HUANG Chao, LAO Qibin, CHEN Fajin, *, ZHANG Shuwen, CHEN Chunqing, BIAN Peiwang, and ZHU Qingmei

Distribution and Sources of Particulate Organic Matter in the Northern South China Sea: Implications of Human Activity

HUANG Chao1), 2), LAO Qibin3), CHEN Fajin1), 2), *, ZHANG Shuwen4), CHEN Chunqing1), 2), BIAN Peiwang1), 2), and ZHU Qingmei1), 2)

1) College of Ocean and Meteorology, Guangdong Ocean University, Zhanjiang 524088, China 2) Key Laboratory of Climate, Resources and Environment in Continental Shelf Sea and Deep Sea of Department of Education of Guangdong Province, Guangdong Ocean University, Zhanjiang 524088, China 3) Marine Environmental Monitoring Center of Beihai, State Oceanic Administration, Beihai 266031, China 4) Institute of Marine Science, Shantou University, Shantou 515063, China

In order to understand origin and fate of particulate organic matter, the isotopic composition (δ13C and δ15N), total organic carbon content, total nitrogen content, and C/N ratios were measured for suspended particulate organic matter (POM) collected from the northern South China Sea (NSCS) during summer. Our study revealed that δ13C generally decreased from land to sea, and elevated δ13C occurred at the nearshore stations, suggesting that POC was mainly contributed from the eutrophic level and microbial activity. Moreover, the distribution of δ15N values were complicated, and heterotrophic modification was responsible for higher δ15N in the nearshore stations. These distribution patterns of δ13C and δ15N in the nearshore stations may be associated with the intensification of human activity in the coast. Based on the Stable Isotope Analysis in R model, 65% of POM was contributed by marine organic matter in the NSCS, 20% by terrestrial inputs, and 15% by freshwater algae.

particulate organic matter; stable isotope; C/N ratio; isotope mixing model; South China Sea

1 Introduction

Continental margins, acting as a filter and/or sink of both natural and anthropogenic material, are a direct channel between the land and ocean. Tremendous amounts of terrigenous and marine organic matter are trapped in continental margins due to a series of physical, chemical, and biological processes during transport, deposition, and burial (Kennedy, 1984; Thornton and McManus, 1994). Particulate organic matter (POM) is an important form of carbon (C) and nitrogen (N) in marine systems, considering its capacity to sustain high levels of biological activity, which has important implications for regional and global carbon and nitrogen cycles (Chen., 2008; Zhang., 2011, 2012; Guo., 2015; Ye., 2017). Inaddition, POM is sensitive to human activity and eutrophication (Tesi., 2007; Harmelin-Vivien., 2008).Stable carbon and nitrogen isotope compositions and the ratio of total organic carbon to total nitrogen have been widely used to elucidate the source and fate of organic matter in marine environments (Wu., 2003; Hu., 2006;Ye., 2017), which is effective because each environment exhibits a characteristic source-specific signature. Thus, knowledge of the temporal and spatial variation of δ13C and δ15N is necessary to identify the sources of organic matter and factors controlling their distribution in these areas.

The South China Sea (SCS) is one of the largest continental shelves in the world. The Pearl River is the second largest river in China in terms of discharge volume, and discharges into the SCS. The Pearl River transports huge amounts of terrigenous organic matter to the oceans, and most of this terrestrial-derived POM is trapped in the estuary and on the shelf (Owen., 2005; Wang, 2007; Liu., 2014). Previous studies suggested that a percentage of the Pearl River particles was forced northeastward by the Yuedong coastal current and the southwesterly monsoon (Hong., 2009; Chen., 2017). The eastern coast of Guangdong Province are therefore ideal regions for studying the biogeochemical behavior of terrigenous organic matter. Especially in the last 30 years, massive economic growth and urban development has discharged a large amount of anthropogenic nutrients into the coastal areas, which would complicate the source of POM (Huang., 2003; Wang., 2008).Although some studies have been published on carbon and nitrogen stable isotopes in the eastern coast of Guangdong Province, many of them are confined to either the surface sediment or suspended particulate organic matter of Daya Bay with only a relatively small survey area (Hu., 2006; Ke., 2017), and no systematic studies were investigated to elucidate sources and distribution of organic matter in the northern South China Sea (NSCS). Especially, the geochemical characteristics and provenance of suspended particulate matter and the influences of anthropogenic activity in the eastern coast of Guangdong Province are not well constrained.

In this study, we report the concentrations of POC and PN and their isotope ratios (δ13C and δ15N) of POM in the eastern coast of Guangdong Province. The main objectives of our study are to investigate the spatial patterns of variation in δ13C and δ15N of the suspended organic matters during summer and evaluate biogeochemical processes controlling their variations. We also hope the main findings of this study will be beneficial to understand the environment change in the eastern coast of Guangdong Province under anthropogenic influence.

2 Materials and Methods

2.1 Field Sampling

Fig.1 shows the sampling stations over 5 transects in the northern South China Sea (NSCS) during the June 2017 cruise. Each sampling transect is comprised of 3–8 stations, with depths ranging from 1 to 110m. We usually selected 2–3 sampling sites with the water depth above 40m, and 4 sampling sites were collected at the water depth below 40 m. Each station was generally sampled at evenly spaced water depth from the surface to the bottom. Water samples of the vertical profiles were collected using a rosette sampler fitted with 12L Niskin bottles. Temperature and salinity profiles were measured using a conductivity-tempe- rature-depth (CTD) meter (SBE911, Seabird scientific). Once the water samples (approximately 1000mL) were brought onboard the vessel, they were immediately filtered using precombusted (450℃, 4h) and preweighed glass fibre filters (0.7μm, 47mm diameter, Whatman GF/F). After filtration, the filters were rinsed with Milli-Q water to remove any salt and frozen at ?20℃ until further analysis in the laboratory. To facilitate the study, the surveyed sea area was divided into two regions for analysis, including coastal area (station YD1-1–3, YD2-1–4, YD3-1–3, YD4-1–3 and YD5-1–2) and offshore area (the rest of stations).

Fig.1 Map of the study area, sampling transects, and stations.

2.2 Sample Analysis

In the laboratory, the total suspended particulate (TSP) content was calculated using the weight difference between the pre-weighed and reweighed values by the volume of water filtered. Chlorophyll (Chl-) concentrations were determined by fluorometry after grinding the sample in 90% acetone (Lorenzen, 1967). The filters for POC content and δ13C analysis were exposed to concentrated HCl vapor for at least 48h to remove carbonate, and then rinsed thrice with deionized water. After acidification, the filters were freeze-dried and stored in a desiccator until analysis (Yamamuro and Kayanne, 1995). All filters for analysis were tightly packed into tin cans before being measured with an elemental analysis isotope ratio mass spectrometer (EA- IRMS) (EA Isolink series elemental analyser interfaced with MAT 253 plus mass spectrometer). The references for δ13C and δ15N are Vienna Pee Dee Belemnite (VPDB) and atmospheric N2, respectively. The average standard deviations were ±0.2‰ for δ13C, ±0.3‰ for δ15N, and ±0.3% for POC and PN contents.

2.3 Isotope Mixing Model

The proportional contribution of potential POM sources to the NSCS were calculated using isotope mixing modal SIAR (stable isotope analysis in R), which is a software package conduct the Bayesian stable mixing model. More detailed information about using this model to quantify various sources of organic matter can be found in Sarma. (2014) and Lao. (2019).

3 Results

3.1 Hydrographic Properties

The spatial and vertical distributions of temperature and salinity are shown in Fig.2. The vertical distribution of water temperature and salinity exhibited stratification in all transects. The temperature varied from 19.3 to 30.7 ℃ (an average of 25.2℃), and the salinity ranged from 29.5 to 34.7 (an average of 33.4). A decreasing trend of temperature from the sea surface to the bottom was generally observed along the eastern Guangdong coast, and downward-increasing trends were observed for salinity. The relatively low salinity was observed in the surface water of the coastal area, particularly in the transects YD3, YD4 and YD5. The concentrations of TSP varied from 1.7 to 39.0mgL?1(with an average of 15.48mgL?1). Re- latively higher levels of TSP were found in the transects YD1, YD2 and YD3, particularly in the coastal area, which was contrary to the distributions of Chl-.

Fig.2 Spatial distributions of temperature, salinity and TSP in the NSCS during summer.

3.2 POC, PN and Chl-a

As shown in Fig.3, the concentrations of POC and PN ranged from 0.03 to 1.03mgL?1(an average of 0.17mgL?1) and from 0.008 to 0.369mgL?1(an average of 0.052 mgL?1), respectively. The spatial and vertical distribution of POC resembled that of PN. Relatively high concentrations of POC and PN were observed at the nearshore stations of all transects. The POC and PN concentrations were rather low and homogenous in the water column of offshore areas. The PN concentrations in transect YD1 were higher than those in other transects. Chl-, as an indicator of phytoplankton biomass, varied from 0.1 to 3.7μgL?1(with an average of 0. 7μgL?1), and higher levels were observed in the coastal areas of transects YD4 and YD5, and the offshore area of transects YD2 and YD3 (Fig.3).

Fig.3 Spatial distributions of Chl-a, POC and PN in the NSCS during summer.

3.3 δ13C, δ15N and C/N Ratios of POM

The δ13C, δ15N and C/N ratios of POM showed considerable spatial variability (Fig.4). δ13C and δ15N varied from ?25.7‰ to ?18.6‰ (an average of ?22.4‰), and from 2.0‰ to 8.9‰ (an average of 5.5‰), respectively. In the five transects, δ13C generally decreased seawards, and elevated δ13C occurred in the coastal area. δ15N values display a complicated spatial distribution relative to the corresponding δ13C values. In transect YD1, the maximum δ15N was observed at the surface water of station YD1-4 and relatively depleted δ15N was found in the coastal area and both surface and bottom water of offshore stations. For transect YD2, relatively enriched δ15N was observed at stations YD2-1 and YD2-4. In transect YD3, the minimum δ15N was found at the surface water of coastal area and enriched δ15N was observed in the bottom water of stations YD3-2, YD3-5 and YD3-6. In transect YD4, relatively higher δ15N occurred at the nearshore stations, whereas the δ15N values of offshore stations were low. For transect YD5, the surface water was characterized by depleted δ15N and the maximum value occurred in the bottom of station YD5-1.

The C/N ratios varied from 0.6 to 24.1, with an average of 5.3 (Fig.4). In transect YD1, the C/N ratios were rather low and homogenous in the coastal area. The transect YD2, YD4 and YD5 were characterized by relatively low C/N ratios. For transect YD3, the maximum C/N ratios occurred at the surface water of coastal area, whereas the C/N ratios of seaward stations were homogenous and ra- ther low.

4 Discussion

4.1 POC and PN in the NSCS

The POC and PN concentrations in the NSCS were generally lower than that in the Pearl River Estuary (0.30–0.93mgL?1for POC and 0.05–0.26mgL?1for PN) (Ye et al., 2017). The Pearl River Estuary received large quantities of terrestrial organic matter from the Pearl River, and the freshwater discharge controlled the POM sources in the estuary (Ye et al., 2017). In summer,previous studies confirmed that a percentage of the Pearl River particles was forced northeastward by the Yuedong coastal current and the southwesterly monsoon (Hong et al., 2009; Chen et al., 2017). Moreover, rapid population growth and economic development of Guangdong Province have carried high loads of manure and sewage into the Pearl River Estuary and subsequently transported to the east coastal waters (Ye et al., 2015). A large amount of pollutants has also been discharged into the coastal water from cities adjacent to the east coast of Guangdong Province, which are the most rapid developing areas in China (Chen et al., 2019). Thus, the higher POM in the coastal area may be attributed to the anthropogenic activity and eutrophication (Ke et al., 2017). Positive relationship between POC and Chl-a (Fig.5) in the coastal area suggest a significant contribution from in situ production. Additionally, poor relationship between POC and PN with Chl-a was found in the offshore area (Fig.5), suggesting that heterotrophs rather than situ phytoplankton are the dominant contribution to the POM in the NSCS (Hung et al., 2007; Krishna et al., 2018).

Fig.4 Spatial distributions of δ13C, δ15N and C/N ratios in the NSCS during summer.

4.2 Sources of POM to the NSCS

4.2.1 C/N ratios of particulate organic matter

C/N ratios can be used to identify the source of organic matter in aquatic ecosystems (Meyers, 1994; Hedges and Oades, 1997; Graham., 2001; Lamb., 2006). The C/N ratios of higher plants are typically higher than 20 (Meyers, 1994; Hedges., 1979, 1997; Lamb., 2006). The C/N ratios of phytoplankton-derived organic matter vary between 6 and 8 (Onstad., 2000). Bacterial and zooplankton biomass have C/N ratios from 3 to 6 (Hedges., 1997; Koski., 1999). Nevertheless, the C/N ratios could be modified by several diagenetic processes (Sarma., 2014). For example, the bacterial assimilation and retention of dissolved inorganic nitrogen would decrease the C/N ratios of higher plant litter, while preferential removal of nitrogen would increase the ratios in algal detritus upon diagenesis. Though these modifications may hinder to identify source of organic matter, it is possible to understand at least the occurrence of modification (Sarma., 2014). As shown in Fig.4, most of the C/N ratios of POM in the NSCS were below 8. The mean C/N ratios of suspended matter (5.3) in the NSCS were relatively lower than that observed in major world rivers (8.1–12.9;Ittekot and Zhang, 1989), suggesting that diagenetic processes modified the C/N ratios extensively. In addition, given that the C/N ratios indicate the provenance of organic matter, it is expected that C/N ratios would have a significant negative relationship with δ13C,.., organic matter with high C/N ratios correspond to lighter δ13C value (Wu., 2003). However, there is no significant relationship between the C/N ratios and δ13C in this study (Fig.6). All of these suggest that C/N ratios are not preferred as a robust indicator of organic matter. This lack of ability to determine organic matter sources using C/N ratios has been reported in the Pear River Estuary (Jia., 2003; Chen., 2008; Guo., 2015), coastal Bohai Bay (Gao., 2012), Zhejiang coast (Xu., 2017) and elsewhere in the world (Middelburg and Herman, 2007; Sarma., 2012, 2014). This can be attributed to decomposition processes (.., autolysis, leaching and microbial mineralization) of organic matter (Thornton and McManus, 1994; Wu., 2003) and/or anthropogenic disturbances (Wu., 2007). The C/N ratios of higher plant litter tend to decrease because of bacterial assimilation of dissolved inorganic nitrogen and retention of nitrogen, whereas C/N ratios of algal detritus tend to increase upon diagenesis (Hedges., 1997; Herman., 1999). These modifications do not allow the C/N ratios to differentiate organic matter sources.

Fig.5 The relationship of Chl-a with POC and PN in different areas of the NSCS.

Fig.6 the relationship between C/N ratios and δ13C in the NSCS.

4.2.2 Isotopic composition of POM in the NSCS

Values of δ13C are often used to track the predominant source of organic matter (Wu., 2003; Hu., 2006; Ye., 2017). In general, the δ13C of particulate terrestrial organic matter ranges from ?30‰ to ?23‰ and that of freshwater phytoplankton δ13C ranges from ?33‰ to ?25‰, whereas that of marine phytoplankton tends to be more positive, ranging from ?22‰ to ?19‰ (Meyers, 1994; Middelburg and Nieuwenhuize, 1998;Lamb., 2006).Nitrogen isotope of organic matter (δ15N) is also another tracer to identify sources of organic matter. For example, the δ15N of marine organic matter ranges from 3‰ to 12‰, with an average of 5‰–7‰, whereas those of nitrogen fixing land plants have δ15N values around zero (Brandes and Devol, 2002; Lamb., 2006). In general, riverine δ15N values are mostly lower than those of the marine end-member (Maksymowska., 2000; Gaye-Haake., 2005). Low δ15N of fluvial suspension could be ascribed to contributions from forest and soil nitrogen as terrestrial plant ecosystems have low δ15N. Wastewater and livestock usually have the δ15N values of 10‰–22‰ (McClelland., 1997).

The δ13C and δ15N displayed a wide range (?25.7‰– ?18.6‰ and 2.0‰–8.9‰, respectively) in the NSCS, suggesting variable sources of organic matter (Fig.4). δ13C generally decreased seawards, and elevated δ13C occurred at the nearshore stations. This distribution pattern has also been reported in other coastal areas (Gao., 2014; Ke., 2017). For example, Gao. (2014) observed a seaward decrease of δ13C in the adjacent western East China Sea; a decreasing trend of surface δ13C from land to sea in Daya Bay was generally found (Ke., 2017). These results suggest that the distribution of δ13C cannot be attributed to a seaward increase in marine photosynthetic organic matter relative to terrigenous organic matter. Moreover, the lowest salinity was found in the surface water and an increasing trend of salinity from the land to sea was generally observed (Fig.2), suggesting the influence of the local diluted fresh water. Guangdong province is strongly influenced by the East Asian summer monsoon (.., Southeast monsoon) (Wang, 2007). Large amounts of water discharge and suspended particulates are accordingly transported into the oceans during wet seasons (from April to September) due to the warm and humid summer monsoon (Zong., 2009). Moreover, rapid population growth and economic development along the eastern coast of Guangdong Province have carried high loads of manure and sewage into coastal waters. This is also supported by relatively high NO3–concentrations and δ15N values of NO3–, which are believed to be associated with the diluted water from the adjacent cities (Chen., 2019). The land runoff and coastal sewage carry amounts of bacteria into the seawater. The low-salinity water environment near the shore and the existence of a relatively high concentration of suspended particulate matter is beneficial for the breeding and reproduction of microorganisms, resulting in severe microbial activity and intensive organic matter decomposition of suspended particles (Chen., 2008; Guo., 2015). This process can be invoked to explain the observed enrichment in δ13C of the nearshore stations. Notably, the relatively depleted δ13C values were observed at the offshore stations of these five transects (Fig.4), but C/N ratios were extremely low. This suggest that the terrigenous organic matter is not the main factor controlling the source of POM in outer waters. Degens. (1968) found significant isotopic differences between the different components of organic matter. In general, lipid and lignin fractions have more negative δ13C compared with proteins and carbohydrates (Cifuentes., 1996). The depleted δ13C of the outer waters at these transects possibly result from the greater degradation of proteinaceous and carbohydrate components relative to lipid and lignin fractions, and the selective loss of the former fractions would result in lighter carbon isotope ratios (Benner., 1990; Cifuentes., 1996; Wu., 2003).

Fig.7 The relationship of Chl-a with δ15N and δ15N with δ13C in different areas of the NSCS.

The extent of the δ15N values (2.0‰–8.9‰) suggests that POM is a mixture of terrigenous and marine organic materials. The δ15N values showed poor correlation with the Chl-concentrations in both coastal area and offshore area (Fig.7). It is well-known that the terrigenous detrital organic matter has a relatively low δ15N values while marine organic matter is generally characterized by high δ15N values. However, the spatial distribution of δ15N in this study showed a contrary trend, with higher δ15N values distributed in the coastal area. A similar pattern has been observed in the surface sediment from the Zhejiang coast (Xu., 2017). As shown in Fig.4, δ15N values displayed a complicated spatial distribution relative to the corresponding δ13C values. The δ15N values showed no significant correlation with δ13C (Fig.7). This suggests that additional factors have a significant influence on the distributions of δ15N. Previous studies have confirmed that heterotrophic bacterial processes extensively modified the nitrogen composition, generating isotopically enriched δ15N values (Caraco., 1998; Sarma., 2014; Ye., 2017). The isotopically heavy δ15N values in the coastal area may be attributed to heterotrophic microorganisms, as evidenced by relatively high NO3–concentrations and δ15N values of NO3–(Chen., 2019). We speculated the rapid development of local economiesresulted from increase of human activities in the coast of the northern South China Sea, which can intensify heterotrophic bacterial processes modified the particulate organic carbon composition.Enriched δ15N was observed in the bottom water (.., stations YD3-2, YD3-5, YD3-6, and YD5-1). The depth-related distribution of δ15N was similar to previous observations made in the East China Sea and the Bering Sea (Wu., 2003; Lin., 2014). High δ15N in the bottom water mainly due to two reasons: 1) degradation of suspended matter itself causing the preferential loss of14N, leaving the remaining PON enriched in15N (Kumar., 2004); 2) mineralization of organic matter (Wu., 2003; Ye., 2017). In addition, although nitrogen stable isotopes have been successfully used to trace source of organic matter in aquatic systems, we suggest that δ15N must be interpreted with caution as an organic tracer since nitrogen isotopic compositions can be easily modified by a series of complex biogeochemical processes (Wu., 2003; Zhang., 2014; Xu., 2017; Ye., 2017).

4.2.3 Quantification of Organic Matter Sources to the NSCS

The relative contribution of POM from different sources, including terrestrial organic matter, freshwater algae and marine organic material in the NSCS (Fig.8), was evaluated using SIAR model (Parnell., 2008, 2010). δ15N must be interpreted with caution as an organic tracer since the characteristics of the δ15N in this study area tend to indicate the dynamic cycling of nitrogen (Wu., 2003; Zhang., 2014; Xu., 2017; Ye., 2017). We assumed ?30.0‰±2.0‰ as the δ13C value of freshwater phytoplankton. The marine end member was taken as ?20.0‰±2.0‰. The terrestrial end member was taken as ?25.3‰±3.3‰, which is the average of the δ13C values of different soils (?28.3‰ to ?21.7‰) from the Pearl River basin (Yu., 2010) and sewage (?28.5‰ to ?21.0‰) (Liu., 2007). The results suggest a dominant marine contribution of 65% to the POM within the NSCS, followed by the terrestrial organic matter, accounting for 20%, while the contribution from freshwater algae was 15% (Fig.9).

Fig.8 Mixing plots for δ13C and δ15N values of POM from three potential sources for all sampling stations in the NSCS. End members for POM sources including freshwater phytoplankton, marine sources and terrestrial OM.

Fig.9 The contribution of potential POM sources in the NSCS. The credibility intervals of the model (95%, 75% and 25%) were shown in different shades of grey colour. To indicates terrestrial organic matter, FP denotes freshwater phytoplankton, and MO denotes marine organic matter.

5 Conclusions

In this study, spatial and vertical distributions of particulate organic carbon and nitrogen were investigated in the NSCS during summer. POC and PN showed a similar spatial distribution. Relatively high concentrations of POC and PN were observed at the nearshore stations of all transects. The POC and PN concentrations were rather low and homogenous in the water column of offshore areas. The δ13C generally decreased seawards, and elevated δ13C occurred in the coastal area, likely reflecting the potential influence of the eutrophic level and microbial activity. δ15N values display a complicated spatial distribution relative to the corresponding δ13C values. The isotopically heavy δ15N values in the coastal area may be attributed to heterotrophic modification. Elevated δ15N in the bottom water may result from degradation and mineralization of suspended matter. The characteristics of the δ15N in particulate organic matter suggest that it appears to indicate the extent of biogenic alteration rather than the source of the organic matter. The δ13C and δ15N values in the coastal area indicate the significant influences of human activity and eutrophication.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Nos. U1901213, 41466010, 41676008), the China National Key Research and Deve- lopment Plan Project (No.2016YFC1401403),the Guang- dong Natural Science Foundation of China (Nos. 2016A030312004; 2020A1515010500), the Project of Enhan- cing School with Innovation of Guangdong Ocean University (Nos. GDOU2016050260, 230419097), and the Marine Science Research Team Project of Guangdong Ocean University (No. 002026002004). We would like to thank Professors Jie Xu and Zhen Shi for their support in collecting water samples.

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. Tel: 0086-759-2396037 E-mail: fjchen@gdou.edu.cn

October 19, 2020;

April 8, 2021;

June 1, 2021

? Ocean University of China, Science Press and Springer-Verlag GmbH Germany 2021

(Edited by Ji Dechun)