Program

Dissolved organic nitrogen (DON) is the largest nitrogen pool (second to nitrate) in the ocean, in which nitrogen limits primary productivity. In coastal seas, pristine nitrogen cycle had been significantly altered by increasing riverine inputs of nitrate and DON in past decades. The spatial distribution of inorganic nitrogen had been focused for many years, however, the biogeochemical roles and behaviors of DON, including sources, fates and biological alteration in addition to conservative physical mixing, was less explored, particularly, from isotope perspective. We investigated the spatial distribution and isotopic compositions of DON and PN in the Changjiang plume during summer 2011 under the influence of large freshwater influx and intensive biological activities. In total nitrogen pool, DIN accounted for 66.3 1.7-150.9 mmol/L), followed by DON (25.6±12.1%, 2.2-32.7 mmol/L), and PN (8.1±9.1%, 0.2-140.8 mmol/L). The highest DON and PN concentrations were found in the river mouth due to sediment resuspension and riverine input. The d¹⁵N values varied significantly among nitrogen species.  Differing from previous studies in the upper oceans in the North Atlantic and North Pacific Oceans, substantially lower d¹⁵N_DON values (-9.0‰ to 5.6‰ with a mean of -0.1±2.7‰, n=58) relative to d¹⁵N_PN (-2.9‰ to 5.0‰ with a mean of 2.0±1.6‰, n=101) were observed suggesting remineralized DON was not the major nitrogen source for phytoplankton growth alues were lower than 5.0‰, which was the typical value of marine origin PN, clearly indicated the importance of local formation and recycle rather than marine origin in PN.