A box model based on nitrogen balance for fish-shellfish-macrophytes polycul-ture in coastal embayment was developed, and applied for Xiangshan Harbor, China. Simulated phytoplankton, zooplankton and nitrogen concentration were consistent with the measured data of Xiangshan Harbor. The results showed that the seasonal variation of biological production rate led to the mutual transformation of nutrients (such as nitrogen) between dissolved inorganic form and detritus form, and the transformation showed an obvious annual oscillation. The phytoplankton reached the peak amount in summer and showed the lowest amount in winter, while the DIN exhibited an opposite variation pattern. Total nitrogen input into Xiangshan Harbor was about 2 182 tonN/a, coming equally from aquaculture and sewage. The nitrogen output from the bay was,by decreasing order,water exchange with ambient water, aquaculture harvest and fishing. At present, the nitrogen input is larger than the output (about 11%) , causing the DIN concentration increase of about 42jμgN/L per year in the bay. The model was used to ana-lyze long-term (20 years) influence of different polyculture modes on the DIN concentration in the bay. It shows that DIN concentration in Xiangshan Harbor will slowly increase and the input of nitrogen tends to be balanced with output for the coming several years under current aq-uaculture mode. If the fish culture is reduced to 40% of the current scale or the macrophytes culture is increased about 20 times of the current scale, the DIN concentration in the bay would gradually decrease and fit the national Grade Ⅲ water quality standard in about 14 years.%建立了基于氮通量物质平衡的象山港海湾养殖生态系统箱式模型.模型模拟的象山港主要生态变量的变化动态与实测数据资料相吻合.模型计算结果表明,生物生产过程的季节变化导致了营养要素(如氮)在可溶性无机状态和有机碎屑状态中的相互转化并呈现周年振荡.象山港总氮输入约2 182 tonN/a,其中养殖输入和排污输入约各占一半；氮输出途径按比例依次为水交换扩散、养殖业收获和渔业捕捞收获.目前象山港氮输入大于氮输出(约11％),造成海湾无机氮浓度年均升高约42μgN/L.利用模型对不同多品种养殖模式下无机氮浓度的长期变化趋势进行了预测.当前养殖模式下,象山港内无机氮浓度增长幅度较小且逐渐趋于输入输出平衡,而要使象山港海水水质达到国家Ⅲ类标准则需要减少鱼类养殖规模至当前规模的约40％或增加藻类养殖规模至当前养殖规模的20倍.
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