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Occurrence, polarity and bioavailability of dissolved organic matter in the Huangpu River, China Qianqian Dong1 , Penghui Li1 , Qinghui Huang1,⁎, Ahmed A. Abdelhafez1,2 , Ling Chen1 1. Key laboratory of Yangtze River Water Environment of the Ministry of Education, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, China. E-mail: [email protected] 2. Soils, Water and Environment Research Institute, Agricultural Research Center, Giza, Egypt
AR TIC LE I N FO
ABS TR ACT
Article history:
Dissolved organic matter (DOM) plays an important role in biogeochemical cycles in aquatic
Received 22 October 2013
ecosystem. To investigate the characteristics of DOM in Huangpu River (the last tributary of
Revised 27 December 2013
the Yangtze River), surface water samples were collected along the river from December
Accepted 10 January 2014
2011 to June, 2013. The concentrations of dissolved organic carbon (DOC), the absorbance
Available online 2 July 2014
and fluorescence spectrum of DOM in water samples were measured. Fluorescent DOM in the Huangpu River was decomposed into four components by the parallel factor analysis
Keywords:
(PARAFAC), including one humic-like substance and three protein-like substances. It
Chromophoric dissolved organic
showed that high spatial variability of DOC concentration was observed in the upstream
matter
water compared to the downstream water, and so did the absorbance coefficients of
Dissolved organic carbon
chromophoric dissolved organic matter and the total fluorescence intensities of different
Polarity
PARAFAC components of DOM. Furthermore, there was a large difference between the
Bioavailability
polarity and bioavailability of DOM in the Huangpu River. Polar compounds dominated
Absorbance coefficients
tyrosine-like component of fluorescent DOM in all seasons. Tryptophan-like and humic-like
Fluorescence
substances had more polar fraction in summer and autumn than those in winter, while aromatic protein-like materials had the highest polar fraction in winter. Almost all of fluorescent DOM components were refractory in spring, while less than 20% of fluorescent DOM in average were biodegradable within 4 weeks in other seasons. We concluded that the spatial variation in the abundance of DOM in the Huangpu River is mainly affected by the water discharges from the Hangjiahu Plain and the seasonal difference in polarity and bioavailability of DOM is largely determined by its origins. © 2014 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.
Introduction The riverine export of dissolved organic matter (DOM) through estuaries has a major impact on the physical features and biogeochemical cycles in the coastal ocean (Benner, 2004). The bioavailable fraction of DOM in lakes and rivers varied from 1% of dissolved organic carbon (DOC) in brown water to 14% of DOC in clear water within 1–2 weeks of incubation (Koehler et al., 2012). Furthermore, the value of bioavailable DOM reaches 50% of the
DOC by increasing the timescale incubation (Søndergaard and Middelboe, 1995; Vähätalo and Wetzel, 2008). Heterotrophic bacteria depend on DOM as a major source for carbon and energy; therefore, bioavailability of DOM directly affects the bacterial production and nutrient cycles in natural waters. The majority of non-bioavailable DOM fractions are characterized by a high molecular weight, which is resistant to biological degradation and therefore, is called recalcitrant DOM (Jiao et al., 2011). However, Amon and Benner (1996) found that some of the
⁎ Corresponding author. E-mail: [email protected] (Qinghui Huang).
http://dx.doi.org/10.1016/j.jes.2014.06.020 1001-0742/© 2014 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.
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recalcitrant DOM in the dark global ocean was of relatively low molecular weight (< 1 kDa) and that the higher the average molecular weight, the higher the bioavailability. DOM also plays an important role for the function of aquatic ecosystems by changing the bioavailability of pollutants through its effects on the biogeochemical cycles of heavy metals, organic pollutants and emerging contaminants (Aiken et al., 2011; Hassett, 2006). Riverine DOM is highly spatiotemporally dynamic in the composition, reactivity and abundance due to its physiochemical properties and some other extrinsic factors, i.e., storm events, agricultural runoff and wastewater effluent. For example, the dominant DOM compounds were generally composed of aromatic/carboxylic compounds near shore to aliphatic/carbohydrate materials offshore (Stephens and Minor, 2010). The optical properties of chromophoric dissolved organic matter (CDOM) in terms of absorbance, and fluorescence had been widely used to investigate the major origins of DOM (Fichot and Benner, 2012; Huguet et al., 2009; Zsolnay et al., 1999). The production, loss and transport of DOM are important terms in the carbon budget within freshwater and marine ecosystems. Combining chemical and optical properties of DOM has proven to be an effective method to differentiate various allochthonous inputs (Osburn and Stedmon, 2011) and demonstrate the impacts of these inputs on the quality of rivers and streams. Water discharged from Jiangsu, Zhejiang and Shanghai into Huangpu River (the last tidal river tributary of the Yangtze River estuary), which is one of the major water sources of Shanghai Metropolis. With the population growth in the river basin, the water quality of the Huangpu River has degraded over time. It has been reported that the concentrations of biological oxygen demand, total nitrogen and phosphorus, oils, phenol and suspended solids increased along the river flowing through Shanghai as a result of the continuous discharge of wastewater from Shanghai in the past decades (Yang et al., 2007a). Our previous study identified the microbially-derived tryptophan and protein-like materials on the part of Yangtze estuary (about 2 to 25 km downstream from the mouth of Huangpu River), and they attributed the origin of these compounds from Huangpu River rather than the mainstream of the Yangtze River (Yang et al., 2007b). Consequently, Huangpu River may have a great impact on the quality of receiving water of the Yangtze estuary. However, the sources and fate of DOM in the Huangpu River are still poorly understood. The main objectives of the present study were to (1) determine the spatiotemporal distribution of DOM abundance in the Huangpu River; and (2) discuss the sources of DOM components and the influence factors on the spatial distribution of DOM and potential bioavailability of DOM in Huangpu River.
through 0.7 μm glass fiber filters which are pre-combusted at 450°C for 4 hr (Whatman GF/F, GE Healthcare Life Sciences, Buckinghamshire, Ireland), and then sent to the laboratory within 24 hr. The samples were prepared as shown in Fig. 2, and the preparation steps can be summarized as follows: (1) the 1000 mL GF/F-filtrate passed through a solid-phase extraction (SPE) C18 column (Supelclean™ LC-18 Cartridges, Supelco., Bellefonte, PA USA) which was activated by methylene dichloride, methanol and Milli-Q water (Milli-Q A10, Merck KGaA, Darmstadt, Germany). An aliquot of SPE-filtrate was further filtered by mixed cellulose ester filters, which were immersed in diluted HCl for 24 hr and then rinsed with deionized water to measure its absorbance, fluorescence and DOC content. Another aliquot of SPE-filtrate was conserved in a dark place for incubation under room-temperature to measure its bioavailability. The incubation lasted for 28 days. (2) The 500 mL GF/F-filtrate was separated into two parts: an aliquot was filtered by mixed cellulose ester filters for further measurement as described before; another aliquot was stored in a dark place for incubation. (3) At the end of incubation, the samples passed through mixed cellulose ester filters for the measurement of absorbance, fluorescence and DOC. The GF/F filters can remove most particles and eukaryotic organisms, but the filtrate still included the dominant decomposers (e.g., active heterotrophic bacteria) of organic matter in similar aquatic environments (Benner et al., 1986).
1.2. Measurements and data processing DOM abundance can be characterized by DOC concentration, UV absorption coefficient and fluorescence intensity. DOC was measured using a total organic carbon analyzer (TOC-VCPN, Shimadzu Corp., Kyoto, Japan) with high-temperature combustion technique after the acidifying water samples (pH 2–3). Absorption spectra were obtained between 200 and 800 nm at 1-nm intervals using a double-beam spectrophotometer (TU-1901, Peking Purkinje General, co., Beijing, China) equipped with 5-cm quartz cell. The absorption coefficients (aλ, m−1) were calculated from the absorbance (A(λ)) at the wavelength λ (nm) using the following equation: aλ ¼ 2:303 AðλÞ=r
1. Material and methods 1.1. Site description and sampling strategy Huangpu River, about 115 km long with an average of 400 m wide and 9 m deep, serves as an important drinking water source for Shanghai. Its mainstream receives water from Dianshan Lake (S11) and three major tributaries in the upper Huangpu River, including the Taipu Stream (S10), Stream (S8) and Damaogang Stream (S7), which are flowing through the Hangjiahu Plain in Zhejiang Province (Fig. 1). Surface water samples were collected from 11 sites (S1 to S11, Fig. 1) along the Huangpu River from December 2011 to June 2013 in every two months. Samples were not collected in June, 2012 for the limited condition. A Magellan GPS instrument (eXplorist 310, Magellan, Taiwan, China) was used to determine the positions and record the latitude and longitude of the sampling sites. The collected water samples of 1.5 L for each study site were filtered
where, r (m) is the cuvette path length. A(λ) was corrected by averaged absorbance from 680 to 700 nm. SUVA254 is the ratio of UV absorbance at 254 nm measured in inverse meters (m−1) to the DOC concentration measured in milligrams per liter (mg/L) that well indicates DOM aromaticity (Weishaar et al., 2003). Three-dimensional fluorescence excitation–emission matrices (EEMs) of DOM samples from the Huangpu River were collected using a fluorescence spectrophotometer (F-4500, Hitachi, Japan) with excitation wavelength from 241 to 400 nm at 3 nm steps and emission wavelength from 250 to 550 nm at 2 nm steps, and Milli-Q water was used as a blank. Parallel factor (PARAFAC) modeling was conducted on 274 EEMs of water samples by using MATLAB (Ver. 7.7.0, 2008b, the Math Works Inc., Natick, Massachusetts, USA) with the DOMFluor toolbox (Stedmon and Bro, 2008). In this study, Raman peaks of the Milli-Q water were used to normalize the fluorescence signals to make them comparable in different batches of the measurement. Raman peaks were confined by
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31.5°N
Ya Taipu Stream TP ngt ze YXJ Yuanxiejing Stream est uar DMG Damaogang Stream y Sampling Sites S1 Downtown of Shanghai Jiangsu
Dianshan lake
Suz hou rive r
S11 TP
31.0°N
S2 S3 S4
Huangpu river
Shanghai S5
S9
S10 YXJ S8 S7 Hangjiahu Plain DMG Zhejiang
S6
(m) 75 50 25
121.0°E
121.5°E
122.0°E
Fig. 1 – Map of sampling sites (S1–S11) along the Huangpu River, China.
the emission wavelengths of 380–420 nm at 350 nm excitation wavelength (Murphy, 2011). Moreover, the inner-filter effect was eliminated following the tutorial by Murphy et al. (2010). The humification and biological indices have been employed to determine the relative degree of humification and autotrophic productivity of fluorescent DOM, respectively. Biological index (BIX) was calculated from the ratio of emission intensities at a shorter (λem = 380 nm) and longer (λem = 430 nm) wavelengths using a fixed excitation (λex = 310 nm). Humification index (HIX) was determined from the ratio of two integrated regions of an emission scan (average of λem = 435–480 nm divided by the average of λem = 300–345 nm) collected with excitation at 254 nm as a method for comparing the relative humification of DOM samples (Huguet et al., 2009; Zsolnay et al., 1999). Statistical analyses including mean values, standard deviation and one way analysis of variance were performed for social sciences 16.0 software. Difference in parameters along the upstream to downstream of Huangpu River and seasons was assessed with one way analysis of variance with difference noted at p < 0.05.
2. Results 2.1. Absorption characteristics of DOM DOC concentrations varied from 1.95 to 6.20 mg/L in the Huangpu River during one and a half years (Fig. 3a). The lowest value of DOC was found at S10, where the mainstream of Huangpu River received clean water through Taipu Stream from the eastern Taihu Lake in Jiangsu Province. This result is in agreement with our previous study (Chen et al., 2010), where the a355 and CDOM abundance in Taihu Lake and Taipu Stream were lower than those of Dianshan Lake. The peak values of DOC and a355 were recorded at S7 (Fig. 3a,3b), which is the convergence area of the mainstream and the water discharged from Yuanxiejing (YXJ) and Damaogang (DMG) streams in Zhejiang Province. At the downstream sites of the river from the river mouth of DMG stream, the Huangpu River flows through the urban area of the Shanghai metropolis. However, the a355 and DOC values at the mainstream sites
MCE filter
GF/F filter
Incubator
0.2 μm filtrate
Measurement
GF/F filter
SPE filtrate
Fig. 2 – Sample treatment and experiment process. GF/F filter refers to Whatman glass microfiber filter Grade GF/F of 0.7 μm particle retention; SPE refers to solid phase extraction; MCE filter refers to mixed cellulose ester filter of 0.2 μm particle retention.
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(S1–S6) came down to constant values compared to the peak values at S7. DOM abundance was higher in summer than in winter showing a significant difference between these two seasons (Table 1), while a non-significant difference of DOM abundance presenting between winter and spring. Obviously, the primary productivity is exuberant and high precipitation brings a lot of exogenous inputs in summer. On the contrary, these influence factors become weak in winter. The values of SUVA254 indicating the aromaticity and reactivity of DOM were different in four seasons (Fig. 3d, Table 1). It represented that the DOM of the Huangpu River in four seasons may have different sources. During winter season, SUVA254 values were low, and BIX values were high, indicating low DOM aromaticity and a “microbial” source signature. Spring SUVA254 values were similar to winter, but lower BIX values indicating DOM with less “microbial” source signature. In summer, SUVA254 values indicate high aromaticity, and low BIX values, suggesting that DOM was dominated by terrestrial plant sources. In autumn, SUVA254 values were similar to those of the summer season, and BIX values were similar to those of spring.
2.2. Fluorescence characteristics of DOM Fig. 4 shows the identified components from the EEMs of 274 DOM samples by PARAFAC analysis (Stedmon and Bro, 2008). The results showed that three protein-like components (C1, C2 and C4) and one humic-like component (C3) were identified. Tyrosine-like C1 (Ex/Em = 280/322 nm) and tryptophan-like C2 (Ex/Em = 238, 295/350 nm) (Henderson et al., 2009; Hudson et al., 2007) were recalcitrant materials (Chen et al., 2003; Murphy et al., 2011). In addition, C4 (Ex/Em = 232/346 nm) was related to aromatic proteins and amino acids (e.g., tryptophan) (Chen et al., 2003; Teale and Weber, 1957). Humic-like C3 (Ex/Em = 247, 310/410–470 nm) has both microbial and terrestrial signals in high nutrient and wastewater impacted environments (Murphy et al., 2011). In the Huangpu River, the fluorescence intensities of tyrosinelike C1 and tryptophan-like C2 components were much higher than humic-like components (Fig. 5a). The presence of three protein-like components in December was higher than those of other months. However, it was the opposite for humic-like C3, which had the highest fluorescence intensity in August.
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Fig. 3 – Spatial–temporal distribution of DOM abundance and properties in the Huangpu River water (line median, box upper and lower bound = 25% and 75%, whiskers = 10% and 90%). (a) spatial variation of DOC concentrations; (b) spatial variation of absorption coefficients at 355 nm (a355); (c) bimonthly variation of a355; (d) seasonal variation of specific absorption coefficients at 254 nm.
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Table 1 – Seasonal variation of DOM absorbance a355, SUVA254, fluorescence intensity of DOM components, HIX and BIX.
SUVA254 (L/(mg.m)) a355 (m−1) C1 C2 C3 C4 HIX BIX
Spring c
5.14 ± 1.02 4.18 6.16 4.35 0.84 1.95 0.50 1.37
± ± ± ± ± ± ±
0.89ab 2.09a 1.57a 0.12a 0.42a 0.14b 0.17a
Summer c
5.02 ± 0.27 3.45 4.23 3.96 0.57 0.57 0.41 1.25
± ± ± ± ± ± ±
0.78b 0.81bc 0.88ab 0.11b 0.07c 0.06b 0.07a
b
8.59 ± 1.81 4.73 3.29 2.72 0.92 0.76 0.87 1.06
± ± ± ± ± ± ±
0.77a 1.02c 0.74b 0.15a 0.21bc 0.37a 0.08b
Autumn 9.18 ± 2.22a 4.39 4.86 3.73 0.78 1.07 0.59 1.24
± ± ± ± ± ± ±
0.70ab 2.03b 1.59ab 0.12a 0.27b 0.15b 0.13a
Data are expressed as mean ± S.D. (n = 11). Results from the Turkey test indicating the significant (p < 0.05) differences among the seasons. Data followed by the same letters are not significantly different.
HIX and BIX are relatively stable to reflect the source of DOM regardless of the changes of the time. HIX values varied between 0.33 and 0.72 while BIX values varied between 0.94 and 1.44, which suggested that there was a predominantly autochthonous/biological origin of DOM (Huguet et al., 2009; Zhang et al., 2010; Zsolnay et al., 1999) in Huangpu River. High HIX and low BIX at S7 revealed that, DOM is terrigenous there (Fig. 5b). Significant negative correlations were obtained between the fluorescence intensity of C1, C2 and HIX value (r = − 0.79, p < 0.001; r = −0.62, p < 0.001; r = −0.75, p < 0.001). However, C3 had a positive correlation with HIX (r = 0.21, p < 0.001) and negative correlation with BIX (r = − 0.21, p < 0.001), suggesting its humification characteristics. The fluorescence intensity of C3 and HIX values were the highest at S7 with average values of 0.70 (R.U.) and 0.69, respectively. The result revealed that the fluorescence characters of DOM components also showed large spatial variations in the upper Huangpu River, while they almost kept constant in the downstream as shown in Fig. 5b.
3. Discussion 3.1. Effect of headstream inputs on DOM variability in the Huangpu River There are several major tributaries namely, Taipu, Yuanxiejing and Damaogang streams (Fig. 1) from Hangjiahu Plain discharging into the upper Huangpu River; however, the water quality is quite different in terms of DOM characteristics. The DOC concentration, DOM absorbance and fluorescence of the water discharged from Taihu Lake through Taipu Stream into the upper Huangpu River are very low (Chen et al., 2010). As a result, the abundance of DOM in the river water at S11 was low due to the dilution effect. YXJ Stream runoff brings much protein-like DOMs (C1, C2 and C4), while DMG Stream runoff brings much humic-like DOM into the Huangpu River. The fluorescence intensity of DOM showed less variable in downstream than in upstream of Huangpu River as well as DOC concentration. The DOC concentrations, DOM absorbance and humic-like
2.3. Polarity and bioavailability of fluorescent DOM Microbial bioavailability of the DOM showed a significant difference among the four fluorescent components and among the four seasons (Fig. 6). In winter season, bioavailable fraction of C1, C2, C3 accounted for 5.4%–16%, 15%–43%, 0%–5.2%, respectively. However, most of the DOM became refractory in spring. In summer and autumn, the bioavailable fraction of DOM accounted
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for an average of 20% above with an exception of refractory C2 in autumn. This may be contributed to the different sources of DOM in different the seasons. During winter, DOM has apparent aromaticity and a “microbial” source signature. Relatively spring DOM is with less “microbial” source signature. In the summer, there were high SUVA254 values and low BIX values (Fig. 3d), suggesting that DOM dominated by terrestrial plant sources. In autumn, SUVA254 values were similar to those of summer, and BIX values were similar to those of spring. Therefore, the source difference may contribute to the variation of biodegradation of DOM components during the year (Fig. 6). Furthermore, in summer, the activity of microorganism is more active than other seasons. Some refractory components may also be transferred to the labile ones in the strong sunlight irradiation. Generally, non-polar organic compounds and some bio-macromolecules in the water can be retained in the C18 column (Kieber et al., 2006). In this study, the filtrates were passed through the C18 column to study the polarity properties of the studied DOM components. It is shown in Fig. 7 that about 99% of C1 was not retained in the column, which indicated that almost all of the tyrosine-like material was polar fraction. On average, approximately 49% of the tryptophan-like material (C2) was polar fraction, while the polar fractions in humic-like C3 and protein-like C4 accounted for 71% and 76%, respectively. The specific values of polar fraction percentage also changed with the season.
0.05 450
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Fig. 4 – EEM contour maps of four fluorescent DOM components decomposed by PARAFAC. C1, C2 and C4 showed the fluorescence characteristics of protein-like material and C3 showed the fluorescence characteristics of humic-like material.
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Fig. 5 – Characterization of fluorescence properties of DOM in the Huangpu River. (a) bimonthly change of maxima fluorescence intensities of four PARAFAC components of DOM; (b) spatial variation of humification index (HIX) and biological index (BIX) of DOM.
fluorescence intensities in the water from the YXJ Stream are lower than those from the DMG Stream, while it was just the opposite for the fluorescence intensities of the three protein-like DOM components. The runoff of YXJ and DMG Streams flowing into the upper Huangpu River was characterized by high DOM abundance. This could be attributed to the high concentrations of organic pollutants (e.g., veterinary antibiotics) present in the upper Huangpu River due to the agricultural and livestock activities in the suburban area (Jiang et al., 2011). Therefore, the spatial variation in the abundance of DOM in the Huangpu River can be mainly attributed to the headstream inputs influenced by the social-economic development in the Hangjiahu Plain.
3.2. Influence of precipitation on DOM variability in the Huangpu River Generally, DOM abundance in the streams and rivers was highly correlated with water discharge, which indicates that the rainfall
events could dominate the export of DOM from the watershed (Shen et al., 2012; Xu et al., 2012). The a355 values of DOM in the Huangpu River (especially at S7 and S8) in August were much higher than those in other months, which could be attributed to the input of surface runoff enhanced by a heavy storm event before the sampling period. In fact, there was heavy rain before sampling in February 2012, which may lead to the high humic-like substance input (Fig. 5a). The result is consistent to the observations in the surrounding rivers of Lake Taihu (Zhang et al., 2011a) and the catchment of Lake Tianmuhu (Zhang et al., 2011b). For example, higher CDOM absorption was observed in the flood season and decreased along the distance from the river mouth into the lake. However, the fluorescence intensities of DOM components in summer are lower than those in other seasons. On the one hand, high runoff dilutes the DOM which part comes from autochthonous. These parts are mainly protein-like components either from microorganism activity or the primary productivity. On the other hand, due to the
Before incubation
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Fig. 6 – Microbially-derived variation in fluorescence intensities of four PARAFAC components of DOM among different seasons.
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Polar DOM (%)
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Fig. 7 – Temporal variation in percentage of polar DOM accounting for each PARAFAC component of fluorescent DOM.
high temperature and strong light in summer, DOM can be easily transferred. It could be concluded that, allochthonous inputs which enhanced by the storm events had a great influence on the components of C3. On the contrary, the high runoff due to heavy precipitation (e.g., in August) played an important role in diluting C1 and C2 components.
3.3. Influence of microbial transformation on DOM variability in the Huangpu River In the Huangpu River, most of the tyrosine-like and tryptophanlike substances were biologically recalcitrant (Fig. 6), while some of the aromatic protein-like fraction was semi-labile as well as polysaccharides (Gogou and Repeta, 2010). It was reported that sewage-derived material contained high abundance of tryptophan-like material, which is dominant by organic matter originating from microbial activity (Hudson et al., 2007). It could be concluded that recalcitrant DOM fractions (e.g., C1 and C2) in the Huangpu River originated from the production of microbial activities affected by sewage effluent (Chen et al., 2003). Furthermore, C4 was mainly related to the new production (e.g., catabolite) of plankton and aquatic plants and their derivatives from microbial transformation (Chen et al., 2003; Murphy et al., 2011), and C3 was derived from soil erosion or the degradation of protein-like material (Murphy et al., 2011). It has been revealed that large size organic matters were more bioavailable than the smallest size one (Amon and Benner, 1996). Moreover, the microorganisms preferred to utilize the newborn DOM rather than the old ones. Microbial transformation of some labile DOM would contribute to the accumulation of humic-like substance.
polarity and bioavailability in different seasons. Polar compounds dominated tyrosine-like DOM component in all seasons but accounted for 30%–100% of other components in different seasons. Less than 20% of the fluorescent DOM components in the Huangpu River are bioavailable while most of them are biologically refractory, especially in spring season. The tyrosine-like and tryptophan-like DOM mainly derived from microbial activity. However, the biodegradable fractions of DOM are different with time change. Therefore, the bioavailability of DOM was mainly determined by its origin while the variability of DOM is largely determined by the land use in the headstream catchments. In the past decades, the urbanization of Shanghai had a great impact on the water quality of the Huangpu River. Therefore, Zhejiang Province should also be partly or even extremely to be blamed for the organic pollution in the Huangpu River. In order to prevent the water resources of the Huangpu River from pollution, the best management practice in the whole catchments should be initiated, taken into consideration, the cooperation between the research institutes in Shanghai and Zhejiang.
Acknowledgment We thank the staff at Shanghai Environmental Monitoring Station for their help in the field work. This study was supported by the National Natural Science Foundation of China (Nos. 41071301, 40601095) and the Fundamental Research Funds for the Central Universities (No. 0400219216).
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4. Conclusion A much higher variability in DOM abundance observed in the upstream of the Huangpu River is mainly attributed to the inputs of different headstreams flowing through the Hangjiahu Plain. Four fluorescent DOM components identified in the water samples of the Huangpu River have great difference in the
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Benner, R., 2004. Export of young terrigenous dissolved organic carbon from rivers to the Arctic Ocean. Geophysical Research Letters 31, L05305. http://dx.doi.org/10.01029/02003GL019251. Benner, R., Moran, M.A., Hodson, R.E., 1986. Biogeochemical cycling of lignocellulose carbon in marine and freshwater ecosystems: relative contributions of prokaryotes and eukaryotes. Limnol. Oceanogr. 31, 99–100. Chen, W., Westerhoff, P., Leenheer, J.A., Booksh, K., 2003. Fluorescence excitation–emission matrix regional integration to quantify spectra for dissolved organic matter. Environ. Sci. Technol. 37, 5701–5710. Chen, X.C., Ji, Y.L., Hu, Q., Huang, Q.H., 2010. Characteristics and sources of CDOM in the drainage system of water diversion from Yangtze River to Lake Tai. J. Lake Sci. 22, 63–69. Fichot, C.G., Benner, R., 2012. The spectral slope coefficient of chromophoric dissolved organic matter (S275–295) as a tracer of terrigenous dissolved organic carbon in river-influenced ocean margins. Limnol. Oceanogr. 57, 1453–1466. Gogou, A., Repeta, D.J., 2010. Particulate-dissolved transformations as a sink for semi-labile dissolved organic matter: chemical characterization of high molecular weight dissolved and surface-active organic matter in seawater and in diatom cultures. Mar. Chem. 121, 215–223. Hassett, J.P., 2006. Dissolved natural organic matter as a microreactor. Science 311, 1723–1724. Henderson, R.K., Baker, A., Murphy, K.R., Hamblya, A., Stuetz, R.M., Khan, S.J., 2009. Fluorescence as a potential monitoring tool for recycled water systems: a review. Water Res. 43, 863–881. Hudson, N., Baker, A., Reynolds, D., 2007. Fluorescence analysis of dissolved organic matter in natural, waste and polluted waters — a review. River Res. Appl. 23, 631–649. Huguet, A., Vacher, L., Relexans, S., Saubusse, S., Froidefond, J.M., Parlanti, E., 2009. Properties of fluorescent dissolved organic matter in the Gironde Estuary. Org. Geochem. 40, 706–719. Jiang, L., Hu, X.L., Yin, D.Q., Zhang, H.C., Yu, Z.Y., 2011. Occurrence, distribution and seasonal variation of antibiotics in the Huangpu River, Shanghai, China. Chemosphere 82, 822–828. Jiao, N., Herndl, G.J., Hansell, D.A., Benner, R., Kattner, G., Wilhelm, S.W., et al., 2011. The microbial carbon pump and the oceanic recalcitrant dissolved organic matter pool. Nat. Rev. Microbiol. 9, 555. Kieber, R.J., Whitehead, R.F., Reid, S.N., Willey, J.D., Seaton, P.J., 2006. Chromophoric dissolved organic matter (CDOM) in rainwater, southeastern North Carolina, USA. J. Atmos. Chem. 54, 21–41. Koehler, B., von Wachenfeldt, E., Kothawala, D., Tranvik, L.J., 2012. Reactivity continuum of dissolved organic carbon decomposition in lake water. Journal of Geophysical Research — Biogeosciences 117, G01024. http://dx.doi.org/10.01029/02011JG001793. Murphy, K.R., 2011. A note on determining the extent of the water raman peak in fluorescence spectroscopy. Appl. Spectrosc. 65, 233–236. Murphy, K.R., Butler, K.D., Spencer, R.G., Stedmon, C.A., Boehme, J.R., Aiken, G.R., 2010. Measurement of dissolved organic matter fluorescence in aquatic environments: an interlaboratory comparison. Environ. Sci. Technol. 44 (24), 9405–9412. Murphy, K.R., Hambly, A., Singh, S., Henderson, R.K., Baker, A., Stuetz, R., et al., 2011. Organic matter fluorescence in municipal water recycling schemes: toward a unified PARAFAC model. Environ. Sci. Technol. 45, 2909–2916.
Osburn, C.L., Stedmon, C.A., 2011. Linking the chemical and optical properties of dissolved organic matter in the Baltic–North Sea transition zone to differentiate three allochthonous inputs. Mar. Chem. 126, 281–294. Shen, Y., Fichot, C.G., Benner, R., 2012. Floodplain influence on dissolved organic matter composition and export from the Mississippi–Atchafalaya River system to the Gulf of Mexico. Limnol. Oceanogr. 57, 1149–1160. Søndergaard, M., Middelboe, M., 1995. A cross-system analysis of labile dissolved organic carbon. Mar. Ecol. Prog. Ser. 118, 283–294. Stedmon, C.A., Bro, R., 2008. Characterizing dissolved organic matter fluorescence with parallel factor analysis: a tutorial. Limnol. Oceanogr. Method 6, 572–579. Stephens, B.M., Minor, E.C., 2010. DOM characteristics along the continuum from river to receiving basin: a comparison of freshwater and saline transects. Aquat. Sci. 72, 403–417. Teale, F.W.J., Weber, G., 1957. Ultraviolet fluorescence of the aromatic amino acids. Biochem. J. 65, 476–482. Vähätalo, A.V., Wetzel, R.G., 2008. Long-term photochemical and microbial decomposition of wetland-derived dissolved organic matter with alteration of 13C:12C mass ratio. Limnol. Oceanogr. 53, 1387–1392. Weishaar, J.L., Aiken, G.R., Bergamaschi, B.A., Fram, M.S., Fujii, R., Mopper, K., 2003. Evaluation of specific ultraviolet absorbance as an indicator of the chemical composition and reactivity of dissolved organic carbon. Environ. Sci. Technol. 37 (20), 4702–4708. Xu, N., Saiers, J.E., Wilson, H.F., Raymond, P.A., 2012. Simulating streamflow and dissolved organic matter export from a forested watershed. Water Resources Research 48, W05519. http://dx.doi.org/10.01029/02011WR011423. Yang, H.J., Shen, Z.M., Zhang, J.P., Wang, W.H., 2007a. Water quality characteristics along the course of the Huangpu River (China). J. Environ. Sci. 19, 1193–1198. Yang, F., Huang, Q.H., Li, J.H., Zhu, X.M., 2007b. Characterization of chromophoric dissolved organic matter in the Yangtze Estuary by absorption and fluorescence spectroscopy. J. Environ. Sci. Sustainable Soc. 1, 55–60. Zhang, Y., Zhang, E., Yin, Y., van Dijk, M.A., Feng, L., Shi, Z., et al., 2010. Characteristics and sources of chromophoric dissolved organic matter in lakes of the Yungui Plateau, China, differing in trophic state and altitude. Limnol. Oceanogr. 55, 2645–2659. Zhang, Y., Yin, Y., Feng, L., Zhu, G., Shi, Z., Liu, X., et al., 2011a. Characterizing chromophoric dissolved organic matter in Lake Tianmuhu and its catchment basin using excitation–emission matrix fluorescence and parallel factor analysis. Water Res. 45 (16), 5110–5122. Zhang, Y., Yin, Y., Liu, X., Shi, Z., Feng, L., Liu, M., et al., 2011b. Spatial–seasonal dynamics of chromophoric dissolved organic matter in Lake Taihu, a large eutrophic, shallow lake in China. Org. Geochem. 42 (5), 510–519. Zsolnay, A., Baigar, E., Jimenez, M., Steinweg, B., Saccomandi, F., 1999. Differentiating with fluorescence spectroscopy the sources of dissolved organic matter in soils subjected to drying. Chemosphere 38, 45–50.
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Occurrence, polarity and bioavailability of dissolved organic matter in the Huangpu River, China Qianqian Dong1 , Penghui Li1 , Qinghui Huang1,⁎, Ahmed A. Abdelhafez1,2 , Ling Chen1 1. Key laboratory of Yangtze River Water Environment of the Ministry of Education, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, China. E-mail: [email protected] 2. Soils, Water and Environment Research Institute, Agricultural Research Center, Giza, Egypt
AR TIC LE I N FO
ABS TR ACT
Article history:
Dissolved organic matter (DOM) plays an important role in biogeochemical cycles in aquatic
Received 22 October 2013
ecosystem. To investigate the characteristics of DOM in Huangpu River (the last tributary of
Revised 27 December 2013
the Yangtze River), surface water samples were collected along the river from December
Accepted 10 January 2014
2011 to June, 2013. The concentrations of dissolved organic carbon (DOC), the absorbance
Available online 2 July 2014
and fluorescence spectrum of DOM in water samples were measured. Fluorescent DOM in the Huangpu River was decomposed into four components by the parallel factor analysis
Keywords:
(PARAFAC), including one humic-like substance and three protein-like substances. It
Chromophoric dissolved organic
showed that high spatial variability of DOC concentration was observed in the upstream
matter
water compared to the downstream water, and so did the absorbance coefficients of
Dissolved organic carbon
chromophoric dissolved organic matter and the total fluorescence intensities of different
Polarity
PARAFAC components of DOM. Furthermore, there was a large difference between the
Bioavailability
polarity and bioavailability of DOM in the Huangpu River. Polar compounds dominated
Absorbance coefficients
tyrosine-like component of fluorescent DOM in all seasons. Tryptophan-like and humic-like
Fluorescence
substances had more polar fraction in summer and autumn than those in winter, while aromatic protein-like materials had the highest polar fraction in winter. Almost all of fluorescent DOM components were refractory in spring, while less than 20% of fluorescent DOM in average were biodegradable within 4 weeks in other seasons. We concluded that the spatial variation in the abundance of DOM in the Huangpu River is mainly affected by the water discharges from the Hangjiahu Plain and the seasonal difference in polarity and bioavailability of DOM is largely determined by its origins. © 2014 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.
Introduction The riverine export of dissolved organic matter (DOM) through estuaries has a major impact on the physical features and biogeochemical cycles in the coastal ocean (Benner, 2004). The bioavailable fraction of DOM in lakes and rivers varied from 1% of dissolved organic carbon (DOC) in brown water to 14% of DOC in clear water within 1–2 weeks of incubation (Koehler et al., 2012). Furthermore, the value of bioavailable DOM reaches 50% of the
DOC by increasing the timescale incubation (Søndergaard and Middelboe, 1995; Vähätalo and Wetzel, 2008). Heterotrophic bacteria depend on DOM as a major source for carbon and energy; therefore, bioavailability of DOM directly affects the bacterial production and nutrient cycles in natural waters. The majority of non-bioavailable DOM fractions are characterized by a high molecular weight, which is resistant to biological degradation and therefore, is called recalcitrant DOM (Jiao et al., 2011). However, Amon and Benner (1996) found that some of the
⁎ Corresponding author. E-mail: [email protected] (Qinghui Huang).
http://dx.doi.org/10.1016/j.jes.2014.06.020 1001-0742/© 2014 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.
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recalcitrant DOM in the dark global ocean was of relatively low molecular weight (< 1 kDa) and that the higher the average molecular weight, the higher the bioavailability. DOM also plays an important role for the function of aquatic ecosystems by changing the bioavailability of pollutants through its effects on the biogeochemical cycles of heavy metals, organic pollutants and emerging contaminants (Aiken et al., 2011; Hassett, 2006). Riverine DOM is highly spatiotemporally dynamic in the composition, reactivity and abundance due to its physiochemical properties and some other extrinsic factors, i.e., storm events, agricultural runoff and wastewater effluent. For example, the dominant DOM compounds were generally composed of aromatic/carboxylic compounds near shore to aliphatic/carbohydrate materials offshore (Stephens and Minor, 2010). The optical properties of chromophoric dissolved organic matter (CDOM) in terms of absorbance, and fluorescence had been widely used to investigate the major origins of DOM (Fichot and Benner, 2012; Huguet et al., 2009; Zsolnay et al., 1999). The production, loss and transport of DOM are important terms in the carbon budget within freshwater and marine ecosystems. Combining chemical and optical properties of DOM has proven to be an effective method to differentiate various allochthonous inputs (Osburn and Stedmon, 2011) and demonstrate the impacts of these inputs on the quality of rivers and streams. Water discharged from Jiangsu, Zhejiang and Shanghai into Huangpu River (the last tidal river tributary of the Yangtze River estuary), which is one of the major water sources of Shanghai Metropolis. With the population growth in the river basin, the water quality of the Huangpu River has degraded over time. It has been reported that the concentrations of biological oxygen demand, total nitrogen and phosphorus, oils, phenol and suspended solids increased along the river flowing through Shanghai as a result of the continuous discharge of wastewater from Shanghai in the past decades (Yang et al., 2007a). Our previous study identified the microbially-derived tryptophan and protein-like materials on the part of Yangtze estuary (about 2 to 25 km downstream from the mouth of Huangpu River), and they attributed the origin of these compounds from Huangpu River rather than the mainstream of the Yangtze River (Yang et al., 2007b). Consequently, Huangpu River may have a great impact on the quality of receiving water of the Yangtze estuary. However, the sources and fate of DOM in the Huangpu River are still poorly understood. The main objectives of the present study were to (1) determine the spatiotemporal distribution of DOM abundance in the Huangpu River; and (2) discuss the sources of DOM components and the influence factors on the spatial distribution of DOM and potential bioavailability of DOM in Huangpu River.
through 0.7 μm glass fiber filters which are pre-combusted at 450°C for 4 hr (Whatman GF/F, GE Healthcare Life Sciences, Buckinghamshire, Ireland), and then sent to the laboratory within 24 hr. The samples were prepared as shown in Fig. 2, and the preparation steps can be summarized as follows: (1) the 1000 mL GF/F-filtrate passed through a solid-phase extraction (SPE) C18 column (Supelclean™ LC-18 Cartridges, Supelco., Bellefonte, PA USA) which was activated by methylene dichloride, methanol and Milli-Q water (Milli-Q A10, Merck KGaA, Darmstadt, Germany). An aliquot of SPE-filtrate was further filtered by mixed cellulose ester filters, which were immersed in diluted HCl for 24 hr and then rinsed with deionized water to measure its absorbance, fluorescence and DOC content. Another aliquot of SPE-filtrate was conserved in a dark place for incubation under room-temperature to measure its bioavailability. The incubation lasted for 28 days. (2) The 500 mL GF/F-filtrate was separated into two parts: an aliquot was filtered by mixed cellulose ester filters for further measurement as described before; another aliquot was stored in a dark place for incubation. (3) At the end of incubation, the samples passed through mixed cellulose ester filters for the measurement of absorbance, fluorescence and DOC. The GF/F filters can remove most particles and eukaryotic organisms, but the filtrate still included the dominant decomposers (e.g., active heterotrophic bacteria) of organic matter in similar aquatic environments (Benner et al., 1986).
1.2. Measurements and data processing DOM abundance can be characterized by DOC concentration, UV absorption coefficient and fluorescence intensity. DOC was measured using a total organic carbon analyzer (TOC-VCPN, Shimadzu Corp., Kyoto, Japan) with high-temperature combustion technique after the acidifying water samples (pH 2–3). Absorption spectra were obtained between 200 and 800 nm at 1-nm intervals using a double-beam spectrophotometer (TU-1901, Peking Purkinje General, co., Beijing, China) equipped with 5-cm quartz cell. The absorption coefficients (aλ, m−1) were calculated from the absorbance (A(λ)) at the wavelength λ (nm) using the following equation: aλ ¼ 2:303 AðλÞ=r
1. Material and methods 1.1. Site description and sampling strategy Huangpu River, about 115 km long with an average of 400 m wide and 9 m deep, serves as an important drinking water source for Shanghai. Its mainstream receives water from Dianshan Lake (S11) and three major tributaries in the upper Huangpu River, including the Taipu Stream (S10), Stream (S8) and Damaogang Stream (S7), which are flowing through the Hangjiahu Plain in Zhejiang Province (Fig. 1). Surface water samples were collected from 11 sites (S1 to S11, Fig. 1) along the Huangpu River from December 2011 to June 2013 in every two months. Samples were not collected in June, 2012 for the limited condition. A Magellan GPS instrument (eXplorist 310, Magellan, Taiwan, China) was used to determine the positions and record the latitude and longitude of the sampling sites. The collected water samples of 1.5 L for each study site were filtered
where, r (m) is the cuvette path length. A(λ) was corrected by averaged absorbance from 680 to 700 nm. SUVA254 is the ratio of UV absorbance at 254 nm measured in inverse meters (m−1) to the DOC concentration measured in milligrams per liter (mg/L) that well indicates DOM aromaticity (Weishaar et al., 2003). Three-dimensional fluorescence excitation–emission matrices (EEMs) of DOM samples from the Huangpu River were collected using a fluorescence spectrophotometer (F-4500, Hitachi, Japan) with excitation wavelength from 241 to 400 nm at 3 nm steps and emission wavelength from 250 to 550 nm at 2 nm steps, and Milli-Q water was used as a blank. Parallel factor (PARAFAC) modeling was conducted on 274 EEMs of water samples by using MATLAB (Ver. 7.7.0, 2008b, the Math Works Inc., Natick, Massachusetts, USA) with the DOMFluor toolbox (Stedmon and Bro, 2008). In this study, Raman peaks of the Milli-Q water were used to normalize the fluorescence signals to make them comparable in different batches of the measurement. Raman peaks were confined by
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Ya Taipu Stream TP ngt ze YXJ Yuanxiejing Stream est uar DMG Damaogang Stream y Sampling Sites S1 Downtown of Shanghai Jiangsu
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Fig. 1 – Map of sampling sites (S1–S11) along the Huangpu River, China.
the emission wavelengths of 380–420 nm at 350 nm excitation wavelength (Murphy, 2011). Moreover, the inner-filter effect was eliminated following the tutorial by Murphy et al. (2010). The humification and biological indices have been employed to determine the relative degree of humification and autotrophic productivity of fluorescent DOM, respectively. Biological index (BIX) was calculated from the ratio of emission intensities at a shorter (λem = 380 nm) and longer (λem = 430 nm) wavelengths using a fixed excitation (λex = 310 nm). Humification index (HIX) was determined from the ratio of two integrated regions of an emission scan (average of λem = 435–480 nm divided by the average of λem = 300–345 nm) collected with excitation at 254 nm as a method for comparing the relative humification of DOM samples (Huguet et al., 2009; Zsolnay et al., 1999). Statistical analyses including mean values, standard deviation and one way analysis of variance were performed for social sciences 16.0 software. Difference in parameters along the upstream to downstream of Huangpu River and seasons was assessed with one way analysis of variance with difference noted at p < 0.05.
2. Results 2.1. Absorption characteristics of DOM DOC concentrations varied from 1.95 to 6.20 mg/L in the Huangpu River during one and a half years (Fig. 3a). The lowest value of DOC was found at S10, where the mainstream of Huangpu River received clean water through Taipu Stream from the eastern Taihu Lake in Jiangsu Province. This result is in agreement with our previous study (Chen et al., 2010), where the a355 and CDOM abundance in Taihu Lake and Taipu Stream were lower than those of Dianshan Lake. The peak values of DOC and a355 were recorded at S7 (Fig. 3a,3b), which is the convergence area of the mainstream and the water discharged from Yuanxiejing (YXJ) and Damaogang (DMG) streams in Zhejiang Province. At the downstream sites of the river from the river mouth of DMG stream, the Huangpu River flows through the urban area of the Shanghai metropolis. However, the a355 and DOC values at the mainstream sites
MCE filter
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Fig. 2 – Sample treatment and experiment process. GF/F filter refers to Whatman glass microfiber filter Grade GF/F of 0.7 μm particle retention; SPE refers to solid phase extraction; MCE filter refers to mixed cellulose ester filter of 0.2 μm particle retention.
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(S1–S6) came down to constant values compared to the peak values at S7. DOM abundance was higher in summer than in winter showing a significant difference between these two seasons (Table 1), while a non-significant difference of DOM abundance presenting between winter and spring. Obviously, the primary productivity is exuberant and high precipitation brings a lot of exogenous inputs in summer. On the contrary, these influence factors become weak in winter. The values of SUVA254 indicating the aromaticity and reactivity of DOM were different in four seasons (Fig. 3d, Table 1). It represented that the DOM of the Huangpu River in four seasons may have different sources. During winter season, SUVA254 values were low, and BIX values were high, indicating low DOM aromaticity and a “microbial” source signature. Spring SUVA254 values were similar to winter, but lower BIX values indicating DOM with less “microbial” source signature. In summer, SUVA254 values indicate high aromaticity, and low BIX values, suggesting that DOM was dominated by terrestrial plant sources. In autumn, SUVA254 values were similar to those of the summer season, and BIX values were similar to those of spring.
2.2. Fluorescence characteristics of DOM Fig. 4 shows the identified components from the EEMs of 274 DOM samples by PARAFAC analysis (Stedmon and Bro, 2008). The results showed that three protein-like components (C1, C2 and C4) and one humic-like component (C3) were identified. Tyrosine-like C1 (Ex/Em = 280/322 nm) and tryptophan-like C2 (Ex/Em = 238, 295/350 nm) (Henderson et al., 2009; Hudson et al., 2007) were recalcitrant materials (Chen et al., 2003; Murphy et al., 2011). In addition, C4 (Ex/Em = 232/346 nm) was related to aromatic proteins and amino acids (e.g., tryptophan) (Chen et al., 2003; Teale and Weber, 1957). Humic-like C3 (Ex/Em = 247, 310/410–470 nm) has both microbial and terrestrial signals in high nutrient and wastewater impacted environments (Murphy et al., 2011). In the Huangpu River, the fluorescence intensities of tyrosinelike C1 and tryptophan-like C2 components were much higher than humic-like components (Fig. 5a). The presence of three protein-like components in December was higher than those of other months. However, it was the opposite for humic-like C3, which had the highest fluorescence intensity in August.
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Fig. 3 – Spatial–temporal distribution of DOM abundance and properties in the Huangpu River water (line median, box upper and lower bound = 25% and 75%, whiskers = 10% and 90%). (a) spatial variation of DOC concentrations; (b) spatial variation of absorption coefficients at 355 nm (a355); (c) bimonthly variation of a355; (d) seasonal variation of specific absorption coefficients at 254 nm.
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Table 1 – Seasonal variation of DOM absorbance a355, SUVA254, fluorescence intensity of DOM components, HIX and BIX.
SUVA254 (L/(mg.m)) a355 (m−1) C1 C2 C3 C4 HIX BIX
Spring c
5.14 ± 1.02 4.18 6.16 4.35 0.84 1.95 0.50 1.37
± ± ± ± ± ± ±
0.89ab 2.09a 1.57a 0.12a 0.42a 0.14b 0.17a
Summer c
5.02 ± 0.27 3.45 4.23 3.96 0.57 0.57 0.41 1.25
± ± ± ± ± ± ±
0.78b 0.81bc 0.88ab 0.11b 0.07c 0.06b 0.07a
b
8.59 ± 1.81 4.73 3.29 2.72 0.92 0.76 0.87 1.06
± ± ± ± ± ± ±
0.77a 1.02c 0.74b 0.15a 0.21bc 0.37a 0.08b
Autumn 9.18 ± 2.22a 4.39 4.86 3.73 0.78 1.07 0.59 1.24
± ± ± ± ± ± ±
0.70ab 2.03b 1.59ab 0.12a 0.27b 0.15b 0.13a
Data are expressed as mean ± S.D. (n = 11). Results from the Turkey test indicating the significant (p < 0.05) differences among the seasons. Data followed by the same letters are not significantly different.
HIX and BIX are relatively stable to reflect the source of DOM regardless of the changes of the time. HIX values varied between 0.33 and 0.72 while BIX values varied between 0.94 and 1.44, which suggested that there was a predominantly autochthonous/biological origin of DOM (Huguet et al., 2009; Zhang et al., 2010; Zsolnay et al., 1999) in Huangpu River. High HIX and low BIX at S7 revealed that, DOM is terrigenous there (Fig. 5b). Significant negative correlations were obtained between the fluorescence intensity of C1, C2 and HIX value (r = − 0.79, p < 0.001; r = −0.62, p < 0.001; r = −0.75, p < 0.001). However, C3 had a positive correlation with HIX (r = 0.21, p < 0.001) and negative correlation with BIX (r = − 0.21, p < 0.001), suggesting its humification characteristics. The fluorescence intensity of C3 and HIX values were the highest at S7 with average values of 0.70 (R.U.) and 0.69, respectively. The result revealed that the fluorescence characters of DOM components also showed large spatial variations in the upper Huangpu River, while they almost kept constant in the downstream as shown in Fig. 5b.
3. Discussion 3.1. Effect of headstream inputs on DOM variability in the Huangpu River There are several major tributaries namely, Taipu, Yuanxiejing and Damaogang streams (Fig. 1) from Hangjiahu Plain discharging into the upper Huangpu River; however, the water quality is quite different in terms of DOM characteristics. The DOC concentration, DOM absorbance and fluorescence of the water discharged from Taihu Lake through Taipu Stream into the upper Huangpu River are very low (Chen et al., 2010). As a result, the abundance of DOM in the river water at S11 was low due to the dilution effect. YXJ Stream runoff brings much protein-like DOMs (C1, C2 and C4), while DMG Stream runoff brings much humic-like DOM into the Huangpu River. The fluorescence intensity of DOM showed less variable in downstream than in upstream of Huangpu River as well as DOC concentration. The DOC concentrations, DOM absorbance and humic-like
2.3. Polarity and bioavailability of fluorescent DOM Microbial bioavailability of the DOM showed a significant difference among the four fluorescent components and among the four seasons (Fig. 6). In winter season, bioavailable fraction of C1, C2, C3 accounted for 5.4%–16%, 15%–43%, 0%–5.2%, respectively. However, most of the DOM became refractory in spring. In summer and autumn, the bioavailable fraction of DOM accounted
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for an average of 20% above with an exception of refractory C2 in autumn. This may be contributed to the different sources of DOM in different the seasons. During winter, DOM has apparent aromaticity and a “microbial” source signature. Relatively spring DOM is with less “microbial” source signature. In the summer, there were high SUVA254 values and low BIX values (Fig. 3d), suggesting that DOM dominated by terrestrial plant sources. In autumn, SUVA254 values were similar to those of summer, and BIX values were similar to those of spring. Therefore, the source difference may contribute to the variation of biodegradation of DOM components during the year (Fig. 6). Furthermore, in summer, the activity of microorganism is more active than other seasons. Some refractory components may also be transferred to the labile ones in the strong sunlight irradiation. Generally, non-polar organic compounds and some bio-macromolecules in the water can be retained in the C18 column (Kieber et al., 2006). In this study, the filtrates were passed through the C18 column to study the polarity properties of the studied DOM components. It is shown in Fig. 7 that about 99% of C1 was not retained in the column, which indicated that almost all of the tyrosine-like material was polar fraction. On average, approximately 49% of the tryptophan-like material (C2) was polar fraction, while the polar fractions in humic-like C3 and protein-like C4 accounted for 71% and 76%, respectively. The specific values of polar fraction percentage also changed with the season.
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S1
Fig. 5 – Characterization of fluorescence properties of DOM in the Huangpu River. (a) bimonthly change of maxima fluorescence intensities of four PARAFAC components of DOM; (b) spatial variation of humification index (HIX) and biological index (BIX) of DOM.
fluorescence intensities in the water from the YXJ Stream are lower than those from the DMG Stream, while it was just the opposite for the fluorescence intensities of the three protein-like DOM components. The runoff of YXJ and DMG Streams flowing into the upper Huangpu River was characterized by high DOM abundance. This could be attributed to the high concentrations of organic pollutants (e.g., veterinary antibiotics) present in the upper Huangpu River due to the agricultural and livestock activities in the suburban area (Jiang et al., 2011). Therefore, the spatial variation in the abundance of DOM in the Huangpu River can be mainly attributed to the headstream inputs influenced by the social-economic development in the Hangjiahu Plain.
3.2. Influence of precipitation on DOM variability in the Huangpu River Generally, DOM abundance in the streams and rivers was highly correlated with water discharge, which indicates that the rainfall
events could dominate the export of DOM from the watershed (Shen et al., 2012; Xu et al., 2012). The a355 values of DOM in the Huangpu River (especially at S7 and S8) in August were much higher than those in other months, which could be attributed to the input of surface runoff enhanced by a heavy storm event before the sampling period. In fact, there was heavy rain before sampling in February 2012, which may lead to the high humic-like substance input (Fig. 5a). The result is consistent to the observations in the surrounding rivers of Lake Taihu (Zhang et al., 2011a) and the catchment of Lake Tianmuhu (Zhang et al., 2011b). For example, higher CDOM absorption was observed in the flood season and decreased along the distance from the river mouth into the lake. However, the fluorescence intensities of DOM components in summer are lower than those in other seasons. On the one hand, high runoff dilutes the DOM which part comes from autochthonous. These parts are mainly protein-like components either from microorganism activity or the primary productivity. On the other hand, due to the
Before incubation
After incubation
8
Winter
Spring
Summer
Autumn
Flourescence intensity (R.U.)
6 4 2 0 6 4 2 0
C1
C2
C3
C4
C1
C2
C3
C4
Fig. 6 – Microbially-derived variation in fluorescence intensities of four PARAFAC components of DOM among different seasons.
JO U R N A L OF EN V I RO N M EN T A L S CI E NC ES 2 6 ( 20 1 4 ) 1 8 4 3–1 8 5 0
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Polar DOM (%)
120 100
C1
C2
C3
C4
80 60 40 20
Polar DOM (%)
0 120 100 80 60 40 20 0
Dec Feb Apr Aug Oct Dec 2011 2012 2012 2012 2012 2012
Dec Feb Apr Aug Oct Dec 2011 2012 2012 2012 2012 2012
Fig. 7 – Temporal variation in percentage of polar DOM accounting for each PARAFAC component of fluorescent DOM.
high temperature and strong light in summer, DOM can be easily transferred. It could be concluded that, allochthonous inputs which enhanced by the storm events had a great influence on the components of C3. On the contrary, the high runoff due to heavy precipitation (e.g., in August) played an important role in diluting C1 and C2 components.
3.3. Influence of microbial transformation on DOM variability in the Huangpu River In the Huangpu River, most of the tyrosine-like and tryptophanlike substances were biologically recalcitrant (Fig. 6), while some of the aromatic protein-like fraction was semi-labile as well as polysaccharides (Gogou and Repeta, 2010). It was reported that sewage-derived material contained high abundance of tryptophan-like material, which is dominant by organic matter originating from microbial activity (Hudson et al., 2007). It could be concluded that recalcitrant DOM fractions (e.g., C1 and C2) in the Huangpu River originated from the production of microbial activities affected by sewage effluent (Chen et al., 2003). Furthermore, C4 was mainly related to the new production (e.g., catabolite) of plankton and aquatic plants and their derivatives from microbial transformation (Chen et al., 2003; Murphy et al., 2011), and C3 was derived from soil erosion or the degradation of protein-like material (Murphy et al., 2011). It has been revealed that large size organic matters were more bioavailable than the smallest size one (Amon and Benner, 1996). Moreover, the microorganisms preferred to utilize the newborn DOM rather than the old ones. Microbial transformation of some labile DOM would contribute to the accumulation of humic-like substance.
polarity and bioavailability in different seasons. Polar compounds dominated tyrosine-like DOM component in all seasons but accounted for 30%–100% of other components in different seasons. Less than 20% of the fluorescent DOM components in the Huangpu River are bioavailable while most of them are biologically refractory, especially in spring season. The tyrosine-like and tryptophan-like DOM mainly derived from microbial activity. However, the biodegradable fractions of DOM are different with time change. Therefore, the bioavailability of DOM was mainly determined by its origin while the variability of DOM is largely determined by the land use in the headstream catchments. In the past decades, the urbanization of Shanghai had a great impact on the water quality of the Huangpu River. Therefore, Zhejiang Province should also be partly or even extremely to be blamed for the organic pollution in the Huangpu River. In order to prevent the water resources of the Huangpu River from pollution, the best management practice in the whole catchments should be initiated, taken into consideration, the cooperation between the research institutes in Shanghai and Zhejiang.
Acknowledgment We thank the staff at Shanghai Environmental Monitoring Station for their help in the field work. This study was supported by the National Natural Science Foundation of China (Nos. 41071301, 40601095) and the Fundamental Research Funds for the Central Universities (No. 0400219216).
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