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Tracing the sources of nutrients through the Tsushima/Korea Strait
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Jing Zhang1, 2, Xinyu Guo3, Lei Zhu2, Jianlong Feng1, 2, Liang Zhao1, 2, *
Acta Oceanologica Sinica | 2024, 43(6) : 142 - 152
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Acta Oceanologica Sinica | 2024, 43(6): 142-152
• Articles •
Tracing the sources of nutrients through the Tsushima/Korea Strait
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Jing Zhang1, 2, Xinyu Guo3, Lei Zhu2, Jianlong Feng1, 2, Liang Zhao1, 2, *
Affiliations
  • 1 Key Laboratory of Marine Resource Chemistry and Food Technology, Ministry of Education, Tianjin 300457, China
  • 2 College of Marine and Environmental Science, Tianjin University of Science and Technology, Tianjin 300457, China
  • 3 Center for Marine Environmental Studies, Ehime University, Matsuyama 790-8577, Japan
Published: 2024-06-25 doi: 10.1007/s13131-024-2372-1
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The nutrients from the East China Sea (ECS) through the Tsushima/Korea Strait (TS) strongly impact the ecosystem of the Japan Sea (JS). The complex origins of the Tsushima Warm Current and the various nutrient sources in the ECS result in complex spatial-temporal variations in nutrients in the TS. Using a physical-biological model with a tracking technique, we studied the effects of nutrient sources from the ECS on the TS. Among all the nutrient sources, the Kuroshio has the highest nutrient concentrations in the TS. Its maximum concentration occurs at the bottom, while those of rivers and atmospheric deposition occur at the surface, and that of the Taiwan Strait occurs in the middle layer. The nutrient transport through the TS exhibits similar seasonal variations, as does the volume transport. The transport of nutrients from the Kuroshio accounts for more than 85% of the total. The transport of nutrients from the Taiwan Strait is greater during autumn and winter. The transport of dissolved inorganic nitrogen (DIN) from both rivers and atmospheric deposition through the TS peak in August. Nutrient transport cannot be equated with volume transport. The DIN in the less saline zone originates not only from rivers but also from atmospheric deposition and the Kuroshio. The transport of nutrients from the Taiwan Strait is not as significant as its volume transport in the TS.

dissolved inorganic nitrogen  /  dissolved inorganic phosphate  /  East China Sea  /  Japan Sea  /  nutrient transport
Jing Zhang, Xinyu Guo, Lei Zhu, Jianlong Feng, Liang Zhao. Tracing the sources of nutrients through the Tsushima/Korea Strait[J]. Acta Oceanologica Sinica, 2024 , 43 (6) : 142 -152 . DOI: 10.1007/s13131-024-2372-1
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1 Introduction
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The Tsushima/Korea Strait (TS) is the connection between the East China Sea (ECS) and the Japan Sea (JS), two highly productive marginal seas in the western North Pacific Ocean (Kwak et al., 2014; Yamada et al., 2005; Lee et al., 2009). Tsushima Island divides the strait into eastern and western channels (ETS and WTS). Currents through the TS, mainly the Tsushima Warm Current (TWC), transport large amounts of water, heat, and materials from the ECS to the JS (Yanagi, 2002; Kodama et al., 2015). The nutrients supplied from the TS, especially, support 70%−80% of primary production in the coastal regions of the JS and have a great impact on the ecological environment in the JS (Onitsuka et al., 2007; Shibano et al., 2019).
The currents through the TS show strong spatial and temporal variations. The average volume transport is 2.64 × 106 m3/s according to long-term ADCP measurements (Shin et al., 2022). The properties in the TS are influenced by many water masses upstream. Due to the connectivity among the straits over the western North Pacific Ocean, the Taiwan Warm Current and the Kuroshio are the main sources of the water flowing through the TS (Teague et al., 2003). Isobe (1999) suggested that the TWC originates from the Taiwan Strait, except in the fall when the Kuroshio branch southwest of Kyushu is strong and takes its place. By passive tracer experiments in a numerical model, Guo et al. (2006) found that in summer, the proportions of water from the Kuroshio and the Taiwan Strait in the TS are equal, but in winter, the ratio becomes 80% and 20%, respectively. Cho et al. (2009) also obtained similar ratios for the Kuroshio and the Taiwan Warm Current. Thus, the circulation patterns here are complex and characterized by the interaction between shelf water and the Kuroshio.
The TWC brings large amounts of nutrients from the ECS to the JS. Based on a box model, Zhang et al. (2007) estimated that the dissolved inorganic nitrogen (DIN) fluxes through the TS are 14.2 kmol/s and 18.1 kmol/s in summer and winter, respectively. The DIN fluxes transported through the ETS in summer-autumn are approximately 4.5–4.7 kmol/s, accounting for more than 60% of the total transport in a year (Morimoto et al., 2009). The transport of DIN through the western channel is comparable to that through the eastern channel, with an even greater flux density (Wang et al., 2019).
Complex water sources result in a wide variety of nutrient sources. In addition to the Kuroshio and Taiwan Strait, nutrients are also supplied from rivers and atmospheric deposition. Furthermore, several rivers from China and Korea’s coasts enter the JS through the TS (Isobe et al., 2002). The main river is the Changjiang River, which contributes approximately 1% of the volume transport across the TS, with an annual mean discharge of 0.03 × 106 m3/s. Approximately 68% of the Changjiang Diluted Water (CDW) extends to the TS during summer (Chang and Isobe, 2003). Because riverine nutrients usually exhibit high concentrations, the CDW is thought to be a significant nutrient source for nutrients in the middle and outer shelves of the ECS in summer (Kim et al., 2009; Bi et al., 2018). To what extent the riverine nutrients affect the TS and even the JS is unknown, atmospheric deposition is thought to be crucial to the health of ecosystems in the western North Pacific Ocean and has recently gained increasing attention (Kim et al., 2011). With a dramatically high N/P ratio, the excess DIN from atmospheric deposition in the ECS can be transported to the JS through the TS. While, Kim et al. (2013) thought the nutrients from Changjiang River contributed more than the atmospheric deposition.
Both complex hydrodynamics and various nutrient sources result in complex spatial-temporal variations in nutrients in the TS. It is also unknown which and to what extent the nutrient sources from the ECS affect the JS through the TS. In this study, we investigated the nutrient sources in the TS by a tracking technique with a physical-biological model. Section 2 provides a description of the method and model validation. Section 3 presents the distributions of the nutrient concentrations and fluxes at the TS and the seasonal variations in nutrient transport through the TS. Section 4 discusses the role of different nutrient sources in the TS and the JS. Section 5 presents a summary of this study.
2 Methods
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2.1 Physical-biological coupled model with a tracking technique
The model domain spans the Bohai Sea, the Yellow Sea, and the ECS between 24°N and 41°N and between 117.5°E and 131.5°E (Fig. 1). The model is composed of a hydrodynamic module and a biological module. The hydrodynamic module is based on the Princeton Ocean Model, with a horizontal resolution of (1/18)° and a sigma level of 21. The low-trophic-level biological module was constructed for the ECS based on NORWECOM (Skogen et al., 1995; Skogen and Søiland, 1998). The configurations of the model in this study are generally the same as those of Zhao and Guo (2011) and Wang et al. (2019). The differences lie in the hydrological boundary conditions, which are changed to update the regional model results. The biological module takes into account three nutrients—DIN, dissolved inorganic phosphorus (DIP), and silicate; two phytoplankton species (CHL)—diatom (DIA) and flagellate (FLA); and two biogenic organic materials—dead organic matter (DET) and biogenic silica. Phytoplankton growth depends on nutrients, light and temperature. Detritus species include dead phytoplankton. The nutrients are restored via detritus remineralization and phytoplankton respiration. The dissolved organic phosphorus is not included in the biological model, which may affect the utilization of the phytoplankton (Jin et al., 2024).
The physical-biological coupled model is forced under climatological conditions. When the steady state of the model is reached, the tracking model is loaded. The tracking model was established based on Ménesguen et al. (2006) and has been applied to areas such as the ECS (Zhang et al., 2019, 2021; Große et al., 2020) and the JS (Onitsuka et al., 2007). The biological state variables share the same governing equations as the biological module but are calculated separately from different external sources. The state variables from each source are handled by a whole subset of equations (Eqs (1)−(4)). Since the physical processes for the state variables are approximately linear, the advection and diffusion terms are separated for each source of variables. The biological processes were divided according to the proportion of the nutrient concentration of each source to the overall concentration.
The primary four external sources of DIN and DIP in the ECS are atmospheric deposition (A), the Taiwan Strait (T), the Kuroshio (K), and rivers (R). The two limiting nutrients in the ECS, DIN and DIP, are tracked in two cases. The nitrogen or phosphate cycle of each source is handled separately via separate equations. As an illustration, DINK represents the DIN concentration from the Kuroshio. By assimilating DINK, phytoplankton create CHLK. CHLK turns into DETK after mortality. Further decomposition of DETK results in the regeneration of DINK.
$ \begin{split}\frac{\partial {\mathrm{DIN}}_{i}}{\partial t}=& {\mathrm{diff}}\left({\mathrm{DIN}}_{i}\right)-{\mathrm{adv}}\left({\mathrm{DIN}}_{i}\right)+{\mathrm{resp}}\left(\mathrm{FLA}\right)\times \frac{{\mathrm{FLA}}_{i}}{\mathrm{FLA}}+\\&{\mathrm{resp}}\left(\mathrm{DIA}\right)\times \frac{{\mathrm{DIA}}_{i}}{\mathrm{DIA}}-{\mathrm{pp}}\left(\mathrm{FLA}+\mathrm{DIA}\right)\times\\&\frac{{\mathrm{DIN}}_{i}}{\mathrm{DIN}}+{\mathrm{remi}}\left({\mathrm{DET}}_{i}\right),\end{split} $
$ \begin{split}\frac{\partial {\mathrm{DIA}}_{i}}{\partial t}=&{\mathrm{diff}}\left({\mathrm{DIA}}_{i}\right)-{\mathrm{adv}}\left({\mathrm{DIA}}_{i}\right)+{\mathrm{pp}}\left(\mathrm{DIA}\right)\times \\&\frac{{\mathrm{DIN}}_{i}}{\mathrm{DIN}}-{\mathrm{resp}}\left(\mathrm{DIA}\right)\times \frac{{\mathrm{DIA}}_{i}}{\mathrm{DIA}}-{\mathrm{mort}}\left({\mathrm{DIA}}_{i}\right) ,\end{split} $
$ \begin{split}\frac{\partial {\mathrm{FLA}}_{i}}{\partial t}=& {\mathrm{diff}}\left({\mathrm{FLA}}_{i}\right)-{\mathrm{adv}}\left({\mathrm{FLA}}_{i}\right)+{\mathrm{pp}}\left(\mathrm{FLA}\right)\times \frac{{\mathrm{DIN}}_{i}}{\mathrm{DIN}}-\\&{\mathrm{resp}}\left(\mathrm{FLA}\right)\times \frac{{\mathrm{FLA}}_{i}}{\mathrm{FLA}}-{\mathrm{mort}}\left({\mathrm{FLA}}_{i}\right) ,\end{split} $
$ \begin{split}\frac{\partial {\mathrm{DET}}_{i}}{\partial t}=& {\mathrm{diff}}\left({\mathrm{DET}}_{i}\right)-{\mathrm{adv}}\left({\mathrm{DET}}_{i}\right)+{\mathrm{mort}}({\mathrm{FLA}}_{i})+\\&{{\mathrm{mort}}(\mathrm{DIA}}_{i})-{\mathrm{remi}}\left({\mathrm{DET}}_{i}\right) , \end{split}$
where the subscript i represents the DIN from the ith source. The adv and diff represent the physical terms advection and diffusion. The resp, pp, remi, and mort indicate the biological terms respiration, primary production, remineralization, and mortality. The advection and diffusion terms are separated for each source of variables. The terms for biological processes are separated following the ratio of each source of DIN concentration to the total DIN concentration.
The lateral boundary conditions for nutrients from the Taiwan Strait were provided by Prof. Chen-Tung Arthur Chen (personal communication), and those for the Kuroshio east of Taiwan Island were obtained from the Japan Meteorological Agency observation data. The nutrient concentrations for ten rivers were obtained from observations at 1980s (Zhang, 1996; Liu et al., 2009). The nitrogen concentration of wet atmospheric deposition and dry atmospheric deposition flux were from observations around 2000s (Zhang et al., 2011), and for phosphate were averaged from limited observed data in the 1960s−2000s (Zhang and Liu, 1994; Chung et al., 1998; Liu et al., 2000; Zhang et al., 2004). The model area is divided into three regions: the Bohai Sea, Yellow Sea and ECS, and a single value for phosphorus atmospheric deposition without temporal and spatial variations is given for each region. So, the simulation presents a climatological state of the ecosystem in the ECS before the 2000s. At the start of the tracking model run, the DIN or DIP concentration, phytoplankton concentration, and detrital concentration of the four external sources are all set to zero. Until the tracking state variables reached a stable state, the tracking module was run for more than 5 years. The results from the last year are examined in this study.
2.2 Validation of the model results in the TS
The ability of the physical-biological coupled model to reproduce the seasonal variations in physical and biological variables across the ECS has been fully confirmed by Zhao and Guo (2011), Wang et al. (2019), and Shen et al. (2021). Wang et al. (2019) reported good agreement between the modeled and measured vertical distributions of DIN and CHL concentrations across the ETS (Morimoto et al., 2009). The concentrations of DIN and CHL in a section in the northern ECS (i.e., the section upstream of the TS) from the model results shown by Zhang et al. (2019) were similar to previous observations (Umezawa et al., 2014). The tracking model has also been used to evaluate the roles of nutrients from different sources in ecosystems of the ECS (Zhang et al., 2019, 2021).
The section of the TS (Fig. 1b) is designed to be the same as the observation line in Takikawa et al. (2005). The red pentagram separates the eastern and western channels. Here, we mainly focus on the validation of the model results in section TS. A comparison of the volume transport through section TS from the model results and observations (Shin et al., 2022) is shown in Fig. 1c. Both of them are highest in October and lowest in January, suggesting similar seasonal variations. The annual means of the volume transport from the model results and observations are 2.62 × 106 m3/s and 2.64 × 106 m3/s, respectively, whose differences are less than 1%.
Then, we compare the vertical distributions of the velocity that is normal to the section, as well as the temperature, salinity, DIN concentration, DIP concentration, and CHL concentration in the TS section from the model results (Fig. 2) with the observations in the ETS in August and October from Morimoto et al. (2009). The velocities from both the model and observation results in August are the greatest (more than 20 cm/s) where the water depth is the greatest in the ETS (Fig. 2a). On the western side of this current core, a countercurrent appears with similar strength. The pattern of currents in October is comparable to that in August but with a weaker countercurrent (Fig. 2b). The core regions of both the WTS and ETS have two northeastward current maximums, with the maximum velocity of the WTS being greater than that of the ETS. The temperature in August is stratified according to either the observations or the model results (Fig. 2c). The surface temperature is higher than 24℃, and the bottom temperature is less than 12℃. In October (Fig. 2d), the stratification in the upper 40-m layer of the ETS decays, and the bottom temperature increases compared to that in August. In August, the salinity is relatively low in the surface layer on the western side of the TS, with observations and model results of approximately 31 (Fig. 2e). The highest salinity is greater than 34.2. The low-salinity zone narrows in October and is confined only to the upper WTS (Fig. 2f). The discharge of freshwater from the Changjiang River in the ECS is the most likely cause of the low salinity in the WTS from August to October (Chen et al., 1994; Takikawa et al., 2005). The vertical variations in salinity in the ETS are small and are also relatively consistent with the observations.
The chlorophyll concentration in August clearly reaches a maximum in the subsurface layer according to both the observation and model results, with concentrations greater than 1 mg/m3 (Fig. 2g). However, in October, the chlorophyll concentrations above a depth of 50 m are relatively high according to both the observation and model results, with a maximum in the ETS coastal zone (Fig. 2h). The minimum DIN concentration in August is less than 1 mmol/m3, and the maximum DIN concentration is greater than 10 mmol/m3. Both the variation and the vertical structure are consistent with the observations (Fig. 2i). The range of change in the DIN concentration in October is comparable to that in August, except that the gradient is less pronounced in the vertical direction; in particular, there is little variation in the low-concentration zone in the upper 40 m layer (Fig. 2j). The distributions of DIP in the TS are generally comparable to those of DIN. The minimum DIP concentration is lower than 0.1 mmol/m3, and the maximum DIP concentration is higher than 0.8 mmol/m3 (Figs 2k and l).
3 Results
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3.1 Vertical distributions of DIN and DIP concentrations in the TS section
Figure 3 displays the vertical distribution of DIN from each source. The four columns, which correspond to the four seasons, are February, May, August, and November. One DIN source appears in each row, and the total DIN (DINW) appears in the final row.
The DINR appears in the first row. From winter to spring, the concentration is quite low across the whole section. The highest DINR concentration (up to 2 mmol/m3) is found in the upper 50-m layer of the WTS in summer, mainly resulting from the strong northeastward CDW. The DINR concentration in the surface layer of the middle ETS is also greater than 0.50 mmol/m3. In autumn, the DINR high-concentration zone is located in the coastal WTS, and its concentration decreases compared to that in summer with the retreating CDW.
The concentration of the DINA is slightly greater than that of the DINR. In winter, the high-value region is concentrated in the lower layer of the WTS. These DINA are not newly supplied from the sea surface but instead are locally regenerated or/and remotely conveyed. In spring, the DINA has an unapparent three-layer structure in both the WTS and ETS, with a greater concentration in the top 10 m layer and around the bottom layer. In summer, there is a noticeable increase in the DINA concentration in the upper layer of the entire TS section due to the strong atmospheric deposition input. The patterns of the DINA throughout the fall are identical to those of the DINR.
The DINK concentration was the highest of the four DINs. Its content in winter is an order of magnitude greater than that of the other DINs, ranging from 3 mmol/m3 to 6 mmol/m3. The DINK is vertically uniform throughout the whole section due to strong vertical mixing. In the spring, the concentration disparity between the top and bottom layers increases in both channels, especially in the ETS. The maximum concentration of DINK (at least 8 mmol/m3) in spring occurs in the bottom layer of the ETS. In summer, the bottom of both channels exhibits a high concentration of DINK. In autumn, the DINK again mixes well within the upper 50-m layer in the vertical direction. The DINK in the lower layer is always greater than that in the upper layer, in contrast to the DINR, DINA, and DINT. However, the nutrients at the Kuroshio main axis show the similar patterns (Guo et al., 2012). The Kuroshio intrusion through the TS section is also stronger at the bottom (Guo et al., 2006).
The DINT has little seasonal or vertical fluctuation and is on the same level as the DINR and DINT. Its concentration and distribution pattern during the winter and spring are comparable to those of DINA. The main difference between them is that they lack a high value in the surface layer in spring. During the summer, the DINT concentration does not substantially differ across the entire section. In the WTS, a weak, highly concentrated core can be observed at a depth of approximately 50 m rather than at the surface layer, as in the DINR and DINA, or at the bottom layer, as in the DINK. In the autumn, the content of the DINT summer core increases to more than 0.9 mmol/m3. In contrast, the DINT is typically vertically homogenous in the ETS. The DINT concentration reaches its peak in autumn, whereas DINT transport through the Taiwan Strait is the greatest in summer following volume transport.
The proportion of DIN from a single source to DINW is shown by the black contour lines in the first four rows. The DINR reaches more than 20% in only limited areas of the upper 20-m layer in summer. Otherwise, its proportion is low, especially during the winter, when it decreases to less than 5%. Although DINA is low in winter as well, it makes up more than half of the DINW in the upper 10-m layer in spring and the upper 20-m layer in summer. The percentage of DINA drops as the water depth increases.
Almost the entire section is under DINK dominance during the winter. This dominance is also present in the spring at a layer deeper than 20 m in the ETS and in the summer and fall at the bottom 50 m layer. As the water depth increases, the proportion of DINK also increases. The percentage of DINT is only approximately 5% in winter and spring and is as low as that of DINR. In the middle layer of the ETS during summer and in the middle layers of the WTS and ETS during fall, there is a tiny cluster of DINT with more than 20% occupancy. However, temporally and spatially, it makes minimal contributions to the DINW during the rest of the period.
The DINW concentration exhibits nearly identical seasonal variations to those of DINK, which is the predominant concentration. The DINW mixes well in the top 50-m layer throughout the winter. As a result of the vigorous primary production in spring, the DINW concentration in the upper 30-m layer decreases. The DINW concentration increases in the bottom layer and peaks in summer. The DINW develops a core with a greater concentration in the upper 10-m layer due to the combined impact of the DINR and DINA near the surface of the WTS in summer. In autumn, the DINW in the upper layer barely changes in the vertical direction. The DINW concentration in the western channel is greater than that in the eastern channel due to the greater water depth and the intrusion of a water mass with high DIN concentrations from the JS (Shibano et al., 2019).
An N/P ratio of 16 is indicated by the blue contours in the final row for reference. The letters “H” and “L” represent N/P ratios higher than 16 and lower than 16, which correspond to the limiting nutrients, phosphate and nitrogen, respectively. In winter and spring, the N/P ratios in the whole section range from 2 to 12, indicating a nitrogen limitation. However, in the upper 20~30 m layer in summer and the upper 20 m layer of the WTS in autumn, the N/P ratio is greater than 16. Overall, nitrogen is the potential limiting factor for primary production in section TS.
The DIP concentrations of the different sources in section TS are shown in Fig. 4. Both the DIPR and DIPA concentrations in section TS are very low, with values less than 0.005 mmol/m3 due to the high N/P ratios of both atmospheric deposition and riverine sources (Zhang et al., 2007).
However, DIPK is the complete opposite of DIPR and DIPA; hence, DIPK occupancy is substantially greater than DINK occupancy. DIPK is still vertically homogenous in winter and accounts for approximately 90% of the total occupancy. In comparison to DINK in spring, the low-concentration zone of DINK found in the surface layer does not develop for DIPK. The surface layer is a nitrogen-limited area, and phosphorus is not used efficiently. The DIPK in the ETS also had a concentration of more than 90% DIPW. In summer, the concentration of DIPK is less than half of the DIPW in the upper 10 m at the WTS and upper 30 m at the ETS. DIPK still comprises more than 90% of the total at depth below approximately 50 m in both channels. By autumn, the DIPK concentration barely changes in the lower layer, while the DIPK concentration in the upper layer rises and returns to springtime levels.
In winter, spring, and fall, the DIPT concentration in the WTS is greater than that in the ETS. The maximum value is observed at the same location as that of DINT, which is at a depth of approximately 50 m in the coastal zone adjacent to the WTS, and the DIPT accounts for more than 20% of the DIPW in this area. However, in summer, the DIPT concentration is greater in the ETS than in the WTS. It dominates the upper 40 m layer of the ETS with more than 50% occupancy, which is very different from the weak state of DINT.
While DIPW and DINW exhibit different patterns, they are both characterized by vertical homogeneity in the winter and increasing concentrations with depth in the spring, summer, and fall. Differences between DIPW and DINW are found in the various phosphorus and nitrogen limitation zones. The DIPW concentration does not significantly differ between the upper layer and the lower layer in spring when the entire section is nitrogen limited. At the same time, the DINW concentration decreases substantially in the euphotic layer due to primary production. Similarly, the DIPW concentration greatly decreases throughout the summer in the phosphorus-limited area in the upper layer, where the DINW is in excess.
3.2 Vertical distributions of DIN and DIP fluxes in the TS section
To evaluate the effects of different nutrient sources on the ecosystem of the JS, the nutrient concentration in section TS alone is not sufficient. Hence, we present the vertical distributions of the DIN and DIP downstream fluxes in Figs 5 and 6. The nutrient flux is calculated as the product of the nutrient concentration and the velocity component normal to the section, demonstrating the vertical patterns of nutrient transport. A positive value indicates a northeastward nutrient flux entering the JS. The current structures in section TS exhibit weak seasonal variation. In the middle of the eastern and western channels, there are two maximums of the northeastward current (Figs 2e and f). The maximum velocity is greater in the WTS than in the ETS. Between the WTS and ETS, a southwestward countercurrent is present, which is associated with two cyclonic and anticyclonic eddies (Takikawa et al., 2005). Additionally, there are weak countercurrents at the bottom of the WTS in summer and autumn and in the eastern ETS.
The DIN flux is primarily in the JS direction since its positive area is wider and has greater flux values than does its negative area (Fig. 5). The DINR and DINA fluxes share similar vertical distributions. Their maximums (>0.5 mmol/(m2·s)) appear in summer at the surface of the WTS, where both the velocity and concentration are high.
During the winter, the DINK flux remains largely uniform in the vertical direction, with greater values in the center of the two channels. The DINK flux increases below a depth of 50 m after winter. The surface has a higher velocity, while the bottom has a higher DINK concentration. Hence, the core DINK flux lies between 50 m and 100 m deep, with the maximum occurring at a depth of 80 m in the WTS in autumn. The flux of DINT is at the same level as that of DINR and DINA. A weak core of the DINT flux appears at the 40-m depth of the WTS in autumn, which corresponds to the largest DINT concentration. The distribution of DINW is mostly similar to that of DINK, except for the upper layer in summer, which is superimposed on DINR and DINA and hence has larger values. The positive DINW flux is the strongest in autumn, and the negative value between the two channels is also the largest due to the eddy effect.
The vertical distributions of DIP fluxes from different sources are shown in Fig. 6. Because of the low concentrations of DIPR and DIPA, their fluxes are similarly quite small. The patterns of the DIPK flux are comparable to those of DINK. Although there is not much difference in DIPK concentrations between the WTS and ETS, the WTS has a stronger flux because of the higher velocity. The DIPT flux is slightly greater than that of DINR and DINA and peaks at the 50-m layer of the WTS in autumn as DINT. With the relatively minor effects of the other three DIP sources, the pattern of DIPW fluxes is more consistent with that of DIPK.
3.3 DIN and DIP transports across the TS section
The transport across the TS section is obtained by integrating the entire flux, as illustrated in Figs 5 and 6, which indicate the combined effect of volume transport and nutrient concentrations (Fig. 7). Figure 1c shows the seasonal fluctuations in volume transport, with a minimum in January and a maximum in October.
As shown in Fig. 7a, the transports of DINR, DINT, and DINA are at the same level. The mean values of these parameters, which correlate to the axis on the left, are 0.33 kmol/s, 0.87 kmol/s, and 0.97 kmol/s. In the winter and spring, the DINR transport is quite low. It starts to rise after May and reaches its peak in August. Freshwater transport through the TS increases noticeably in the summer and fall, two or three months after river discharge (Isobe et al., 2002). The seasonal trend of DINA transport is fairly similar to that of DINR, although the DINA values are almost three times greater. The DINT transport remains at a low level in spring and rapidly increases from June to a maximum in December. While volume transport from the Taiwan Strait is highest in summer, DINT transport peaks later.
The annual mean transport of DINK is 11.86 kmol/s, which is an order of magnitude greater than those of the other three sources and accounts for 85% of the total transport. Its seasonal oscillation is consistent with that of the volume transport, which is greatest in October and smallest in January. The DINW transport is 14.03 kmol/s, which is comparable to the estimated value of 16.20 kmol/s (Zhang et al., 2007). In winter and spring, almost all of the DINW transport comes from the Kuroshio, and in summer and fall, it is supplemented partly by atmospheric deposition, rivers, and the Taiwan Strait.
The DIPW transport has an annual mean of 1.15 kmol/s, which is similar to the value of 0.96 kmol/s from Zhang et al. (2007). The seasonal fluctuation in DIPW is fairly similar to that in DINW, except that there is no slight peak in summer due to the small contributions from the atmosphere and rivers. The transports of DIPA and DIPR make up less than 0.50% of the total. With an annual mean of 1.04 kmol/s, the proportion of DIPK transport reaches 90%. Its seasonal variations are similar to those of DINK. Similar to the DINT flux, DIPT transport also begins to increase after June, and maintains high levels from September through December.
4 Discussions
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4.1 DIN nutrient sources in the less saline zone
The less saline zone is found in the upper layer in summer and autumn (Figs 2e and f) and is considered to be associated with the CDW (Chang and Isobe, 2003). The CDW flows into the JS under the southeast monsoon in summer (Lie and Cho, 2016). The less saline zone corresponds to the zone with a high N/P ratio, where primary production is limited by DIP. The excess DIN in the TWC is thought to be affected by rivers in East Asia (Kodama et al., 2015, 2017).
According to our model results (Fig. 3), the DIN in the less saline zone originates not only from rivers (~20%) but also from atmospheric deposition (~50%) and the Kuroshio (~20%). The DINR does not have as much of an impact as the DINA does. The total input of DINR is 1.20 kmol/s, and the DINR transport through the TS is 0.33 kmol/s, accounting for only 28% of the input. Approximately 68% of the Changjiang River discharge is transported through the TS according to tracer experiments (Chang and Isobe, 2003). These two distinct ratios show that nutrient transport and freshwater transport cannot be equated.
While in this study, the nutrient input fluxes from the rivers remain consistent with our previous study (Zhang et al., 2019). The nutrient levels from the Changjiang River are observed value in the 1980s (Zhang, 1996), which is less than the values in recent years reported by Wu et al. (2023). A sensitivity experiment was designed specifically that changing the DIN concentration in Changjiang River from 110.20 mmol/m3 to 33.50 mmol/m3 (Zhang et al., 2021). With the increasing loading of Changjiang River DIN from the 1980s to the 2010s, the produced PON does not increase proportionally due to limited primary production. Consequently, residual DINR from the Changjiang River is transported further from the ECS to the JS, thereby increasing the environmental stress in the adjacent areas. The contribution of the DINR may be underestimated due to the low level of the input nutrient concentrations in the 1980s.
4.2 Effects of the Kuroshio and the Taiwan Strait on nutrients in the TS
The Taiwan Strait and the Kuroshio have a competing relationship when they act as the source waters of the Tsushima Warm Current. The Kuroshio branch southwest of Kyushu is the main source of the TWC in winter, while the Taiwan Warm Current also contributes to the TWC in summer (Ichikawa and Beardsley, 2002). According to a tracer experiment conducted by Guo et al. (2006), the Taiwan Strait contributes almost half of the tracer entering the Tsushima Strait during the summer, with the Kuroshio contributing the other half. The contribution from the Taiwan Strait decreases to 20%, and that from the Kuroshio increases to 80% from summer to winter. Although the volume transport in the Taiwan Strait is comparable to that in the Kuroshio onshore intrusion, nutrient transport is only approximately one-tenth of that in the Kuroshio due to low nutrient concentrations.
Compared to those of other sources, the nutrient transport from the Kuroshio Current is substantially greater. The Kuroshio branch southwest of Kyushu is an important component of the Kuroshio onshore intrusion and is also a significant source of the Tsushima Warm Currents, particularly in the fall (Isobe, 1999). The Kuroshio contributes the most to nutrient transport in the TS due to its high nutrient concentrations and proximity to direct intrusion.
4.3 Which nutrient source contributes significantly to the JS ecosystem?
The results of our model can provide an explanation for the origins of the nutrients in the TS. The impact of nutrients through the TS on the JS has been the subject of some related research. We therefore tried to clarify the effects of nutrient sources from the ECS on the ecosystem of the JS. Judging from the quantity of nutrient transport, it is clear that the nutrients from the Kuroshio through the TS have the greatest impact, particularly the DIP. However, the roles of the nutrients through the TS in the JS may have been somewhat influenced by the timing and location of the transports.
Using a similar tracking technique within a physical-biological coupled model, Onitsuka et al. (2007) studied the effect of nutrients through the TS on the JS. They found that the DIN flux through the WTS supports high primary productivity in the southwestern JS. The nitrogen flux through the ETS determines the surface nutrient conditions in the nearshore area of the Japanese coast. By examining the biogeochemical responses to various DIN fluxes through the TS, Shibano et al. (2019) demonstrated that the DIN through the TS regulates almost 70% of the DIN in the nearshore zone of the JS. The summertime DIN flux change had the greatest influence on the ecosystem of the JS. Since stratification prevents DIN from being absorbed locally at the TS, DIN through the TS can instead be delivered to a larger region of the JS.
The sources of summer DIN transport are abundant (Fig. 7a), with both DINR and DINA reaching their maximum. However, these two compounds are primarily concentrated in the upper layer (Fig. 5), suggesting that they may be utilized quickly after entering and have limited impact on the JS. The summer transport of DINK is approximately 15 kmol/s (Fig. 7a). Most of the DINK is transported between the 50-m and 100-m depth layers (Fig. 5). The elevated chlorophyll concentration in the center of the JS is mostly caused by nutrient transport in this subsurface layer (Shibano et al., 2019). In addition to being completely dominant in terms of transport quantity, the nutrients from the Kuroshio also distribute fairly in the vertical direction to play further roles in the JS.
5 Conclusions
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Using a physical-biological model with a tracking technique, we studied the effects of DIN and DIP from ECS sources on nutrients in the TS. The model can reproduce the variations in velocity, temperature, salinity, chlorophyll a concentration, DIN concentration, and DIP concentration in the TS. The volume transport through the TS is also comparable to the observations.
Primary production is generally limited by DIN, except in the upper layer, during summer and fall. Among all the DIN sources, the Kuroshio water has the highest concentration, accounting for 90% of the total DIN in most of the section. The maximum Kuroshio-origin nutrient concentration is located at the bottom, while those of nutrients from the atmosphere and rivers are on the surface during summer and autumn. The concentration of DIN from the Taiwan Strait reaches its highest value in the middle layer in autumn. For DIP sources, the dominance of the Kuroshio is more pronounced.
The nutrient flux is primarily northeastward, and the countercurrent is not strong in the TS. The cores of the nutrient fluxes from the rivers and atmospheric deposition occur in the upper layer. The nutrient fluxes in the Kuroshio current are the greatest between depths of 50 m and 100 m under the combined effect of nutrient concentration and velocity. The nutrient transport through the TS exhibits similar seasonal variations, as does the volume transport. The transports of DIN and DIP from the Kuroshio water account for 85% and 90%, respectively, of the total. The transport of nutrients from the Taiwan Strait is greater during autumn and winter. The transport of DIN from both the atmosphere and rivers peaks in August.
Nutrient transport cannot be equated with volume transport. The DINs in the less saline zone originate not only from rivers but also from atmospheric deposition and the Kuroshio. The transport of nutrients from the Taiwan Strait is not as significant as its volume transport in the TS. The nutrients from the Kuroshio Current, which have the greatest nutrient transport capacity and are mainly distributed in the subsurface layer, have a profound impact on the JS.
Funding
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  • The National Natural Science Foundation of China under contract Nos 42006018, 41876018 and 42176198; the Grants-in-Aid for Scientific Research [MEXT KAKENHI] under contract No. 22H05206; the Tianjin Municipal Education Commission Scientific Research Project under contract No. 2019KJ219.
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Year 2024 volume 43 Issue 6
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doi: 10.1007/s13131-024-2372-1
  • Receive Date：2023-12-25
  • Online Date：2025-11-19
  • Published：2024-06-25
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  • Received：2023-12-25
  • Accepted：2024-05-07
Funding
The National Natural Science Foundation of China under contract Nos 42006018, 41876018 and 42176198; the Grants-in-Aid for Scientific Research [MEXT KAKENHI] under contract No. 22H05206; the Tianjin Municipal Education Commission Scientific Research Project under contract No. 2019KJ219.
Affiliations
    1 Key Laboratory of Marine Resource Chemistry and Food Technology, Ministry of Education, Tianjin 300457, China
    2 College of Marine and Environmental Science, Tianjin University of Science and Technology, Tianjin 300457, China
    3 Center for Marine Environmental Studies, Ehime University, Matsuyama 790-8577, Japan

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表12种不同金属材料的力学参数
科
Family		 属数
Number of
genus		 种数
Number of
species		 占总种数比例
Percentage of
total species (%)		 属
Genus		 种数
Number of
species		 占总种数比例
Percentage of total
species (%)
鹅膏菌科Amanitaceae 2 11 5.26 鹅膏菌属 Amanita 10 4.78
小菇科 Mycenaceae 2 12 5.74 丝盖伞属 Inocybe 5 2.39
多孔菌科 Polyporaceae 8 14 6.70 蜡蘑属 Laccaria 5 2.39
红菇科 Russulaceae 3 23 11.00 小皮伞属 Marasmius 6 2.87
小菇属 Mycena 11 5.26
光柄菇属 Pluteus 5 2.39
红菇属 Russula 17 8.13
栓菌属 Trametes 5 2.39
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