Light condition and water temperater are the key parameters controlling the growth of eelgrass,Zostera marina L. In this study, we investigated the effect of these two parameters on the net growth of Z. marinaby the mesocosm experiment facility and a numerical simulation model of seasonal changes in the biomass.After 200 shoots of Z. marina being transplanted into the mesocosm pool, the leaf length and the shootdensity were surveyed every 1-2 month. There was a seasonal fluctuation in the Z. marina leaf length andshoot density; horizontally heterogeneous distribution was observed in the shoot density but not in the leaflength from the mesocosm experiment. Calculated horizontal distribution of relative light period was consistentwith the horizontal distribution of shoot density, which was greater at longer light period areas. A newestimated value of minimum light requirement for Z. marina, which takes into account for renewal of theleaves, was proposed.