Seasonal variations and sources of sedimentary organic carbon in Tokyo Bay

MPB-08105; No of Pages 7 Marine Pollution Bulletin xxx (2016) xxx–xxx

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Seasonal variations and sources of sedimentary organic carbon in Tokyo Bay Atsushi Kubo ⁎, Jota Kanda Department of Ocean Sciences, Tokyo University of Marine Science and Technology, 4-5-7 Konan, Minato-ku, Tokyo 108-8477, Japan

a r t i c l e

i n f o

Article history: Received 19 July 2016 Received in revised form 7 October 2016 Accepted 12 October 2016 Available online xxxx Keywords: Organic carbon Organic nitrogen Stable isotope Sediment Bayesian mixing model

a b s t r a c t Total organic carbon (TOC), total nitrogen (TN) contents, their stable C and N isotope ratio (δ13C and δ15N), and chlorophyll a ([Chl a]sed) of surface sediments were investigated monthly to identify the seasonal variations and sources of organic matter in Tokyo Bay. The sedimentary TOC (TOCsed) and TN (TNsed) contents, and the sedimentary δ13C and δ15N (δ13Csed and δ15Nsed) values were higher in summer than other seasons. The seasonal variations were controlled by high primary production in the water column and hypoxic water in the bottom water during summer. The fraction of terrestrial and marine derived organic matter was estimated by Bayesian mixing model using stable isotope data and TOC/TN ratio. Surface sediments in Tokyo Bay are dominated by marine derived organic matter, which accounts for about 69 ± 5% of TOCsed. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Sediments from coastal regions play an important role in global biogeochemical carbon cycle because they are the dominant reservoir for organic carbon burial in marine environments (Berner, 1989; Hedges and Keil, 1995). Sedimentary organic carbon in coastal waters can be supplied from both terrestrial and marine sources (Heip et al., 1995; Artemyev, 1996). In addition, anthropogenic input aided by sewage treatment plants and waste water treatment plants also play a significant role in the sedimentary organic matter budget in coastal waters (Pradhan et al., 2014). Despite numerous studies have been carried out, the fate of terrestrial and marine organic matter are still under debate (Krishna et al., 2013). Some studies have shown that it is primarily deposited on the coastal waters (e.g., Hedges and Parker, 1976) while the other demonstrated lateral transport over shelf to the open ocean (Keil et al., 1998; Galy et al., 2007). Therefore, knowledge of the sources of sedimentary organic carbon in coastal water and the factors controlling its distribution is essential for better understanding the global carbon cycle. To elucidate the source and fate of organic matter in the marine environment, the elemental ratio of total organic carbon (TOC) to total nitrogen (TN) contents and stable carbon and nitrogen isotope ratios (δ13C and δ15N, respectively) have been widely applied (Krishna et al., 2013; Koziorowska et al., 2016). In general, terrestrial organic matter can have a wide range of TOC/TN ratios (ca. 12–400; Hedges et al., 1986), while TOC/TN ratios of marine organic matter are less variable (ca. 4–9; Meyers, 1994; Tyson, 1995; Hedges et al., 1997). Evaluation of isotope ratios indicates that terrestrial particulate organic matter ⁎ Corresponding author. E-mail address: [email protected] (A. Kubo).

(POM) has a depleted carbon isotope ratio (δ13CPOM) and nitrogen isotope ratio (δ15NPOM) when compared with marine POM (Vizzini et al., 2005). Typical isotope compositions of terrestrial POM have δ13CPOM and δ15NPOM values ranging from − 33 to − 25‰ (e.g., Barth et al., 1998; Middelburg and Nieuwenhuize, 1998) and from 0 to 4‰ (Thornton and McManus, 1994), respectively. Typical δ13CPOM and δ15NPOM values of marine POM range from −22 to −18‰ (e.g., Peters et al., 1978; Wada et al., 1987; Middelburg and Nieuwenhuize, 1998) and 3 to 12‰ (e.g., Wada et al., 1987; Thornton and McManus, 1994), respectively. It is clear that δ13CPOM and δ15NPOM can vary seasonally (Cifuentes et al., 1988; Ogawa et al., 1994), however while eutrophication could be a factor of variation is not always a seasonal phenomenon (as could be vertical mixing). The isotope ratio of surface sediments (δ13Csed) may reflect seasonal variations in δ13CPOM. Reliable estimates of sedimentary organic carbon sources probably take into account such seasonal variations; however, sedimentary organic carbon in coastal waters did not adequately account for temporal variability. In addition, sedimentary chlorophyll a ([Chl a]sed) is also possible indicator of primary production in the water column. Coastal waters support high primary production due to the availability of inorganic nutrients. Especially, human activity has greatly accelerated the flows of nutrients to coastal waters over the past half century, causing widespread cultural eutrophication (Nixon, 1995). Excessive nutrient loads can cause higher rates of algal production, sometimes leading to exceptional algal blooms (e.g., van der Zee and Chou, 2005; Minaudo et al., 2015). Subsequently, organic matter deposition to the bottom water was increased. However, [Chl a]sed are affected by seagrass meadows and benthic microalgae if the basin is shallow enough for light to reach the seafloor (Paula et al., 2001). In this study, seasonal variations in TOCsed, TNsed, δ13Csed, δ15Nsed, and [Chl a]sed in the surface sediments of Tokyo Bay were observed. In

http://dx.doi.org/10.1016/j.marpolbul.2016.10.030 0025-326X/© 2016 Elsevier Ltd. All rights reserved.

Please cite this article as: Kubo, A., Kanda, J., Seasonal variations and sources of sedimentary organic carbon in Tokyo Bay, Marine Pollution Bulletin (2016), http://dx.doi.org/10.1016/j.marpolbul.2016.10.030

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A. Kubo, J. Kanda / Marine Pollution Bulletin xxx (2016) xxx–xxx

addition, suspended POM in freshwater and seawater were measured to determine isotope values of terrestrial and marine POM throughout the year. These data were then used to assess the sedimentary organic carbon sources based on Bayesian mixing models.

throughout the year (Shinpo, 2007). Accordingly, we can reasonably assume that observed sedimentary organic carbon and nitrogen represent terrestrial and marine origin and that Chl a of surface sediment mainly represent water column production.

2. Materials and methods

2.2. Sample collection

2.1. Study area

Surface sediment and surface seawater samples of Tokyo Bay were collected monthly from May 2012 to April 2013 on the research vessel Seiyo-maru of Tokyo University of Marine Science and Technology. Surface sediment samples were collected using a multiple-corer (Rigo Co., Ltd., Tokyo, Japan) at station F3 and a gravity core sampler (HR type core sampler; Rigo Co., Ltd., Tokyo, Japan) at station F6 (Fig. 1). The sediment core samples were collected triplicate, cut into 0–1 cm sections, put into polyethylene bags, and stored at −25 °C until analysis. Surface seawater samples were collected at stations F3 and F6 in Teflon-coated 8 L Niskin bottles mounted on a conductivity–temperature–depth (CTD) rosette (Falmouth Scientific Inc., Bourne, MA, USA) with a chlorophyll fluorometer (SCF; Seapoint Sensors Inc., Exeter, NH, USA) and a dissolved oxygen (DO) sensor (RINKO-3 ARO-CAV; JFE Advantech Co., Ltd., Hyogo, Japan). Freshwater samples were collected monthly from the lower Arakawa River station and effluent of the Shibaura sewage treatment plant (STP) between May 2012 and April 2013 (Fig. 1). Both seawater and freshwater samples were transferred into HCl acidwashed 1-L polyethylene bottles and kept in the dark until being processed in the laboratory. Samples for suspended POM measurement were filtered through precombusted (450 °C, 3 h) GF/F filters, after which the filters were stored at −80 °C until analysis. Water samples for chlorophyll a (Chl a) measurement were filtered through precombusted (450 °C, 3 h) GF/F filters. After filtration, chlorophyllous pigments were extracted using N,N-dimethylformamide (Suzuki and Ishimaru, 1990) and stored at −25 °C until analysis.

Tokyo Bay is a semi-enclosed embayment with an area of approximately 920 km2 and a mean water depth of 19 m. The bay is bounded by highly urbanized areas. The bay has suffered from severe cultural eutrophication since the late 1950s, along with rapid development in catchment areas including the Tokyo metropolis. Nutrient concentrations in the bay decreased from 1990s to present due to reduced load of nutrients and yet phytoplankton blooms persist throughout the year (Kanda et al., 2008). Moreover, anoxic bottom waters consistently appear during summer in northwest part of the bay. About 80% of the freshwater flowing into the bay occurred from the northwest part of the bay. Therefore, higher concentrations of anthropogenic substances (e.g., alkylbenzenens (Ambe, 1973); coprostanol (Ogura and Ichikawa, 1983)) were observed at the northwest part of the bay sediment. In contrast, anthropogenic substance concentrations at other parts of the bay were lower than that of northwest part of the bay and homogeneous distributions (Ambe, 1973). Then, we selected two sampling stations at the northwestern part of the bay and the central bay (Fig. 1). Although the benthic fauna in the bay is mainly composed of Polychaeta, the development of hypoxia from June to October led to mass mortality of macrobenthos with complete defaunation (Kodama et al., 2012). Seagrass meadows also occupy b 2% of the total area of the bay (Central Environmental Council, 2010). There are no benthic microalgae in the bay sediment because the light did not reach the sediment

2.3. Sample analysis

35.8

Latitude

35.6

The sediment samples for TOCsed, TNsed, δ13Csed, and δ15Nsed analyses were freeze-dried, homogenized, and powdered using a mortar and pestle. The analyses were preceded by treatment of samples with 1 mol L−1 HCl to remove carbonates. The water samples for suspended POC and δ13CPOM analyses were dried at 60 °C, then acidified with vapor at 12 mol L−1 HCl to remove carbonate before analysis. Sediment samples for TOCsed, TNsed, δ13Csed, δ15Nsed, and water samples for suspended POC and δ13CPOM were measured using a Hydra 20–20 isotope ratio mass spectrometer coupled to an ANCA-GSL elemental analyzer (SerCon Ltd., Crewe, UK). Analytical precision for δ13C and δ15N were 0.08‰ and 0.22‰, respectively. The sediment samples were measured in triplicate. To prepare sediment samples for [Chl a]sed analysis, about 0.2 g of freeze-dried homogenized sediments were sonicated with 8 mL of N,Ndimethylformamide (Suzuki and Ishimaru, 1990) and pigments were extracted for 24 h at −25 °C in the dark until analysis. Sediment samples for [Chl a]sed and water samples for Chl a was then measured using a fluorometer (TD-700, Turner Designs, Sunnyvale, CA, USA).

The lower Arakawa River

Shibaura STP F3 F6

35.4

2.4. Data analysis

35.2

35.0 139.6

139.8

140.0

Longitude Fig. 1. Sampling locations in Tokyo Bay.

140.2

The Bayesian mixing model, Stable Isotope Analysis in R (SIAR) developed by Parnell et al. (2010), was used to assess the relative contributions of sedimentary organic carbon sources (river, sewage treatment plant, and marine organic carbon) based on the δ13C, δ15N, and TN/TOC signatures. We choose TN/TOC rather than TOC/TN ratios in the model because TN/TOC ratios were statistically more robust; the higher TOC concentration is the denominator and behaves linearly in end-member mixtures (Goñi et al., 2003; Perdue and Koprivnjak, 2007). For each source, we report the mean value and the 95% confidence interval of the estimate of the proportional contribution of each source to the observed value.

Please cite this article as: Kubo, A., Kanda, J., Seasonal variations and sources of sedimentary organic carbon in Tokyo Bay, Marine Pollution Bulletin (2016), http://dx.doi.org/10.1016/j.marpolbul.2016.10.030

A. Kubo, J. Kanda / Marine Pollution Bulletin xxx (2016) xxx–xxx

3. Results 3.1. Sediment data A positive linear relationship between TOCsed and TNsed was observed in Tokyo Bay, and the intercept of the regression line passed very close to the origin ([TNsed] = 0.113 × [TOCsed] + 0.01, R2 = 0.90). The contents of ammonium ions adsorbed onto clay minerals in surface sediments were low (Mackin and Aller, 1984). Consequently, the measured TNsed represents a reasonable estimate of organic nitrogen, and TOCsed/TNsed ratios can be used as a better proxy to estimate the sources of organic matter. Seasonal variations of surface sediments TOCsed, TNsed, δ13Csed, δ15Nsed, and [Chl a]sed in Tokyo Bay are presented in Fig. 2. High values of TOCsed, TNsed, δ13Csed, and δ15Nsed were observed during summer, while low values were observed during other seasons. TOCsed and TNsed at station F3 were higher than or similar to that of station F6, while δ13Csed and δ15Nsed were slightly lower than that of station F6. High values of [Chl a]sed were observed during summer and autumn at station F3 and during summer at station F6. Seasonal variations of [Chl a]sed at station F3 differed slightly from the results of TOCsed because the maximum [Chl a]sed was reached in November. In contrast, seasonal variations in [Chl a]sed at station F6 were similar to the results of TOCsed.

3

The TOCsed/TNsed ratios in the bay ranged from 9.2 to 11.8 at station F3 and from 8.8 to 10.5 at station F6 (Fig. 3). The TOCsed/TNsed ratios were lower than those of terrestrial organic matter and slightly higher than those of marine organic matter. These findings indicate that surface sediments in the bay mainly dominated by marine derived organic matter rather than terrestrial organic matter. 3.2. Tokyo Bay water and freshwater data Suspended POC, δ13CPOM, PON, and δ15NPOM in surface water of the bay (stations F3 and F6) and freshwater sites (the lower Arakawa River station and the Shibaura STP) are presented in Fig. 4. In surface water of the bay, high values of suspended POM, δ15NPOM and δ13CPOM were observed during spring and summer, while low values were observed during autumn and winter. In freshwater sites, suspended POC and PON concentrations were similar to those of the bay, while relatively low values of δ13CPOM and δ15NPOM were observed. At the lower Arakawa River station, there was no distinct seasonal pattern in suspended POC, PON, δ13CPOM and δ15NPOM values. At the Shibaura STP, there was no distinct seasonal pattern in suspended POC and PON, while δ13CPOM and δ15NPOM was constant throughout the year. 4. Discussion 4.1. Seasonal and spatial variations in sedimentary organic carbon

δ13Csed (‰)

TOCsed (%)

(a)

[Chla]sed (μg g-1)

TNsed (%)

δ15Nsed (‰)

(b)

(c)

Surface sediments in Tokyo Bay were dominated by marine derived organic matter, especially high values of TOCsed and TNsed were observed in summer. This was because of high organic matter supply derived from high primary production. In the water column, high values of chlorophyll fluorescence (Fig. 5), POC, and δ13CPOM (Fig. 4) were observed during summer. The large amount of organic carbon generated by primary production led to an increased flux of organic carbon to the sediments. Oxygen at the bottom water was used to decompose organic matter; however, the supply of oxygen from the surface was hindered when density stratification occurs, resulting in bottom hypoxia (Sato et al., 2012) (Fig. 6). Sedimentary organic carbon decomposition is mediated by a variety of aerobic and anaerobic microbial processes, which could progressively modify the bulk organic carbon composition because different fractions of organic carbon degrade at different rates (Zonneveld et al., 2010). Although protein was degraded at a much the same rate in the presence or absence of oxygen, degradation rate under anoxic condition of other organic materials, which were mainly produced by phytoplankton as lipid and carbohydrate with heavy δ13CPOM, were about 5 times lower than oxic conditions (Harvey et al., 1995). Hence, sedimentary organic carbon accumulates mainly derived from high primary production during summer. Increasing freshwater inflow with high terrestrial organic carbon during summer is also a possible reason for increasing TOCsed in the bay due to rainy season in Japan. However, terrestrial organic carbon load was relatively low because the δ13Csed were also increased significantly. Vertical mixing was caused by atmospheric cooling in October, which resulted in a well-mixed water

Month Fig. 2. Seasonal variations of (a) TOCsed (%) (F3; black square with solid line, F6; white square with solid line) and δ13Csed (‰) (F3; black square with dotted line, F6; white square with dotted line) (b) TNsed (%) (F3; black square with solid line, F6; white square with solid line) and δ15Nsed (‰) (F3; black square with dotted line, F6; white square with dotted line) (c) [Chl a]sed (μg g−1) in surface sediments at station F3 (black square) and F6 (white square). Error bars indicate standard deviation.

Fig. 3. Seasonal variations of TOCsed/TNsed in surface sediments at stations F3 and F6.

Please cite this article as: Kubo, A., Kanda, J., Seasonal variations and sources of sedimentary organic carbon in Tokyo Bay, Marine Pollution Bulletin (2016), http://dx.doi.org/10.1016/j.marpolbul.2016.10.030

A. Kubo, J. Kanda / Marine Pollution Bulletin xxx (2016) xxx–xxx

250

:F6

:River

(a)

200 150

5

100 50 0 -10

10

15

-15 -20

20

-25 -30

0

-35

(b)

40

5 30

Depth (m)

PON (μmol L-1)

M J J A S O N D J F M A Month

chlorophyll fluorescence

δ13CPOM (‰)

0

:STP

chlorophyll fluorescence

:F3

Depth (m)

POC (μmol L-1)

4

20 10 0 15

10 15 20

δ15NPOM (‰)

12 9

25

6 3

M J J A S O N D J F M A Month

Fig. 5. Contour plot of chlorophyll fluorescence at (a) station F3 in 0–20 m layer and (b) station F6 in 0–25 m layer during observation period.

0 M

J

J

A

S

O

N

D

J

F

M

A

Month Fig. 4. (a) Particulate organic carbon concentrations (POC) and (b) carbon isotope ratio values (δ13CPOM) in surface seawaters (F3; black square, F6; white square) and freshwaters (the lower Arakawa River station; light gray square, Shibaura STP; dark gray square).

column during autumn and winter. As a result, organic materials with heavy δ13CPOM decomposed gradually owing to oxygen levels in the bottom waters in autumn. Seasonal variations in [Chl a]sed at station F3 differed slightly from the TOCsed values (Figs. 2), with anaerobic degradation rates of [Chl a]sed being about 10 times lower than aerobic degradation rates (Sun et al., 1993). Hence, [Chl a]sed at station F3 did not decompose during summer and early autumn, resulting in continued accumulation of [Chl a]sed until November. TOCsed, TNsed, [Chl a]sed at station F3 were higher than or similar to that of station F6 because of higher primary production, longer period of hypoxic water, and higher input of terrestrial organic matter at station F3. In contrast, δ13Csed and δ15Nsed at station F3 were slightly lower than that of station F6. Hence, the primary production in the water column is probably the main factor to control the spatial variations of organic matter at the sediment. 4.2. Sedimentary organic carbon sources estimated from Bayesian mixing model The values of δ13Cterr and δ15Nterr can be estimated by suspended POM concentrations and the δ13CPOM and δ15NPOM of the lower Arakawa

River station and the Shibaura STP while considering the freshwater discharge ratio. The discharge of Arakawa River, which is the largest river flowing into the bay, accounts for about 30% of the freshwater discharge (Nihei et al., 2007a). Most rivers flowing into the bay have similar water quality because of similar land use in the drainage basin (Nihei et al., 2007b); accordingly, it can reasonably be assumed that the data at the lower Arakawa River station represent concentrations of river water flowing into the bay. The total discharge ratio of the rivers and the STP effluents into the bay is 11:2 (Matsumura and Ishimaru, 2004); hence, the equation of δXterr is expressed as: δXterr ¼

½POMRIV   11  δXRIV ð½POMRIV   11 þ ½POMSTP   2Þ ½POMSTP   2  δXSTP þ ð½POMRIV   11 þ ½POMSTP   2Þ

ð1Þ

where [POMRIV] and [POMSTP] are suspended POM concentrations of the lower Arakawa River station and the Shibaura STP, respectively, and δXRIV and δXSTP are the carbon and nitrogen isotope ratios of the lower Arakawa River station and the Shibaura STP, respectively. We next assessed the suspended POM derived from terrestrial organic carbon and nitrogen in surface water of the bay. There were positive linear relationships between suspended POM and Chl a in stratified and mixing seasons at stations F3 and F6, respectively (Fig. S1 and S2). The POM concentration derived from terrestrial organic carbon ([POCterr] and [PONterr]) at stations F3 and F6 can be estimated from the y-intercept of each linear regression equation as 18.1, 17.2, 14.8, and 12.9 (μmol L−1) for POC (Fig. S1) and 2.4, 2.4, 2.5, and 2.6

Please cite this article as: Kubo, A., Kanda, J., Seasonal variations and sources of sedimentary organic carbon in Tokyo Bay, Marine Pollution Bulletin (2016), http://dx.doi.org/10.1016/j.marpolbul.2016.10.030

A. Kubo, J. Kanda / Marine Pollution Bulletin xxx (2016) xxx–xxx

0

measured at stations F3 and F6 is expressed as:

(a)

δXsample ¼

DO (mL L-1)

5

Depth (m)

5

10

½POMmar   δXmar þ ½POMterr   δXterr ð½POMmar  þ ½POMterr Þ

ð3Þ

The following equation can then be obtained from the above:

δXmar

  POMsample  δXsample −½POMterr   δXterr ¼ ½POMmar 

ð4Þ

15

20

M J J A S O N D J F M A Month

0

(b) DO (mL L-1)

Depth (m)

5 10 15 20 25

M J J A S O N D J F M A Month

Fig. 6. Contour plot of DO (mL L−1) at (a) station F3 in 0–20 m layer and (b) station F6 in 0–25 m layer during observation period.

(μmol L−1) for PON (Fig. S2), respectively. The equation of [POMsample] as measured at stations F3 and F6 is expressed as:   POMsample ¼ ½POMmar  þ ½POMterr 

ð2Þ

where [POMmar] is the POM concentration derived from marine POM. The isotope ratio of each sample δXsample (δ13Csample and δ15Nsample) as

The estimated values of monthly δ13Cmar, and δ15Nmar determined from the above method are shown in Table 1. The estimated δ13Cmar and δ15Nmar value was −14.8 ± 1.8‰ and 8.8 ± 1.7‰ (average ± standard deviation) at station F3 and −15.4 ± 1.7‰ and 8.8 ± 1.4‰ at station F6, respectively. The estimated values of δ13Cmar (Table 1) were comparable to phytoplankton δ13C in Tokyo Bay (−15.7‰; Ogawa et al., 1994). The estimated maximum δ13Cmar value was − 11.7‰. The higher phytoplankton growth rate and lower CO2 concentration in the cells causes less isotope fractionation; hence, the δ13CPOM levels in phytoplankton deviate less from those in sea water and may reach a theoretical level of − 11‰ (O'Leary, 1988). In addition, remarkably high δ13CPOM values of around −13‰ have been reported in various eutrophic environments owing to rapid growth of phytoplankton (Ogura et al., 1986; Cifuentes et al., 1988; Ogawa and Ogura, 1997; Savoye et al., 2003; Kubo et al., 2015). Isotopic and elemental signatures of organic matter sources (river, sewage treatment plant, and marine organic carbon) and sedimentary organic matter sampled are shown in Fig. 7. The estimated contribution of river, sewage treatment plant, and marine organic carbon in the bay sediments from Bayesian mixing model are shown in Fig. 8. The contributions of terrestrial organic carbon derived from river and sewage treatment plant were 21.5 ± 6.0% and 10.0 ± 4.5%, respectively. The contributions of marine organic carbon were 68.5 ± 4.5% in the bay. Overall, the surface sediments in the bay were dominated by marine derived organic matter. The sedimentation rates derived from terrestrial and marine organic matter was 0.98 ± 0.47 × 1010 and 2.12 ± 0.14 × 1010 gC year−1 respectively, assuming that the organic carbon sedimentation rates was 3.1 × 1010 gC year−1 in the bay (Kubo, 2015). Sedimentation rates derived from marine organic matter was about 11 ± 1% of the net community production in the bay (19 × 1010 gC year− 1 ; Kubo, 2015). In contrast, sedimentation rates derived from terrestrial organic matter was 52 ± 25% of the terrestrial POC flowing into the bay (1.9 × 1010 gC year− 1 ; Kubo, 2015). Terrestrial POC flowing into the bay was degraded about 40% in the bay (Kubo, 2015). These findings suggest that most of terrestrial POC were deposited and degraded before being discharged to the open ocean.

Table 1 Estimated carbon and nitrogen isotope values of marine POM. Marine end-member (‰) F3 13

F6 15

Date

δ C

δ N

δ13C

δ15N

May-2012 Jun-2012 Jul-2012 Aug-2012 Sep-2012 Nov-2012 Dec-2012 Jan-2013 Feb-201 Mar-2013 Apr-2013

−11.7 −17.0 −15.4 −12.9 −12.8 −16.4 −16.9 −17.4 −13.7 −14.1 −14.5

6.3 10.6 11.5 10.4 11.3 8.5 7.6 7.7 8.6 7.7 6.8

−13.9 −16.2 −12.2 −13.3 −14.7 −16.2 −18.0 −17.8 −15.2 −15.8 −15.7

9.0 9.4 10.5 11.0 9.7 9.2 7.9 7.4 9.2 6.4 7.2

Please cite this article as: Kubo, A., Kanda, J., Seasonal variations and sources of sedimentary organic carbon in Tokyo Bay, Marine Pollution Bulletin (2016), http://dx.doi.org/10.1016/j.marpolbul.2016.10.030

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A. Kubo, J. Kanda / Marine Pollution Bulletin xxx (2016) xxx–xxx

12

Sedimentary organic carbon derived from terrestrial organic matter was 52 ± 25% of the terrestrial POC flowing into the bay. Terrestrial POC flowing into the bay was degraded about 40% in the bay (Kubo, 2015). As a result, the most amount of terrestrial POC was deposited and degraded in the bay before being discharged to the open ocean.

δ15N (‰)

9

6

Acknowledgments

3

0 -32

-27

-22

-17

-12

We thank Mr. Kentaroh Sugiyama, as well as other scientists, officers and crewmembers on board the R/V Seiyo-maru for their help in sampling. This work was supported by a Grant-in-Aid for Scientific Research (C) (24510009) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan and by a Canon Foundation grant. The authors are grateful to the anonymous reviewer who provided valuable comments on the manuscript.

12 Appendix A. Supplementary data

TOC/TN

9

Supplementary data to this article can be found online at doi:10. 1016/j.marpolbul.2016.10.030.

6

3

0 -32

References

POMF3 POM F6

POMSTP POMriv

OMSed@F3 OMSed@F6

-27

-22

-17

-12

δ13C (‰) Fig. 7. Isotopic and elemental signatures of organic matter sources and sedimentary organic matter sampled. Error bars indicate standard deviations of each source.

5. Conclusions Surface sediments in Tokyo Bay were dominated by marine derived organic matter (68.5 ± 4.5%) which was estimated by Bayesian mixing model using isotopic and elemental signatures. High values of TOCsed and TNsed were observed in summer because of high organic matter supply derived from high primary production. In contrast, the contributions of organic carbon derived from river and sewage treatment plant were minor proportions (21.5 ± 6.0% and 10.0 ± 4.5%, respectively).

Source contribution (%)

100

80

60

40

20

0

Riv STP Mar F3

Riv STP Mar F6

Fig. 8. Composition of the sediment organic carbon (results of Bayesian mixing model) for each source (river, sewage treatment plant, and marine sources). Error bars indicate standard deviations of each source.

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Please cite this article as: Kubo, A., Kanda, J., Seasonal variations and sources of sedimentary organic carbon in Tokyo Bay, Marine Pollution Bulletin (2016), http://dx.doi.org/10.1016/j.marpolbul.2016.10.030