September 2025 | Oceanography
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INTRODUCTION
Macronutrients, essential for fundamental life processes, have been
traditionally categorized in marine sciences as primarily encom-
passing carbon, nitrogen, phosphorus, and silicon. However,
micronutrients, such as iron, also play significant roles in marine
ecosystems (Emerson and Hedges, 2008; Hutchins and Boyd,
2016). The regulation of nutrient concentrations in the ocean
depends on the interplay of the geophysical and biogeochemical
processes governing the introduction of these elements into sea-
water and their subsequent dispersion, advection, and removal
(K.-K. Liu et al., 2000; J. Zhang et al., 2007; Chen, 2009; Umezawa
et al., 2014; W. Wang et al., 2016).
When viewing the ocean as a system, the primary exter-
nal sources of most nutrients can be attributed to rock weather-
ing, organic matter decomposition, and land-based waste dis-
charge. However, on an annual global scale, the recycling and
internal transport of nutrients within the ocean provide the vast
majority of the nutrients required to sustain primary production,
greatly exceeding the contributions from terrestrial sources such
as rivers and atmospheric deposition (Schlesinger and Bernhardt,
2020). This can be exemplified by considering the East China
Sea (ECS), one of the most productive marine regions globally
(K.-K. Liu et al., 2010; J. Zhang et al., 2019). It has been reported
that the supply of phosphorus from the subsurface waters of the
Kuroshio, a powerful western boundary current, outweighs the
contributions from all rivers, including the mighty Changjiang
River (Yangtze River; C.T.A. Chen, 1996; Zhao and Guo, 2011).
Similarly, in another western boundary current system, upwelling
driven by the Loop Current serves as the primary source of nutri-
ents for the South Atlantic Bight and the West Florida Shelf in the
Gulf of Mexico (Ishizaka, 1990; Lee et al., 1991; Y. Liu et al., 2016;
Weisberg and Liu, 2025).
Phytoplankton play a central role in the biological removal of
inorganic macronutrients, namely nitrogen, phosphorus, and sili-
con, from seawater. While coastal and benthic algae also contrib-
ute to nutrient removal, their impact is relatively minor (Cloern,
2001). This biological removal primarily occurs within the
euphotic layer, which tends to be thin on continental shelves due to
high turbidity caused by terrestrial particle inputs (He et al., 2013,
2014). The marine food web, which includes zooplankton, bacte-
ria, and higher consumers, recycles nitrogen, phosphorus, and sili-
con by breaking down organic matter and releasing these nutrients
as dissolved inorganic forms, thereby sustaining primary produc-
tion and completing the oceanic nutrient cycle. In addition to
macronutrients, the growth and metabolic functioning of phyto-
plankton also depend on a suite of trace elements known as micro-
nutrients. Micronutrients such as iron, nickel, copper, and zinc are
essential cofactors in phytoplankton metabolic processes, includ-
ing photosynthesis and nitrogen fixation, despite their trace con-
centrations in seawater (Morel and Price, 2003). In the Kuroshio
region, these trace metals originate predominantly from anthro-
pogenic aerosol deposition, sediment resuspension on the ECS
shelf, and terrestrial inputs via riverine discharge, with additional
contributions from intermediate waters and lateral transport
from marginal seas (W.H. Liao and Ho, 2018; Takano et al., 2022;
Hsieh and Ho, 2024).
The ECS has been a focal point of numerous studies investi-
gating biological productivity, particularly in relation to nutri-
ent availability. Five external sources of nutrients have been iden-
tified for the ECS: Kuroshio Current, Taiwan Strait, river inputs,
submarine groundwater discharge, and atmospheric deposi-
tion (C.T.A. Chen and Wang, 1999; Gong et al., 2003; S.L. Wang
et al., 2018; J. Zhang et al., 2019). The Changjiang River, especially
during the summer, significantly contributes to nutrient inputs in
its estuary (Sun et al., 2023). Nevertheless, it is widely accepted
that the primary source of nutrients for the ECS is the subsurface
waters of the Kuroshio (C.T.A. Chen, 2008; Liu et al., 2010; X. Guo
et al., 2012; Umezawa et al., 2014). Various processes, including
upwelling, Kuroshio frontal eddies, and filaments extending onto
the shelf, result in supply of nutrients to continental shelf waters
(Li et al., 2016; Meng et al., 2020; Su and Pan, 1987; Yuan et al.,
2015; Zheng and Zhai, 2021; Zhou et al., 2015; Jiang et al., 2023).
The Kuroshio is characterized by its oligotrophic nature in the
euphotic zone but nutrient-rich characteristics in subsurface lay-
ers. Furthermore, due to the weakening of the Kuroshio Current
with depth, the nutrient flux exhibits a subsurface maximum,
referred to as the “nutrient stream” (Pelegri and Csanady, 1991;
C.T.A. Chen et al., 1995; X. Guo et al., 2012). Kuroshio Intermediate
Water (KIW), originating from North Pacific Intermediate Water
(NPIW), is the primary contributor to the Kuroshio nutrient
stream and has been identified as the major source of nutrients
for the ECS continental shelf (C.T.A. Chen and Huang, 1996;
Nagai et al., 2019; S.M. Liu et al., 2020; C.T.A. Chen et al., 2021;
Long et al., 2022).
To shed light on this intricate nutrient flow, we track macro-
nutrients along their journey from the source of the Kuroshio east
of the Philippines and onto the ECS continental shelf, after receiv-
ing contributions from the South China Sea (SCS). This research
aims to enhance our understanding of the origin and transforma-
tion of nutrient-rich water masses that sustain the vibrant primary
productivity of the ECS.
STUDY AREA AND METHODS
In this study, we analyzed data sourced from various loca-
tions proximal to the Kuroshio Current, within and adjacent to
the ECS (Figure 1). Our data originated from multiple research
cruises, arrayed from south to north as follows: east of Philippines
(INDOPAC leg II, May 1976), southeast of Taiwan (ORI-462,
September 1996), southeastern ECS (ORI-179, September
1988), eastern ECS (TPS-24, June 1985), and northeastern ECS
(KEEP-MASS, July 1992).
These cruises were selected to offer a comprehensive perspec-
tive on the Kuroshio Current’s dynamics across different locales
and temporal stages, particularly before and after the current veers