Impact of atmospheric wet deposition on phytoplankton community structure in the South China Sea

Estuarine, Coastal and Shelf Science 173 (2016) 1e8

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Impact of atmospheric wet deposition on phytoplankton community structure in the South China Sea Dong-Yang Cui, Jiang-Tao Wang*, Li-Ju Tan, Ze-Yi Dong Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266100, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 August 2015 Received in revised form 20 January 2016 Accepted 15 February 2016 Available online 18 February 2016

The South China Sea (SCS), which is the largest marginal sea in East Asia, plays a significant role in regional climate change. However, research on the phytoplankton community structure (PCS) response to atmospheric wet deposition remains inadequate. In this study, field incubation experiments were performed to survey the impact of atmospheric wet deposition on the PCS in the SCS in December 2013. Results indicate that the mean dissolved inorganic nitrogen/dissolved inorganic phosphorous (DIN/DIP) ratio in rainwater was 136, which was higher than that in seawater. Under low initial nutrient concentrations, rainwater inputs not only significantly increased total chlorophyll a (Chl a) concentrations but also potentially altered the PCS. The total Chl a concentration increased 1.7-, 1.9-, and 1.6-fold; microphytoplankton increased 2.6-, 3.2-, and 1.7-fold with respect to their initial values in the 5%, 10% addition, and 10% addition (filtered) treatment samples, respectively. Finally, microphytoplankton contributed 61% to the total Chl a concentration in 10% addition treatment samples. Differences in the nutrients induced by atmospheric wet deposition resulted in a shift in the advantage from picophytoplankton to microphytoplankton. Diatoms became the predominant species, accounting for 55% of the total abundance after rainwater addition. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Rainwater inputs Phytoplankton community structure Chl a size-fractionation South China Sea

1. Introduction Atmospheric deposition is considered a significant source of nutrients in the marine ecosystem (Van Jaarsveld, 1993; Zhang, 1994; Migon and Sadroni, 1999; de Leeuw et al., 2003; Jickells et al., 2005; Boulart et al., 2006; Okin et al., 2011). Atmospheric nutrient inputs, which are normally equal to or greater than river inputs, are recognized as the second largest source of major and trace elements in seawater (Duce and Tindale, 1991; Jickells, 1995). Atmospheric wet deposition is the main source of nutrients, especially in oligotrophic oceans (Owens et al., 1992; Jickells, 1995). A previous study showed that the atmospheric nutrient inputs are estimated to increase in the future (Duce et al., 2008). Marine phytoplankton mainly includes cyanophyta, bacillariophyta, pyrrophyta, euglenophyta, and chrysophyta. Phytoplanktons are classified into three size classes: microphytoplankton (>20 mm), nanophytoplankton (2e20 mm), and picophytoplankton (<2 mm) (Paerl et al., 1990, 2002; Markaki et al., 2003). In general, the size of

* Corresponding author. Tel./fax: þ86 532 66782506. E-mail address: [email protected] (J.-T. Wang). http://dx.doi.org/10.1016/j.ecss.2016.02.011 0272-7714/© 2016 Elsevier Ltd. All rights reserved.

cyanophyta is less than 2 mm, whereas diatoms and dinoflagellates are larger 20 mm (Brotas et al., 2013). Atmospheric wet deposition provides external nutrients that support marine phytoplankton growth, impacts primary production (Zou et al., 2000; MartínezGarcía et al., 2015), and may even trigger biogenic blooms in oceans (Huo et al., 2001; Wang et al., 2011). New nutrients derived from atmospheric wet deposition can promote marine phytoplankton biomass and nitrogen fixation and enhance marine capacity in absorbing CO2. Atmospheric wet deposition could also change the paths of carbon and nitrogen cycles in oceans and potentially impact regional environmental change (Paerl, 1997; Bishop et al., 2002; Jo et al., 2007; Duce et al., 2008; Guo et al., 2012; Shi et al., 2012). The South China Sea (SCS), which is located in the tropicalesubtropical rim of the Northwest Pacific Ocean, is one of the largest marginal seas in the world (Chen et al., 2001). The climate in the SCS is part of the Asian monsoon system. In this system, the northeast monsoon prevails in winter and spring, whereas the southwest monsoon occurs in summer and autumn (http://www. sciencedirect.com/science/article/pii/S0967064513003512Liu et al., 2002). The SCS is essentially oligotrophic, especially in the central basin. In the SCS, chlorophyll a (Chl a) concentration is usually low,

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and concentrations of N and P in the euphotic layer are also under detection limits (Ning et al., 2004). Thus, the new nutrients imported by atmospheric deposition may be an important environmental factor to control phytoplankton growth (McGillicuddy et al., 2003; Chen et al., 2006). Guo et al. (2012) used a direct experimental approach to test the effects of atmospheric dry deposition on the PCS and photosynthetic efficiency. However, data that could reveal the response of phytoplankton growth to atmospheric wet deposition in the SCS are limited. This paper aims to evaluate the effects of atmospheric wet deposition on the PCS in the SCS. 2. Materials and methods 2.1. Rainwater and seawater sample collection Rainwater addition experiments were conducted aboard on December 18e25, 2013 in the SCS. Rainwater samples were collected within 32 h before the experiments in the areas extending from 2 5900 N, 109 5500 E to 41100 N, 110 4300 E (Fig. 1). A wet deposition collector made of a polyethylene bottle (10 L) was connected to a polyethylene funnel placed on the front deck at 7 m above sea level. Samples were frozen at 20  C, and subsequently thawed and mixed with seawater samples before the experiments. A 200 mL subsample was stored for chemical analysis. Surface seawater samples were collected using acid-cleaned bottles below the surface (~0.5 m depth) and then screened through a 200 mm mesh to exclude most zooplankton. The in situ temperature was 27.5  C. 2.2. Experimental design The rainwater addition experimental design included an incubation series for the following treatment samples in10 L bottles prepared in duplicate: (1) control treatment: no rainwater addition; (2) 5% addition: 5% (v/v) rainwater addition; (3) 10% addition: 10% (v/v) rainwater addition; (4) 10% addition (filtered): 10% (v/v) addition of filtered rainwater through nuclepore filters (pore size: 0.7 mm). The exact initial volume for each experimental treatment was 8 L. Experimental bottles were placed in plastic containers filled with in situ surface seawater to control the incubation temperature. The experimental samples were incubated under in situ lightedark conditions for 8 days.

2.3. Nutrient analysis The filtered water sample were collected in 50 mL polyethylene bottles and immediately frozen at 20  C until subsequent analysis for nutrients. Sampling was conducted once every two days during the incubation period. Concentrations of ammonium (NHþ 4 ), nitrite e (NOe 2 ), nitrate (NO3 ), phosphate (dissolved inorganic phosphorous, DIP), and silicate (SiO44) were analyzed by spectrophotometry using a Bran þ Luebbe Auto Analyzer 3 (Zhu et al., 2006). The detection e e 4 limits of NHþ 4 , NO2 , NO3 , DIP, and SiO4 were 0.04, 0.003, 0.015, 0.024, and 0.03 mM, respectively. Dissolved inorganic nitrogen (DIN) e e included NHþ 4 , NO2 , and NO3 . For the rainwater analysis to determine total nitrogen (TN) and total phosphorus (TP), sample was collected in polyethylene bottles and analyzed using persulfate digestion and standard colorimetric methods (Zhu et al., 2006). The precision values of TN and TP determinations were 3% and 5%, respectively. 2.4. Chl a analysis Water samples each with a volume of 250 mL were taken once every four days during the incubation period. However, Chl a, a proxy for phytoplankton biomass, presents certain problems (Domingues et al., 2008); nevertheless, it was used as the biomass index in numerous references (Hall et al., 2013; Baek et al., 2015; Jakobsen et al., 2015). Chl a was determined in accordance with the work of http://www.sciencedirect.com/science/article/pii/ S0304420399000420 Parsons et al. (1984). For the sizefractionted microphytoplankton (20e200 mm), nanophytoplankton (2e20 mm) and picophytoplankton (0.7e2 mm), the samples were filtered sequentially through 20, 2, and 0.7 mm filters, respectively. The filters were immediately wrapped in tin foil and then frozen at 20  C. Pigments were extracted with 25 mL of 90% acetone (v/v) at 4  C in the darkroom overnight and then measured using a fluorescence spectrophotometer (Hitachi Fe4500). The fluorescence spectrophotometer was calibrated with a Chl a standard (Aladdin, HPLC). The detection limit of Chl a was 0.01 mg L1. 2.5. Phytoplankton analysis For microphytoplankton, and nanophytoplankton, 500 mL water samples were fixed with Lugol's iodine soultion. The samples €hl method were analyzed under inverted microscope by the Utermo €chter, 2010). For picophytoplankton, 4.5 mL of (Edler and Elbra samples were fixed with PþG (1% paraformaldehyde þ 0.05% glutaraldehyde final), followed by deep-freezing in liquid nitrogen. Picophytoplankton analysis was conducted using the protocol proposed by Maria et al. (2000) in the laboratory. 2.6. Data analysis

Fig. 1. The sampling areas in the SCS. Quadrangle represents seawater collection site and circles represent the rainwater collection area in December 2013.

Nutrient consumption rate was calculated according to Xu et al. (2008) as following: nutrient consumption rate (m) ¼ (Nt  N0)/t, where Nt and N0 are nutrient concentrations at days t and 0. In addition to Chl a, net phytoplankton growth rate is also the important parameter to judgment of phytoplankton growth state. The phytoplankton growth rate (R) was calculated according to Tang (2010) as: R ¼ ln([Chl a]t/[Chl a]0)/t, where [Chl a]t and [Chl a]0 are Chl a concentrations at days t and 0. The specific growth rate (r) was determined according to the following equation: r ¼ (lnXtelnX0)/t, where Xt and X0 are cell numbers at days t and 0. Significant differences between treatments were determined using ANOVA with Duncan comparisons (software used: SPSS 19.0) and were accepted at p < 0.05.

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3. Results 3.1. Nutrient concentrations in samples The initial concentrations of nutrients in seawater and rainwater are shown in Table 1. Evidently, the initial dissolved inorganic nutrient concentrations in rainwater were higher than those in surface seawater. The major DIN species in rainwater were NHþ 4 and NOe 3 . The concentration of particulate nitrogen (PN) was less than that of DIN in rainwater. Phosphorus in rainwater existed mainly in the form of particulate phosphorus (PP) with a concentration of 1.63 mM. The DIN/DIP ratio in rainwater was high, averaging approximately 136. In surface seawater, the mean DIN concentration was 2.91 mM, and the mean DIN/DIP ratio was 15. Nutrient concentrations declined to near initial concentrations in surface seawater samples aside from control treatment samples during the latter part of the incubation period (Fig. 2). However, the nutrient concentrations in the samples subjected to 10% addition treatments and 10% addition (filtered) treatments vary. The initial mean cone e 4 centrations of NHþ 4 , NO3 , NO2 , DIP, and SiO4 in the 10% addition treatment samples were 5.27, 2.61, 0.48, 0.29, and 3.7 mM, respectively; the corresponding concentrations in the 10% addition (filtered) treatment samples were 4.77, 2.49, 0.29, 0.16, and e 3.24 mM, respectively. The initial mean ratios of NHþ 4 /DIP and NO3 / DIP in the 10% addition treatment samples were 25 and 12, respectively; the corresponding initial mean ratios in the 10% addition (filtered) treatment samples were 24 and 13, respectively 4 (Fig. 3). During the first four days, more NOe were 3 and SiO4 consumed in the 10% addition treatment samples, whereas the concentration of NHþ 4 decreased faster in the 10% addition (filtered) e treatment samples (Fig. 2). The ratios of NHþ 4 /DIP and NO3 /DIP in the 10% addition treatment samples were 12 and 8, respectively; the corresponding ratios in the 10% addition (filtered) treatment samples were 10 and 9, respectively (Fig. 3). 3.2. Response of total Chl a concentrations and phytoplankton sizes to rainwater addition At the beginning of the experiment, the mean Chl a concentration was 0.17 mg L1 (SD ¼ 0.06). On the fourth day, the total Chl a concentrations increased 1.7-, 1.9-, and 1.6 efold; microphytoplankton increased 2.6-, 3.2-, and 1.7-fold; and picophytoplankton and nanophytoplankton increased 1.4-,1.4-, and 1.5-fold with respect to their initial values in the 5%, 10% addition, and 10% addition (filtered) treatments, respectively. Initially, the percentage contributions of picosize(Pico%),nanosize(Nano%),andmicrosize(Micro%)classesto totalChlaconcentrationwere48%,24%,and28%,respectively (Fig.4), and the contribution order was Pico% > Micro% > Nano%. Rainwater inputs changed the percentage of different size-fractionated Chl a. In the 10% addition treatment samples, Micro% reached 61%, Whereas Pico% decreased to 12% (Fig. 4), and the contribution order became Micro% > Nano% > Pico%. 3.3. Response of PCS to rainwater additions A total of 117 phytoplankton species were identified in the experiment. Bacillariophyta was the most diverse group, with 74

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diatom species described. Pyrrophyta, cyanophyta, chrysophyta, and euglenophyta were also identified. The phytoplankton community was dominated by cyanophyta (47%) in surface seawater. The average abundances of Synechococcus and Prochlorococcus were 21.4  103 and 16.8  103 cells dm3. Nutrient inputs by rainwater had a distinct effect on phytoplankton species composition. Diatoms became the most abundant species, accounting for 55% of the total abundance on the fourth day in the 10% addition treatment samples (Fig. 5), with Pseudo-nitzschia spp., Navicula spp., Chaetoceros spp., Asterionellopsis spp., and Skeletonema spp. as the dominant species. The phytoplankton growth rate (R) and diatom net growth rate increased in the 5% and 10% addition treatment samples, but the values in 10% addition (filtered) treatment samples was less than those in the 10% addition treatment samples. This change was 4 consistent with the trend of NOe 3 and SiO4 net consumption rates. However, the increase of cyanobacteria net growth rate and NHþ 4 net consumption rate were higher in the 10% addition (filtered) treatment samples than those in the 5% and 10% addition treatment samples (p < 0.05) (Table 2).

4. Discussion Atmospheric wet deposition is an important source of nutrients for marine ecosystems (Mahowal et al., 2005; Krom et al., 2010; Okin et al., 2011). Regional variations in atmospheric deposition may exert a significant effect on PCS and biogeochemical cycles (Jickells et al., 2005; Duce et al., 2008; Hein et al., 2013). Justic et al. (1995) suggested that DIN ¼ 1 mM, DIP ¼ 0.1 mM, and Si ¼ 2 mM were selected as the threshold values for phytoplankton growth. The initial nutrient concentrations in surface seawater are close to these threshold values (Table 1), resulting in a lower total Chl a concentration. In the SCS, the northeast monsoon prevails in winter and may carry considerable amounts of dust particles, including macronutrients (Chameides et al., 1999). The Takelamagan Desert and China's central Gobi Desert are among the main source regions of dust emission, and the total dust emission in these regions is roughly 2.9  106 ton year1 in winter (Xuan et al., 2000). Satellite data showed that dust from the Gobi and Taklimakan deserts could cover large areas of the SCS with the strong northeast monsoon in winter (Lin et al., 2007). The DIN/DIP ratio in rainwater was considerably higher than the Redfield ratio (Redfield, 1958). Phosphorus in the atmosphere, which exists mainly in the form of PP, mainly originates from mineral dust (Ridame and Guieu, 2002; Anderson et al., 2010). With the rapid development of human activities, the deposition of atmospheric nitrogen gradually increases (Galloway et al., 2004). In addition, the source of N is more abundant than P, especially due to soil erosion. Therefore, the DIN/DIP ratio is high. Phytoplankton growth is mainly limited by N in the SCS (Chen et al., 2006). Evidently, the nutrient condition in rainwater is considerably different from that in surface seawater. Wet deposition not only increases nutrient concentrations but also alters the DIN/DIP ratio in seawater. In addition, wet deposition affects the growth and community structure of phytoplankton. For example, the gradual of DIN/DIP ratio increase from west to east in the wet deposition in the Mediterranean Sea area corresponds to

Table 1 Initial nutrient concentrations (mM) in seawater and rainwater in the SCS. (DIP: dissolved inorganic phosphorus; DIN: dissolved inorganic nitrogen, including nitrate, nitrite and ammonium); PN: particulate nitrogen; TN: total nitrogen; PP: particulate phosphorus; TP: total phosphorus; n.d. represents no data).

Seawater Rainwater

NOe 3

NOe 2

NHþ 4

DIP

SiO4e 4

DIN

PN

TN

PP

TP

1.52 ± 0.17 11.37 ± 0.23

0.16 ± 0.10 3.24 ± 0.21

1.23 ± 0.08 41.37 ± 1.47

0.19 ± 0.03 0.41 ± 0.05

2.47 ± 0.19 14.31 ± 0.73

2.91 ± 0.87 55.88 ± 14.82

n.d. 14.37 ± 0.41

n.d. 70.25 ± 2.35

n.d. 1.63 ± 0.33

n.d. 2.02 ± 0.48

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e þ 4 Fig. 2. Temporal variations of NOe 3 (a), NO2 (b), NH4 (c), DIN (d), SiO4 (e) and DIP (f) during the incubation period. DIN: dissolved inorganic nitrogen; DIP: dissolved inorganic phosphorus; control: no addition; 5% addition: 5% rainwater addition; 10% addition: 10% rainwater addition; 10% addition (filtered): 10% filtered rainwater addition.

the change in DIN/DIP ratio in seawater. This finding indicated that atmospheric wet deposition was closely related to the DIN/DIP ratio in seawater (Markaki et al., 2010). Zhang et al. (2011) suggested that nutrient inputs from atmospheric wet deposition could raise Chl a concentration by 30.6% on average after 85 rainfall events. Previous studies have demonstrated that nutrient concentrations increase could stimulate large phytoplankton growth (Ning et al., 2008; Che et al., 2012) and phytoplankton community succession progresses from small cells to large cells (Chisholm, 1992; Li, 2002; Chen and Liu, 2010). Compared with microphytoplankton and nanophytoplankton, picophytoplankton presents a higher metabolic

rate, which is associated with a smaller cell structure and higher surface-to -volume ratio; a higher nutrient uptake efficiency; and a faster growth rate (Raven and Kübler, 2002). In this study, picophytoplankton was found to be predominant species in the phytoplankton community. This finding is consistent with those of previous studies conducted in the ocean areas (Ning et al., 2004; Li et al., 2012). In the experiment, with an increase in nutrient availability caused by rainfall, larger-sized cells of phytoplankton (micro) showed a competitive advantage and grew faster. Generally, in the experiments, phytoplankton of every size fraction was stimulated, but the impact of rainwater additions on

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e Fig. 3. Temporal variations of the ratios of NHþ 4 /DIP (a) and NO3 /DIP (b) during the incubation period.

Fig. 4. Temporal variations of composition of size-fractionated Chl a and total Chl a concentrations (mg L1) during the incubation period. a: no addition; b: 5% rainwater addition; c: 10% rainwater addition; d: 10% filtered rainwater addition.

picophytoplankton was not as clear as those on nanophytoplankton and microphytoplankton. Diatoms (micro) became the dominant species after rainwater additions (Fig. 4), as new nutrient were  inputted. However, more NHþ 4 than NO3 inputs may result in an increasing in cyanobacteria (pico) rather than diatoms, because NHþ 4 is the preference of green algae and cyanobacteria, which rely  only on NHþ 4 as their N-source, whereas diatoms prefer NO3 and þ thus do not respond to NH4 inputs (Domingues et al., 2011). This contradicting result may be attributed to the presence of

compounds inhibiting cyanobacteria growth in rainwater or the phytoplankton competition for nutrients. For instance, diatoms show an inherent growth rate that is 5e50% faster than that of flagellate group when the SiO44 concentration exceeds 2 mM (Egge and Aksnes, 1992). This faster growth rate results in the faster NOe 3 and DIP consumption. DIP concentrations could drop to a value that could restrain massive picophytoplankton growth, even if their preferred N-source (NHþ 4 ) is available in excess. In addition, zooplankton that grazes large phytoplankton species had been

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Fig. 5. Temporal Percentage composition of the PCS by taxonomic group during the incubation period. PCS: phytoplankton community structure; a: no addition; b: 5% rainwater addition; c: 10% rainwater addition; d: 10% filtered rainwater addition.

Table 2 Mean nutrient net consumption rate (mM d1), phytoplankton net growth rate (Chl a, d1), and specific net growth rates (d1) of cyanobacteria (CYA) and diatoms (DIA) over the first four days in the incubation experiment.

Control 5% Addition 10% Addition 10% Addition (filtered)

NOe 3

NOe 2

NHþ 4

DIP

SiO4e 4

Chl a

CYA

DIA

0.09 0.22 0.47 0.34

0.00 0.02 0.05 0.05

0.02 0.53 0.52 0.88

0.02 0.03 0.05 0.03

0.27 0.44 0.58 0.29

0.07 0.06 0.08 0.04

0.03 0.11 0.11 0.22

0.09 0.09 0.15 0.03

removed through the 200 mm mesh. Therefore, the density of diatoms increased faster than that of cyanobacteria after rainwater addition. This result is consistent with those in previous studies that showed that diatoms become the dominant species after addition experiments (Boyd et al., 2007; Guo et al., 2012). The impacts of the different rainwater additions tested on nutrient consumption rates and phytoplankton net growth rates could reflect temporary changes of the major phytoplankton groups. Cyanobacteria are more supported by low nutrient demand in oligotrophic ocean than larger phytoplankton groups (Raven, 1998). Compared with the 5% addition treatment samples, more nutrients were input into the 10% addition treatment samples.

Phytoplankton would uptake more nutrients in the 10% addition treatment samples. The faster growth rate of diatoms would uptake 4 more NOe 3 , DIP, and SiO4 , whereas this would restrain cyanobacteria growth. The net growth rates of phytoplankton and diatoms increased with increasing nutrient concentrations (Table 2). Diatoms became the predominant species except the control treatment samples, particularly accounting for 55% of the total abundance in the 10% addition treatment samples on the eighth day (Fig. 5). Pseudo-nitzschia spp., Navicula spp., Chaetoceros spp., Asterionellopsis spp., and Skeletonema spp. became the dominant species. In the 10% (filtered) addition treatment samples, particulate matters were filtered out. Under the action of biological chemistry, PP would be biodegradable in the 10% addition treatment samples if added to the DIP in seawater. The net growth rates of phytoplankton and diatoms were higher in the 10% addition treatment samples than those in the 10% addition (filtered) treatment samples. Compared with the 10% addition treatment samples, e picophytoplankton absorbed more NHþ 4 than NO3 , and cyanobacteria net growth rate and NHþ net consumption rate were higher in 4 the 10% addition (filtered) treatment samples (Table 2). Influenced by such factors as atmospheric circulation and topography, atmospheric wet deposition (rainfall) presents typical seasonal variation characteristics. The average annual rainfall in the

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SCS exceeds 2500 mm. In summer and fall, the southwest monsoon brings heavy rainfall, and the average monthly rainfall is higher than 300 mm. Under the influence of the dry and cold northeast monsoon in winter and spring, the monthly rainfall in the SCS is approximately 200 mm (Chen et al., 2005). Aside from atmospheric deposition, freshwater (from the Pearl River and Mekong) also brings considerable amounts of terrestrial inorganic nutrients in the SCS. Tropical cyclones, through oceanic eddies near their paths, increase Chl a concentrations and primary productivity rates, accounting for 20e30% of the annual new production in the SCS (Lin et al., 2003). Seasonal upwelling also brings nutrients from deep layers closer to the surface and even results in phytoplankton blooms (Chen et al., 2006; Wang et al., 2010). Paytan et al. (2009) suggested that atmospheric deposition introduces inhibitory compounds. However, the concentrations of copper and other elements were not analyzed in this experiment, and toxic effects maybe possible. Compared with those of tropical cyclones and upwelling, the contribution of atmospheric wet deposition on phytoplankton growth in the SCS is less. However, the sudden and massive nutrient inputs by wet deposition have a significant influence on phytoplankton growth and community structure. Martínez-García et al. (2015) reported that rainwater additions could increase Chl a concentration by 68% and primary production by 169% with respect to the control values and that mixed diatom biomass is considerably increased after rainwater additions in 5% urban treatment. Zou et al. (2000) indicated that Chl a concentration could increase by 2.6 times in 10% rainwater addition treatment after 24 h. The low nutrient levels in surface seawater indicate that the marine phytoplankton growth is dependent on the mode of nutrient input into the ocean. Allan and Soden (2008) suggested that the rainfall frequency is expected to increase because of global warming. New nutrient inputs potentially increase CO2 consumption on the basis of the increased total Chl a concentration, which could change the carbon cycle and affect pH of oceans. Microphytoplankton dominates the PCS in terms of Chl a and phytoplankton compositions because of atmospheric wet deposition inputs. If nutrient availability is sustained, phytoplankton could increase steadily. The increase in phytoplankton biomass could impact zooplankton growth and even marine ecosystems. 5. Conclusion The present study shows that the initial nutrient concentrations in surface seawater are low and that the N/P ratio in surface seawater is lower than that in rainwater. Rainwater additions to surface seawater potentially alter the existing balance in the phytoplankton community as phytoplankton species compete for nutrients and subsequently change the PCS. With the addition of new nutrients, microphytoplankton exhibits higher sensitivity response to nutrient uptake than picophytoplankton, shifting the size-fractionation proportion in favor of the microphytoplankton. As the nutrient inputs brought by rainwater into the SCS are increasing rapidly, our study could contribute to a better understanding of the effects of atmospheric wet deposition on the PCS in the SCS. Acknowledgments We are very grateful to the captain and crew of the “Dong Fang Hong 2” for help during field incubation experiments. We thank two anonymous reviewers for constructive comments and suggestions which greatly improved the manuscript. We gratefully acknowledge the support of the National Programme on Global Change and AireSea Interaction (GASI-03-01-02-01).

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