We evaluate the possibility of the deterioration of the coastal marine ecosystem based on the following "silica deficiency hypothesis". While the loadings of nitrogen (N) and phosphorus (P) are enhanced by the human activities, dissolved silicate (DSi) supplied naturally 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). We develop it further to the "extended silica deficiency hypothesis": 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. Such a change will cause the retention of the surplus nutrients and biogenic substances in the upper layer and will be significant in the coastal and shelf seas of the east Asia
