Modeling of the spring bloom in Funka Bay, Japan

Effects of vertical stability and concentration of nutrient on the diatom bloom in Funka Bay, south Hokkaido, in spring were investigated using one-dimensional and three-dimensional ecosystem models. In the model, six compartments: two for phytoplankton (diatom and dinoflagellate), two for nutrients (silicate and nitrate), one for zooplankton and one for detritus were considered. Vertical stability depended on the net heat flux through the sea surface. Calculated results are compared with the observational results for 1981 (Tsunogai and Watanabe, Journal of the Oceanographical Society of Japan 39 (1983) 231–239). This comparison shows that the rapid spring diatom bloom corresponds to the timing when the net heat flux through the sea surface changes from cooling to heating. This result suggests that the stability of the water column due to warming play an important role in the onset of the diatom bloom in Funka Bay. In addition, the limiting factor of the diatom bloom during the spring bloom is not the silicate, but nitrate. The model can reproduce the observed data by changing the nutrient uptake rate. This suggests a dramatic increase in the assimilation rate of silicate after the consumption of nitrate.