Above- and below-ground productivities and tissue N content were measured monthly to quantify N incorporation to sustain eelgrass growth in Koje Bay on the south coast of Korea from January to December 2002. N acquisition was also estimated through measurements of N uptake kinetics, tissue biomass, and in situ inorganic N concentrations in water column and sediments. Above- and below-ground productivities were highest in summer and lowest in late fall and winter. Leaf tissue N content was highest in December and lowest in July, while rhizome tissue N content was highest in October and lowest in April. Estimated monthly N incorporation by leaf tissues based on the leaf productivity and N content ranged from 0.4 g N $m^{-2}$ $month^{-1}$ in November to 2.0 g N $m^{-2}$ $month^{-1}$ in May. N incorporation by below-ground tissues ranged from 0.1 g N $m^{-2}$ $month^{-1}$ in February to 0.2 g N $m^{-2}$ $month^{-1}$ in October. Annual whole plant N incorporation was 14.5 g N $m^{-2}$ $y^{-1}$, and N incorporation by leaf tissues accounted for about 87 % of total N incorporation. Maximum uptake rate ($V_{max}$) and half saturation constant ($K_m$) of leaf $NH_4{^+}$ uptake were significantly lower than those of root $NH_4{^+}$ uptake. Above- and below-ground biomass ranged from 20.8 g DW $m^{-2}$ and 8.6 g DW $m^{-2}$ in winter to 350.0 g DW $m^{-2}$ and 81.3 g DW $m^{-2}$ in spring, respectively. $NH_4{^+}$ concentrations varied from 0.2 to 4.3 mM in water column and from 93.0 to 551.7 mM in sediment pore water. Based on these measurements, annual N acquisition by root tissues contributed slightly higher than that by leaf tissues to total plant N acquisition. During winter, monthly leaf N acquisition was lower than monthly leaf N incorporation. This implies that Z. marina has internal nitrogen retention system to offset the shortage and excess of nitrogen.