphorus, and meteorological conditions observed a few times a month at a buoy station in Mikawa Bay (Japan) are averaged on a monthly basis, and monthly transitions of chlorophylla at the station are predicted by GMDH (Group Method of Data Handling, one of the methods of nonlinear system identification) using these monthly mean data. The prediction is successful. GMDH is useful in describing the ecosystem and in predicting transitions within the bay.

ZOOPLANKTON AND PHYTOPLANKTON IN ALL ANOXIC LAYERS OF LAKE FUKAMI-IKE

Lake Fukami-ike is a small monomictic and eutrophic lake, located in southern Nagano Prefecture in Central Japan. An uncommon phenomenon of dissolved oxygen 0-1 mgL -1 from the surface to the bottom layer occurred on 16 November, 2013. On November 2, Trichocerca similis (Rotatoria) etc. dominantly distributed from 0-5 m depth depending on distribution DO concentration. Tintinnopsis lacustris (Protozoa) was found in the bottom layer (anoxic condition). The number of cells (Fragilaria rumpens (Bacillariophyceae) etc. abounded from the 0-5 m layer. On November 16, T. lacustris distributed from the surface to the bottom layer. Then, T. lacustris continued to distribute from the surface to the bottom layer, and the genus Synedra was found to have distributed uniformly from the surface to 6 m depth in the next investigation on 21 December, when the DO concentration was about 10 mgL -1 from the surface to the bottom layer. The changes in vertical distribution of T. lacustris, seemed to indicate that lake water was lifted from the bottom layer when shift stagnation periods to circulation periods.

CHANGES IN DEPTH AND THE SEDIMENT RATE BEFORE AND AFTER THE LAKESHORE DEVELOPMENT IN LAKE FUKAMI-IKE, JAPAN

Changes in depth and decrease of the sediment rate before and after a building breakhead maintenance construction were studied in small monomictic and eutrophic Lake Fukami-ike in central Japan. The maintenance of farm village drainage and the waterfront function was carried out for the activation of the town in 1992, and the water quality improvement was expected. However, variations of transparency were observed and no blue-green algal bloom outbreak had occurred before recently observed. Maximum depth changed from 9.3 m (1951), 8.5 m (1979), 8.1 cm (1992) to 7.8 m (2012), respectively. Changes in the autochthonous and allochthonous matters in the lake were thus considered. Sediment rates of 19.5 ± 10.19 gm -2 d - 1 (1978 to 1979) and 4.40 ± 2.27 gm -2 d -1 (2007 to 2008) were observed, and decreased 22.6%. These deposition rate data corresponded to 3.1 cm year -1 (1979) and 1.2 cmyear -1 (2009), respectively. The decreased percentage of organic matter and the reduced deposition rate were because rice fields and forest around the lake give way to take concrete roads. It was considered to be because the inflow of sediment stopped when it rained, and allochthonous inorganic matter was significantly reduced.