September 2025 | Oceanography
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For instance, a cyclonic eddy impinging on the Kuroshio east of
Taiwan weakened poleward transport and reduced pycnocline
slopes across the Kuroshio (Jan et al., 2017). As a result, the nitrate
inventory changed, increasing when the pycnocline was uplifted
and decreasing when it was depressed.
The 20°C isotherm (Figure S4) was used as a reference to com-
pare uplift occurrences at different latitudes. At 18°N and 21.2°N,
it lies at 200 m. However, at 21.5°N and 21.75°N, it tilts westward
to a depth of 150 m. Off the east coast of Taiwan (22.8°–25.2°N),
it uplifts to a depth of around 100 m. Further along the PN Line
(28°N), the 20°C isotherm reaches approximately 60 m during
September and October 2000. The depth difference of the 20°C
isotherm between the WPS- and SCS-like water near the LS is
around 50 m at 21.5°N and 21.75°N, and this disparity is simi-
lar to data collected from the WPS and the LS (Shaw, 1991). As
the latitude increases, the difference also increases to about 100 m
between 22.8°N and 24.2°N (east of Taiwan); the colder water is
also discernible in satellite images (Figure 1 in Z. Huang et al.,
2021). This uplift in the coastal area is influenced by both the inter-
action of different water masses and terrain effects, with half of the
uplift resulting from the mixing of these water masses before ter-
rain interactions. At 28°N on the PN Line, the depth difference of
the 20°C isotherm between the western and eastern sides increases
to approximately 120 m, as the main Kuroshio current is located at
the shelf break (Y. Liu and Yuan, 1999; Yuan et al., 1998). In addi-
tion to the mechanisms described, year-round upwelling in north-
east Taiwan contributes to bringing cold water to the surface layer
(at least 60 m depth; Wu et al., 2008).
Seawater temperatures are generally inversely correlated with
nutrient concentrations (Kamykowski and Zentara, 1986). Table 1
provides a summary of the uplifted features observed in the five
cross sections studied. Nutrient concentrations are the most strik-
ing differences among these cross sections, especially when com-
paring data at depths of 100 m and 200 m. They are lowest in the
southernmost cross section. The most extreme case is nitrate at
100 m, which is only about 1 µM, compared to around 9 µM in
the northernmost cross section. At 200 m, nitrate increases from
5 µM at 14°N to 16 µM at 32°N (Table 1). In the southernmost
section (14°N in Table 1), phosphate, the limiting nutrient in the
inner ECS, registers the lowest concentrations, with values of
0.2 µM at 100 m and 0.5 µM at 200 m. In the northernmost sec-
tion (32°N in Table 1), phosphate increases to 0.7 µM at 100 m
and 1.2 µM at 200 m, playing a critical role in supplying essential
nutrients to the ECS (T.-H. Huang et al., 2019). Similarly, silicate
is abundantly available in the ECS, and its concentration shows a
four- to fivefold increase in the northernmost section compared to
the origin area of the Kuroshio, likely contributing to the silicate
supply to the ECS.
NUTRIENT INVENTORIES AND
RESEARCH IMPLICATIONS
The uplifted cold waters interact and mix with the surrounding
waters, but nitrate behaves conservatively below 50 m on the shelf
due to limited biological activity, as decreased sunlight inhibits the
light-sensitive nitrifying bacteria (Guerrero and Jones, 1996a,b;
Merbt et al., 2012). Nitrite in the euphotic zone primarily origi-
nates from phytoplankton metabolism (Kiefer et al., 1976), under-
lining the importance of biological factors in its distribution.
For spatial comparison, nitrate concentration is integrated over
depths of 120 m and 300 m in the repeat transects—the PN Line
and 21.75°N (Table S1, Figure 1B,C). The shallowest area of the
PN Line (Area I of Figure 1B, the ECS shelf) is around 120 m,
so the depth-integrated nitrate concentrations represent the whole
water column nitrate inventory. At a depth of 120 m in Areas I and
II of Figure 1B,C, the nitrate inventory values along the PN Line
are more than twice those along 21.75°N. This phenomenon
results from the deeper nutricline depth at 21.75°N and the addi-
tion of nitrates from decomposed organic material on the ECS
shelf (Y. Chen et al., 2016; J. Guo et al., 2018). The nitrate inventory
values remain at similar levels from Areas III to V in Figure 1B,C
between 0 m and 120 m, as the dominant water mass is WPS-like
water. At a depth of 300 m in the PN Line (Area III of Figure 1B),
the integrated nitrate concentration is higher than at 21.75°N. This
is because the PN Line (Area III of Figure 1B) is close to the con-
tinental slope, where deeper water is uplifted by the terrain and
mixed with high-nutrient concentration water from the SCS. The
nitrate inventory values are at comparable levels in Areas IV and V
of Figure 1B,C between 0 m and 300 m.
Table S1 shows the depth-integrated nitrate inventories between
0 m and 120/125 m and 0–300 m at different latitudes during
September and October 2000. Overall, the SCS-like seawater has
two to six times higher nitrate inventories than the WPS-like sea-
water, and these inventories increase with latitude. From 16°N to
25°N, the SCS-like nitrate inventories double at 125 m, whereas the
values remain similar at 300 m. This phenomenon suggests that
uplifting only influences the seawater at the 120 m layer but does
not affect the deeper layer. In contrast, the WPS-like nitrate inven-
tories are stable at 125 m. At the 300 m layer, the values rise sig-
nificantly at 24.2°N, which is close to a ridge (about 800–1,000 m
depth). This suggests that the terrain effect also increases the nitrate
inventories of WPS-like seawater. The SCS-like nitrate inventory at
TABLE 1. Concentrations of nitrate, phosphate, and silicate at different
depths in various cross-sections.
Nitrate
(µM)
Phosphate
(µM)
Silicate
(µM)
100 m
200 m
100 m
200 m
100 m
200 m
32°N, KEEP-MASS
16
0.7
1.2
15
25
28°N, TPS-24
13
0.6
0.9
12
20
25°N, OR1-179
10
0.5
0.7
11
15
21.75°N, OR1-462
0.4
0.6
13
14°N, INDOPAC
0.2
0.5
SCS
15
0.8
1.0
12
25