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The size-fractionated
composition of particulate
biogenic silica and its ecological
significance in the Changjiang
Estuary area
Xizhen Liu
1,2
, Bin Wang
2,3
*, Siyang Chen
1
, Haiyan Jin
2,4
,
Yanpei Zhuang
5
, Zhibing Jiang
2
, Hongliang Li
2,4
*
and Jianfang Chen
2,4
1
Marine Monitoring and Forecasting Center of Zhejiang Province, Hangzhou, China,
2
Key Laboratory
of Marine Ecosystem Dynamics, Second Institute of Oceanography, Ministry of Natural Resources,
Hangzhou, China,
3
Donghai Laboratory, Zhoushan, China,
4
State Key Laboratory of Satellite Ocean
Environment Dynamics, Second Institute of Oceanography, Ministry of Natural Resources,
Hangzhou, China,
5
Polar and Marine Research Institute, Jimei University, Xiamen, China
The concentrations and distributions of particulate biogenic silica (PBSi) and its size-
fractionated composition (>20 mm, 0.8–20 mm) of the Changjiang Estuary and its
adjacent area were investigated during the summer of 2011. PBSi, primarily
produced by diatoms in the surface waters of oceans, was examined for
correlations with hydrographic conditions, nutrients, particulate organic carbon,
and dissolved oxygen. The distribution of PBSi showed distinct patterns: high levels
in nearshore, but relatively low further offshore; low concentrations in the surface
layer, whereas relatively high concentrations in the bottom layer. Large-sized PBSi
(>20 mm) prevailed in the surface layer, whereas small-sized PBSi (0.8–20 mm)
dominated in the bottom layer. Temperature and nutrients were crucial factors
controlling the grain size structure and distribution of PBSi. Further, we observed
that the distinct zones of high PBSi values in the surface waters were affected by the
Changjiang freshwater flushing, and those in the bottom waters were affected by the
Yellow Sea Cold Water masses. Moreover, in the area where >20-mmPBSiprevailed,
the silicate-to-nitrate ratio was less than1atmostsamplingstations,rendering
silicate the limiting nutrient in this area. The PBSi/particulate organic carbon values in
the surface waters of the study area ranged from 0.01 to 0.3. Areas exhibiting values
exceeding 0.13 primarily clustered in nearshore waters, which was characterized by
a dominance of large-sized (>20 mm) PBSi. The nearshore benthic waters exhibited
anoxic conditions, where diatoms predominantly comprised the phytoplankton
biomass and organic matter featured marine phytoplankton. Consequently, the
proliferation of diatoms (siliceous phytoplankton) in the midupper water significantly
contributed to the hypoxic conditions at the bottom, as diatoms underwent
dissolution during sedimentation, leading to oxygen depletion.
KEYWORDS
Changjiang Estuary, particulate biogenic silica, size-fractionated composition,
particulate organic carbon, hypoxia
Frontiers in Marine Science frontiersin.org01
OPEN ACCESS
EDITED BY
Daniel Puppe,
Leibniz Center for Agricultural Landscape
Research (ZALF), Germany
REVIEWED BY
Yongquan Yuan,
Chinese Academy of Sciences (CAS), China
Bin Zhao,
Ministry of Natural Resources, China
*CORRESPONDENCE
Bin Wang
wangbin@sio.org.cn
Hongliang Li
lihongliang@sio.org.cn
RECEIVED 28 July 2024
ACCEPTED 09 January 2025
PUBLISHED 29 January 2025
CITATION
Liu X, Wang B, Chen S, Jin H, Zhuang Y,
Jiang Z, Li H and Chen J (2025) The size-
fractionated composition of particulate
biogenic silica and its ecological significance
in the Changjiang Estuary area.
Front. Mar. Sci. 12:1471650.
doi: 10.3389/fmars.2025.1471650
COPYRIGHT
© 2025 Liu, Wang, Chen, Jin, Zhuang, Jiang, Li
and Chen. This is an open-access article
distributed under the terms of the Creative
Commons Attribution License (CC BY). The
use, distribution or reproduction in other
forums is permitted, provided the original
author(s) and the copyright owner(s) are
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which does not comply with these terms.
TYPE Original Research
PUBLISHED 29 January 2025
DOI 10.3389/fmars.2025.1471650
1 Introduction
Diatoms play an important role in the marine biological pump
process, contributing approximately 30% of global oceanic primary
productivity (Nelson et al., 1995;Treguer et al., 1995) and up to 75%
in nearshore and eutrophic water (Liu et al., 2008). Biogenic silica
(BSi) is produced in the surface waters of oceans, mostly by diatoms,
but also by radiolarians and silicoflagellates (Nelson et al., 1995).
The particulate biogenic silica (PBSi) in the water column
represents the standing stock of BSi in the photic zone and can
be used as an indicator of the instantaneous biomass of siliceous
phytoplankton (Nelson et al., 2002).
ThemarginalseaisthemainplaceofBSiburialand
transformation and the main reservoir of BSi in the ocean, and it
plays an important role in the global biogeochemical cycle of silica
(Conley, 1997). As one of the most crucial estuarine-marginal sea
regions worldwide, the Changjiang Estuary and its adjacent waters
are the ideal place to study the distributional characteristics of BSi
and its ecological effects. This is because of the unique hydrographic
characteristics and abundant nutrient inputs. Diatoms dominate
biomass over the East China Sea (ECS), particularly during
the summer months, because of the Changjiang plume. The
presence of diatom blooms resulted in significantly higher
concentrations of PBSi in the surface layer in summer than in
other seasons (Cao et al., 2013). The biogenic organic particulate
matter from the upper layers of the water column is transformed,
degraded, settled, and buried in the ocean sediments by the action of
a silicate pump (Buesseler et al., 2001). The biogeochemistry of Si in
the marine environment is of global significance. Further, the
behavior of Si can be used as a proxy to understand the carbon
cycle in the ocean, which drives the biological pump in the water
column (Treguer and Pondaven, 2000).
The BSi content of different size fractions varies significantly in
ECS as it is largely controlled by the species of diatoms and their cell
sizes. Thus, the grain size distribution of the bulk sediment should be
considered when using BSi as a proxy for the primary production of
diatom (Wang et al., 2014). The size-fractionated composition of
PBSi is an important factor controlling the particulate organic carbon
in marine areas where diatoms are the dominant population, and it is
a good indicator of the biological pump efficiency in this area.
However, the previous studies on ECS are mainly about the
concentration of PBSi in water column and BSi in sediments (Liu
et al., 2005;Wang et al., 2014;Li et al., 2018). Studies on the
contribution of PBSi with different sizes of the standing stock of
BSi in water columns and the ecological significance of the size-
fractionated composition of PBSi are very limited. The occurrence of
hypoxia off the Changjiang Estuary during summer can be traced
back to the 1950s (Li et al., 2002), and the expansion of the affected
area has been documented (Li et al., 2002;Zhu et al., 2011). Seasonal
hypoxia off the Changjiang Estuary is an increasingly recognized
environmental issue of global concern to both the scientific
community and the public. It is caused by the decomposition of
newly produced marine and riverine-borne biogenic substances
deposited in the bottom water (Chen et al., 2007). Therein, marine-
sourced organic matter is the dominant oxygen consumer in the
subsurface hypoxia zone (Wang et al., 2016). Diatoms typically
prevail in the phytoplankton community of nutrient-rich aquatic
environments (Nelson et al., 1995;Treguer and Pondaven, 2000).
However, they exhibit rapid reproduction and opportunistic
behavior, thriving in relatively cool temperatures (<20°C) and areas
with turbulent mixing (Lomas and Glibert, 1999). Therefore, the
factors affecting the distribution of BSi and the relationship between
BSi and hypoxia formation are of equal interest. This article elucidates
the size-fractionated composition of PBSi in the water column, its
influencing factors, its relationship with particulate organic carbon
(POC), and the formation of a hypoxia zone. Further, this study
provides insights into the size-fractionated (>20 mm, 0.8–20 mmand
total) composition of PBSi in the ECS and its biogeochemical
cycling processes.
2 Materials and methods
2.1 Study site
The Changjiang Estuaryand its adjacent area are among the most
crucial estuarine-coastal ocean continuum worldwide. The region’s
water system is complex, including the cold, fresh, and nutrient-rich
Changjiang Diluted Water and ECS Coastal Water; the warm and
nutrient-poor Taiwan Warm Water Current and Kuroshio Water;
and the relatively cold, fresh, and nutrient-rich Yellow Sea Coastal
Water and the Yellow Sea Cold Water Mass (Zhang et al., 2007;
Chen, 2009). During summer, southwest winds prevail and the
Changjiang Diluted Water extends northeast toward the Tsushima/
Korea Straits (Beardsley et al., 1985;Liu et al., 2016a). Extensive water
exchange between the ECS and Kuroshio occurs across the shelf
break through upwelling and frontal processes. The incursion of the
Kuroshio sustains the upwelling conditions, which are enhanced by
the summer southerly winds (Su, 1998;Zhang et al., 2007).
2.2 Sample collection
This study was conducted from July 5 to July 25, 2011, onboard
the scientific research vessel Dongfanghong 2 of the Ocean
University of China for in situ observations in the Yellow Sea and
ECS. As shown in Figure 1, three sections (D, F, and PN) were
investigated from the Changjiang Estuary to the outer shelf area.
Water samples were collected using a precalibrated conductivity,
temperature, and depth sensors unit (Sea-Bird Electronics, SBE-911
Plus) attached to 12-L Niskin bottles. After seawater was collected,
0.5–2.0 L of seawater was filtered through a 20-mm pore-size nylon
membrane to obtain PBSi samples with particle sizes of >20 mm.
Subsequently, the seawater was filtered through a 0.8-mm pore-size
polycarbonate membrane to obtain PBSi samples with particle
sizes ranging from 0.8 to 20 mm. Nutrient samples were filtered
through precleaned cellulose acetate membranes (47 mm, pore size
0.45 mm), and 0.3 mL of 35 g/L HgCl
2
was added into each 100 mL
sample to preserve the samples for further analysis.
For POC and POC stable carbon isotope (d
13
C) collection, 0.5–
2.0 L of seawater was filtered using GF/F filters (47 mm, pore size
0.7 mm) that had been precombusted for 4 h at 450°C. For the
Liu et al. 10.3389/fmars.2025.1471650
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Chlorophyll a(Chl a) collection, 150 mL of seawater was filtered
through GF/F membrane filters under low pressure and dim light.
2.3 Analytical methods
Seawater temperature and practical salinity scale (without
units) were measured in situ using the conductivity, temperature,
and depth sensors. The samples for dissolved oxygen (DO) analyses
were collected, fixed, and titrated onboard, following the classic
Winkler titration procedure (Bryan et al., 1976). The precision of
the DO measurements was ±0.02 mg/L.
The PBSi concentrations in the water were determined using
the extraction method of Ragueneau et al. (2005). The filters were
dried at 60°C and extracted twice with 4 mL of 0.2 mol/L NaOH in
polyethylene centrifuge tubes at 100°C for 40 min two times. After
centrifugation (3,036.8 × g), the concentrations of Si and Al in the
supernatant were determined for both the first step ([Si]
1
and [Al]
1
)
and the second step ([Si]
2
and [Al]
2
). Silicate concentration was
determined by spectrophotometry using a continuous nutrient flow
analyzer (model Skalar San++, Skalar, Holland; Grasshoff et al.,
2009), and Al concentration was measured on a Varian Vista Pro
inductively coupled plasma atomic emission spectrometer. Since all
BSi were extracted in the first step, the [Si]
2
:[Al]
2
ratio was specific
to the sample’s suspended silicate minerals. The accurate BSi
concentration could be calculated using the following formula:
PBSi =½Si1−½Al1½Si2:½Al2(1)
Nitrate and silicate concentrations were determined using a
colorimetric method with a Skalar nutrient flow analyzer (Grasshoff
et al., 2009). Meanwhile, nitrite, ammonium, and phosphate
concentrations were determined manually in situ according to
standard methods using a spectrophotometer (model 723,
Shanghai Jingke, China). The data quality was monitored by
intercalibration, and the detection limits for nitrate, nitrite,
ammonium, phosphate, and silicate were 0.1, 0.05, 0.05, 0.03, and
0.1 mmol/L, respectively.
The filters for detecting POC and d
13
C were dried at 50°C until
constant weight; thereafter, they were decarbonated by fumigating
with hydrochloric acid. Carbon concentrations were analyzed using
an Elementar Vario MICRO cube elemental analyzer, and d
13
C was
analyzed using a Thermo MAT 253 isotope ratio mass
spectrometer. Chl adata were provided by Yuming Cai (personal
communication, 2011). Further, filtered phytoplankton was
jammed and extracted with 90% acetone overnight; thereafter,
fluorescence was measured using a Turner Designs model 10.
The relationships between PBSi and other variables (e.g., POC
and DO) were assessed using correlation analysis. Spearman’srank
correlation was applied to evaluate the relationship between PBSi and
POC, as it is suitable for non-normally distributed data and can
effectively capture non-linear correlations. The relationship between
PBSi and DO was examined using a logarithmic regression model,
and the coefficient of determination (R
2
) and p-value were calculated
to assess the model’sgoodnessoffit. To compare the differences in
temperature, salinity, nitrate, phosphate, silicate, N/P and silicate/
nitrate ratios, DO, and POC between two depth layers (above 10 m
and below 10 m) across the three regions (I, II, and III), statistical
tests were selected based on data distribution. For datasets that
met the assumptions of normality and homoscedasticity, the
independent samples t-test was used. For datasets that did not
meet these assumptions, the non-parametric Mann-Whitney U test
was employed. Additionally, to compare the distribution of PBSi
FIGURE 1
The distribution of sampling stations and current systems in the investigated sea area, where CDW is the Changjiang Diluted Water, TWC is the Taiwan Warm
Current, ECSCoW is the East China Sea Coastal Current, YSCoW is the Yellow Sea Coastal Water, and KSW is the Kuroshio Surface Water.
Liu et al. 10.3389/fmars.2025.1471650
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concentrations in different size fractions (<20 mm and 0.8–20 mm)
among regions I, II, and III, as well as the total PBSi distribution
between surface and bottom layers, the same statistical approach
was applied. The significance level was set at p < 0.05, with
statistical significance denoted as follows: *p < 0.05, **p < 0.01, and
***p < 0.001, while ‘ns’indicated no significant difference. All
statistical analyses were conducted using SPSS Statistics 27.
3 Results
3.1 Hydrographic conditions
The temperature and salinity in the study region ranged from 10.4°
C to 28.9°C and 16.1 to 34.8, respectively. According to the
temperature–salinity analysis of the investigated area, five water
masses in the study area were identified (Figure 2A), which were
similar to those described in previous studies (Chen, 2009;Zhang et al.,
2007). They include (1) Changjiang Diluted Water, with salinities of
22.0–31.0 and temperatures of 20.0°C–30.0°C, (2) Taiwan Warm
Current Water, with characteristic salinities of 31.0–34.0 and
temperatures of 25.0°C–30.0°C, (3) Shelf Mixed Water (SMW), with
salinities of 31.0–34.0 and temperatures of 15.0°C–25.0°C, (4) Yellow
Sea Cold Water, with salinities of 31.0–34.0 and temperatures of 10.0°
C–15.0°C, and (5) Kuroshio Surface with salinities of 34.0–35.0 and
temperatures of 22.5°C–28.0°C. The investigated area was divided into
three parts based on the characteristics of different water masses
(Figure 2B). Region I was mainly affected by the Changjiang Diluted
Water, and Region II was affected by the relatively low-temperature
and moderately saline SMW and Yellow Sea Cold Water Mass. Region
III was mainly affected by the high-temperature and highly saline
Taiwan Warm Current and the Kuroshio surface water, which
included the stations east of P05 in Section PN. Owing to the big
difference in the temperature and salinity of the surface and bottom
waters in the investigated area in summer, as shown in Figure 2,the
temperature and salinity characteristics of the waters above 10 m and
waters below 10 m were divided and further analyzed to obtain the
following results. For Region I, the salinity of the water column above
10 m was <31, which was the characteristic value of the Changjiang
Diluted Water. The water column below 10 m was caused by the water
masses of the Changjiang Diluted Water, the coastal current of the
Yellow Sea, and the Taiwan Warm Current, which defined the SMW
(Zhang et al., 2007). For Region II, the water column above 10 m
showed the thermohaline characteristics of SMW. Further, the
temperature of the water column below 10 m was significantly low,
all less than 15°C, under the influence of the expansion of the Yellow
Sea Cold Water Masses to the south in summer. In Region III, the
waters above 10 m were mainly affected by the Taiwan Warm Current
with high temperature and subhigh salinity characteristics.
Furthermore, the deep waters below 10 m with high salinity
characteristics could result from the intrusion of Kuroshio Surface
Water (Zhou et al., 2018).
3.2 Distribution of seawater constituents
The distributions of DO in surface and bottom water are shown in
Figure 3. A bottom hypoxia area (with DO concentration less than 3
mg/L) was found near the mouth of the Changjiang, centered at station
P01 (St.P01, DO = 2.6 mg/L) and station F02 (St.F02, DO = 3.0 mg/L).
However, this region did not exhibit high values of surface DO, with
areas of high surface values occurring in St.D03 (DO = 8.4 mg/L) and
FIGURE 2
Scatter plot of potential temperature vs. salinity (A) and spatial division of the investigated area based on water mass characteristics (B).(A) The
scatter plot illustrates the characteristics of different water masses: CDW (Changjiang Diluted Water), TWC (Taiwan Warm Current), SMW (Shelf Mixed
Water), YSCW (Yellow Sea Cold Water Mass), and KSW (Kuroshio Surface Water). The colors represent the spatial divisions: blue corresponds to
Region I, green corresponds to Region II, and red corresponds to Region III. (B) The investigated area is divided into three regions based on water
mass characteristics: Region I, Region II, and Region III. The station colors are consistent with panel (A): blue represents Region I stations, green
represents Region II stations, and red represents Region III stations.
Liu et al. 10.3389/fmars.2025.1471650
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St.D05 (DO = 8.0 mg/L). Due to sampling limitation, the surface DO
sample for St.F02 was not obtained; therefore, the DO value of the 10 m
layeratthisspecific station is depicted in Figure 3A.ThesurfacePOC
concentration showed a large variation, ranging from 2.0 to 20.1 mmol/
L(8.6±6.3mmol/L, mean ± standard deviation, as used hereinafter)
(Figure 4A). Two areas of relatively high POC concentrations were
observed: the northern part outside the Changjiang Estuary was at
St.D01 (POC = 26.0 mmol/L) and St.F02 (POC = 20.1 mmol/L),
whereas the southern part was at St.P05 (POC = 13.8 mmol/L). In
the vast majority of stations, the bottom POC values were greater than
the surface values, ranging from 1.7 to 154.9 mmol/L (24.7 ± 38.0 mmol/
L, Figure 4B). Furthermore, DO and POC concentrations were
calculated across the three regions (Table 1). The results revealed
that DO levels were generally lowerindeeperwaters(below10m),
with the most pronounced decrease observed in Region I. In contrast,
POC concentrations exhibited high variability, with a significant
increase in bottom waters of Region II, likely due to sediment
resuspension or organic matter accumulation. This study examined
nutrient concentrations in the upper and lower water column and
analyzed them according to the various regions, as shown in Table 1.In
the upper water column, nitrate concentration decreased offshore, with
high concentration in Region I and low concentrations in Regions II
and III. As the depth increased, nitrate levels decreased in Region I,
whereas they increased in Regions II and III. The minimum average
concentration for phosphate was observed in the water column below
10 m in Region II. Meanwhile, the average phosphate levels in the water
column from other regions varied slightly. The nearshore exhibited
higher silicate concentrations in Region I, with mean values of 19.0
mmol/L and 21.1 mmol/L in the upper and lower waters, respectively,
than in Regions II and Region III.
3.3 Characteristic of particulate biogenic
silica distribution and size-
fractionated compositions
The PBSi concentration in the surface layer ranged from 0.01 to 2.3
mmol/L (Figure 5A), with a mean concentration of 0.7 ± 0.8 mmol/L.
FIGURE 3
Distribution of dissolved oxygen concentration (mg/L) in the surface (A) and bottom (B) waters of the investigated area.
FIGURE 4
Distribution of the particulate organic carbon (POC) concentration (mmol/L) in the surface (A) and bottom (B) waters of the investigated area.
Liu et al. 10.3389/fmars.2025.1471650
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The peak concentration of 2.3 mmol/L was recorded at St.P05, whereas
the second highest concentration of 2.1 mmol/L was observed at St.F02,
located at the mouth of the Changjiang. The levels of PBSi maxima were
comparable to those of previous studies in the region (Liu et al., 2005).
These two stations showed a dominance of PBSi in the >20-mmsize
fraction, representing 69.7% and 79.4% of the total PBSi concentration,
respectively. The PBSi concentration in the bottom layer ranged from
0.05 to 6.8 mmol/L (Figure 5B), with a mean concentration of 2.0 ±
2.1 mmol/L. The highest and second highest values of 6.8 and 6.2 mmol/
L in the bottom layer were observed at stations D05 and D07, which
were offshore and dominated by the PBSi of the 0.8–20 mm size fraction.
Further, they accounted for 73.6% and 84.6% of the total PBSi
concentration, respectively. Additionally, the bottom PBSi
concentration at nearshore St.P01 was relatively high, at 5.6 mmol/L,
showing a predominance of the >20- mm size fraction, representing 85%
of the total PBSi concentration. The lowest concentrations in the surface
and bottom layers were determined atoffshoreSt.P12.Overall,the
distribution of the surface PBSi showed a pattern of high concentration
near the coast and low concentration further offshore, with high values
concentrated in the Changjiang Estuary area. The bottom PBSi
concentration was higher than that of the surface layer, and the high-
value area in the bottom layer was concentrated in the offshore D05 area
and the nearshore P01 area. Significant differences (p<0.05) were
observed between the bottom PBSi and the surface PBSi. For size-
fractionated compositions, in surface waters, the PBSi concentration in
the >20-mm size fraction ranged from 0.004 to 1.7 mmol/L, accounting
for 0.5% to 84.4% of the total PBSi, with an average proportion of 51.2%.
In the bottom waters, the PBSi concentration in the 0.8–20 mmsize
fraction ranged from 0.01 to 5.3 mmol/L, accounting for 11.2% to 98.5%
of the total PBSi, with an average proportion of 66.3%. Overall, the
large-particle-size PBSi (>20 mm) predominated in the surface layer,
accounting for 55.6% of the survey stations, primarily concentrated in
the Changjiang Estuary. For the offshore area, the predominance of the
large-particle-size PBSi gradually decreased, and the small-particle-size
PBSi (0.8–20 mm) predominated in the bottom layer, accounting for
72.2% of the survey stations.
InSectionD,theconcentrationofPBSiinthe>20-mm size fraction
ranged from 0.01 to 1.8 mmol/L, with an average value of 0.4 ± 0.5
mmol/L, accounting for 24.7% of the total PBSi on average. Further, the
PBSi concentration of particle size 0.8–20 mmrangedfrom0.08to5.3
mmol/L, with an average value of 1.6 ± 1.9 mmol/L, accounting for
75.3% of the total PBSi on average (Figure 6A). We found that in the
majority of stations in Section D, the PBSi of particle size 0.8–20 mm
dominated. In Section F, the concentrationofPBSiinthe>20-mmsize
fraction at all depths was in the range of 0.003–0.8 mmol/L, with an
average value of 0.3± 0.5 mmol/L, accounting for 44.6% of the total PBSi
on average. The concentration of PBSi in the 0.8–20 mm size fraction
ranged from 0.02 to 2 mmol/L, with an average value of 0.4 ± 0.5 mmol/
L, accounting for 55.4% of the total PBSi on average (Figure 6B).
We found that in Section F, the PBSi concentration in the 0.8–20 mm
sizefractionwashigherthanthatinthe>20-mm fraction. However, the
dominance of the PBSi in the >20-mm size fraction was concentrated in
the surface and middle waters, and this dominance weakened with the
increase in the depth of the water column. In Section PN, the
concentrationofPBSiinthe>20-mm size fraction at each depth
ranged from 0.004 to 8.4 mmol/L, with an average value of 0.7 ± 1.8
mmol/L. It accounted for 0.4% to 88.8% of the total PBSi, with an
average of 43.0%, and the >20-mm PBSi size fraction dominated mainly
in the surface water of the frontal zone at stations P03 and P05
(Figure 6C). The concentration of PBSi in the 0.8–20 mm size fraction
ranged from 0.01 to 9.6 mmol/L, with an average value of 0.9 ± 2.1
mmol/L, and the proportion of the total PBSi ranged from 11.2% to
99.6%, with an average of 57.0%. We considered that the PBSi in the
0.8–20 mm size fraction was dominant, concentratingin the surface and
middle water column at nearshore station P01 and in the bottom water
column at most other stations. Furthermore, we calculated the size-
fractionated PBSi concentration across the three regions (Figure 7).
Region I and Region II exhibited higher PBSi levels in the 0.8–20 mm
size fraction compared to >20-mm fraction, while Region III showed
minimal differences between the two size fractions. In Region I, there
was a significant difference (p<0.05) in PBSi concentration between
0.8–20 mm and >20-mm, whereas in Regions II and III, there was no
statistical significance.
4 Discussion
4.1 Factors affecting the distribution of
particulate biogenic silica
The concentrations of PBSi ranged from 0.01 to 18.03 mmol/L,
with an average concentration of 1.3 ± 2.6 mmol/L. These values were
TABLE 1 Temperature (°C), salinity, nutrient concentrations (mmol/L), and nutrient ratios in the water column at different depths of the
investigated area.
Water column Temperature Salinity Nitrate Phosphate Silicate N/P Silicate/
nitrate
DO POC
I (above 10 m) 23.8 ± 2.0 27.9 ± 2.7 18.7 ± 10.0 0.4 ± 0.3 19.0 ± 11.1 92.0 ± 74.6 1.0±0.4 6.9±1.5 11.1±5.8
I (below 10m) 18.1±2.1*** 33.0±1.3*** 12.7±3.5* 0.8±0.5** 21.1±5.0ns 22.9±13.8*** 1.7±0.3*** 4.6±1.4*** 25.0±39.5ns
II (above 10 m) 22.3 ± 1.6 31.8 ± 0.4 0.9 ± 0.4 0.03 ± 0.05 10.9 ± 1.4 21.5 ± 8.7 13.7±4.9 7.9±0.3 4.8±0.8
II (below 10m) 12.8±3.1*** 32.9±0.4*** 10.1±2.9*** 0.5±0.2** 16.7±1.5*** 33.5±26.7ns 1.8±0.6*** 7.2±0.5** 32.8±33.7**
III (above 10 m) 28.3 ± 0.5 32.7 ± 0.8 0.8 ± 0.5 0.2 ± 0.2 3.8 ± 1.6 17.1 ± 7.4 22.2±55.3 6.7±0.2 5.3±3.9
III (below 10m) 22.7±3.2*** 34.2±0.5*** 4.0±3.0** 0.4±0.3** 10.3±6.3** 15.7±14.8ns 4.0±3.1ns 5.8±0.8** 4.1±1.8ns
The numbers in the table represent the quantitative data as mean ± standard deviation (SD).
***p < 0.001, **p < 0.01, *p < 0.05, ns, no statistical significance.
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comparable to those observed in the Bohai Sea and Yellow Sea during
spring (1.1 ± 1.4 mmol/L) but lower than the concentrations recorded
in the Bohai Sea and Yellow Sea during autumn (2.6 ± 2.1 mmol/L)
(Liu et al., 2016b). To analyze the influence of different water masses
on PBSi distribution, we calculated temperature, salinity, nitrate,
phosphate, silicate, as well as N/P and silicate/nitrate ratios in the
surface waters (0-10 m) and bottom waters (>10 m depth to the
seabed) across the three regions (Table 1). Except for silicate in
Region I, N/P ratios in Region II and III, and silicate/nitrate ratios in
Region III, all other parameters showed significant differences
between depths. High values of surface PBSi were mainly
concentrated in Region I. Region I is affected by the diluted water
FIGURE 5
Distribution of the particulate biogenic silica (PBSi) concentration (mmol/L) in different size fractions in the surface (A) and bottom (B) waters of the
investigated area. The triangle indicates the location of Station P01, and the pie chart marked by an arrow indicates the size-fractionated PBSi
concentration (mmol/L) in Station P01.
Liu et al. 10.3389/fmars.2025.1471650
Frontiers in Marine Science frontiersin.org07
from the Changjiang, which brings abundant nutrients to the ECS,
thereby providing favorable conditions for the growth of diatoms.
Freshwater inputs might contribute to the development of high PBSi
values. Further, Conley (1997) reported that the PBSi levels in the
world’s rivers, including the Amazon, Mississippi, and Congo, ranged
from (2.7 ± 0.52) to (74 ± 17.60) mmol/L, with an average level of 28
mmol/L. Diatoms thrive in nearshore waters, and compared with
dinoflagellates, diatoms thrive better in environments with low
salinity, turbidity, and high nutrients (Jiang et al., 2015). In
circumstances of sufficient nutrients, diatoms have a competitive
growth advantage, whereas in situations of insufficient nutrient
supply later on, the growth of motile large dinoflagellates takes
precedence (Savidge et al., 1995). The growth of diatoms depends
not only on nitrate and phosphate but also on the supply of
FIGURE 6
Distribution of PBSi concentration (mmol/L) in different size fractions in Sections D (A),F(B),andPN(C) of the investigated area. The dark blue shading
indicates the PBSi concentration in the >20-mm size fraction, whereas the stripes indicate the PBSi concentration in the 0.8–20 mm size fraction.
Liu et al. 10.3389/fmars.2025.1471650
Frontiers in Marine Science frontiersin.org08
monomeric silicic acid, which can be used for building up frustules
consisting of amorphous silica. In this area, the concentrations of
nitrate and silicate reached 18.7 ± 10.0 and 19.0 ± 11.1 mmol/L,
respectively, providing sufficient nutrients for diatom growth. In the
bottom water of Region II, a high-value area of PBSi was measured
because of the summer-extended Yellow Sea Cold Water Mass,
lowering the water temperature to below 15°C, which diatoms
prefer for growth (Xiao et al., 2018). In the surface water of Area
II, the temperature was similarly lower than that in the other two
areas. Under low-temperature conditions, diatom cells successfully
divide by developing a tree-like pattern (low silicification), affording
high cell density (Javaheri et al., 2015). As the temperature increases,
the algal community shifts from a typical community with both
diatoms and dinoflagellates to a community dominated by micro-
flagellates (Anderson et al., 1994).
From the perspective of size-fractionated compositions, PBSi in the
0.8–20 mm size fraction predominantly existed in the D, F, and PN
sections overall. However, the >20-mm PBSi size fraction predominated
in the surface and middle waters in the nearshore of Section F and PN
(i.e., region I). Previous studies have shown that the nano-diatoms are
the dominant species in ECS, with dominant cells of 2–14 mm, resulting
in a high BSi content in fraction <16 mm(Wang et al., 2014). The
convergence of the Taiwan Warm Current and freshwater results in
water stratification. Coastal and offshore water systems play a crucial
role in controlling the composition of phytoplankton particle sizes
(Ho et al., 2015). Small phytoplankton species tend to dominate in the
low-nutrient regions of the ocean, whereas large species tend to thrive
in nutrient-rich waters (Maranon et al., 2001;Li, 2002;Kostadinov
et al., 2010). Chain-formed, small-sized (<20 mm) diatoms have a
relatively high surface-area-to-volume ratio, facilitating the absorption
of nutrients, particularly phosphates (Karp-Boss et al., 1996). The ratio
at which phytoplankton absorb nitrogen and phosphorus nutrients is
16:1 (Redfield ratio), and phosphorus is a limiting factor that influences
phytoplankton growth in most parts of the ECS (Harrison et al., 1990).
The ratio of silicate to nitrogen required for diatom growth is
conventionally close to 1 (Brzezinski, 1985). However, nitrates are
depleted first in waters where the silicate/nitrate ratio is >1, and silicates
are depleted first in waters where the ratio is <1 (Levasseur and
Therriault, 1987). In our research, most stations had a silicate/nitrate
ratioof<1intheregionwherePBSilargerthan20mmdominated,
making silicate the limiting factor. The growth of diatoms requires
monomeric silicic acid to synthesize their frustules, indicating that the
diatoms with large individuals (>20 mm) have a relatively high demand
for silicate. However, the region is rich in nutrients with nitrate and
silicate, which can provide essential nutrients for the growth of large-
sized PBSi. However, as diatoms proliferate further, silicate might be
depleted first.
4.2 Relationship between particulate
biogenic silica and particulate
organic carbon
The primary mechanism for ocean carbon storage is the action of
the “biological pump.”Diatoms are the most significant contributors to
global marine primary productivity, and their growth process involves
the utilization of dissolved silica to synthesize their siliceous shells.
Thus, the “biological carbon pump”in the ocean is mainly driven by
the “biological silica pump.”Without silicate limitation, 27 marine
diatom cultures yielded an average BSi/POC ratio of 0.13 (Brzezinski,
1985). The PBSi/POC ratios exceeded 0.3 during algal blooms in the
Western Ross Sea and the Weddel-Scotia Sea regions (Treguer et al.,
1988;Leynaert et al., 1991). In the investigated area, the values of PBSi/
POC ranged from 0.01 to 0.3 in surface waters (Figure 8A). The PBSi/
POC values at St.F02 and St.F03 were close to the ratio of PBSi/POC in
the pure diatom culture samples. Further, the ratios of PBSi/POC in the
sea west of P07 were all greater than 0.13, with the highest value of 0.3
occurring at St.P03, which suggested that this was an area where
phytoplankton were dominated by siliceous organisms. We observed
that the regions where the PBSi/POC values exceed 0.13 are mainly
FIGURE 7
Distribution of PBSi concentration (mmol/L) in different size fractions across Regions I, II, and III. The dark blue bars represent the PBSi concentration
in the >20-mm size fraction, while the striped bars represent the PBSi concentration in the 0.8–20 mm size fraction.
Liu et al. 10.3389/fmars.2025.1471650
Frontiers in Marine Science frontiersin.org09
concentrated in the nearshore areas of the F and PN sections, with a
predominance of large-sized (>20 mm) PBSi. Therefore, the differences
in PBSi size-fractionated compositions may be one of the reasons for
the variations in the PBSi/POC distribution. Large-size diatoms
enriched a large amount of dissolved silicate in the water column,
resulting in a high PBSi/POC zone.Changesintheparticlesizeor
dominant species composition of phytoplankton can lead to differences
in Si/C ratios (Treguer et al., 1988;Boyd and Newton, 1995).
In the bottom waters, the values of PBSi/POC ranged from 0.02
to 0.29 (Figure 8B). Unlike the distribution of high-value areas in
the surface layer, the high-value regions of the bottom layer are
mainly concentrated in Section D. The values of PBSi/POC at
St.D05, St.D07, and St.D01 were 0.30, 0.24, and 0.22, respectively.
The Changjiang Diluted Water extends to the northeast toward the
Tsushima/Korea Straits during summer, and the transportation and
deposition of organic matter from terrestrial sources might
contribute to developing high-value areas. The POC in the ECS is
rapidly exported from euphotic waters, and the cross-shelf flux of
POC from shelf regions to the open ocean is significant (Zhu et al.,
2009). In Sections F and PN, PBSi/POC ratios showed varying
decreases in depth, with the PBSi/POC ratio decreasing to 0.22 at
St.P03 and below 0.13 at all other stations. This could reflect the
high rates of BSi dissolution or the low rates of organic matter
remineralization in the water column. According to the model
estimation, approximately 75% of the BSi was dissolved in the water
column, and 11% of the BSi was buried in the sediment (Liu et al.,
2008). Particle size directly governs the settling velocity of particles
(Berelson, 2001), and large-particle PBSi (>20 mm) might have
higher sinking rates than small-particle PBSi (0.8–20 mm).
4.3 Relationship between particulate
biogenic silica and the formation
of hypoxia
Hypoxia is defined as DO levels below 2 or 3 mg/L (Diaz, 2001;
Dai et al., 2006;Chen et al., 2007). The seasonal hypoxic zone off the
Changjiang Estuary is one of the most important ecological
problems in China’s shelf waters, particularly in August. A trend
of the gradual expansion of the hypoxic area has been observed in
recent years (Li et al., 2002;Zhu et al., 2011). In order to discuss the
relationship between PBSi, DO, and POC, we draw plots of PBSi
versus DO and PBSi versus POC using all the data collected along
sections D, F, and PN (Figure 9). The fitting line shows the negative
relationship between PBSi and DO. Spearman’s analysis shows a
positive correlation between POC and PBSi. Stations P01 and F02
showed bottom hypoxia characteristics, with the bottom DO values
distributed as 2.6 and 3.0 mg/L, respectively, and the surface and
subsurface DO values in the range of 4.0–5.9 mg/L (Figure 10A).
The DO saturation of each layer ranged from 16.8% to 39.4%. It is
generally believed that water hypoxia is related to water column
stratification, as well as oxygen depletion and the decomposition of
bottom organic matter. The former is the external physical
condition for hypoxia formation, and the latter is the
biogeochemical endogenous cause for the formation and
development of hypoxia (Justicet al., 2003;Rabalais et al., 2010).
To characterize the vertical variation of Chl aand PBSi in water
column, the concentrations of Chl aand PBSi at St.P01 and St.F02
were analyzed (Figure 10B). At St.F02, PBSi and Chl a
concentrations both reached maximum values of 2.1 mmol/L and
11.6 mg/L in the surface layer, respectively. Further, with increasing
depth, the concentrations of PBSi and Chl adecreased sharply, and
DO dropped to 3 mg/L. At St.P01, closer to the coast, the values of
PBSi and Chl aconcentrations in the surface layer were 1.7 mmol/L
and 0.96 mg/L. Owing to light limitation (Zhu et al., 2009), surface
phytoplankton biomass was not high at St.P01; as the depth
increased, the PBSi concentration reached a maximum value of
18.0 mmol/L in the 20-m layer. The bottom water of St.P01
exhibited hypoxic characteristics with a DO value of 2.6. The
PBSi (diatom) dissolution is accompanied by DO depletion, and
under stratified conditions in the water column, the bacterial
degradation of organic substrates accelerates the dissolution of the
underlying siliceous cell membranes (Abe et al., 2014). The
phytoplankton of the St.P01 and St.F02 taxa were identified and
FIGURE 8
The ratios of PBSi/POC distribution in the surface (A) and bottom (B) waters of the investigated area.
Liu et al. 10.3389/fmars.2025.1471650
Frontiers in Marine Science frontiersin.org10
counted using a light microscope. The proportions of diatom
density in the phytoplankton community both occupied an
absolute dominance of more than 98% (Figure 10C). The
abundance of other phytoplankton species was limited; the
combined cell densities of dinoflagellates, haptophytes and
chrysophytes accounted for less than 2% of the total.
Additionally, the bulk stable isotope compositions (d
13
C) of
organic materials have been used to identify the source of organic
matter. The d
13
C-POC can discriminate POC sources between
marine origins (−18‰to −22‰) and terrestrial origins (−23‰to
−34‰from C3 plants and −9‰to −17‰from C4 plants)
(Gearing, 1988;Kumar et al., 2022). In this study, stable carbon
isotope analyses, except the surface of St.P01, revealed that marine
phytoplankton predominantly contributed to POC in St.P01 and
St.F02. The vertical carbon isotope profiles displayed high
consistency, showing less negative d
13
C(−20.97‰to −23.11‰)
throughout the water column at St.P01 below 10 m and at St.F02
(Figure 10D), indicating the sinking of marine particulate
organic matter.
The Changjiang carries a large amount of terrestrial organic matter
intothesea;however,land-sourcedorganicmatterisgenerallyahighly
degraded product. The organic matter contains a limited quantity of
nitrogenous compounds required by oxygen-consuming bacteria,
making the organic matter relatively “inert”with limited oxygen-
consumption capabilities (Tan et al., 1991;Tian et al., 1992).
Resultantly, the formation of hypoxic zones in the bottom layer may
have limited contributions from terrestrial organic matter. However,
the degradation of a large amount of marine-derived organic matter,
caused by the flourishing of phytoplankton, plays a significant role in
the formation of hypoxic zones. Previous studies have demonstrated
that the proliferation of surface diatoms can contribute 70% to 80% of
bottom-layer hypoxia, as estimated based on the silicate release-to-
uptake ratio (Wang et al., 2017). However, the dissolution dynamics of
PBSi during sedimentation and its relationship with DO remain poorly
understood. In this study, we examined the vertical distribution of PBSi
in the water column and its concurrent changes with DO, providing
further evidence that PBSi dissolution is accompanied by oxygen
depletion during sedimentation. Our findings suggest that the
proliferation of diatoms (siliceous phytoplankton) in the upper to
middle water layers at stations St.P01andSt.F02servesastheprimary
driver of bottom-layer hypoxia, with diatom dissolution during
sedimentation closely tied to oxygen consumption.
5 Conclusion
This study systematically describes the horizontal distribution
of particulate biogenic silica (PBSi) across different size fractions
(>20 mm, 0.8–20 mm, and total) and its vertical distribution within
the water column of the Changjiang Estuary and its adjacent waters
during summer. PBSi in the 0.8–20 mm size fraction was generally
dominant across sections D, F, and PN, while PBSi in the >20 mm
fraction was primarily concentrated in the surface and middle
waters of nearshore areas in sections F and PN (i.e., Region I).
Temperature and nutrient availability were identified as critical
factors influencing the size composition and spatial distribution of
PBSi. For the first time, this study systematically analyzed the size
composition (>20 mm, 0.8–20 mm) of PBSi in the Changjiang
Estuary and adjacent areas, identified the factors affecting PBSi
distribution, and examined the relationships between PBSi,
particulate organic carbon (POC), and hypoxia. A more detailed
size-fractionated classification provided greater accuracy in
revealing the spatial distribution patterns and ecological
significance of PBSi. Our findings indicated that high PBSi/POC
ratios in the surface layer were predominantly concentrated in the
nearshore areas of sections F and PN, with a significant
contribution from large-sized (>20 mm) PBSi. However, with
increasing depth, the PBSi/POC ratio declined sharply. Large-
FIGURE 9
Relationships between PBSi and DO (A) and correlation between PBSi and POC (B).
Liu et al. 10.3389/fmars.2025.1471650
Frontiers in Marine Science frontiersin.org11
sized PBSi, which is capable of enriching more silicate and exhibits
higher sedimentation rates, underwent substantial degradation
during the sedimentation process. The grain-size structure of
biogenic silica serves as a reliable indicator of biological pump
efficiency. However, due to the lack of data on POC settling flux in
this study, we could not address this aspect in detail. Future studies
should investigate the relationships between the grain-size structure
of phytoplankton populations, POC flux, and biological pump
efficiency. Additionally, this study analyzed the spatial variations
of dissolved oxygen (DO), PBSi, and d
13
C in the water column at
stations St.P01 and St.F02. It confirmed that PBSi dissolution
during sedimentation was accompanied by oxygen depletion. The
findings highlight that the proliferation of surface diatoms and the
associated oxygen consumption during their dissolution in the
water column were the main drivers of bottom-layer hypoxia.
This study provides new regional evidence to deepen the
understanding of the interaction mechanisms between diatoms
and hypoxia. In the future, greater attention should be paid to
FIGURE 10
Vertical profiles of PBSi, Chlorophyll a, and dissolved oxygen concentrations (St.P01 and St.F02). (A) Dissolved oxygen concentration in the water
column. (B) Chlorophyll a(blue line) and PBSi (black line) concentrations at St.P01 and St.F02. (C) Density ratio of diatoms and dinoflagellates in the
water column (Jiang Zhibing, unpubl); the gray shading indicates diatoms, and the black shading indicates dinoflagellates. (D) POC carbon isotopes
(d
13
C-POC) in the water column of St.P01 and St.F02 (Haiyan Jin, unpubl).
Liu et al. 10.3389/fmars.2025.1471650
Frontiers in Marine Science frontiersin.org12
quantifying the relationships between PBSi dissolution, silicate
regeneration, and oxygen depletion. Systematic analyses are
needed to evaluate the contribution of PBSi to hypoxia formation.
Data availability statement
The datasets presented in this study can be found in online
repositories. The names of the repository/repositories and accession
number(s) can be found in the article/Supplementary Material.
Author contributions
XL: Writing –original draft, Formal analysis, Investigation. BW:
Writing –review & editing, Formal analysis, Investigation. SC:
Resources, Visualization, Writing –original draft. HJ: Investigation,
Writing –review & editing, Methodology. YZ: Investigation, Validation,
Writing –original draft. ZJ: Formal analysis, Investigation, Writing –
review & editing. HL: Funding acquisition, Methodology, Supervision,
Writing –review & editing. JC: Project administration, Writing –review
& editing, Conceptualization, Resources.
Funding
The author(s) declare financial support was received for the
research, authorship, and/or publication of this article. This study
was supported by the Natural Science Foundation of Zhejiang Province
(No. LDT23D06023D06), the Open Research Program of the Key
Laboratory of Marine Ecosystem Dynamic (MED), MNR (No.
MED202202), the Scientific Research Fund of the Second Institute of
Oceanography, MNR (No. SZ2403;JG2213), the Key R&D Program of
Zhejiang (No.2023C03011), the National Key Research and
Development Program of China (No.2023YFC3108000) and the
National Basic Research Program of China (No.2010CB428900).
Acknowledgments
We would like to thank Prof Zhao Liang’s group from ocean
university of China for providing the hydrographic data. Thanks to
all the staff of the research vessel “Dofanghong2”for their help
during the cruise. We appreciate the data-collection assistants of
Yuming Cai, Haiyan Jin, and Zhibing Jiang, all affiliated with
the Second Institute of Oceanography, Ministry of Natural
Resources, China.
Conflict of interest
The authors declare that the research was conducted in the
absence of any commercial or financial relationships that could be
construed as a potential conflict of interest.
The author(s) declared that they were an editorial board
member of Frontiers, at the time of submission. This had no
impact on the peer review process and the final decision.
Publisher’s note
All claims expressed in this article are solely those of the authors
and do not necessarily represent those of their affiliated organizations,
or those of the publisher, the editors and the reviewers. Any product
that may be evaluated in this article, or claim that may be made by its
manufacturer, is not guaranteed or endorsed by the publisher.
Supplementary material
The Supplementary Material for this article can be found online at:
https://www.frontiersin.org/articles/10.3389/fmars.2025.1471650/
full#supplementary-material
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