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Continental Shelf Research
Volume 25, Issue 16, October 2005, Pages 1996-2007

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doi:10.1016/j.csr.2005.07.003    How to Cite or Link Using DOI (Opens New Window)  
Copyright © 2005 Elsevier Ltd All rights reserved.

Spatial–temporal distribution of dimethylsulfide in the subtropical Pearl River Estuary and adjacent waters

Min Hua, Corresponding Author Contact Information, E-mail The Corresponding Author, Lingli Liua, Qiju Maa, Tong Zhua, Xudong Tiana and Minhan Daib
aState Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences, Peking University, Beijing 100871, PR China
bMarine Environmental Laboratory, Xiamen University, Xiamen 361005, PR China
Received 5 July 2004;  revised 22 February 2005;  accepted 15 July 2005.  Available online 30 August 2005.



Abstract

Three cruises were carried out in the Pearl River Estuary (PRE) and adjacent northern South China Sea (SCS) in July 2000, May 2001 and November 2002 to collect seawater samples. Concentrations of dimethylsulfide (DMS), chlorophyll a (chl a), nutrients (N, P, Si), salinity and temperature in seawater were measured. The spatial and temporal distribution of DMS concentrations showed larger fluctuations compared with other estuaries reported in the literature. The mean DMS concentrations in three cruises ranged from 0.05 nM (nM=10−9 mol l−1) to 52.7 nM (n=76). The higher concentrations of DMS were observed at the mouth of the estuary. In wet season, high variations of environmental salinity might stimulate algae to release more DMSP to adjust osmotic pressure. Most of these DMS ‘hotspots’ were coincident with the area of high chl a concentrations, although no significant correlation between DMS and chl a was found. The values of DMS/chl a showed a clear trend along the north to the south transect, increasing sharply from estuary to shelf and open sea. There was no significant correlation between DMS and salinity in the wet season (July and May), but a significant positive correlation in the dry season. High primary production and more iron deposition implied that the NE monsoon might influence DMS production in the dry season.

Keywords: Dimethylsulfide; Spatial–temporal distribution; Phytoplankton; Nutrients; Pearl River Estuary; South China Sea



1. Introduction

Dimethylsulfide (DMS) is the most abundant volatile sulfide emitted from the ocean. Once DMS is oxidized in the atmosphere, the by-products can contribute to the acidity of rain, and also the formation of sulfate aerosols, which are a major source of cloud condensation nuclei over remote oceans (Andreae and Crutzen, 1997). DMS is formed from its precursor dimethylsulfoniopropionate (DMSP), which is produced by marine phytoplankton. DMSP is involved in osmoregulation in algae and bacteria, and cryoprotection in algae (Liss et al., 1997; Simó, 2001). The production of DMSP has been found to vary with phytoplankton species. Prymnesiophytes and dinoflagellates are phytoplankton groups with high intracellular DMSP concentrations (Liss et al., 1997). Generally, DMS concentration is high when high DMSP-producing groups dominate the phytoplankton population. Diatoms have low intracellular DMSP concentrations and it is generally believed that diatoms are less important than Prymnesiophytes and dinoflagellates in DMS production. However, in some diatom-dominated waters with high biomass, DMS concentrations were as high as in those dominated by those major DMSP-producing groups (Iverson et al., 1989). In addition, several investigators are now showing that microzooplankton grazing of particulate DMSP to dissolved DMSP, and bacterial conversion of this to DMS is also important. Jones et al. (1998) found that in polar waters increased microzooplankton grazing in diatom-dominated waters, may lead to above-average concentrations of DMS. This does not appear to be the case when the biomass was dominated by dinoflagellates in subantarctic waters.

Due to the growing human population and urbanization processes, coastal waters are heavily disturbed. The amount of natural and anthropogenic materials, which are carried from land to coast, has increased by a factor of 1.5–2 over the past 50 years (Christophe and Fred, 2001). Discharges of industrial and municipal waste have contributed to the eutrophication of the coastal areas of oceans on a global scale. Red tide outbreaks are hence most frequent in near-shore waters. For example, 53 red tides were recorded from 1981 to 1992 in the Pearl River Estuary (PRE) (Qian and Liang, 1999; Yan et al., 2001).

The eutrophication process changes the chemical and physical characteristics of seawater. These changes may affect the phytoplankton biomass, species composition and physiological processes. In the northwestern Black Sea, eutrophication induced a shift in plankton speciation from diatoms to flagellate-dominated populations (Mee, 1992; Lancelot et al., 1987). VandenBerg et al. (1996) suggested that the trend in eutrophication could be explained by a shift of plankton population to Phaeocystis species in the coastal zones of the North Sea, leading to an increase in DMS emissions. A model about the effects of eutrophication on DMS production in the southern part of the North Sea suggested that the anthropogenic eutrophication of the southern North Sea had caused an increase of a factor 2.5 in the mean annual emission of DMS to the atmosphere in the period of the 1900–1980 (VandenBerg et al., 1996). Andreae (1990) observed high DMS concentration in oligotrophic areas of the Sargasso Sea. The culture experiments conducted by Turner et al. (1988) suggested that the cocclithophore Emiliania huxleyi produces less internal DMSP in higher nitrate medium. Furthermore, if nitrate is added to a culture medium with low nutrients, the intracellular DMSP concentration will decrease within 24 h. Gage et al. (1997) outlined a pathway of DMSP biosythesis in marine algae, and explained why nitrogen deficiency enhances DMSP production.

Estuaries are transition zones between rivers and marine ecosystems. Pritchard (1967) defined an estuary as a semi-enclosed coastal body of water which has free connection with the open sea, and within which seawater is measurably diluted with fresh water derived from land drainage. High concentrations of DMS have been reported from several estuary plumes (Turner et al., 1996; Simó et al., 1997; Amouroux et al., 2002). A few studies about DMS in American and European estuaries have been reported (Iverson et al., 1989; Cerqueira and Pio, 1999; Sciare et al., 2002), and all the researches were focused on the temperate zone. Estuaries in the subtropical zone are quite different from the temperate ones due to abundant precipitation and higher temperature. Studying spatio-temporal distribution of DMS in subtropical estuaries will help us to understand the biogeochemical characteristics of DMS in estuarine waters. The PRE has long suffered from heavy eutrophication pressures (Huang et al., 2003). Studying DMS spatial and temporal distribution in this area may also give us some insight in how human activities can modify the natural cycle of DMS.

2. Study area

2.1. Background information of the study area

The Pearl River is considered the second largest river in China, next to the Yangtze River, and 13th largest in the world by discharge volume (3.3×1011 m3 yr−1). The PRE is situated at the south of the Tropic of Cancer with an annual average temperature of 22 °C and annual average precipitation of 1470 mm. The Pearl River has three principal tributaries, namely the Xijiang River, Beijiang River and Dongjiang River, and the waters discharge into the South China Sea (SCS) through eight outlets. About 50–55% of the water enters Lingdingyang Bay through the four eastern outlets (Humen, Jiaomen, Hongqimen and Hengmen). Lingdingyang Bay is the biggest estuary bay of the Pearl River Delta (PRD), and also the area where this present study was carried out (Fig. 1) (Han, 1998).


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Fig. 1. Map of study area and locations of sampling stations.

The PRD includes several municipalities such as Guangzhou, Shenzhen, Hong Kong, Macao, with a population approaching 50 million. PRD has been the fastest-growing economic area of China during the past two decades. Environmental pollution in the region has become worse in these years due to rapid industrialization and urbanization (Zhang et al., 1999). Already, a large amount of domestic, industrial and agricultural sewage is discharged into the estuary. The major pollutants include inorganic nitrogen, phosphate and lead. The PRE has become one of the most heavily polluted marine areas in China. A large area of the northern SCS can be affected by the discharges of the Pearl River due to the combination of large discharge volume and the high nutrient concentrations (Huang et al., 2003; Cheung et al., 2003).

2.2. Description of study area

The study was conducted in Lingdingyang Bay commonly called PRE, and its outer waters within 20–23°N and 113–116°E (Fig. 1). Following the criteria of Iverson et al. (1989), the study area was be divided into three sections: (1) estuarine section, where salinity is lower than 30, from sta. E1 to sta. E18; (2) shelf section, which is between the 200 m isobath and the seaward boundary of the estuary, from sta. S1 to sta. S7; (3) open sea section, which is located at the south of 200 m isobath, from sta. O1 to sta. O4.

3. Sampling and analyses

Samplings were conducted during three cruises on board R/V Yanping II in July 2000, May 2001 and November 2002, respectively. Sampling stations are shown in Fig. 1. According to the topography of study area, the inner Lingdingyang Bay is in the north of 22.30°N and sampling stations E1 to E18 are located in this area; Sta. S2 lies in the connection of the PRE and the northern SCS; the stations in south of Sta. S2 are in northern SCS. The cruise of July 2000 lasted for 13 days from July 12 to July 25. The 2001 cruise lasted for 16 days from May 14 to May 30. The 2002 cruise lasted for 13 days from November 3 to November 16. Generally, during the three cruises, the vessel first got Sta. S2, and then sampled water in the estuary from south to north until arriving at the mouth of Pearl River (from sta. E18 to sta. E1). After finishing the sampling in the PRE, the vessel went back to Sta. S2, and then measurements were made from north to south until to the open sea.

Surface seawater was collected at the depth of not, vert, similar1 m with Goflo samplers on a rosette equipped with CTD and then seawater was stored in polyethylene bottles (100 ml) without headspace. Then samples were immediately stored in the dark at −18 °C in a freezer until analysis at the continental lab (within 1 week after the cruise). The recovery of the storage test was 80±6% (n=5). Seawater samples were analyzed for DMS using the cryogenic purge-and-trap technique (Andreae and Barnard, 1983; Bates et al., 1987; Burgermeister et al., 1990). The determination method was described in detail in Hu et al. (1997). In brief, a seawater sample of 5–25 ml was pre-concentrated by purging with pure nitrogen at the flow of 35–40 ml min−1 for 30 min, and then the cryogenic was trapped by passing through a U-tube packed with Chromosorb R in liquid nitrogen. DMS was analyzed by a Shimadzu 8A gas chromatography, equipped with Carbopack B packed column (2 m×3 mm Teflon column, Carbopack B/1.5% XE-60/1.0% H3PO4 60–80) and flame photometric detection. Temperature ramp program (initial 50 °C for 5 min and final 130 °C with 32 °C min−1 ramp) gave a retention time for DMS around 7.1 min. Peak areas were recorded with a HP 3395 integrator. The detection limit for DMS was 0.5 ng DMS. The linear detection range is from 0.5 to 15 ng DMS. The precision of this method was within 10%.

The shipboard temperature and salinity were determined with an SBE-19-plus Conductivity–Temperature–Depth/Pressure (CTD) unit. The determination of chlorophyll a (chl a) was carried out according to the spectrophotometric method (Parsons et al., 1984). Phosphate (PO43−) and Silicate (H2SiO3, hereafter SiO32−) were determined colorometrically using a flow injection analyzer (Tri-223 autoanalyzer) provided by S.C. Pai of the National Taiwan University. Nitrate (NO3) plus NO2 was measured by reducing nitrate to nitrite using the same Tri-223 autoanalyzer with an on-line Cd coil. The precision is 1.9% for PO43−, 0.1% for NO3, 0.2% for NO2 (Cai et al., 2004) (The data of surface temperature, salinity, chl a, phosphate, silicate and nitrate were provided by Xiamen University).

4. Results

4.1. Hydrographic parameters

A strong seasonal variation of salinity is observed in the estuary. Due to a large volume of freshwater in May and July (wet season), the estuary is dominated by water of high temperature and low salinity. In November (dry season), the influence of fresh water from the Pearl River decreases, and more offshore high-salinity water intrudes into the estuary. The estuary in that period is dominated by water of high salinity and low temperature. In the shelf and open sea area, the seasonal variations of salinity and temperature are small (Fig. 2a and b).


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Fig. 2. (a) Surface salinity and (b) surface temperature as function of latitude.

The Pearl River transports large amounts of nutrients in dissolved or particulate form. Three main nutrients, dissolved inorganic nitrogen (DIN), phosphate and silicate were measured during the cruises. The concentrations of the three nutrients in the estuary were very high upstream and decreased rapidly in the mouth of estuary (near 22°N) (Fig. 3a–c). The DIN, phosphate and silicate show significantly negative correlations with salinity (Table 1). The average mole ratio of N to P ranged from 66 to 204 in different seasons, indicating P limitation. The average mole ratio of N to Si ranged from 0.5 to 0.6, indicating potential N limitation, especially in the oceanic waters.


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Fig. 3. Surface nutrient concentrations vs. latitude of the three cruises: (a) DIN, (b) phosphate and (c) silicate.

Table 1.

Correlation coefficients between parameters of nutrients and salinity
Correlation coefficients July 2000 May 2001 Nov. 2002
r (DIN) −0.93 (n=23) −0.74 (n=24)
r (Phosphate) −0.88 (n=23) −0.82 (n=25) −0.72 (n=29)
r (Silicate) −0.80(n=23) −0.92 (n=25) −0.98 (n=24)

All correlations are significant at 0.01 level (2-tailed).

4.2. Spatial and temporal distribution of DMS concentration in surface water

As described in Section 2.2, the study transect is divided into three sections: estuarine section, shelf section and an open sea section. Fresh water discharged from Pearl River influences the location of the estuarine section's seaward boundary, which is closer to the land in November (dry season) than in May and July (wet season). The surface water values of DMS concentration, chl a concentration and salinity are plotted vs. latitude for the three cruises are given in Fig. 4a–c. The average concentrations of DMS and chl a in the three sections (estuary, shelf and open sea) are given in Table 2.


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Fig. 4. DMS concentration and salinity vs. latitude in three cruises: (a) July 2000, (b) May 2001 and (c) November 2002.

Table 2.

The average concentrations of DMS and Chl a in estuary, shelf, and open sea of the three cruises
Area DMS (nM)
Chl a (μg l−1)
July 2000 May 2001 Nov 2002 July 2000 May 2001 Nov 2002
Estuary Mean 4.5 8.6 3.0 7.1 3.8 1.1
Std. deviation 6.8 14.8 1.1 8.5 2.1 1.3
N 15 15 17 15 15 17
Shelf Mean 3.8 14.4 6.3 0.2 2.9 0.2
Std. deviation 1.7 17.0 1.4 0.2 5.9 0.2
N 5 6 7 5 6 7
Open sea Mean 0.9 2.9 5.6 0.1 0.2 0.1
Std. deviation 0.6 2.0 1.9 0.01 0.1 0.02
N 3 4 4 3 4 3
Total Mean 3.9 9.1 4.2 4.7 3.0 0.7
Std. deviation 5.6 14.2 2.0 7.6 3.4 1.1
N 23 25 28 23 25 27

Fig. 4a shows the data collected during July 2000 cruise. DMS concentration in surface water ranged from 0.05 to 25.9 nM. The highest concentrations of DMS were observed at the mouth of the estuary, with a sharp increase from about 2 to 22 nM. The maxima of DMS concentrations coincided with the maxima of the surface chl a at the mouth of the estuary. However, this trend does not hold in the low salinity region. The results from the cruise of May 2001 are shown in Fig. 4b. The highest average DMS concentration (9.1 nM) was found during the three cruises, with the largest variation (0.7–52.7 nM). The maxima of both DMS and chl a concentrations were observed at the mouth of estuary; the pattern of DMS, however, was not entirely consistent with that of chl a. The data distribution from the cruise of November 2002 is shown in Fig. 4c. The whole cruise exhibited less variation of DMS concentration between stations than those in May and July, and lower DMS concentrations were measured, with an average of 4.2 nM (ranged from 1.36 to 8.24). As in the previous cruise, a maximum of 5.6 nM was also found at the south boundary of the estuary.

Comparing the salinity of three cruises, in the estuarine section, a more irregular distribution is found along the estuary in the wet season (May and July) than that in the dry season (November). The DMS concentration in the estuary section was higher in average with greater variation in May and July (Table 2). However, in November DMS levels increased steadily from north to south in the estuary, along with an increase in salinity. Spearman's correlation analysis suggested that there was no significant correlation between DMS and salinity in July and May in the estuarine section. However, in November DMS showed a significant positive correlation with salinity (r2=0.79, N=15).

The average DMS concentrations in the shelf section are about the same or higher than those in the estuary, although the average concentrations of chl a are much lower compared to the estuary, which means that the DMS production in the shelf section is probably more influenced by phytoplankton species than by biomass. This situation is different from the results of the estuarine section, where generally most DMS ‘hotspots’ coincide with high chl a concentration areas.

In May and July, the lowest average DMS concentration was found in the open sea section, where chl a concentration is much lower, probably because of less productivity due to very low nutrient concentrations. Nevertheless phytoplankton species exist in the open sea section that can produce high DMS emissions. For instance, pretty high DMS concentration was observed in the open sea section in November, and the chl a concentrations pattern of this cruise is similar to those in July and May. This effect may also be due to the monsoon, as will be discussed later.

4.3. DMS and phytoplankton

Liss (1999) found that there was a rough correspondence between areas of high DMS concentrations and the blooming of algae with high intracellular DMSP concentrations, such as coccolithophorids. However, a statistical analysis of almost 16,000 measurements of DMS in surface seawater showed only a weak correlation between DMS and chl a (Kettle et al., 1999). Simó and Dachs (2002) have published an empirical algorithm between DMS, chl and MLD (climatological mixed layer depth). It's very interesting to compare our data with their results. But it's very pitiful that the MLD data were not available for us now. So we just discuss the correlation between DMS and chl a. In our study, no significant correlation between DMS and chl a was found during the wet reason, but in the dry season a strong negative correlation between DMS and chl a was observed (r2=-0.79, N=27). Such a correlation, however, may reflect the spatial variation of salinity from the estuary to the open sea rather than the relation between the DMS concentration and the phytoplankton. And no correlation was found between DMS and chl a (r2=0.19, N=27) (corrected for the effects of salinity).

The large spatial and seasonal variability of DMS concentration can be caused by changes in the biomass and taxonomic composition of phytoplankton (Turner et al., 1988). Chl a is used as a standard measure for phytoplankton biomass. To find out the spatial distribution of the DMS production ability per unit biomass, we used the chl a-normalized DMS concentration (DMS/chl a).

The values of DMS/chl a showed a clear trend along the north to south transect, increasing sharply from the estuary to the shelf and the open sea (Fig. 5a–c). The highest DMS/chl a values were found in the shelf section in all seasons. The variation of DMS/chl a was also greatest in the shelf area (Table 3). The average DMS/chl a of the November 2002 cruise exceeded those of other two cruises in wet season by a factor of 1.9–3.2, and also showed larger variability. DMS/chl a ratios in the shelf water exceeded by 4.5–20 times those values in estuarine water, and 1.3–2.2 times those in open seawater. These results are not consistent with those found in North American estuaries (Iverson et al., 1989), where the highest DMS/chl a ratios were found at open sea stations, and the average value were much lower than PRE (Table 3), which may be a reflection of the difference in phytoplankton population and nutrient conditions.


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Fig. 5. DMS/chl a vs. salinity: (a) July 2000, (b) May 2001 and (c) Nov. 2002.

Table 3.

Average DMS/Chl a values in the three sections of each cruise
PRE, July 2000 PRE, May 2001 PRE, Nov 2002 Delaware Bay (Iverson et al., 1989) Chesapeake Bay (Iverson et al., 1989)
Estuary 1.6±2.4 6.2±14.0 4.5±2.9 0.3±0.2 0.8±0.6
Shelf 32.0±18.8 28.6±30.9 60.5±79.1 3.6±3.4 2.4±1.9
Open sea 14.5±8.0 20.1±11.4 47.3±6.4 15.1±4.7 24.0±13.7

DMS/chl a values in the estuary were larger and more variable, in the cruise of May 2001 compared to the cruise of November 2002, where DMS/chl a ratios increased steadily with the salinity. Significant correlations between DMS/chl a and salinity were found in November (r2=0.826, P=0.00, N=16) and July (r2=0.588, P=0.017, N=16), but not in May (r2=0.226, P=0.339, N=16).

5. Discussion

5.1. The influence of fresh water in the Pearl River Estuary

Limited studies on DMS production in estuarine ecosystems are found in the literature, and most of them have been carried out in temperate regions (Iverson et al., 1989; Cerqueira and Pio, 1999; Sciare et al., 2002). Previous studies regarding DMS distribution in the SCS were conducted in open sea regions (Yang et al., 1999; Yang, 2000). In order to investigate DMS production in a subtropical estuary, our work is focused on the spatial and seasonal distribution of DMS in the PRE and adjacent SCS. We found that higher DMS concentrations and greater seasonal variations are the distinctive features in Pear River Estuary compared to other studies in temperate zones (Table 4).

Table 4.

Comparing average DMS concentration in PRE with other estuaries
Region Concentration range (nM) Average concentration (nM) Reference
Estuary in North America 1–18 Iverson et al. (1989)
Scheldt estuary 0–2.5 0.4–0.6 Sciare et al. (2002)
Gironde estuary 0–1.7 0.2–0.7
Elbe Estuary 0–2.5 0.9
Ems 0–1.5 0.2
Rhine estuary 0–10
Loire estuary 0.5–3.6 1.3
Canal de Mira 0–18 2.9–5.3 Cerqueira and Pio (1999)
Pearl River Estuary 0.05–56.7 3.0–8.6 This work

The salinity of the PRE is strongly influenced by seasonal patterns of rainfall. Because of the complex topography of the PRE, fresh water enters the SCS from eight channels with various volumes. The salinity distribution of the surface water is thus complicated and fluctuates greatly. The ‘hotspots’ of DMS production are located at the southern boundary of the estuary, where the salinity increased sharply from about 15–30 and a salinity front can be observed. On the continental shelf of New Zealand, the same phenomenon is also observed: a sharp increase of DMS coincides with a surface lens of freshwater (Walker et al., 2000). In Zuari estuary, Goa (India), Shenoya and Patilb (2003) observed the highest DMS concentrations from July to August; meanwhile, the salinity showed highest variations during their 14-month study. Laboratory studies of Vairavamurthy et al. (1985) suggested that the DMS production from algae increased with increasing salinity of medium. If algal cells, growing in a high-salinity medium, were transferred to a low-salinity medium, DMS was released into the medium, and the DMS amount increased with an increasing salinity difference between the mediums.

The variation of environmental salinity in an estuary may act as a kind of stimulation to DMS production. In response to the change in salinity, algae release more DMS to adjust the osmotic pressure. A total of 70–80% of the total annual water of the Pearl River discharge enters the SCS in the wet season (April–September), and 20–30% in dry season (October–March). July is normally the river's flood season, but the salinity front in July 2000 was closer to the river exit than in May 2001. This may be explained by heavy precipitation experienced in the watershed during May 2001 (Zhai et al., 2005). The greatest variation of DMS concentrations in the estuary as well as the highest average DMS concentration and highest DMS/chl ratios were all found in May 2001. Such large variations of DMS distribution in the wet season may be due to the influence of large variations in salinity. However, since we have no continuous measurements of both DMS and salinity in any station, it is impossible to draw a final conclusion.

The volume of fresh water may also affect the speciation of the phytoplankton. In the Swan River estuary, Western Australia, dinoflagellates and marine diatoms dominate during periods of low flow, whereas the faster-growing freshwater diatoms were associated with higher flow rates (Chan and Hamilton, 2001). Most of the algae with high DMS production rates belong to the marine phytoplankton, such as dinoflagellates. With the influence of fresh water diminishing in the dry season, DMS producing algae will possibly enter the estuary. This may explain why in dry season DMS/chl a values are high even with low biomass and lower temperatures.

5.2. High DMS concentrations and high DMS/chl a values in the shelf section

The average DMS concentrations in the shelf section are higher than those in the estuary, and the highest DMS/chl a values with the greatest variation were observed in both the wet and dry season. Various processes could cause the observed patterns of DMS concentrations on the shelf. Firstly, the phytoplankton population structure and biomass play an important role in DMS production. In shelf water the composition of the phytoplankton community is different from that in the PRE, where normally it is dominated by fresh water, and brackish water algae, such as Chaetoceros affinis Laude, Skeletonema costatum (Huang et al., 1997). These species are low DMS production algae. While in the shelf water of the SCS, the biomass of high DMS production phytoplankton, such as dinoflagellates, is increased.

Secondly, the transparency of the estuary is lower than 1 m (Han, 1998). The suspended materials may significantly affect the photosynthetic process due to radiation limitation. Experiments indicate that exposure to UV radiation will lead to a 10–25% increase in the per-cell amount of DMSP of E. huxleyi. And the intracellular DMSP concentration is always higher in photosynthetically active radiation (PAR)+UV-exposed E. huxleyi than in PAR-exposed E. huxleyi (Slezak and Herndl, 2003). Due to the lower concentration of suspended material, the UV radiation intensity is higher in the shelf than in the estuary, and hence the primary production in shelf water is still higher than in the estuary (Zhang et al., 1999). These processes may lead to higher DMS/chl a values in the shelf area.

Thirdly, DMSP is involved in osmoregulation in algae. Both laboratory studies and fieldwork indicate that high nitrate concentrations may reduce DMSP/DMS production, while low nitrate concentration will stimulate DMSP/DMS synthesis in algae (Turner et al., 1988; Andreae, 1990; Leck et al., 1990). Because of human emissions, the average DIN concentration in the PRE is much higher than in the shelf section. In July 2000 and May 2001, respectively, the DIN concentration in the estuary was 14 and 4 times higher than in the shelf. The lower nitrate in the shelf section might induce higher DMSP production in the algae, and then higher DMS emission.

5.3. The influence of the monsoon in the open sea

Seasonal monsoons are a climatic feature of the PRE and SCS. The NE monsoon prevails (with stronger winds) in winter, and the SW monsoon (weaker winds) dominates in summer (Han, 1998; Yin, 2002) The highest DMS and DMS/chl a in the open sea were observed in November, although the average chl a concentration was lower than in May or July. This indicates that more active DMS production process occurred in November. Zhang et al. (1999) suggested that Fe2+ is a limiting factor for phytoplankton photosynthesis in SCS. Generally, most Fe is carried from land by atmospheric transport. Several studies indicate that iron fertilization will significantly increase the DMS production in the open sea (Martin et al., 1994; Turner et al., 1996). Fieldwork indicated that, biological activity in surface water in the SCS is enhanced by monsoon winds, for instance, the highest primary production was observed in winter. This may due to the NE monsoon carries suspended dust from the northern Chinese mainland (Chen et al., 1998). When the high Fe containing dust is deposited into the area, phytoplankton metabolism and DMSP/DMS production were stimulated. Higher iron deposition causing greater primary production may offer an explanation as to how the monsoon can influence the DMS production.

6. Conclusions

PRE and adjacent areas display different DMS and DMS/chl a distribution patterns compared with other estuaries, possibly due to different climatic influences, and anthropogenic pollution. The large amount of precipitation during the wet season causes high salinity variation in the estuary and the fluctuation may stimulate DMS emissions by the phytoplankton. To regulate the osmotic equilibrium, algae have to produce or release DMSP, the precursor of DMS. Both phytoplankton biomass and species composition influence DMS concentration in the surface water. High nutrient content may lead to high DMS concentrations in PRE and shelf area, while the NE monsoon may enhance DMS production in the open sea area.


Acknowledgments

We would like to Dr. Zhai, Weidong, Huang, Tao and their colleagues for supplying the salinity, temperature and nutrient data. We are grateful to the Ocean Carbon group from Xiamen University for the help during sampling. We are indebted to China National Nature Science Foundation for financial support (#20177002, #20131160731, #30230310). The sampling cruise was supported by CNNSF through Grants # 49825111, #40176025 and #49976021. We thank Dr. Slanina (Professor of the Netherlands Wageningen University, Professor of Peking University) for his suggestions and comments.


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Continental Shelf Research
Volume 25, Issue 16, October 2005, Pages 1996-2007
 
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