ABSTRACT: This study was designed to identify a sustainable Zostera marina population based on the relationships between the distributional patterns of shoots and light conditions in the population. Population structures and light conditions on 22 April 2003 (season of reducing shoot density), 27 September 2003 (season of the annual minimum density and biomass) and 9 April 2004 (season of the annual maximum density and biomass) were examined. On 22 April 2003, the frequency distribution in shoot length was almost even. The spatial pattern is characterized by small clumps within 2–5 cm radius. On 27 September 2003, the lengths of all shoots were less than 40 cm, and the distributional patterns were similar to 22 April. On 9 April 2004, the spatial pattern is characterized by larger clumps within 25 cm radius. The reproductive shoots had a regular distribution. The relative light intensities on the population floor of the sea surface on 27 September and 9 April were 53.3 and 10.2%, respectively. The light intensity on 9 April 2004 was not sufficient for growth. The results suggest that the competition for harvesting solar radiation is caused by the shoot length and the spatial pattern among shoots in the population.