Seasonal variations in the nitrogen isotopic composition of dissolved nitrate(δ 15 NO 3) were investigated in the surface waters of the Changjiang River estuary in 2009 and 2010. δ 15 NO 3 varied between-4.6‰ and 8.9‰ with changes in temperature, salinity, dissolved oxygen concentrations, and the composition of the dissolved inorganic nitrogen pool. In February, biological processes decreased because of low temperature, and the mean δ 15 NO 3 near the river mouth was 2.4‰. In May, δ 15 NO 3 was the highest in the surface waters among all seasons. Analysis on the conservative mixing revealed assimilation, and this fi nding is supported by positive relationship between Chl a and δ 15 NO 3. The fractionation factor of assimilation was estimated to be 2.0‰ by the Rayleigh equation. Nitrifi cation was supported based on the mixing behaviors in November 2010 and the low δ 15 NO 3 values in May and November 2009. The high ammonium concentrations in the adjacent marine area and positive relationships between total organic nitrogen and δ 15 NO 3 in November 2010 indicated that mineralization was taking place. Seasonal variations in the nitrogen isotopic composition of dissolved nitrate (δ 15 NO 3) were investigated in the surface waters of the Changjiang River estuary in 2009 and 2010. δ 15 NO 3 varied between-4.6 ‰ and 8.9 ‰ with changes in temperature, salinity , dissolved oxygen concentrations, and the composition of the dissolved inorganic nitrogen pool. In February, biological processes decreased because of low temperature, and the mean δ 15 NO 3 near the river mouth was 2.4 ‰. In May, δ 15 NO 3 was the Analysis on the conservative mixing revealed assimilation, and this fi nding is supported by the positive relationship between Chl a and δ 15 NO 3. The fractionation factor of assimilation was estimated to be 2.0 ‰ by the Rayleigh equation . Nitrifi cation was supported based on the mixing behaviors in November 2010 and the low δ 15 NO 3 values ​​in May and November 2009. The high ammonium concentrations in the adjacent marine area and p ositive relationships between total organic nitrogen and δ 15 NO 3 in November 2010 indicated that mineralization was taking place.