Imbalance between vertical nitrate flux and nitrate assimilation on a continental shelf: Implications of nitrification

The nitrate assimilation rate and diapycnal nitrate flux were simultaneously determined on the continental shelf of the East China Sea (ECS). Further, the archaeal amoA gene was quantified to examine the potential distribution of nitrification activity. Nitrate assimilation rates and distribution of the archaeal amoA gene were also investigated in the Philippine Sea and in the Kuroshio Current. At all the stations, while the surface nitrate was depleted (<0.1 μ M), active nitrate assimilation was observed with mean rates of 1400, 270, and 96 μ molN m −2 d −1 in the ECS, the Philippine Sea, and the Kuroshio Current, respectively. Archaeal amoA was observed at shallower light depths, namely at or above 10% light depth, in the ECS than in other regions, suggesting that nitrification occurred within the euphotic zone in the ECS, especially on the shelf. Moreover, a station on the continental shelf of the ECS exhibited a considerable discrepancy between the nitrate assimilation rate (1500 μ molN m −2 d −1 ) and vertical nitrate flux (98 μ molN m −2 d −1 ). Here, 6.7 ± 3.1 × 10 3 and 2.5 ± 0.7 × 10 5 copies mL −1 of archaeal amoA were detected at 10% and 1% light depths relative to the surface, respectively. Thus nitrification within the euphotic zone would be attributed at least in part to the observed discrepancy between nitrate assimilation and vertical flux. These observations imply that the assumption of a direct relationship between new production, export production, and measured nitrate assimilation is misplaced, particularly regarding the continental shelf of the ECS.