Abstract: We studied sexual reproductive characteristics of a population of eelgrass (Zostera marina L.) in Shuangdao Bay, Weihai, China. At population scale, reproductive shoots were first observed in late March or early April. Initial anthesis occurred in early May, and seed release peaked in late June. Senescent reproductive shoots were detached around early July and the fertile season ended in late July. The phenophases of this species were closely correlated with water temperature and 15 ℃ was required for initial anthesis. At individual scale, the maximum numbers of reproductive branch and rhipidium were recorded in the initial appearance of reproductive shoots, and they were 4.4 and 4.9 per shoot, respectively. The maximum number of spathes per shoot (18.6) and the maximum height of reproductive shoots (96.2 cm) were observed during the initial seed release with a mean abortion rate of 40%. The average mature seed production of eelgrass was 44.8 per shoot. The maximum density of reproductive shoot (64 shoots·m^-2) was found in initial anthesis. The reproductive shoot ratio was only 12.6%. The vegetative shoot density and total population density decreased more than 40% in reproductive season. The biomass of reproductive shoots was the largest (27.9 g·m^-2) in the initial appearance of visible fruit and the mean biomass allocation percentages of underground, vegetative shoots and reproductive shoots were 38.4%, 52.4% and 9.2% respectively. The mature seed production of eelgrass was 1859 seeds·m^-2.

Key words: phosphorus use efficiency, phosphorus balance, oasis irrigation region., Chinese cabbage, phosphorus threshold