Changes in seawater N : P ratios in the northwestern Pacific Ocean in response to increasing atmospheric N deposition: Results from the East (Japan) Sea

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 5 13 6 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.

Long‐term decrease in phosphate concentrations in the surface layer of the southern Japan Sea

“…The increase in atmospheric nitrogen deposition [ Kim et al ., ], lowering the nutrient supply from deeper layers [ Aoyama et al ., ; Ono et al ., ; Tadokoro et al ., ; Watanabe et al ., ], and horizontal advection [ Kim et al ., ; Kim and Kim , ] were recognized as possible mechanisms behind the declining phosphate trend in the JS. Initially, the increase in the atmospheric nitrogen supply should enhance phytoplankton growth and phosphate consumption and, as the result, the phosphate concentration should decrease.…”

“…Therefore, the phosphate decline is unlikely to be controlled by atmospheric deposition. Kim and Kim [] also rejected the effect of atmospheric nitrogen deposition in the JS, and the wet nitrogen deposition only increased by 2% per year (on average) from 1991 in the south‐western JS [ Kitayama et al ., ]. Therefore, the impact of atmospheric deposition on the JS's long‐term nutrient trends is very limited in the Tsushima Warm Current.…”

“…[] indicated that the Tsushima Strait water is advected to the offshore region of the Tsugaru Strait, based on the distribution of the subsurface nutrient inversion structure within one stratified season (May–October) by the Tsushima Warm Current. Moreover, Kim and Kim [] pointed out the importance of horizontal advection in long‐term nutrient variations in the JS, but the effects of horizontal advection were not demonstrated in the previous studies mentioned above. Therefore, in this study, the long‐term trends of nutrient concentrations in the surface Tsushima Warm Current (southern JS) were reinvestigated, and the impact of horizontal advection, atmospheric deposition, and the development of stratification on nutrient dynamics were evaluated.…”

“…However, Kim and Kim [] used a box model and nutrient concentration data sets to conclude that the increase in excess nitrate concentrations is not predominantly due to atmospheric deposition in the JS. Additionally, nutrient conditions in the East China Sea (ECS) strongly affect the surface layer of the southern JS because of horizontal advection of the Tsushima Warm Current from the Tsushima Strait to the Tsugaru and Soya straits [ Kodama et al ., ; Takikawa et al ., ].…”

Macroscale‐wide nutrient inversions in the subsurface layer of the Japan Sea during summer

“…In our observations, the nutrient concentration at the surface was depleted or low (Figure ), and the surface chlorophyll fluorescence was not high (Figure d), indicating that the nutrient concentration was not supplied from the surface. Thus, although atmospheric deposition is an important nutrient source in the Japan Sea [ Kim and Kim , , ; Kim et al ., ; Onitsuka et al ., ], nutrient vertical transport, atmospheric inputs, and photochemical processes are not considered to be the causes of nutrient inversions in the Japan Sea.…”