The number of shoots, shoot density, leaf-blade size (e.g. length and width), elongation rate, LMA (Leaf Mass of Area), PI (Plastochron Interval) and life-span in the eelgrass, Zostera marina L., were measured by mesocosm experiments from April 2005 to May 2006.
　Turion length and the number of shoots in the mesocosm pool were the greater from April to June, and the lesser from September to October. Although the number of shoots increased from 341 (April 2005) to 579 (March 2006), the shoot density did not increase at the low light plot. PI and life-span were correlated with water temperature. The length, width and elongation rate of leaf blades were the greatest in April and lessest in September. LMA was the highest in September and the lowest in March. Specific growth rate and defluxion rate were estimated from these parameters. As a result, the life strategy of eelgrass was evaluated.