Riverine Sources and Estuarine Fates of Particulate Organic Carbon from North China in Late Summer

Estuarine, Coastal and Shelf Science (1998) 46, 439–448

Riverine Sources and Estuarine Fates of Particulate Organic Carbon from North China in Late Summer J. Zhanga, S. M. Liua, H. Xua, Z. G. Yua, S. Q. Laib, H. Zhangb, G. Y. Gengb and J. F. Chenb a

Department of Marine Chemistry, College of Chemistry and Chemical Engineering, The Ocean University of Qingdao, 5 Yushan Road, Qingdao 266003, People’s Republic of China b State Key Laboratory of Gas Geochemistry, Lanzhou Institute of Geology, Chinese Academy of Sciences, Donggangxilu No. 198, Lanzhou 730000, People’s Republic of China Received 6 January 1997 and accepted in revised form 10 July 1997 Suspended sediments from three North China estuaries, the Luanhe, Shuangtaizihe and Yalujiang, were collected during August in 1991–93 to determine the organic carbon concentrations. Distributions of particulate organic carbon (POC) demonstrate distinctive features in each estuary: a rather simple dilution of the terrigenous population by organic poor particles for the Luanhe, relatively low and stable levels of POC in the Shuangtaizihe, and an important contribution from the organic pool at the turbidity maximum zone of the Yalujiang. Comparison of POC between large Chinese rivers shows a general trend of a rapidly decreasing POC content with increasing amounts of suspended sediments. On a logarithmic scale, a negative relationship between the concentration of POC and total suspended sediment has been found for large Chinese systems. ? 1998 Academic Press Limited Keywords: particulate organic matter; suspended sediment

Introduction The scientific data base of organic carbon regimes for large world rivers has been greatly enhanced in last 10–20 years, particularly by the SCOPE/UNEP supported programme: ‘ transport of carbon and minerals by major world rivers ’ (cf. Degens et al., 1991; Spitzy & Ittekkot, 1991). Indeed, the organic materials transported by large rivers can exert a significant impact on the carbon cycle and ecosystems in coastal and shelf regions. Taking the Amazon as an example, the particulate organic carbon (POC) carried by the river can be identified even at a distance of 1000 km off the river mouth, and this material has been supplied mainly by the natural weathering sources in the drainage basin (Hedges et al., 1986; Cai et al., 1988). Organic material carried by small and middle-size rivers, however, may affect the marine ecosystems and environment on a more regional scale (Cauwet et al., 1990). For the Chinese estuaries, organic carbon data are available from the international literature on some large systems, such as the Changjiang (Yangtze River) and Huanghe (Yellow River), which were mainly obtained during several international joint studies in the 1980s (Milliman et al., 1984; Zhang et al., 1988, 1992; Tan et al., 1991; Cauwet & Mackenzie, 1993). Given the information 0272–7714/98/030439+10 $25.00/0/ec970277

on world rivers in general and on specific features of Chinese systems, we expect that riverine POC regimes are greatly modified by the hydrodynamics in the estuary, and the fate of POC might vary under different conditions. Here we report the data of particulate organic carbon (POC) from three estuaries in North China, the Luanhe, Shuangtaizihe and Yalujiang. This study was aimed towards a more complete understanding of the organic material transported in riverine suspended matter and the impact of continental organic carbon sources on coastal element geochemistry and the environment. Finally, the POC data from this study are put in a context of nation-wide rivers, in order to reach a synthesis of POC regimes at this NW Pacific land–sea boundary region.

The study area, sample collection and analysis The Luanhe, Shuangtaizihe and Yalujiang are three major freshwater sources from North China draining into the Bohai and Huanghai (Yellow Sea), a semienclosed area of the NW Pacific Ocean (Figure 1). The long-term averaged water discharge of these rivers together is c. 45–50#109 m3 year "1, with a sediment load of 30–35#106 tons year "1 (Table 1). In Table 2 the general features of these three estuaries ? 1998 Academic Press Limited

440 J. Zhang et al.

N

(b) Luanhe Estuary

120°

125°

China

Shenyang

Rive Luan r he

Jinzhou

Wangzhuangzi Jiujianfang

Ya Riv lu er jia ng

Sh au Ri ng ver ta iz ih e

(a)

10

N

01

Dandong

Korea

N 40°

15

Qinhuangdao

07

08

09 02

12 03

13 14

40°

04

Tangshan

06 05 Bohai

0

40

Dalian

Bahai

Huanghai

80 120 km

120°

(c)

0

3 km

125°

N

e

ngh

oya

Ra

N

(d)

05

Panshan 04

China

02

Sh

ua

03

ng

01 zih e

tai

06

22

Panjin

29

21

Dandong 23

24

30 20

07

Sinuiju

17 27 16

Yalujiang Estuary

Langtou 13

15

Shuanglaizihe Estuary

Korea

08

e

09

l

Da

26

Donggou

20

10

05 04

19

24 11 12

15 16

06

Erjiegou 25

14

07

h ios

17 09 02 18 10

11

18 22 21 23

08 03

19 01

Yingkou

13

14

Bohai

0

5 km

Yellow Sea

0

8 km

F 1 Study area (a) and location of estuaries for the Luanhe (b), Shuangtaizihe (c) and Yalujiang (d). The arrows show the general water circulations in the Bohai and Huanghai (Yellow Sea).

are summarized, including the tides, currents and the river course affected by the tides. Moreover, these estuaries suffer from anthropogenic perturbations to various extents, including waste drainage from domestic, industrial and agricultural activities (cf. Zhang et al., 1994). The field observations for these three estuaries were undertaken in flood periods (i.e. in August) between 1991 and 1993, in order to catch high water and sediment discharges, because water and sediment

carried out in July–September may account for 70– 80% of annual river load. Water depth is usually <2 m in the river course and extends to 25 m at the delta front. Sample collection was limited to the fluvioestuarine zones beginning from typical fresh water (salinity: 20) and extending into marine waters at in situ salinity of 25–30 (chlorinity: 215‰). Water and suspended sediments were collected at random tides with 5 l Niskin bottles at the surface (0–1 m) in the river course and at the surface plus near-bottom in

Sources and fates of particulate organic carbon 441 T 1. Drainage area, water discharge and the sediment load for the Luanhe, Shuangtaizihe, and Yalujiang in this study

River Luanhe Shuangtaizihe Yalujiang

Drainage area (km2)

Water discharge (109 m3 year "1)

Sediment load (106 ton year "1)

Observation period

54 412 57 104 62 630

6 5·1 37·8

20 9·1 4·8

August 1991 August 1993 August 1992

Data are from the Ministry of Hydraulics (1980).

T 2. Description of estuarine features for the Luanhe, Shuangtaizihe and Yalujiang Estuary

Description

Luanhe

The estuary has several shoals and sand-bars. The tidal range is 1–2 m with currents of 0·5–1 m s "1. The tide is limited to 5–10 km inland from the river mouth. Shuangtaizihe The estuary consists of two branches that join at the upper estuary. The tidal range is 3–4 m on average, with currents of 1·0–2·0 m s "1. The area influenced by the tides is limited to 30–40 km inland from the river mouth. Yalujiang The tidal range reaches 4–5 m and the tidal influence reaches 40 km upstream of the river mouth. A turbidity maximum zone occurs at salinity of 0–10, with currents of 1·5–2 m s "1 in the estuary.

the estuary while the boats were drifting. The sampling strategies were designed following the salinity gradients. All samples were filtered on precleaned (by heating to 450 )C over night) glass-fibre Whatman GF/F filters. The filtration was carried out within in situ laboratories under a clean plastic tent. After filtration the suspended matter trapped on the filters was kept frozen (2"20 )C) until analysis. Duplicate samples were collected to determine chlorinity and suspended sediment concentrations (i.e. turbidity). In the laboratory, the filters were dried at 50 )C and weighed to determine the amount of total suspended matter (TSM). For the particulate organic carbon (POC) analysis, the samples were immersed in dilute HCl (5%) to remove inorganic carbon (e.g. carbonates). The organic carbon in the suspended sediment was determined with a LECO CS-344 carbon and sulphur analyser. The detection limit is 1#10 "5 with a precision of <5–10% estimated by repeated analyses. The POC data are expressed as weight-percent of suspended sediment (%) and as absolute mass per volume of water (mg l "1). The chlorinity of water samples was determined by AgNO3 titration with precisions with 5% estimated by repeated analyses. There is, however, no available data of chlorophyll pigments from these estuaries.

Results Suspended sediment regimes The distribution of total suspended matter (TSM) in the three North China estuaries is shown in Figure 2. Clearly, the suspended sediments have various distribution characteristics depending upon the estuary. The most striking features in the Yalujiang estuary are the very low suspended sediment concentrations upstream (<5 mg l "1), and a turbidity maximum zone in the upper estuary at a chlorinity range of 0–5‰, where the suspended sediment concentrations can be up to a factor of 100–200 times higher than in the river course, followed by a rapid decrease further seaward (Figure 2). Both the Luanhe and Shuangtaizihe show a rapid removal of terrigenous suspended sediments in the mixing zone, indicating a dilution by particle poor marine waters, or more possibly, by rapid deposition of riverine particles at early stage of mixing processes (Figure 2). The amount of total suspended matter in the estuary indicates that freshwater turbidity in the Shuangtaizihe can be two orders of magnitude higher than in the Luanhe (Figure 2). In all three estuaries, concentrations of suspended matter in the lower estuary (e.g. chlorinity: 14–15‰) are still high relative

442 J. Zhang et al. Luanhe Estuary 160 (a) TSM (mg l–1)

140 120 100 80 60 40

0

5

15

10 Chlorinity (‰)

Yalujiang Estuary 600 (b) TSM (mg l–1)

500 400 300 200 100 2

0

4

8 6 Chlorinity (‰)

10

12

14

Shuangtaizihe Estuary 12 (c) TSM (mg l–1)

10 8 6 4 2 0

5

10

15

Chlorinity (‰)

F 2 Longitudinal distribution of suspended sediment (mg l "1) in the estuaries of Luanhe, Shuangtaizihe and Yalujiang. Note that the suspended sediment concentrations indicate highly variable levels between rivers and in the estuary. (a) Luanhe; (b) Yalujiang; and (c) Shuangtaizihe.

to typical level for the Bohai (5–10 mg l "1) (cf. Wiseman et al., 1986). Particulate organic carbon (POC) In the Luanhe Estuary, the organic carbon in suspended sediments shows an overall decrease with increasing chlorinity, indicating a rather simple dilution of riverine POC (1·3–2·4%) by local organic

poor particle populations, although the data show considerable scatter (Figure 3). A similar distribution has been found for the absolute concentration of POC (mg l "1), which reduces from 22 mg l "1 in the river to 0·5 mg l "1 in the lower estuary (Figure 3). It is difficult, however, to determine the real marine POC end-members since the sampling was ended at a chlorinity of 14215‰. Clearly, the POC levels in weight percentage (%) in the Shuangtaizihe Estuary remain low and relatively stable, varying between 1·1% and 1·6% over the whole salinity range sampled, except for one sample collected upstream of the Raoyanghe which has a POC value of 6·7% indicating a different freshwater organic pool with low total suspended matter of c. 50 mg l "1 before joining the main stream (Figure 4). A plot of absolute POC concentration in solution (i.e. mg l "1) against chlorinity indicates a rapid removal of particulate organic materials at early stages of mixing between fresh and marine waters (Figure 4). However, a value of 5–10 mg l "1 for POC is typical at the lower estuary with chlorinity 10–15‰, corresponding to a concentration of suspended sediments well above 100 mg l "1. The absolute POC concentration (i.e. mg l "1) in the Shuangtaizihe Estuary can be up to a factor of 100 higher than in the Luanhe Estuary (compare Figure 4 with Figure 3). The riverine concentration of POC in the Yalujiang was as high as 15–20% of the weight for suspended matter during the field observations (Figure 5). A rapid decrease of POC in percentage took place in the upper estuary, corresponding to the early stages of fresh and saline waters mixing, and POC was found to be <2·5% when chlorinity reached 1–2‰ (Figure 5). The POC remained relatively stable (1·5–2·5%) further down the estuary until an in situ salinity of 25·0 (Figure 5). in the Yalujiang Estuary, the POC in solutions (mg l "1) shows a maximum at the upper estuary followed by a rather rapid reduction further offshore (Figure 5). The POC maximum coincides with the estuarine high turbidity zone. The absolute POC level in the lower estuary can still be as high as 2–3 mg l "1. Given that riverine and estuarine POC are quite different, the distribution of POC in the Yalujiang Estuary can be sub-divided into two sections with three end-members, namely, a freshwater organic pool, the turbidity maximum zone and a marine organic pool from the coastal ocean (Yellow Sea). Since the suspended matter concentration is extremely high compared with both upstream and coastal waters and the sample collection was not extended far enough to the Yellow Sea, the real marine end-member can’t be identified from the estuarine curve.

Sources and fates of particulate organic carbon 443 Luanhe Estuary

Luanhe Estuary

3

2.5 (a)

(b) POC (mg l–1)

POC (%)

2.5 2 1.5

1.5 1

1 0.5

2

5

0

10

0.5

15

5

0

Chlorinity (‰) Luanhe Estuary 2.5 (c)

(d)

2.5

2 POC (mg l–1)

POC (%)

15

Luanhe Estuary

3

2 1.5 1 0.5 40

10 Chlorinity (‰)

60

80 120 100 140 Suspended sediment (mg l–1)

160

1.5 1 0.5 40

60

80 120 100 140 Suspended sediment (mg l–1)

160

F 3 Distribution of POC in the Luanhe estuary. (a) POC (%) vs chlorinity; (b) POC (mg l "1) vs chlorinity; (c) POC (%) vs turbidity; and (d) POC (mg l "1) vs turbidity.

Relationship between POC and suspended sediments

Discussion

The plots of POC in weight-percent of suspended matter vs total suspended matter concentrations in the estuaries demonstrate a general reverse relationship between these two parameters, at least at the low end of the turbidity range (Figures 3–5). However, the absolute concentrations of POC (mg l "1) in these three estuaries show a strongly positive relationship with suspended matter loads (Figures 3–5). The average POC to suspended sediment ratio ranges from 0·013 for the Shuangtaizihe Estuary to 0·020 for the Yalujiang Estuary, with the Luanhe at an intermediate position (0·015). The correlation coefficient (r2) for the relationship between POC and TSM can be ordered:

Source of POC in the estuary

Shuangtaizihe (0·99)=Yalujiang (0·99)>Luanhe (0·60) which, again, indicates that POC distribution is regulated by the concentration of suspended sediments in these North China estuaries.

In this study, the POC can be reduced to 1·5–2·5% or lower while the suspended sediment concentrations exceed 50–100 mg l "1 (Figure 3–5). A value of 1·5–2·0% is typical of organic carbon concentrations in soils (1·3–1·8%) from drainage areas of these rivers (National Environmental Monitoring Centre, 1990). We hypothesize, however, that in low turbidity waters (e.g. Yalujiang), photosynthesis can be an important contributor to the observed POC. The data sets obtained show that at a concentration greater than 50–100 mg l "1 for suspended sediments, photosynthesis is probably strongly reduced, and the POC in the river is hence regulated by the organic materials supplied from soil erosion, with in situ primary production being in a state of radiation limitation (Figures 3–5). While photosynthetic carbon may play a role in regulating POC in the estuary, there is still no existing data sets nor other evidence to examine this hypothesis.

444 J. Zhang et al. Shuangtaizihe Estuary

Shuangtaizihe Estuary

8

200 (a)

(b) 150 POC (mg l–1)

POC (%)

6 4 2

5

0

10

100 50

0

15

5

Chlorinity (‰) Shuangtaizihe Estuary 200 (c)

(d)

6

150 POC (mg l–1)

POC (%)

15

Shuangtaizihe Estuary

8

4 2

0

10 Chlorinity (‰)

2

6 4 8 Suspended sediment (mg l–1)

10

12

100 50

0

2

4 8 6 10 Suspended sediment (mg l–1)

12

F 4. Distribution of POC in the Shuangtaizihe estuary. (a) POC (%) vs chlorinity; (b) POC (mg l "1) vs chlorinity; (c) POC (%) vs turbidity; and (d) POC (mg l "1) vs turbidity.

Apparently, the relatively high level of POC upstream in the Yalujiang and Raoyanghe is a result of low turbidity and a high photosynthetic contribution to the organic pool, whereas low percentages of POC in the estuarine mixing zone underline a significant dilution of phytoplanktonic and riverine POC by the tremendous suspended sediment load there, dominated by the terrigenous organic pool, especially in the case of the high turbidity Shuangtaizihe Estuary. Our data provide no definite evidence on the biological oxidation or metabolism of river-origin POC taking place in the estuary. The positive POC–TSM relationship in these three estuaries suggests presumably a common terrigenous source for most of the POC. Further offshore the POC value of 22 mg l "1 indicates a rather important input from the marine pool. If we assumed that the estuarine distribution of POC is controlled by the mixing of two different population of particles, a linear mixing model can be written to approach the POC distributions in these North China estuaries: f=a/(a+b) TSM=TSMa #f+TSMb #(1"f )

(1) (2)

POC=[(TSMa #POCa #f )+(TSMb #POCb #(1"f ))]/TSM (3) where a, b represent two end-members of mixing; f is the fraction of end-member a in total particle population (f: 0]1·0); TSM is the total suspended matter in g l "1; POC is the particulate organic carbon in % of suspended matter. These equations model a river/ estuary system where particulate components (TSM and POC) are conservative (i.e. the particulate population is not significantly affected by deposition and resuspension, nor by chemical and biological reactions in the estuary). In this study we consider two extreme cases. In the first case, fresh water of high turbidity and low POC is diluted by a less turbid seawater with a higher POC value. The second case is the mixing of two freshwater end-members, one with high turbidity and low POC, and the other with low turbidity and high POC. In Table 3 the end-member compositions for the two cases are summarised. It seems from the theoretical mixing model output that a parabolic distribution can be expected from case 1, the mixing between fresh and marine end-members, in such a simple estuary.

Sources and fates of particulate organic carbon 445 Yalujiang Estuary

Yalujiang Estuary

20

10 (a)

(b) 8 POC (mg l–1)

POC (%)

15 10 5

6 4 2

0

2

4

6 8 Chlorinity (‰)

10

12

0

14

2

Yalujiang Estuary

6 8 Chlorinity (‰)

10

12

14

Yalujiang Estuary

20

10 (c)

(d) 8 POC (mg l–1)

15 POC (%)

4

10 5

0

6 4 2

100

300 500 200 400 Suspended sediment (mg l–1)

600

0

100

200 400 300 500 Suspended sediment (mg l–1)

600

F 5. Distribution of POC in the Yalujiang estuary. (a) POC (%) vs chlorinity; (b) POC (mg l "1) vs chlorinity; (c) POC (%) vs turbidity; and (d) POC (mg l "1) vs turbidity.

T 3. Components of a simple mixing model between two end-members

End-member Case 1 a b Case 2 a b

Salinity (‰)

TSM (g l "1)

POC (%)

0 35

10 0·01

1·5 15

0 0

10 0·01

1·5 15

See the text for the model equations.

Specifically, as shown in Figure 6 a rather linear pattern of POC is observed in the region of salinity <30, followed by a rapid increase in higher salinity areas until the marine end-member is reached. This suggests that in a highly turbid estuary, the particulate organic pool would be dominated by the riverine POC, and the contribution from the marine organic pool should be significant only where the salinity is higher than 30. Such a distribution of POC represents well the Shuangtaizihe and Luanhe Estuaries. For

example, particulate organic carbon in the Luanhe Estuary can be approximated by such a mixing model with a fresh end-member of POC=2% and TSM=0·1 g l "1, and a marine end-member of POC=1% and TSM=0·01 g l "1 (data not shown). Unfortunately, our data do not strenuously test the model in case 1, because we made no measurements at high salinities. It is further noted that a hyperbolic pattern of POC distribution is reproduced by the theoretical mixing model in case 2, with two freshwater end-members (Figure 6). Here the POC decreases very rapidly as turbidity increases from 0·01 to 0·5 g l "1, and then remains rather stable with higher turbidity. This is very similar to the POC distribution in the upper estuaries of Shuangtaizihe and Yalujiang, where the high POC levels from upstream were diluted down the river course by the higher turbidity water masses, although both the absolute concentrations of POC and POC in weight percentage of TSM are quite different between these two rivers. In summary, the model output reproduces well the estuarine features of POC for these systems. The model simulation of case 1 shows the dominance

446 J. Zhang et al.

case 2 reproduces the dilution of organic carbon enriched population from upstream by a organic poor particle pool in the upper estuary, like the Shuangtaizihe and Yalujiang.

Theoretical mixing 20 (a) POC (%)

15 10

Comparison with other Chinese rivers

5

0

10

20 Salinity (‰)

40

30

Theoretical mixing 20 (b) POC (%)

15 10 5

0

2

4

8 6 Turbidity (g l–1)

10

12

F 6. Theoretical model output for the mixing between two end-members. See the text for more details. (a) one fresh and one marine end-member, which shows the predominance of terrigenous source in the mixing zone until salinity up to 30, like the Shuangtaizihe and Luanhe estuaries; (b) two fresh end-members, which indicates a turbidity control over POC when two water masses join with distinct turbidity and POC levels (e.g. Shuangtaizihe and Yalujiang).

of terrigeneous POC in the estuaries of the Shuangtaizihe and to a lesser extent, the Luanhe, although the real marine end-members can’t be distinguished with our longitudinal POC profiles. Moreover, the

Clearly, the TSM and POC in the river are highly variable depending upon water regimes (e.g. dry vs wet seasons), sediment origin and system of interest. A comparison of POC in the context of national river is of great importance in understanding influences from the watersheds weathering and aquatic photosynthesis on the budget or riverine POC in the continuum of land–ocean interactions. Table 4 compares the POC values of this study with those of other large Chinese rivers draining into the NW Pacific Ocean, including the Huanghe and Changjiang (Cauwet & Mackenzie, 1993), the Daliaohe (Yu, 1990), the Jiulongjiang (Guo et al., 1991) and the Zhujiang (Chen et al., 1988). Again, an inverse relationship can be seen between POC and amount of suspended sediments, with higher POC values found in rivers with a lower turbidity. It is, therefore, assumed that the relationship between POC (%) and total suspended matter (mg l "1) can be described by an equation of the form: POC=m#[TSM]n

(4)

with m and n as constants. On logarithmic scales, the data in Table 4 show a linear relationship between POC and TSM (Figure 7). This distribution pattern may not necessarily be interpreted by the dilution of soil particles in the river, because using Equations 1–3 the theoretical mixing model produces a concave distribution of POC against TSM (Figure 7). In Figure 7, the dilution of soil particles was simulated

T 4. Comparison of average particulate organic carbon (%) between large Chinese rivers. Note that Yalujiang provides an extreme case of high POC and low TSM, whereas Shuangtaizihe plus Huanghe represent another end-member of high TSM with low POC for Chinese rivers

Rivers

TSM (mg l "1)

POC (%)

Yalujiang Daliaohe Shuangtaizihe Luanhe Huanghe Changjiang Jiulongjiang Zhujiang

2·56 630 10 300 117·5 22 000 528 60 102

11 0·35 1·32 1·62 0·5 1·15 2·8 1·4

Data sources This study Yu, 1990 This study This study Cauwet and Mackenzie, 1993 Cauwet and Mackenzie, 1993 Guo et al., 1991 Chen et al., 1988

Sources and fates of particulate organic carbon 447 Chinese rivers 3 (a)

ln[POC] (%)

2

1

0

–1

–2

0

1

2

3

4 5 6 7 –1 ln[TSM] (mg l )

8

9

10

11

Theoretical mixing 3 (b)

ln[POC] (%)

2.5 2 1.5

indications of photosynthetic contributions of POC in fresh waters. There is, however, no field evidence to approve this hypothesis. Soil erosion (e.g. plant debris) and in situ organisms (e.g. plankton and bacteria) are generally the two major sources of POC in a river. However, the relationship between POC and TSM may not necessarily imply a simple or linear mixing of these two sources/ processes as is shown by the data in Figures 3–5, probably due to the fact that an apparent inverse correlation between photosynthetic production and water turbidity has not been established for the studied systems, as indicated earlier by data from other rivers (Meybeck, 1982; Milliman et al., 1984; Ittekkot et al., 1985; Cauwet et al., 1990; Zhang et al., 1992; Trefry et al., 1994; Paolini, 1995). Moreover, POC levels in estuaries can be altered by anthropogenic activities such as waste dumping and discharge. A quantitative estimate of the contribution from pollutant drainage on the mass-balance of POC in the estuaries is not possible at present, however. There remain a number of questions still to be addressed, including the seasonal or inter-annual variations of POC in these estuaries, and internal POC cycles relative to chemical and biological reactions and processes, which should be taken into account in future studies.

1

Concluding remarks

0.5 0

2

4 6 –1 ln[TSM] (mg l )

8

10

F 7. Logarithmic plot of POC (%) vs suspended sediment concentrations (mg l "1) for some large Chinese rivers. Note a statistically linear relation between riverine POC and TSM on logarithmic scale (ln [POC]= 2·02"0·28 ln [TSM], r2 =0·62) (a), and the theoretical mixing model output is also given for comparison, indicating a concave flexure rather than linear relationship (b).

with the linear mixing model, described by Equations 1–3, with two end-members of TSM1 =10 g l "1, POC1 =1·5%, and TSM2 =0·01 g l "1, POC2 =15%, respectively, which really covers the TSM and POC ranges for rivers in Table 4. The concave feature of theoretical model in Figure 7 can’t be adopted to interpret POC–TSM relations. This suggests that a situation that usually produces a significant correlation, but may not imply a relationship. However, if one assumes that background levels of POC in soils remain stable in drainage areas, the differences between actual riverine POC distribution and the theoretical mixing model output may provide some

This study reports particulate organic carbon (POC) data from three North China estuaries, namely the Luanhe, Shuangtaizihe and Yalujiang. The field observations were made between 1991 and 1993 during August flood periods, in order to match high water discharges and high sediment loads. The distribution of POC clearly illustrates distinctive processes and events taking place in these three estuaries, with a simple dilution of the terrigenous organic pool by organic poor particles in the Luanhe, a rather stable POC level irrespective of chlorinity in the Shuangtaizihe, and a rapid dilution of a freshwater organic population at the turbidity maximum zone in the Yalujiang. In all these estuaries, the POC remains low and somewhat stable when the amount of suspended sediments is superior to c. 100 mg l "1, presumably indicating a radiation limitation for in situ photosynthesis at these turbidity levels. A logarithmic linear relationship has been found between POC and TSM for large Chinese rivers, which can be explained by the limitation of in situ photosynthesis at higher turbidity and planktonic organic carbon diluted by the organic-poor debris supplied by soil erosion in the river.

448 J. Zhang et al.

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