Increasing nitrogen abundance relative to phosphorus in the northwestern Pacific Ocean due to anthropogenic nitrogen deposition

The increased uses of fossil fuel and agricultural fertilizer have rapidly altered nitrogen availability in many ecosystems over the last several decades. In particular, the coastal waters downwind of highly populated and industrialized areas have been receiving ever higher inputs of anthropogenic nitrogen via rivers and the atmosphere. One of those areas is the coastal and marginal seas of the Northwestern Pacific, which includes the East China Sea, the Yellow Sea and the East/Japan Sea. In this study, we document increases in the availability of nitrogen over phosphorus in these seas by analyzing temporal trend of surface (<50 m) N* (N - R_N:P × P, where N, P and R_N:P are nitrate concentration, phosphorus concentration, and the mean N:P ratio, respectively). The nutrient data used in the study were collected by the National Fisheries Research and Development Institute in Korea and the Japan Meteorological Agency during the past three decades. The observed N* increase, which was mainly due to an increase in nitrate concentration, was attributed to the riverine and atmospheric input of anthropogenic nitrogen. The N* values in two major rivers (Changjiang and Han River) discharging to the East China Sea and Yellow Sea have increased to >100 μM over the last thirty years. The riverine N* values were compared to those in the selected sea areas where riverine influence is probably greatest and found significant correlations in downstream of the Changjiang River (but not in the Han River). However, the influence of nitrogen flux from the Changjiang River appears to be confined to its estuary based on the analysis of plume extent and nutrient distribution in summer 1998 when the Changjiang River discharge was the greatest observed since the 1950s. We estimated atmospheric nitrogen deposition in the three air monitoring stations in Korea and one in Japan using the data measured by the Acid Deposition Monitoring Network in East Asia and the Korea Meteorological Administration. The estimated atmospheric nitrogen deposition had a high temporal correlation with N* in the study area (r = 0.69-0.88), except in the areas wherein riverine N load showed significant positive correlations. This suggests atmospheric nitrogen deposition as a major cause of the N* increase. In particular, total N deposition since 1980s agreed well with the change in N* inventory for the same period in the East/Japan Sea where no major river flows. Finally, the increase in N availability caused by atmospheric deposition and riverine input has switched the extensive parts of the study area from being N-limited to P-limited. These shifts may influence the composition of phytoplankton and, in the long run, the structure of the ecosystem. Our finding implies similar trends in nitrogen availability in the US east coast and the European coast, which have received ever-increasing nitrogen inputs via the atmosphere and rivers.