The purpose of this study was to estimate biological parameters for the ecosystem model especially in cold region estuary. Because the phytoplankton is primary producer in the ecosystem, it is necessary to measure the growth to the accuracy of the ecosystem calculation. Field investigations including Phytoplankton's incubation experiment were conducted at Lake Furen in Hokkaido. From the experiment, we had an equation showing the relation between light-photosynthesis of cold region phytoplankton. The maximum photosynthetic rate and optimum light intensity were 3.3 (1/day) and 34.0 (W/m2) respectively at 17.8°C. They indicate that the phytoplankton grows up in conditions of low temperature and a little quantity of light, compared with the general parameters used in Japan.