Author(s): Takeshi Takemura; Jiro Maruyama; Masamitsu Arita
Keywords: Phytoplankton; Peak of phytoplankton; Thermocline; Stokes low
Abstract: An aggravation of water quality and dissolved oxygen deficiency in a bottom layer by organic pollution in stagnant water area such as lakes are becoming a great problem. Numerical model for water quality prediction is needed to prevent the water pollution. However, it is difficult to determine the parameters relate to the model. This paper focused the mechanism of formation of the peak concentration of phytoplankton around thermocline observed in the fields. That peak was sometime observed in the fields, whereas, mechanism and formation process of it are not clear yet. In this paper, we investigated the process of accumulation of phytoplankton around thermocline based on experiments and numerical model. The experiments were conducted under the control of physical conditions (light intensity, water temperature and thermocline intensity). The experimental results show that the accumulation was triggered by the reduction of settling speed of phytoplankton due to the density change around thermocline. We also investigated the phenomena numerically and tried to develop numerical model to predict the vertical concentration of phytoplankton. In the numerical model, self-production, settling, mortality and the density diffusion of phytoplankton were considered. Specific growth rate of phytoplankton was given by our other experiments (the dominant species was Senedesmus spp.). Proposed numerical model present the good agreement with the experimental results when we gave the smaller settling speed of phytoplankton estimated by Stokes low. These results introduce that the change of settling speed due to the thermocline is dominant parameter of peak concentration of phytoplankton.