Science of the Total Environment 739 (2020) 139999

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Science of the Total Environment
journal homepage: www.elsevier.com/locate/scitotenv

Dynamics of phytoplankton and nutrient uptake following dust additionsin the northwest Paciﬁc
Chao Zhang a,b, Jingyi He a,c, Xiaohong Yao a,b, Yingchun Mu a,d, Xinyu Guo a,e, Xiaokun Ding a, Yang Yu a,
Jinhui Shi a,b, Huiwang Gao a,b,⁎a
Frontiers Science Center for Deep Ocean Multispheres and Earth System, Key Laboratory of Marine Environment and Ecology, Ministry of Education of China, Ocean University of China, Qingdao
266100, Chinab
Laboratory for Marine Ecology and Environmental Sciences, Pilot National Laboratory for Marine Science and Technology, Qingdao 266071, Chinac
Key Laboratory of Environmental Protection in Water Transport Engineering Ministry of Communications, Tianjin Research Institute for Water Transport Engineering, M.O.T., Tianjin 300456,
Chinad
Estuarine and Coastal Environment Research Center, Chinese Research Academy of Environmental Sciences, Beijing 100012, ChinaeCenter for Marine Environmental Studies, Ehime University, Matsuyama, JapanH I G H L I G H T S

G R A P H I C A L

A B S T R A C T• Phytoplankton growth was limited by
N, Fe, or co-limited by Fe and P in the
NWP.
• Dust additions supplied N and Fe tostimulate phytoplankton growth.
• Dust-enhanced shift of phytoplankton
size structure varied in the KE and TR.
• Nutrient uptake ratio converged towards the Redﬁeld ratio.
• Phytoplankton growth and nutrient uptake showed consistent responses in the
KE and TR.

a r t i c l e

i n f oArticle history:
Received 18 March 2020
Received in revised form 2 June 2020
Accepted 3 June 2020
Available online 06 June 2020
Editor: Daniel Wunderlin
Keywords:
Dust deposition
Phytoplankton
Size structure
Nutrient concentrationa b s t r a c t
Dust deposition can supply nutrients that affect marine phytoplankton, but changing trophic statuses of the surface ocean increase the complexity of interpreting the process. In this study, four onboard incubation experiments amended with various nutrients and dust were performed in the Kuroshio Extension (KE) andKuroshio–Oyashio transition (TR) of the northwest Paciﬁc (NWP), which are characterised by lower and higher
trophic statuses, respectively. According to the nutrient-addition experiments, phytoplankton were limited bynitrogen (N) in the KE, and limited by iron (Fe) or co-limited by Fe and phosphorus (P) in the TR. Dust additions
supplied a considerable amount of N and Fe but negligible amount of P to stimulate phytoplankton growth, as
indicated by chlorophyll a (Chl a) concentration. In the KE incubations, dust additions enhanced the shift of phytoplankton size structure towards larger cells from dominantly pico-sized (0.2–2 μm) Chl a to comparable contributions from each size class (i.e. pico-, nano-: 2–20 μm, micro-: N20 μm). On the basis of the large shift ofsize structure towards nano- or micro-phytoplankton in the unamended control treatments in the TR, dust additions furtherly promoted the shift towards micro-phytoplankton becoming the dominant contributor to the total⁎ Corresponding author at: Frontiers Science Center for Deep Ocean Multispheres and Earth System, Key Laboratory of Marine Environment and Ecology, Ministry of Education of China,
Ocean University of China, Qingdao 266100, China.
E-mail address: hwgao@ouc.edu.cn (H. Gao).

https://doi.org/10.1016/j.scitotenv.2020.139999
0048-9697/© 2020 Elsevier B.V. All rights reserved.

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C. Zhang et al. / Science of the Total Environment 739 (2020) 139999

Nutrient ratio

Chl a. The collective analysis of the data from experiments in both regions revealed that, the extent of phytoplankton growth stimulation and the shift towards larger cells were enhanced gradually with increasing amountsof nutrient uptake (including N, P, and silicon). The nutrient uptake ratios of phytoplankton converged towards
the Redﬁeld ratio in comparison to the wider range of nutrient ratios in the dust-amended seawater. This study
suggested consistencies in the dynamic of phytoplankton growth, shift of size structure, and nutrient uptake following dust additions in the KE and TR, although the trophic status and limiting nutrient varied between these
two regions.
© 2020 Elsevier B.V. All rights reserved.

1. Introduction
Atmospheric deposition can supply considerable amounts of nutrients such as nitrogen (N), phosphorus (P), and iron (Fe) to the upper
ocean, and thus affects phytoplankton growth and the succession ofcommunity structure (Jickells et al., 2005, 2017; Mahowald et al.,2008). Atmospheric deposition has reportedly increased the N stocks
in the seawater of the north Paciﬁc and has the potential to change
the nutrient limitation status from N to P in the future (Kim et al.,2014). The well-known “Fe hypothesis” emphasises the importance of
Fe in enhancing primary productivity in high-nutrient low-chlorophyll(HNLC) regions, which cover ~25% of the global ocean (Martin, 1990;
Boyd et al., 2007). Atmospheric deposition has been suggested as an important source of Fe to these regions (Boyd and Ellwood, 2010). Dust
particles account for ~50% of the global particle emissions (Forster
et al., 2007) and act as an important carrier of bioavailable nutrients.
The bioavailability of nutrients in dust particles have been shown to increase during their long-range transport from the continents to theoceans (Schroth et al., 2009; Formenti et al., 2011; Schulz et al., 2012)
because of chemical reactions between anthropogenic air pollutantsand dust particles (Li et al., 2017; Qi et al., 2018; Shi et al., 2019). The increased N supply and the enhanced solubility of dust-borne nutrients
through dust deposition exert an important inﬂuence on primary productivity, and thus contribute to regulating the efﬁciency of carbon sequestration in the oceans (Mills et al., 2004; Ridame et al., 2014; Chu
et al., 2018; Zhang et al., 2018).
In conventional wisdom, the input of nutrients via dust deposition
can increase the concentrations of limiting nutrients and stimulate phytoplankton growth. For instance, Saharan-dust addition experiments in
the Mediterranean Sea demonstrated that the supply of N and P stimulated primary productivity (Ridame et al., 2014). In the Yellow Sea adjacent to China, a substantial input of N during Asian dust-storm events
can cause phytoplankton blooms (Shi et al., 2012). Recent studies
have shown that dust deposition can also affect phytoplankton by altering the nutrient ratio in seawater. By combining observations andmodelling, Chien et al. (2016) found that an excess input of N relative
to P via Sahara dust deposition increased the N:P ratio in the surface waters of the western tropical Atlantic, and favoured the growth of a picosized (0.2–2 μm cell diameter) phytoplankton (Prochlorococcus). The
baseline N:P ratio in the surface waters regarding parts of the East
China Sea has been shown to be higher than the Redﬁeld ratio, and additions of Asian dust further increased this N:P ratio and led to a succession of dominant species from diatoms to dinoﬂagellates (Zhang et al.,2019a). In general, as dust deposition simultaneously alters the nutrient
concentration and nutrient ratio of seawater (Mills et al., 2004; Ridame
et al., 2014; Zhang et al., 2018), it remains a challenge to distinguish theimpact of each of these factors on phytoplankton.
The response of the phytoplankton size structure to dust deposition
varies with the trophic statuses of seawater. In HNLC regions that arecharacterised by a low Fe availability, the considerable input of Fe supplied by dust deposition has the potential to favour the growth of all
types of phytoplankton due to the presence of substantially replete
macronutrient stocks, e.g. N, P, and silicon (Si). In this scenario, larger
phytoplankton (N2 μm cell diameter) such as diatoms can become thedominant algae as a result of their higher biomass accumulation rates(Boyd et al., 2007; Zhang et al., 2019a). However, nutrient stocks inmost of the global ocean are generally at relatively low levels and cannot
create an ideal environment for the various types of phytoplankton(Ridame et al., 2014; Zhang et al., 2019b). For instance, in the Yellow
Sea, the combination of P supplied by dust inputs plus the existing
stock in the surface waters can only meet a portion of the requirementsof phytoplankton; thus, phytoplankton remain as dominantly picosized cells even if there is an increase in the contribution of microsized (N20 μm cell diameter) cells to the total Chl a (Zhang et al.,2018). In extremely oligotrophic seawater, dust deposition is insufﬁcient to support the growth of larger phytoplankton, and therefore
only affects the succession of different pico-sized species (Maranon
et al., 2010; Chien et al., 2016). At present, research is fragmented dueto the variety of trophic statuses, and there is still an insufﬁcient understanding of the relevant mechanisms to interpret the general shift pattern that occurs in the phytoplankton size structure under the impactof dust deposition.
The study region of this research included the Kuroshio Extension(KE) and Kuroshio–Oyashio transition (TR) oceanic zones, which covera broad range of trophic statuses (Kitajima et al., 2009; Zhang et al.,2019b) in the northwest Paciﬁc (NWP). Asian dust is deposited frequently in this region, particularly during the spring each year (Schulz
et al., 2012). Accordingly, the atmospheric and oceanic conditions in
the NWP allowed us to pursue the following two objectives: (1) compare the impact of dust additions on phytoplankton in seawater
characterised by different trophic statuses, and (2) explore the relationship between the nutrient dynamics and phytoplankton growth andsize structure.
2. Materials and methods
2.1. Treatment and analysis of dust samples
Soil dust samples were collected in the Tengger Desert (38.79° N,
105.51° E), which is one of the primary sources of Asian dust storms.
We followed a protocol of dust treatment used in previous studies tosimulate the aging process taking place during the long-term atmospheric transport of dust from the continents to the oceans (Guieu
et al., 2010; Zhang et al., 2018, 2019a). Brieﬂy, dust samples were dry
sieved to obtain particles with a diameter of ≤20 μm. Then, the aging
process was simulated by mixing the selected dust particles with synthetic cloud water in a clean fume hood, and the dust samples were ﬁnally obtained after evaporating the aqueous phase. The chemical
analyses (nutrients and soluble trace metals) of the dust followed theprotocol described in Zhang et al. (2018).
2.2. Seawater sampling and incubation experiment
Four incubation experiments were performed in the KE (sites M1
and M1B) and TR (sites E10M and E02) during the RV Dongfanghong
II spring cruise of 2016 (21 March to 26 April, Fig. 1). Surface seawater (2–5 m) was collected using a Sea-Bird CTD rosette equippedwith 24 12-L Teﬂon bottles. Immediately after collection (day 0), a
seawater subsample was used to determine the nutrient concentrations as well as the total and size-fractionated chlorophyll a (Chl a)

C. Zhang et al. / Science of the Total Environment 739 (2020) 139999

3

Fig. 1. Locations of the incubation sites during the 2016 spring cruise in the northwest Paciﬁc.

concentrations. The remaining seawater was ﬁltered through a
200 μm mesh and then evenly decanted to 20 L and 2 L polycarbonate
bottles (Nalgene, each in triplicate, detailed in Table 1). After the addition of dust and nutrients, the bottles were transferred into threelarge plastic vessels and incubated for 5 or 6 days under the condition of a continuous supply of pumped surface seawater. The incubation microcosm system was exposed to ~40% attenuation of onboard
light intensity using a neutral density screen (Guo et al., 2012). Further details of the incubation experiments can be found in (Zhang
et al., 2018, 2019a).

2.3. Measurement of nutrients and total and size-fractionated Chl a
The vacuum ﬁltration method (with a vacuum level of ≤0.02 MPa)
was used to collect samples for the determination of nutrient and Chl
a concentrations (Strickland and Parsons, 1972; Fujiki et al., 2014).
The nutrient samples were collected on day 0, day 3 (only for 20 L bottles), and on the last day (day 5 or 6) of the incubations. Seawater(200 mL) collected from the incubation bottles was ﬁltered through0.45 μm acid-cleaned cellulose acetate ﬁlters (Ximengdou, China), intopre-rinsed (three times with seawater) 125 mL high-density polyethylene bottles, which were subsequently stored at −20 °C awaiting analysis (Liu et al., 2012; Lu et al., 2018). In the laboratory, an AA3 continuous
ﬂow analyser was used to determine the concentrations of nutrients, in−3−
cluding NO−
3 + NO2 (N + N), PO4 , and Si(OH)4. The detection limit ofTable 1Treatments to the microcosm incubation experiments.
Treatmentsa
Control (20 L, 2 L)
Dust low
Dust medium
Dust high
N
P
Fe
N+P
N + P + Feb

Incubation sitescAll
E02, E10M
All
M1B, E02, E10M
All
All
E02, E10M
All
M1, M1B

Amended concentrations–
0.3 mg L−1
0.5 mg L−1
1 mg L−1
1 μmol L−1
0.2 μmol L−1
2 nmol L−1
1 μmol L−1 + 0.2 μmol L−1
1 μmol L−1 + 0.2 μmol L−1 + 2 nmol L−1aDust addition experiments were carried out in 20 L incubation bottles, and nutrient
addition experiments were carried out in 2 L incubation bottles.
b
The substrate added as N, P, Fe is NaNO3, NaH2PO4, and FeCl3, respectively.
c‘All’ represents sites M1, M1B, E02, and E10M.

−1
N + N, PO3−
, 0.02 μmol L−1, and
4 , and Si(OH)4 was 0.02 μmol L
0.03 μmol L−1, respectively.
Samples for Chl a were collected on each day of the incubations. For
the 20 L bottles, seawater (200–300 mL) was sequentially ﬁlteredthrough 20 μm, 2 μm, and 0.2 μm ﬁlters (Millipore nylon ﬁlter with a
20 μm pore size, Whatman polycarbonate ﬁlters with 2 μm and 0.2 μm
pore sizes) to collect micro-sized, nano-sized (2–20 μm cell diameter),and pico-sized Chl a pigments. The total Chl a concentration was determined as the sum of the values of the different size fractions. For the 2 L
bottles, the total Chl a was obtained directly by ﬁltering through a
Whatman GF/F glass ﬁbre ﬁlter. After the extraction treatment in a
90% acetone solution (over 24 h, at −20 °C in the dark), the Chl a concentration was determined directly onboard the research vessel usinga Trilogy ﬂuorometer (Turner Designs, Strickland and Parsons, 1972),
which was pre-calibrated using a pure Chl a standard (Sigma-Aldrich,
Welschmeyer, 1994).

2.4. Data analysis
To distinguish the trophic status at each of the four incubation sites,
the nutrient index (NI, μmol L−1) deﬁned by Otto (1998) was used,which is expressed by Eq. (1):pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
NI ¼ 3 C N  C P  C Si ;

ð1Þ

where CN, CP, and CSi are the concentrations of N + N, PO3−
4 , and Si(OH)4, respectively. The NI can be regarded as the normalised concentration of nutrients including N + N, PO 3−
4 , and Si(OH) 4 . We
further deﬁned two additional terms: the ‘supply NI’ (hereafterNI S) and the ‘decreased NI’ (hereafter NID ) in this study. The NIS
was calculated by regarding C N, C P, and CSi as the concentrationsof N + N, PO3−
4 , and Si(OH)4, respectively, in the incubated seawater on day 0 in the control and dust treatments. The NID was calculated by regarding C N , C P , and C Si as the decrease in theconcentrations of N + N, PO3−
4 , and Si(OH)4, respectively, that occurred between day 0 and the end of the incubations in the control
and dust treatments.
One-way analysis of variance (ANOVA) was used to provide evidence of whether there were signiﬁcant differences in the Chl a concentration during the incubations among various treatments.

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C. Zhang et al. / Science of the Total Environment 739 (2020) 139999

3. Results
3.1. Original seawater characteristics
The trophic status indicated by the NIS in the original seawater generally increased from sites M1B (0.20 μmol L−1) and M1 (0.55 μmol L−1)
in the KE to sites E10M (1.39 μmol L−1) and E02 (2.12 μmol L−1) in the
TR (Table 2). The concentrations of N + N, PO3−
4 , and Si(OH)4 at site E02

were as high as 4.52 μmol L−1, 0.35 μmol L−1, and 6.12 μmol L−1, respectively, and those at site M1B were as low as 0.11 μmol L−1, 0.05 μmol L−1,
and 1.42 μmol L−1, respectively. The N stock in the original seawater
was deﬁcient at sites M1 and M1B according to the lower ratios of N:P(≤8:1) and N:Si (b1:2) in comparison to the Redﬁeld ratios (N:P =
16:1; N:Si = 16:15). In contrast, the ratios of N:P and N:Si in the original
seawater were close to the Redﬁeld ratios at sites E02 and E10M. The
observed total Chl a concentration was relatively low (≤1.20 μg L−1) at

Table 2
Baseline conditions of sampled seawater used for incubations.

Incubation time
NIS (μmol L−1)
−1
−
NO−
)
3 + NO2 (μmol L
−1
PO3−
)
4 (μmol L
−1
SiO3−
)
4 (μmol L
N:P (μmol:μmol)
N:Si (μmol:μmol)
Chl a (μg L−1)
Micro-sized Chl a (%)
Nano-sized Chl a (%)
Pico-sized Chl a (%)

M1

M1B

E02

E10M

2016.3.28–2016.4.2
0.55
0.79
0.10
2.19
~8:1
~1:2.8
0.76
14
21
65

2016.4.18–2016.4.23
0.20
0.11
0.05
1.42
~2:1
~1:12.9
0.50
7
22
71

2016.4.3–2016.4.9
2.12
4.52
0.35
6.12
~13:1
~1:1.4
0.63
2
27
71

2016.4.10–2016.4.16
1.39
2.74
0.23
4.34
~12:1
~1:1.6
1.16
12
21
67

Fig. 2. Changes in the (a) Chl a concentration, and (b) N + N, PO3−
4 , and Si(OH)4 concentrations in the control and nutrient treatments during the incubations. Data are presented as themean and standard deviation of three replicate incubation experiments. The down arrows indicate that the nutrient concentrations were below the detection limit.

C. Zhang et al. / Science of the Total Environment 739 (2020) 139999

all sites. The contribution of pico-sized Chl a to the total Chl a in the original seawater was N65%, and was followed by nano-sized Chl a(21–27%) and micro-sized Chl a (2–14%, Table 2).
Nutrient limitation in the original seawater was explored in the
nutrient-addition experiments (Fig. 2). At sites M1 and M1B, the Chl aconcentration in N alone treatments was signiﬁcantly higher than thatin the control and P alone treatments (days 3–5, p b 0.05), and was comparable to that in the N + P and N + P + Fe treatments (Fig. 2a). In contrast to the amended values on day 0, the N + N concentration in N
alone, N + P, and N + P + Fe treatments decreased by N75% at theend of the incubations (day 4, Fig. 2b). Moreover, the addition of
N + P and N + P + Fe enhanced the uptake of PO3−
4 in comparison tothat of the P alone treatments (Fig. 2b). Hence, phytoplankton at sites
M1 and M1B were primarily limited by N. At site E02, the addition of
Fe alone increased the Chl a concentration (days 3–5) relative to thecontrol and other nutrient treatments, although there was no signiﬁcantdifference between them. The N + N and PO3−
4 concentrations at theend of the Fe-amended incubations (day 5) were lower than those in
the control treatment. This indicates that phytoplankton at site E02
were probably limited by Fe. Different from site E02, the Chl a concentration at site E10M responded positively to the additions of Fe alone
and P alone (days 4–5, p b 0.01). The decreased concentration of
N + N during the incubations for the additions of Fe alone and P alone
was 58% and 14% higher relative to the control treatment, respectively(Fig. 2b); hence, phytoplankton at site E10M were co-limited by Fe
and P.
3.2. Phytoplankton response to dust addition in the KE
In the control treatment at site M1, the total Chl a concentration increased by 69% to the maximum value of 1.28 μg L−1 on day 2, and thenshowed a decreasing trend until the end of the incubations (Fig. 3). The
contributions of pico-sized Chl a to the total Chl a decreased from ~65%
on day 0 to b50% on days 3–5 (Fig. 4). Correspondingly, the contributions of micro-sized and nano-sized Chl a to the total Chl a bothexceeded 20% on days 3–5. At site M1B, however, there was no obvious
increase in the Chl a concentration in the control treatment. The

5contributions of micro-sized and nano-sized Chl a to the total Chl a increased slightly, while pico-sized Chl a was always the dominant contributor to the total Chl a (57%–71%) throughout the incubations(Fig. 4).
At sites M1 and M1B, the medium and high dust additions supplied a
considerable amount of N, while the added amounts of P and Si were
negligible relative to the original seawater (Tables 2 and 3). The N supplied by the medium dust addition was 57% and 401% higher than thebaseline N + N concentration at sites M1 and M1B, respectively(Table 3, Fig. 5). As a result, in contrast to the control treatments, dust
additions led to a signiﬁcant increase in the total Chl a concentration(p b 0.05) and increased the contributions of micro-sized and nanosized cells to the total Chl a. However, the stimulation effect varied at
the two sites (Figs. 3 and 4). At site M1, the maximum Chl a concentration of 1.68 μg L−1 on day 2 in the medium dust treatment was 30%
higher than that in the control treatment (1.28 μg L−1 on day 2). Phytoplankton in each size class made comparable contributions to the total
Chl a on days 4–5. At site M1B, the medium and high dust additions
both increased the maximum Chl a concentration on day 2 by 73% and108%, respectively, relative to the control treatment concentration of
0.55 μg L−1 on day 1. The size shift towards larger cells at site M1B
was not as obvious as that at site M1.
The SN:SP and SN:SSi ratios in all treatments (hereafter only for the
20 L incubations unless otherwise speciﬁed) varied within ~2–18
and ~0.1–0.7 at the two sites, respectively. At the end of the incubations, N + N and PO3−
were almost exhausted by phytoplankton in4
all treatments (Fig. 5), thus the DN :D P ratio ranged between ~14
and ~24 in the dust treatments, and between ~3 and ~10 in the control treatments. The medium dust additions at site M1 enhanced theuptake of Si(OH) 4 by phytoplankton in comparison to the control
treatment, while the DN:DSi ratio only changed slightly both in thecontrol (~0.9) and dust treatments (~0.8). At site M1B, the Si(OH)4concentration in the incubated seawater remained relatively stablein the control and medium dust treatments, and a clear decline in
the Si(OH)4 concentration was only observed at the end of the highdust treatment (on day 5). Consequently, the DN:DSi ratio in all treatments varied from ~2.1 to ~4.2.

Fig. 3. Changes in the Chl a concentration in the control and dust treatments during the incubations. “Dust-L”, “Dust-M”, and “Dust-H” indicate low (0.3 mg L−1), medium (0.5 mg L−1), andhigh (1 mg L−1) dust additions, respectively. Data are presented as the mean and standard deviation of three replicate incubation experiments.

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C. Zhang et al. / Science of the Total Environment 739 (2020) 139999

Fig. 4. Shifting patterns in the contribution of size-fractionated Chl a (micro-size: N20 μm; nano-size: 2–20 μm; pico-size: 0.2–2 μm) to the total Chl a in the control and dust treatments
during the incubations. Data are presented as the mean and standard deviation of three replicate incubation experiments. The up arrows correspond to the date when the maximum total
Chl a concentration was obtained.

Table 3Chemical characteristics of dust and increased nutrient concentrations in the incubated bottles amended with low (0.3 mg L−1), medium (0.5 mg L−1), and high (1 mg L−1) additions ofdust.
Nutrients

Content (μmol g−1)b
Dust low (nmol L−1)c
Dust medium (nmol L−1)
Dust high (nmol L−1)

Soluble trace metals

DINa

N+N

NH+
4

PO3−
4

Si(OH)4

Fe

Mn

Cu

Cr

Co

Zn

882
265
441
882

879
264
440
879

2.66
0.80
1.33
2.66

0.85
0.26
0.43
0.85

1.55
0.47
0.78
1.55

1.24
0.37
0.62
1.24

3.16
0.95
1.58
3.16

0.027
0.008
0.014
0.027

0.039
0.012
0.020
0.039

0.036
0.011
0.018
0.036

0.161
0.048
0.080
0.161

DIN includes N + N and NH+
4 .
Calculated by determining concentrations of nutrients and soluble trace metals released from dust in the pre-ﬁltered oligotrophic seawater (Zhang et al., 2018).
c
Calculated by dividing the released amounts of nutrients and soluble trace metals at different dust loadings by the incubation volume of 20 L.
ab3.3. Phytoplankton responses to dust additions in the TR
Over the duration of the incubations, the Chl a concentration in thecontrol treatment increased to varying degrees at sites E02 and E10M.
The maximum Chl a concentration occurred on day 2 at site E02 andon day 6 at site E10M, and was 81% and 134% higher than that of theoriginal seawater, respectively (Fig. 3). The dominant phytoplankton
size class changed from pico-size in the original seawater to nano-sizeon day 6 at site E02 and to micro-size on days 4–6 at site E10M. Correspondingly, the contribution of pico-sized cells to the total Chl a

C. Zhang et al. / Science of the Total Environment 739 (2020) 139999

7

Fig. 5. Changes in the N + N, PO3−
4 , and Si(OH)4 concentrations in the control and dust treatments during the incubations. Data are presented as the mean and standard deviation of three
replicate incubation experiments. The down arrows indicate that the nutrient concentrations were below the detection limit.

decreased to b37% at the end of the incubations (day 6) at the two sites(Fig. 4).
Dust additions increased the N + N concentration by 6%–20% and10–32% relative to the original seawater concentration at sites E02
and E10M, respectively. The SN:SP and SN:SSi ratios in all treatments varied within ~12–16 and ~0.6–0.9, respectively. The Chl a concentration in
the dust treatments increased signiﬁcantly relative to the control treatment (p b 0.05) and also increased with dust loadings (Fig. 3).
At site E02, the maximum Chl a concentration in the low, medium,and high dust treatments (on day 6) was 71%, 120%, and 222% higher
than that in the control treatment, respectively (Fig. 3). During theﬁrst 4 days of the incubations, the contributions of each sized cells inthe dust treatments were comparable to those in the control treatment.
Dust additions increased the contribution of micro-sized cells obviously
at the end of the incubations (day 6, Fig. 4). Moreover, different from thedominance of nano-sized cells on day 6 in the control treatment, microsized cells dominated in the dust treatments. After the addition of duston day 0, the N + N and PO3−
4 concentrations generally decreased by
N45%, whereas Si(OH)4 decreased by b25%, and all of these nutrientswere still in surplus at the end of the incubations. The DN:DP and DN:
DSi ratios in all treatments varied within ~15–20 and ~2.8–4.4,
respectively.
At site E10M, the maximum Chl a concentration in the three dust
treatments was only 26%–80% higher than that in the control treatment(Fig. 3). Dust additions had a slight impact on phytoplankton size structure relative to the control treatment (days 4–5, Fig. 4). The contributionof micro-sized cells to the total Chl a reached N67% on day 6 in the control and dust treatments. The concentrations of N + N, PO3−
4 , and Si(OH)4 in the dust treatments decreased by more than ~90% on day 6 relative to the amended values on day 0. As a result, the DN:DP and DN:DSiratios in all treatments were ~14–17 and ~0.8–1.4, respectively (Fig. 5).
Collectively, our results demonstrated the following: 1) Dust additions stimulated phytoplankton growth, whereby the extent of thegrowth stimulation increased with dust loadings. 2) Dust additions further promoted the shift of the phytoplankton size structure towards
larger cells on the basis of the spontaneous shift in the control treatment. Although micro-phytoplankton became the dominant contributor to the total Chl a after the addition of dust in the TR experiments,
the extent of dust induced promotion on phytoplankton size structure
at most of incubation periods was not as obvious as that in the KE experiments. 3) Obvious declines in the N + N and PO3−
4 concentrations wereobserved in every dust treatment at all sites. The Si(OH)4 concentration
only clearly declined at site M1 in the KE experiments and site E10M in
the TR experiments.
4. Discussion
4.1. Stimulation mechanisms in the dust addition experiments
The limiting nutrients in the original seawater generally determined
the key nutrients in dust that stimulated phytoplankton growth. At sites
M1 and M1B, which were characterised by N limitation, the8C. Zhang et al. / Science of the Total Environment 739 (2020) 139999

considerable input of N in the dust treatments alleviated N limitationand led to a signiﬁcant increase in the Chl a concentration (Fig. 3). The
enhanced uptake of nutrients (e.g. N + N and PO3−4 ) after the additionof dust also supported the key role of N in stimulating phytoplankton
growth (Fig. 5). The important fertilisation effect of dust-derived N
has been reported widely in the KE and other oceanic regions, where
seawaters were characterised by the low availability of N (Mills et al.,
2004; Shi et al., 2012; Ridame et al., 2014; Zhang et al., 2018). We
note that the decline of the Si(OH)4 concentration in the dust treatments at site M1B was not as clear as that at site M1 (Fig. 5). In general,
Si(OH)4 was primarily absorbed by diatoms, which have a growth advantage in a nutrient-replete environment and are easily limited by
the low availability of nutrients (Tréguer and De La Rocha, 2013).
Hence, in contrast to site M1, the lower nutrient stocks in the seawaterat site M1B more likely limited the growth of diatoms.
Sites E02 and E10M in the TR were inﬂuenced by the intrusion of the
Oyashio current, as indicated by HNLC characteristics (Boyd et al., 2007;
Isada et al., 2019); thus, phytoplankton responded positively to the addition of Fe (Fig. 2a). Dust additions supplied a certain amount of soluble
Fe (0.37–1.24 nmol L−1) and stimulated phytoplankton growth(Table 3, Fig. 3). An enhanced uptake of nutrients was also observed
after additions of Fe alone and dust (Figs. 2b, 5). Although the Fe concentration in the sampled seawater was not determined in this study,
we could deduce the extent of Fe deﬁciency based on the uptake of N
or P to some extent. At site E02, according to the Redﬁeld ratio of N:
Fe = 16:0.0075 (Klausmeier et al., 2008), the Fe supplied by low, medium, and high dust additions could drive phytoplankton to absorb N
at 0.79, 1.32, and 2.64 μmol L−1, which were lower than the N + N concentrations in the dust treatments on day 0 (Table 3, Fig. 5). This wasconsistent with the surplus of N + N at the end of the dust-amended incubations (Fig. 5). A similar condition was also observed for P. On thecontrary, N and P nutrients in the dust treatments were almost
exhausted at site E10M, thus suggesting that the Fe stocks after dust additions were relatively sufﬁcient. Further, a phytoplankton bloom might
have occurred based on the signiﬁcant increase in the Chl a concentration in the control treatment (Fig. 3). Indeed, blooms occur frequently
in the study area under the conditions of favourable irradiance, temperature, and nutrient resources during the spring (Isada et al., 2019; Zhang
et al., 2019a). On the other hand, phytoplankton also responded positively to P additions, even in the relatively P-replete seawater at site
E10M (Fig. 2a). Arrigo (2005) argues that phytoplankton can increase
their P requirement to meet cell proliferation during the bloom period.
Considering the negligible P supplied by dust additions relative to theoriginal seawater (Tables 2 and 3), the impact of dust-derived P would
have been minor in our study.
In contrast to the control treatment, the shift of the phytoplankton
size structure towards larger cells in the dust treatments was evenlarger after the peak Chl a concentration (Fig. 4). At sites M1 and M1Bin the KE, the Chl a concentration in the dust treatments peaked on
day 2, with pico-phytoplankton making the primary contribution(Fig. 4). With the exhaustion of nutrients on day 3 (Fig. 5), the growthof all phytoplankton size classes was limited. However, the luxury nutrient uptake of larger phytoplankton could sustain their growth for a longer time, and thus increased their contribution to the total Chl a on days
3–5 (Figs. 4 and S1, Maranon, 2015). In contrast, as the peak Chl a concentration increased from the KE to the TR, micro-phytoplankton became the dominant contributor at sites E02 (day 6) and E10M (day4) due to their higher upper-limit of growth in nutrient-replete seawater (Maranon, 2015). We note that the impact of dust additions on thephytoplankton size structure at site E10M was not as clear as that atsite E02 (Fig. 4). This was ascribed to a bloom at site E10M, whereby
micro-phytoplankton has made the dominant contribution to the total
Chl a during the incubations in the control treatment (Zhang et al.,2019a). On the other hand, we found a clear decline in the Si(OH)4 concentration when the dominant contributor to the total Chl a changed
from pico-phytoplankton to micro-phytoplankton (days 4–6) at site

E10M (Fig. 5). This was probably due to the presence of diatoms,
which have a higher requirement for Si(OH)4 in comparison to otherphytoplankton (Tréguer and De La Rocha, 2013). The fast growth of diatoms such as Chaetoceros spp. and/or Nitzschia spp. has been reported
previously on the basis of in-situ iron enrichment experiments and onboard bottle incubation experiments in the NWP (Takeda and Tsuda,
2005; Zhang et al., 2019a). Similarly, at site E02, we only found a slightdecline in the Si(OH)4 concentration at the end of the incubations due tothe dominance of pico-phytoplankton on days 0–4 (Fig. 4). Indeed, theshift in the phytoplankton size structure towards larger cells is a stepby-step process in the context of the dominance of pico-sized cells in
the original seawater and the longer generation times of larger cells(Maranon, 2015; Zhang et al., 2018).
In addition to Fe, dust additions also provided a considerable amountof other trace metals that may have also affected phytoplankton growth(Table 3). For instance, the low availability of zinc (Zn) has been reported to limit phytoplankton growth (especially for diatoms) in thesurface seawater of the north Paciﬁc (Crawford et al., 2003; Jakuba
et al., 2012; Sunda, 2012). In some speciﬁc conditions, cobalt (Co) cansubstitute the function of Zn to meet the phytoplankton requirement(Saito et al., 2008). These hypotheses need to be further substantiatedin future studies by determining the trace metal concentrations in thesampled seawater and by carrying out related addition experiments.
In our study, however, the dust-derived N and Fe clearly played important roles in stimulating phytoplankton growth.
4.2. Impact of nutrient concentration on phytoplankton growth and size
structure
To compare the impacts of the different dust additions on phytoplankton growth at the four incubation sites, we proposed the dust conversion index (DCI, in μg mg−1·d−1) to reﬂect the average increase in
the Chl a concentration (relative to the control treatment) as induced
per unit mass of dust on each day of the incubations. The DCI is deﬁned
as follows:tXðChl aDi −Chl aCi Þ

DCI ¼ i¼0

tA

;

ð2Þ

where Chl aCi and Chl aDi represent the Chl a concentration on incubation day i in the control and dust treatments (μg L−1), respectively, t is
the number of incubation days, and A is the amended dust concentration (mg L−1) in the incubated seawater.
As shown in Fig. 6a, the DCI presented a decreasing trend between sites, from 0.96 to 1.39 μg mg−1 ·d −1 at site E10M, 0.90–0.95 μg mg−1 ·d−1 at site E02, 0.67 μg mg−1·d −1 at site M1, to
0.53–0.54 μg mg−1·d−1 at site M1B. These ﬁndings suggest a higher
conversion efﬁciency of dust to Chl a in Fe-limited conditions (sites
E02 and E10M) than in N-limited conditions (sites M1 and M1B).
The trophic statuses at sites E02 and E10M were also higher thanthose at sites M1 and M1B (Table 2). Accordingly, we examined therelationship between i) the NIS and DCI, and ii) the NID and DCI. Although there was no signiﬁcant correlation between the NI S andDCI (Fig. 6a), a good linear correlation was determined between
the NID and DCI (R2 = 0.70, p b 0.01, Fig. 6b). Indeed, the NIS represents the standing nutrient stock in the environment, but it does
not necessarily represent the actual phytoplankton requirements.
In contrast to the NIS, the NID reﬂects the amount of nutrient uptake
and thus directly affects phytoplankton growth. Hence, the nutrient
supply and phytoplankton requirements were likely disconnectedduring the incubation periods. Such a disconnection has also been reported elsewhere, for instance, N:P ratio higher than the Redﬁeld
ratio indicates relative P-starvation, while nutrient-addition experiments have shown that phytoplankton had a more urgent requirement for N rather than P in the surface seawater of the East China

C. Zhang et al. / Science of the Total Environment 739 (2020) 139999

9

Fig. 6. Correlations between the DCI and (a) NIS, and (b) NID in the dust treatments at the four incubation sites. The DCI represents the average increase in the Chl a concentration relative tothe control treatment, as induced per unit mass of dust on each day (Eq. (2)). The NIS (NID) represents the geometric mean of the N + N, PO3−
4 , and Si(OH)4 concentrations on day 0 (thedecreased N + N, PO3−
4 , and Si(OH)4 concentrations between day 0 and the last day of the incubations, Eq. (1)).

Sea and the Mediterranean Sea (Tanaka et al., 2011; Zhang et al.,2019a). Additionally, although macronutrient stocks are replete in
the seawater of HNLC regions, the low availability of Fe restricts thephytoplankton requirements for these nutrients (Boyd et al., 2007;
Isada et al., 2019).
The changes in the phytoplankton size structure can be indicated by
the contributions of pico-sized (RP), nano-sized (RN), and micro-sized(RM) cells to the total Chl a (Maranon, 2015; Li et al., 2018; Zhang
et al., 2019a). However, it is difﬁcult to distinguish the actual amountof nutrient uptake by the phytoplankton in each size class. To explore
the relationship between the phytoplankton size structure and the nutrient concentrations/ratios, we introduced one parameter, the shiftindex (SI, Eq. (3)), to replace the original three parameters, and to better
reﬂect the shift of size structure under the interactions of phytoplankton
in each size class.
SI ¼ ð−1Þτ 

qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
ðRP −RN Þ2 þ ðRM −R N Þ2 ;

ð3ÞFig. 7. Correlations between the SI and NID (μmol L−1) in the control and dust treatments at the four incubation sites. The SI represents the phytoplankton size structure as indicated by thesize-fractionated Chl a on day 3/the last day of the incubations (Eq. (3)). NID represents the geometric mean of the decreased N + N, PO3−
4 , and Si(OH)4 concentrations in the control anddust treatments between day 0 and day 3/the last day of the incubations (Eq. (1)). Data was excluded when the difference between the mean nutrient concentrations was smaller thanzero. The blue open symbols indicate that the data are from this study, and olive crosses indicate that the data are from our previous studies (Zhang et al., 2018, 2019a, 2019b). (Forinterpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

10

C. Zhang et al. / Science of the Total Environment 739 (2020) 139999

where τ is “0” when RP N RM, and τ is “1” when RP ≤ RM. Larger absolutevalues of positive SI correspond to a more obvious shift towards smaller
sized cells (i.e. micro-size → nano-size → pico-size), whereas larger absolute values of negative SI correspond to a more obvious shift towards
larger sized cells (pico-size → nano-size → micro-size). Negative/positive SI values close to 0 correspond to a more even distribution of eachsize class. The relationship between the SI and Chl a-associated size fractions is provided in Fig. S2.
A signiﬁcant negative correlation was determined between the NID
and SI values from the control and dust treatments (R2 = 0.85,
p b 0.01, Fig. 7). In addition, a similar trend was observed when we
added data from our previous studies in the subtropical gyre of the
NWP, Yellow Sea, and East China Sea (Zhang et al., 2018, 2019a,
2019b, Fig. 7). Moreover, such a trend has also been veriﬁed to some extent by other studies. For instance, in the South China Sea, the estimated
NID in high aerosol treatments (0.19 mg L−1) was ~0.83 μmol L−1, whichcorresponded to the dominance of micro-phytoplankton (Guo et al.,2012) and was consistent with the result as shown in Figs. 7 and S2.
During a mesoscale Fe-fertilisation experiment conducted in the polar
Southern Ocean, the estimated NID reached N1 μmol L−1 (after
12 days) when phytoplankton with a cell length of 30–50 μm dominated the community (Boyd et al., 2000). This is consistent with the relationship that the primary contributor of micro-sized cells corresponds
to higher NID values (Figs. 7 and S2).

The NID spread over a relatively wide range under the condition of
SI b 0 when the data from our previous studies were included (Fig. 7).
This likely occurred because larger phytoplankton can dominate thecommunity across a wider range of trophic statuses, whereas picosized phytoplankton dominate in oligotrophic seawaters (Moore et al.,2013; Maranon, 2015; Xiao et al., 2018; Zhang et al., 2019b). On theother hand, Maranon (2015) argues that nano-phytoplankton dominate
the total biomass in medium and high trophic statuses, which does notagree with the dominance of micro-phytoplankton in our study (Figs. 4
and S2). This discrepancy may primarily relate to the variable C:Chl amass ratio under the combined inﬂuence of nutrients, irradiance, andtemperature (Maranon, 2015; Burt et al., 2018; Liu et al., 2019). The different size partitioning methods on the basis of Chl a (pigment collection using ﬁlters of different pore sizes) and biomass (measurement ofcell abundance and volume) could also contribute to this discrepancy.
Besides, due to the enhanced package effect (i.e. light shading by pigment molecules) with increasing cell size, micro-phytoplankton needto synthesise more intracellular pigments to support their growth(Maranon, 2015). Hence, when Chl a is used to indicate the phytoplankton size structure, the dominance of micro-phytoplankton is a common
phenomenon in medium and high trophic statuses across the global
ocean (Guo et al., 2012; Maranon et al., 2012; Maranon, 2015; Li et al.,2018). Overall, the established relationship between the NID and SI
make it possible to monitor the change in the nutrient concentration

Fig. 8. Correlations between the (a) DCI and the SN:SP ratio, and DN:DP ratio, (b) DCI and the SN:SSi ratio, and DN:DSi ratio, (c) SI and the SN:SP ratio, and DN:DP ratio, and (d) SI and the SN:SSi
ratio, and DN:DSi ratio in the dust treatments at the four incubation sites. The dashed lines indicate the Redﬁeld ratios of N:P (16:1) and N:Si (16:15). The DCI reﬂects the average increase in
the Chl a concentration relative to the control treatment, as induced per unit mass of dust on each day (Eq. (2)), and SI reﬂects the phytoplankton size structure as indicated by the sizefractionated Chl a on the last day of the incubations (Eq. (3)). Where SN/P/Si (μmol L−1) is the mean concentration of N + N, PO3−
4 , or Si(OH)4 in the dust treatments on day 0, and DN/P/Si(μmol L−1) is the difference of the mean concentration of N + N, PO3−
4 , or Si(OH)4 in the dust treatments between day 0 and the last day of the incubations.

C. Zhang et al. / Science of the Total Environment 739 (2020) 139999

to reﬂect the phytoplankton size structure, although additional research(e.g. conducting experiments in different seasons and covering more
extensive trophic statuses) is also required to improve this correlation
further.
4.3. Impact of the nutrient uptake ratio on phytoplankton growth and size
structure
In conventional wisdom, phytoplankton generally absorb nutrients
according to the constant proportions of the Redﬁeld ratio. However,an increasing number of studies have found that the nutrient uptake
ratio of phytoplankton varies with changes in the environmental nutrient ratios, especially when the growth rate of phytoplankton is relatively low (Goldman et al., 1979; Arrigo, 2005; Klausmeier et al., 2008;
Hillebrand et al., 2013; Zhang et al., 2018). This is because the nutrient
ratios in seawater are not always equal to the Redﬁeld ratio, and phytoplankton have the potential to acclimatise via mechanisms such as
adjusting their stoichiometry and performing a luxury uptake of one
or more abundant nutrients to cope with environmental changes(Moore et al., 2013). The changing nutrient ratios in a given environment can have a pronounced impact on phytoplankton growth andcommunity structure due to the varying acclimatisation capacities ofeach species (Chien et al., 2016; Xiao et al., 2018; Zhang et al., 2019a).
Hence, on the basis of large amounts of observation and laboratory
data, it can be considered that the Redﬁeld ratio is an average value ofphytoplankton stoichiometry due to the trade-off processes between
phytoplankton requirements and environmental changes, rather thanreﬂecting the actual nutrient uptake ratio (Klausmeier et al., 2004;
Arrigo, 2005; Finkel et al., 2010; Moore et al., 2013; Gruber andDeutsch, 2014; Zhang et al., 2018).
In our study, the increased nutrient concentrations (mainlyN) that occurred after dust additions were always accompanied bya change in the nutrient ratio (i.e. SN:SP and S N:S Si ). The lattermight also positively or negatively affect phytoplankton growthand size structure (Chien et al., 2016; Zhang et al., 2019a). To identify

11

the impact, we examined the relationships of the SN:SP and SN:SSiwith the DCI and SI (Fig. 8). No signiﬁcant correlations were foundwhen pairing these two groups. When the SN:SP ratio was replacedwith the DN:DP ratio, most of the DCI values (~89%) were distributed
within a narrow range of ~14–17, which is close to the Redﬁeld ratio(Fig. 8). The same was true for SI values. We note that DCI values atsites M1B and E02 deviated far from the Redﬁeld ratio when the
DN:DSi ratio was used, which was ascribed to the lower Si(OH)4 requirement of phytoplankton (Fig. 5). On the other hand, when thecontrol and dust treatments were analyzed together, we found thatthe DN:DP and DN:DSi ratios followed the Redﬁeld ratio statistically(Fig. 9).
Therefore, our results demonstrate the selective uptake of various
nutrients by phytoplankton in the study region. Indeed, on a shorttime scale, P and Si stocks are generally substantial relative to the N
stock in the open oceans, thus leading to lower N:P and N:Si ratios incomparison to the Redﬁeld ratio (Moore et al., 2013; Kim et al., 2014;
Martino et al., 2014). Dust deposition, which is characterised by a substantial N input relative to P and Si, tends to increase the N:P and N:Si
ratios in seawater (Shi et al., 2012; Ridame et al., 2014; Zhang et al.,2018). This facilitates phytoplankton to absorb nutrients according to
the Redﬁeld ratio. The fast growth of phytoplankton induced by dust additions has also been found to weaken their dependence on the nutrient
ratios in seawater (Goldman et al., 1979; Klausmeier et al., 2008;
Hillebrand et al., 2013; Zhang et al., 2019a). Moreover, the averagevalue of phytoplankton stoichiometry (as indicated by the Redﬁeld
ratio) also reﬂects a convergent phenomenon in comparison to thehighly variable nutrient uptake ratios reported in different case studies(Guo et al., 2014; Ridame et al., 2014; Chien et al., 2016; Li et al., 2018;
Zhang et al., 2019a).
In this study, the relatively constant nutrient uptake ratio of phytoplankton may indirectly reﬂect biotic control over the oceanic nutrient
ratio to some extent (Falkowski, 2000; Arrigo, 2005). Indeed, over along time scale, the oceanic nutrient ratio has the potential to follow
the Redﬁeld ratio due to the inﬂuence of phytoplankton (Falkowski,

Fig. 9. Correlations between the (a) DN and DP, and (b) DN and DSi during the incubations. Where DN/P/Si (μmol L−1) is the difference of mean concentrations of N + N, PO3−
4 , or Si(OH)4 in
the control and dust treatments between day 0 and day 3/the last day of the incubations. Data were excluded when the difference of mean nutrient concentrations was smaller than zero.
The dashed lines indicate the Redﬁeld ratios of (a) N:P (16:1) and (b) N:Si (16:15). With respect to panel b, the blue symbols correspond to the control and dust treatments at sites M1 andE10M, where the Si(OH)4 concentration showed a clear decline during the incubations. The olive symbols correspond to the control and dust treatments at sites M1B and E02, where the Si(OH)4 concentration showed a slight decline during the incubations (Fig. 5). Note: the Si(OH)4 concentration between day 0 and day 3 at site E10M showed a slight decline or even an
increase, and thus the calculated DN and DSi during this period were excluded from the correlation analysis. (For interpretation of the references to color in this ﬁgure legend, thereader is referred to the web version of this article.)

12

C. Zhang et al. / Science of the Total Environment 739 (2020) 139999

2000; Klausmeier et al., 2008). Meanwhile, due to substantial inputs ofanthropogenic N, the limiting nutrient in coastal sea areas, especiallynearshore regions, changes from N to P and/or Si (Zhou et al., 2017;
Wang et al., 2019). In such conditions, dust deposition can further increase the N:P and N:Si ratios in seawater. Such dramatic environmental
changes can exceed the threshold of the biotic control capacity, thereby
signiﬁcantly affecting phytoplankton growth and community structure(Zhou et al., 2017; Xiao et al., 2018; Zhang et al., 2019a).
5. Conclusions
Previous studies focused mainly on the role of limiting nutrients
supplied by atmospheric deposition in affecting primary production(Mills et al., 2004; Boyd et al., 2007; Duce et al., 2008; Martino et al.,2014; Jickells et al., 2017; Zhang et al., 2018; Isada et al., 2019). However, the impacts of other nutrients supplied by atmospheric deposition as well as the trophic statuses of seawater cannot be neglected(Maranon et al., 2010; Chien et al., 2016; Zhang et al., 2019a). In
this study, four onboard incubation experiments were performed in
the NWP with an ascending order of trophic statuses from the KE to
the TR. In contrast to nutrient standing stocks in the dust-amended
seawater (i.e. NIS), the process of nutrient uptake (i.e. NID) during
the incubations could better reﬂect the phytoplankton response todust addition. This was because the considerable inputs of dustderived N and Fe in the study region, which was characterised by N
or Fe limitation, facilitated phytoplankton to absorb nutrients according to the Redﬁeld ratio rather than the altered nutrient ratios
in the dust treatments. Moreover, we successfully established linear
correlations between i) the NID and phytoplankton growth (as indicated by the DCI), and ii) the NID and size structure (as indicated by
the SI). Such ﬁndings emphasise that the dust-induced alteration of
the nutrient uptake amounts by phytoplankton plays an importantrole in marine primary production. The results also elucidate the response of phytoplankton to external nutrient inputs, irrespective of
the variation in the trophic status. On the other hand, as the phytoplankton response in this study was based mainly on Chl a, further
research could better depict the processes of the marine biogeochemical cycles if we could change Chl a into carbon ﬁxation (i.e. biomass) and establish the corresponding relationship withphytoplankton species.
CRediT authorship contribution statement
Chao Zhang: Data curation, Writing - original draft, Methodology,
Formal analysis. Jingyi He: Validation, Formal analysis. Xiaohong Yao:
Writing - review & editing. Yingchun Mu: Methodology, Investigation.
Xinyu Guo: Writing - review & editing. Xiaokun Ding: Data curation.
Yang Yu: Visualization. Jinhui Shi: Resources. Huiwang Gao: Conceptualization, Writing - review & editing, , Supervision, Funding acquisition.

Declaration of competing interest
The authors declare no competing conﬂict of interest.
Acknowledgments
This work was ﬁnancially supported by National Natural Science
Foundationof China (NSFC, 41876125, 41906119), NSFC-Shandong Joint
Fund (U1906215), and Major State Basic Research Development Program of China (973 Program, 2014CB953701).
Data and materials availability
Relevant data are reported in the text or supplementary material,and data in tables are available on request.

Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.scitotenv.2020.139999.
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