While the loadings of nitrogen (N) and phosphorus (P) are enhanced by the human activities, dissolved silicate (DSi) supplied by natural weathering of minerals tends to be trapped in the eutrophicated still waters such as dammed reservoirs, which are also increasing globally. The consequent change in the N: P: Si stoichiometric ratio of the river water flowing into the coastal sea may be advantageous to flagellates (nonsiliceous and potentially harmful) but not to diatom (siliceous and mostly benign). This is the "silica deficiency hypothesis". We try to develop it further to the "extended silica deficiency hypothesis" as follows. While the diatoms effectively draw down the substances from the upper layer with their sinking after the spring bloom (biological pump), other algae do not. This will cause the retention of the surplus nutrients and biogenic substances in the upper layer and lead further to eutrophication of upper layer substantially. To verify this hypothesis, a simplified marine ecosystem model was developed taking the nutrient stoichiometry and differential behaviors of algae into account based on the comparison with the existing field monitoring results of four aquatic continua, where various types of silica decline are uncertain. Although, this study is still in the stage of improvement, the preliminary results infer that above hypothesis could be verified with the consideration of the behavioral characteristics of algae as well as the stoichiometric consideration. Sinking response to the nutrient depletion, which is specific to each algal group, is one of possible factors.