Oceanography | Vol. 38, No. 3
48
120 m on the PN Line is nearly twice as high as the result at 16°N
due to the significant increase in nitrate concentration near the bot-
tom layer. However, the WPS-like nitrate inventories at 120 m and
300 m are consistently around 50 mmol·m–2 and 600 mmol·m–2.
It is important to note that this analysis is preliminary at best, as
it does not account for seasonal effects, and all the data were col-
lected between May and November. Additionally, the study did not
investigate potential variabilities influenced by factors such as the
El Niño-Southern Oscillation, the Pacific Decadal Oscillation, or
long-term trends (Jan et al., 2015; Kim et al., 2022; Wei et al., 2023).
These factors could introduce significant variations and should be
considered in future research to provide a more comprehensive
understanding of the dynamics in the marine environment studied.
CONCLUSIONS
The Kuroshio Current is well known for its nutrient-poor surface
layer and nutrient-rich deeper layers. Consequently, if only the
surface waters of the Kuroshio were to enter the ECS, they would
contribute little to biological production in the region. The reason
that the ECS is an important fishing ground primarily lies in the
upwelling of nutrient-rich subsurface waters from the Kuroshio,
which originates from the NPEC, known for its oligotrophic con-
ditions. The concentration of nitrate does not exceed 1 µM until a
depth of 160 m, which is too deep and dark for phytoplankton to
efficiently utilize the nutrients.
The subsurface waters rich in nutrients from the NPEC begin to
rise when the NPEC encounters the Philippines. The subsequent
northward-flowing portion is known as the Kuroshio, where the
nitrate concentration reaches 1 µM at a depth of 100 m. This indi-
cates an uplift of nutrients by about 50 m. Some of the Kuroshio
waters enter the SCS, where intensive upwelling and vertical mix-
ing bring up subsurface waters along with nutrients. Consequently,
by the time these waters exit the SCS and rejoin the Kuroshio, the
nutrient content is enriched. In other words, the left-hand part of
the Kuroshio now contains SCS waters with higher nutrient con-
centrations. This phenomenon is clearly observed in the cross sec-
tion southeast of Taiwan.
Further upwelling occurs when the Kuroshio is forced to turn
northeastward as it encounters the continental shelf and slope of
the ECS. Topographically induced upwelling further lifts nutri-
ents from the SCS-influenced left-hand part of the Kuroshio. This
continuous upwelling as the Kuroshio flows along the shelf break
of the ECS provides an additional supply of nutrients to the ECS
shelf, contributing to its significance as a productive fishing area.
The complex interplay of oceanographic processes, upwelling, and
the exchange of water masses significantly impacts nutrient avail-
ability in the ECS, ultimately influencing the region’s biological
productivity and its role as a vital fishing ground.
SUPPLEMENTARY MATERIALS
The supplementary materials are available online at https://doi.org/10.5670/oceanog.
2025.e304.
DATA AVAILABILITY
Data will be made available on request.
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