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Transactions on Ecology and the Environment vol 23, © 1998 WIT Press, www.witpress.com, ISSN 1743-3541 Stable isotope technique for evaluation of organic matter movement and coastal environment status S. Meksumpun, C. Meksumpun, A. Hoshika, Y. Mishima & T. Tanimoto Marine Environment Section, Chugoku National Industrial Research Institute, 2-2-2, Hiroshima 737-0197, Japan Email: [email protected] Abstract Stable carbon and nitrogen isotope techniques were used to clarify the movement of terrestrial organic matter and the status of coastal environment of the Osaka Bay which was known as one of the most polluted coastal areas in Japan. Sedimental cores and particulate matter samples from the Yodo River and the Osaka Bay were collected for examination of 6^C, 6^N, carbon and nitrogen contents. Means 6^C for sediment, large size of participate organic matter (15-149 jum) and small size of particulate organic matter (<15 //m) in the Yodo River were -25.1 ±0.2 %o, -25.3±0.6%o and -25.5±0.4%o, respectively, whereas means of those the inner part of the Osaka Bay were -20.6±0.3 %o, -21.6±1.6%o and -22.3±1.7%o, respectively. In the outer part of the Osaka Bay, mean 6"C for large size of particulate organic matter (-20.7±0.9%c) was slightly increased, but for small size of particulate organic matter (-22.5±1.3%<?) was nearly similar to those in the inner part. Our results suggest that the sediments in this coastal area were mostly derived from primary production in the bay. Almost of the large particle of terrestrial organic matter discharged from the Yodo River into the Osaka Bay had been deposited onto the bottom sediment at the inner part, within a few kilometers close to the river mouth, while the small particle of terrestrial organic matter was more transported through the bay to neighbor sea areas. Nitrogen sources as nutrients for phytoplankton growth in each area of the bay may be different since the values of 6^N for sediment, large and small size of particulate organic matters significantly varied. Such nutrients should be great contributed from marine origin. Transactions on Ecology and the Environment vol 23, © 1998 WIT Press, www.witpress.com, ISSN 1743-3541 190 Applied Sciences and the Environment 1 Introduction Stable carbon and nitrogen isotope technique are a possible way to clarify the movement of terrestrial organic matter and get more understanding of the coastal environment. These techniques have been used by a number of authors to identify the origins of organic matters (Freemanfl], Thornton[2], Guo[3]) and to determine the trophic structure of marine communities (Wada[4], Parson[5], Wu[6]). Moreover, the values of 6^N lower than 6%o and approaching 0%o for particulate nitrogen in oligotrophic regions have been interpreted as indicating a large contribution from nitrogen fixation (Minagawa[7J, Saino[8], Carpenter[9J). Our present study attempted to use stable carbon and nitrogen isotope techniques to clarify the movement of terrestrial organic matter and understand the environmental status of the Osaka Bay. The Osaka Bay is located at the eastern part of the Seto Inland Sea, Japan. This bay has been faced problems of organic matter loading from the land by recent man-made activities and natural runoff of rivers since several decades. Outbreaks of red tides occurred throughout the year in the inner part of the bay since large amounts of industrial and domestic waste water from the Osaka region have entered the inner part of the bay. Nowadays, organic pollution in the bay seem to be a serious problem due to natural primary production rather than the organic materials discharged from the land. Hence, movements of both terrestrial and marine organic matters in the bay and related water areas are needed to be clarified for further assessment of coastal pollution. Here we report a study of the carbon and nitrogen contents and the stable carbon and nitrogen isotope ratios of particu late organic materials in surface sediments and overlying water samples collected from the Osaka Bay and the Yodo River. The goal of this study is to evaluate recent marine environmental status, and to extend the available information on o^Con and 5^N of the Osaka Bay for understanding the source and subsequent movement of particulate matter in the bay. 2 Materials and methods Sedimental cores and water samples were collected from each sampling station in the Yodo River and the Osaka Bay (Fig. 1). Water depth and salinity were also measured with CTD meter (Alec ASTU-1000M). The depth and surface salinity contour lines are shown in Figs. 2 and 3, respectively. Sediment samples were collected by a gravity Transactions on Ecology and the Environment vol 23, © 1998 WIT Press, www.witpress.com, ISSN 1743-3541 19! Applied Sciences and the Environment 50' 40' 30' 20' 134' 50'1C 135' 00' 20' 30' 40' 34'10'N Figure 1. Sampling stations for 6^C and 5^N (#) and additional stations for depth and salinity contours (o)in the Yodo River and the Osaka Bay. core sampler during winter period in 1990 and 1993. The surface (0-2 cm) of the obtained sediment cores was sectioned and immediately frozen at -60 °C until further analysis. The sediment samples were freeze-dried in laboratory. The dried sediment samples were then ground and packed in silver cups. In order to remove carbonate, the sediments in silver cups were treated with 1M-HC1 solution. They were again dried and packed in tin cups prior to analysis. Water samples were collected from discrete depths with a VanDorn bottle sampler in December 1996 and 1997. These samples were sieved through 149 jum mesh sieve first, then they were sieved through 15 jum mesh sieve. Particulate matters remained on 15 /urn sieve were resuspended in filtered seawater. Large (15-149 jum) and small (<15//m) sizes of particulate matters in the water were recollected on Whatman GF/C filters (free of organic matter by ignition at 450 °C for 2 h) and the filters were stored at -80 °CtiJl analysis. Filters were Transactions on Ecology and the Environment vol 23, © 1998 WIT Press, www.witpress.com, ISSN 1743-3541 Applied Sciences and the Environment 192 134* 50'K 135* 00' 20' 30' Figure 2. Depth contour of the Osaka Bay. freeze-dried below -80 °C. These samples were exposed to the vapor of concentrate HCI for one night before packed in tin cups for analysis. The total organic carbon and nitrogen contents, together with carbon and nitrogen isotopic compositions of the sediments and filtered samples were determined using an continuous flow analytical system joining an elemental analyzer (Carlo Erba, NA-1500) with a stable isotope ratio analyzer (Finnigan MAT252 delta S). Measuring standards consisted of CO% gas produced from NBS 18 standard and Nz gas produced from L-alanine. The final results are reported as ft"C and 6*N (%o) relative to the Peedee belemnite (PDB) limestone standard (carbon) and atmospheric Ni (nitrogen), as defined by the following equation: R sample standard X 1000 R standard where R = 'W*N or "C/*C. Data quality control throughout the analysis was ensured by running a reference standard after every 10 runs. The analytical precision for standard preparation and mass spectrometric analysis was less than ± 0.1 %o and ± 0.2 %ofor S^Cand 5^N, respectively. Nutrients in water samples were determined using an autoanalyzer (Bran & Luebbe Traacs 800). Transactions on Ecology and the Environment vol 23, © 1998 WIT Press, www.witpress.com, ISSN 1743-3541 Applied Sciences and the Environment 10 134" 50'K 193 ZOlun 135" 00' Figure 3. Salinity contours of the Osaka Bay in December 1996 (upper) and 1997 (lower). 3 Result 3.1 Variation in 5 C and organic carbon contents The 8"C values of surface sediment in the Yodo River and the Osaka Bay are shown in Table 1. The (V'c values of surface sediments in the Yodo River (-25.3 to -24.9%o), inner (-20.8 to -20.3%o ) and outer (-20.3 to -20.2 %o) parts of the Osaka Bay show relatively narrow ranges. Mean of total organic carbon contents of surface sediments in the inner part of the bay (22.6±1.5 mg/g) was markedly higher than that in the outer part (15.8±0.1 mg/g ). Tables 2 and 3 show the fi^C values and total carbon contents of Transactions on Ecology and the Environment vol 23, © 1998 WIT Press, www.witpress.com, ISSN 1743-3541 Applied Sciences and the Environment 194 Table 1. Stable carbon isotope ratios (o^C), total organic carbon (C) and nitrogen contents (N), and calculated percentages of terrestrial organic matter (Terr-OM) of sediment samples for each sampling station. Station 1 2 3 4 5 6 7 8 9 ft "C -25 .2 -24 .9 -25 .3 -20 .8 -20 .5 -20 .3 -20 .3 -20 .2 -20 .2 ft' 'N (*») C (mg, 'g) 3,.1 5,.5 5,.3 5,,2 6,.2 5.9 6,,3 2. 1 1. 1 27.2 23.4 23.8 22.8 20.4 15.8 15.7 rvf (mj 0.2 0.1 2,.2 2,.7 2,.8 2,.8 2,.5 2,.0 2,.0 Terr- OM (%0 80 .0 75 .4 81 .5 12 .3 7.7 4.6 4.6 3. 1 3. 1 large (15-149 >.im) and small (<15 ^m) sizes of particulate organic matter in December 1996 and 1997, respectively. Means ft C for large (-25.3±0.6%o) and small (-25.5±0.4%o) sizes of particulate organic matters in the Yodo River were almost same value as that of the sediment (-25.1±0.2%^). Means ft^C for the large size of particulate organic matter in the inner and outer parts of the Osaka Bay were -21±1.6%0 and -20.7±0.9%0, respectively, whereas means for the small size of particulate organic matter were -22.3±1.7%« and -22.52±1..3%o, respectively. The ft^C values for large size of particulate organic matter in the outer part of the bay (stations 8-11, Table 2) at the water depth of 10 m were higher than those of surface water. In December 199, highest and lowest values of total carbon content of large size of particulate organic matter of surface water were found at station 10 (97.0 pg/1) and station 6 (21.6 f.ig/1), respectively. The highest values of total carbon content of small size of particulate organic matters in both 1996 and 1997 were found in the surface water of station 4 with the values of 271.5 ng/1 and 311.5 ug/l, respectively (Tables 2, 3). 3.2 Variation in 5^N and organic nitrogen contents The ft^N value of surface sediment in the Yodo River (station 3) was 3.1%o (Table 1), whereas those in the inner and the outer parts of the Transactions on Ecology and the Environment vol 23, © 1998 WIT Press, www.witpress.com, ISSN 1743-3541 Applied Sciences and the Environment 195 Table 2. Stable carbon isotope ratios (cY*C), total organic carbon contents (C), and calculated percentage of terrestrial organic matter (Terr-OM) of the small (<15 /urn) and large (15-149 jam) sizes of particulate matter samples collected in December 1996. <15 fim Station (%o) 4 Surface bottom+0.5m 5 Surface 10 m depth bottom+0.5m 6 Surface 10 m depth bottom+0.5m 7 Surface 10 m depth boltom+0.5m 8 Surface 10 m depth bottom+0.5m 10 Surface 10 m depth bottom+0.5m 11 Surface 10 m depth bottom + 0.5m -27.3 -21.1 -23.5 -22.3 -20.6 -22.7 -21.5 -20.8 -23.5 -22.4 -20.9 -21.1 -20.5 -20.5 -222 -24.8 -23.6 -21.6 -23.7 -24.4 15- 149 /mi C Terr- OM W/I) 271 .5 231 .7 165 .1 133 .7 105 .3 112.8 142.2 138.3 134.7 115 .9 122,.4 151,.1 231. 1 153.9 131. 2 224. 2 92.3 98.8 79.9 100.0 16..9 53 .8 35 .4 9.2 41 .5 23 .1 12.3 53 .8 36.9 13..8 16,.9 7.7 7.'7 33J% 73.;S 55.4 24.6 56.9 67.7 &'"C ( Terr-OM (wj5/1) -25 .1 54 .1 -23 .2 41 .7 -22 .4 72,2 -22.1 -20 .4 -19 .8 21 .6 -20.1 29.0 -19 .8 42.2 -21 .9 42.6 -22.5 72.5 89 1 -7,0 0 -21.,4 84..9 -20. 1 83.6 -21.2 146.16 -21.3 97.0 -20.5 89.8 -19.2 123. 1 -21.8 39.7 -20.4 44.0 -20. 1 69.7 87.7 49.2 36.9 323 6.2 0.0 1.5 0.0 292 38.5 0.0 21.5 1.5 18.5 20.0 7.7 0.0 27.7 6.2 1.5 Osaka Bay ranged from 5.2 to 6.2%o and from 5.9 to 6.3%o, respectively. Mean of total nitrogen contents in sediment in the inner part of the bay was 2.7±0.1 mg/g, whereas that in the outer part was 2.0=1=0 mg/g. In December 1996, the cV^N values for large size of particulate organic matters in the surface water were 3.9-4.6%o in the inner part of the bay and were 6.8- 12.8%o in the outer part (Table 4). At the depth of 10 m, the S^N values for large size of particulate organic matters in the inner and outer parts of the bay varied in 6.2-7.2%o and 6.5-10.5%o, Transactions on Ecology and the Environment vol 23, © 1998 WIT Press, www.witpress.com, ISSN 1743-3541 196 Applied Sciences and the Environment Table 3. Stable carbon isotope ratios (S^C), total organic carbon contents (C), and calculated percentage of terrestrial organic matter (Terr-OM) of the small (<15 /on) and large (15-149 jam) sizes of participate matter samples collected in December 1997. 15-149 jum <15 /nn Station Surface Surface Surface Surface 10 m depth bottom+0.5m 7 Surface 10 in depth 8 Surface 10 m depth 9 Surface 10 m depth bottom+0.5m 1 2 3 4 C 8^ Terr-OM ')" (%6 c%>) (%g/ 0 -25 .4 -26, .0 -25,,1 -23,.1 -20,.4 -22, .9 -22, .5 -22, .1 -22.,3 -22..9 -22, .9 -22.,3 -22, .6 765,.5 215.,8 311,,5 271.,4 148,.2 174,,4 103, .2 85.,0 90.,3 97,2 89.,8 142,,0 83 .1 92.3 78.5 47.7 6.2 44.6 38.5 32.3 35.4 44.6 44.6 35 .4 40.0 cV-3/y C Terr-OM (%) -26 .0 -25,.0 -24,.9 -22,.8 -21,.3 -21,.9 -21,.7 -19,.8 -19,,8 -20,,1 107.1 69.4 71.4 64.2 71.9 56.0 64.6 62.0 30.0 41.5 923 76.9 75.4 43.1 20.0 29.2 26.2 0.0 0.0 1.5 -22, .1 65.6 323 respectively. The ft N values for small size of particulate organic matter in the surface water in the inner part of the bay ranged from 0.3 to 2.8%o and from 5.0 to 9.8%o, respectively. In December 1997, the 6 N of large and small size of particulate organic matters in the Yodo River ranged from 0 to 5A%o and from 2.8 to 4.7%o, respectively (Table 5). The ft N values for the large size of particulate organic matter in the surface water in the inner part of the bay ranged from 8.0 to 8.2%o. The ft N values for small size of particulate organic matter in the surface water in the inner part and outer parts of the bay ranged from 4.7 to 9A%o and 7.0 to 8.6%o, respectively. Table 6 showed concentrations of NH/-N, NCV-N and total inorganic nitrogen (TIN) in the water samples. The concentration of NH/-N found in the surface water in the outer part of the Osaka Bay was less than 12% of those of the Yodo River and the innermost part of the bay (station 4). Transactions on Ecology and the Environment vol 23, © 1998 WIT Press, www.witpress.com, ISSN 1743-3541 Applied Sciences and the Environment 197 Table 4. Stable nitrogen isotope ratios (fi^N)and total organic nitrogen contents (N) of the small (<15 /mi) and large (15-149 //m) sizes of particulate matter samples collected in December 1996. <1 5 jum Station 4 5 6 7 8 10 11 Surface bottom+0.5m Surface 10 m depth bottom+0.5m Surface 10 m depth bottom+0.5m Surface 10 m depth bottom+0.5m Surface 10 m depth bottom+0.5m Surface 10 m depth bottom+0.5m Surface 10 m depth bottom+0.5m ISI 15 - ;149 um 6"N 7/ \ C f4 Vk) (A5/1) 0.3 5.8 2.8 9.3 5.9 2.4 2.8 73 .2 59 .4 41 .5 35 .0 4.6 6.6 29 .8 30 .3 3.9 6.0 1 .3 4,,1 6.7 8. 1 10.!•? 9.9 5.0 8.4 9.0 9.8 9.6 ) 10. ( 39 .0 39 .3 32 1 33,.8 40.,8 55.3 33.5 33.0 54.2 25.0 27.9 23.3 4.0 7.2 7.2 12.8 10.6 8.,7 8.3 6.5 6.0 7.8 18 .7 10 .5 15 .4 97 1 20. 1 17.8 27.3 19.4 19.8 6.8 10.5 9.5 13.6 (/4 14 .5 10 .0 16 .1 4 Discussions Stable carbon and nitrogen isotope compositions of organic matter in sediment have been studied to examine the movement of terrestrial organic matter by several workers (e.g. WadaflO], Thornton|2], Yamada[ll], Mishima[12j). Goering|13] reported that the ft^Cand 6 N of the particulate matter in water column and sedimental samples collected simultaneously from off shore areas had no statistical difference at the a=0.10 level and were thus assumed to be isotopically Transactions on Ecology and the Environment vol 23, © 1998 WIT Press, www.witpress.com, ISSN 1743-3541 198 Applied Science* ami the Environment Table 5. Levels of ammonium nitrogen, nitrate nitrogen, and total inorganic nitrogen concentrations of the water samples collected from each station in December 1996 and 1997. <1J) ium Station 1 2 3 4 7 8 9 10 11 Surface Surface Surface Surface 10m depth bottom+0.5m Surface 10 m depth Surface 10 m depth bottom+0.5m Surface 10 m depth bottom+0.5m Surface 10m depth bottom+0.5m Surface 10 m depth bottom+0.5m 6 "N /\ (9 4.7 3.0 2.8 9.4 7.4 7.2 4.7 9.8 8.6 9.4 9.6 7.1 9.4 14.4 5.0 8.4 9.0 9.8 9.6 10.0 N 4) us1.9 37.8 64.3 65.7 24.6 29.2 20.3 14.1 16.7 13. 1 17. 1 13.7 20.6 33.5 33.0 54.2 25.0 27.9 23.3 15 - 149^ m 6 "N (9%>) N l%&/I) 5.4 4.6 0.0 8.2 1 .5 8.1 8.0 9.1 8.3 6.9 9.6 18.1 8.1 12.5 9.6 13.1 7.6 8.5 14.2 5.5 8.7 7.7 8.3 6.5 7.0 19.4 19.8 6.8 10..5 9.5 13.6 equivalent. Our (Y C data also indicated that the o^C values of particulate organic matter in the Yodo River were almost similar as those of sediments although the samplings had been conducted in different year. Yamada|l 1] reported nearly similar values of mean b ^C for organic matter in surface sediments which collected during February and May 1994 in the Yodo River (-25.2±0.7). These results have revealed that the terrestrial organic matters which were transported from the Yodo River watershed to the Osaka Bay during 1993 to 1997 were similar. The o^C data of this study corresponded with the report of Transactions on Ecology and the Environment vol 23, © 1998 WIT Press, www.witpress.com, ISSN 1743-3541 Applied Sciences and the Environment 199 Gearing! 14] in which the (V'c of Narragansett Bay phytoplankton ranged from -22.2±0.6%o (nanoplankton; primarily microflagellates and non-motile ultraplankton) to -20.3±0.6%« (diatoms; primarily .SW^r^^a cY;^a^/M). Although the 8^C of phytoplankton populations collected from different locations and/or different dates at same location had some variations caused by the differences in species composition (Falkowski[15J, Fry [16]), temperature (Francois) 17], Dehairs[18]), light (Thompson [19]), and the 8^C of inorganic carbon fixed during photosynthesis (Rau[20], [21], Laws[22]), the cf Cof the marine planktonic source was still definitely different from those of terrestrial organic material sources. Several authors found that the 8' C for terrestrial organic matters ranged from -28 to -26%o (e.g. Tan[23],^Thornton[2], MishimaJ 12]). Therefore, the relatively narrow 8 C range of sedimentary organic carbon in the outer part of the Osaka Bay (with a mean value close to -20%%) clearly indicates a Significance of marine planktonic source. In order to evaluate the movement of terrestrial organic matter from the Yodo River into the Osaka Bay, the ratio of terrestrial organic carbon (/) was calculated by the following equation. f /., x ./ (%) = - marine -- & C marine : 8* C of marine organic matter 8 C sample : 8 C of measured sample 8 C ten : 8 C of terrestrial organic matter ^ sample • X 100 Based on our 8"Cdata and that reported by WadaflO], we decided the end members of 8^C mannc and 8^C ,_ to be -20.0 and -26.5%o, respectively. Our calculated data showed that the percentage of terrestrial organic matters in the sediment in the outer part of the Osaka Bay was only 3.1% (Table 1). It can be concluded that the sediments irUhe Osaka Bay were mostly derived from marine source. Data of 8 Cfor particulate materials from each station in different sampling occasion were slightly different. These occurrences were considered to be influenced by variation in water volume loaded from the Yodo River. The data on salinity distribution (Fig. 3) responded well with this consideration. Transactions on Ecology and the Environment vol 23, © 1998 WIT Press, www.witpress.com, ISSN 1743-3541 200 Applied Sciences and the Environment Table 6. Stable carbon isotope ratios (ft^C), total organic carbon contents (C), and calculated percentage of terrestrial organic matter (Terr-OM) of the small (<15/an) and large (15-149 //m) sizes of particulate matter samples collected in December 1997. 199 7 1996 Station 1 2 3 4 5 6 7 8 9 10 11 Surface Surface Surface Surface 10 m depth bottom+0.5m Surface 10 m depth bottom+0.5m Surface 10 m depth bottom+().5m Surface 10 m depth bottom + 0.5m Surface 10 m depth bottom+0.5m Surface 10 in depth bottom+().5m Surface 10 m depth bottom+0.5m Surface 10 m depth bottom+0.5m NH/-N 0M) 38.7 13 .0 10.5 2<).8 9.2 10.6 14.5 6.7 6.2 21 .6 9.6 2.9 3.8 1 .4 1 .6 2.0 1 .4 0.6 4.5 4.2 0.9 2..2 0..4 0.5 NCV-N 41.4 18.6 16.7 33.7 15.9 16.0 20.7 14.7 13.1 24.7 17.2 11.4 13.0 11.0 10.7 9.6 10.7 9.2 129 10.8 10.2 9.8 9.2 9.0 Tl N (jLilVI) 84.6 34.2 29 .7 67.6 27.5 28,.9 38,.1 23,.5 21,.2 49,.5 29,,3 15,,8 18.,5 13.,9 13.,7 13,0 13.6 10.9 19. 1 16.4 12.4 13.3 10.8 10.6 NH NO (XM) (MlM) TIN (uM) 39,,2 39,,9 39,.0 44,,6 9.,3 6,,3 96.,4 45.,3 46. 1 38.,8 9.1 7.8 138.1 89.4 90.7 88.0 21.1 16.6 4.6 2.6 2.6 2.5 2.6 2.5 4.4 2.5 I.5 6.5 5.8 5.7 5.4 5.2 5.5 7.0 6.0 6. 1 13.4 10.6 10.5 10.1 10.1 10.2 13.8 10.8 9.6 Transactions on Ecology and the Environment vol 23, © 1998 WIT Press, www.witpress.com, ISSN 1743-3541 Applied Sciences and the Environment 201 The movement characteristics of participate organic materials loaded from the Yodo River was evaluated based on the changes office and organic carbon content of the material collected in the water column along a distance from the river mouth to the outer part of the bay. For the large size of particulate organic matters, a comparison of organic carbon contents between station 3 and station 8 (1997) was focused. Our results showed that the high levels of organic carbon of 53.8 ug/1 in station 3 gradually decreased along the distance and became 0 ug/l in station 8. Base on this result, it can be concluded that almost of the large size of particulate organic matter discharged from the Yodo River into the Osaka Bay had been deposited onto the bottom sediment at the inner part, within a few kilometers closed to the river mouth. Fort he case of the small size of particulate organic matters, similar comparison in organic carbon contents had been done. Although higher levels of the contents were similarly found in station 3 (169.4 Mg/1), the contents only slightly decreased and can still be observed in a significant level of 44.6 ug/l in station 9 in the outer part of the bay. Thus, the small size of particulate organic matter may be more transported through the bay to neighbor sea areas. The (V*N values of the sediment in the Yodo River were markedly lower than those in the inner and outer parts of the Osaka Bay. Moreover, the fi^N value in the sediment of Yodo River was as low as 3.1%o, although the sewage discharged into this area has been estimated to be about 6%o (Yamada [11]). This characteristic is same as the Tokyo Bay (Wada [10]) and the observation of Yamadafil] in the Yodo River (different sampling points). In this study, we have concentrated on the ft^N values of large size (15-149 urn) of particulate organic matter because this size was the optimum size for representative of dominant phytoplankton in the Osaka Bay. As reported previously by several workers, the 6^N value of phytoplankton in Pacific Ocean was nearly 6%o (e.g. Minagawa[7], Goering[13]) which is similar to the mean (6%o) for cV*N of nitrate in Pacific Ocean surface waters (Cline[24J, Wada[25J). Therefore, the cV'N value of phytoplankton should be similar to the (V'N of nutrient source. Our present study showed that the 5^N values in the large size of particulate organic matter in the bay were varied. These results can be concluded that nitrogen sources as nutrient for phytoplankton growth in each area of the bay were different. Surprisingly, the fi^N values in the sediment between the inner and outer parts of the bay were not significantly different, but those in the particulate organic matter were significantly different. If we focused on the data of (V'N values Transactions on Ecology and the Environment vol 23, © 1998 WIT Press, www.witpress.com, ISSN 1743-3541 202 Applied Sciences ami the Environment of the large size of participate organic matter which composed of terrestrial organic matter less than 10%, we can remarkably observe that the phytoplankton in the outer part of the bay had higher cV*N values than those in the inner part. Since the 6^N of dissolved ammonium in the pore water which produced from bacterial degradation of marine organic matter have been estimated to be 10%o (Robert [26]) and that of nitrate in rain was ca Woo (Wada [10]), the phytoplankton with higher value of 8^N in the outer part of the bay should uptake nutrients which mostly derived from the bottom sediment. More detail study on the distribution of 8^N values of dissolved nitrogen will be further carried out in order to find out the actual ratio of marine-derived and landderived nitrogen sources for phytoplankton production in the bay. References 1. 2. 3. 4. 5. 6. 7. Freeman, K.H., Hayes, J.M., Trendel, J.M. & Albrecht, P., Evidence from carbon isotope measurements for diverse origins of sedimentary hydrocarbons, Nature, 343, 254-256, 1990. Thornton, S.F. & McManus, J. Application of organic carbon and nitrogen stable isotope and C/N ratios as source indicators of organic matter provenance in estuarine systems: Evidence from the Tay Estuary, Scotland. Estuarine, Coastal aW iW/iScfcMcc 38, 219-233, 1994. Guo, L., Santschi, P.H., Cifuentes, L.A., Trumbore, S.E. & Southon, J., Cycling of high-molecular-weight dissolved organic matter in the carbon isotopic (^C and ^C) signatures, Limnol. Occanogr., 41(6), 1242-1252, 1996. 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