Changes in seawater N : P ratios in the northwestern Pacific Ocean in response to increasing atmospheric N deposition: Results from the East (Japan) Sea
Limnol. Oceanogr., 58(6), 2013, 1907-1914 | DOI: 10.4319/lo.2013.58.6.1907
ABSTRACT: An increase in seawater N : P ratios due to enhanced atmospheric N deposition has recently been reported in the northwestern Pacific Ocean, including the East (Japan) Sea. In the Pacific Ocean, the East Sea is an ideal site to examine this effect since it is fed by the Pacific surface water, which has very low N concentrations relative to P concentrations (N : P < 5), and is located in the downwind area of major N emission sources in China and Korea. In addition, the East Sea is semi-enclosed, without any major river inputs, resulting in a long surface-water residence time (∼ 2 yr for the upper 0–200 m layer). Vertical and horizontal distributions of nutrients determined in the East Sea from 2001 to 2009 showed that the N : P ratios were always lower than 10 in the mixed layer (10–30 m) and constant (N : P = 13 ± 1) in the deep ocean (200–3500 m). A simple mass-balance box model shows that the combined effects of physical (the rapid ventilation of the surface water with a N : P ratio of < 10 into the deep ocean) and biological (remineralization of sinking organic matter) processes result in the low N : P ratios (∼ 13) over the deep water column of the East Sea. This model predicts that N : P ratios in the East Sea will be lower than 14 within the predictable range of atmospheric N input in the next 100 yr, indicating that the N-limited condition in the northwestern Pacific Ocean cannot be shifted to the P-limited condition in the near future.