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A Study of Carbon and Nitrogen Stable Isotope and Elemental Ratios as Potential Indicators of Source and Fate of Organic Matter in Sediments of the Forth Estuary, Scotland
Estuarine, Coastal and Shelf Science (2001) 52, 375–380 doi.10.1006/ecss.2000.0742, available online at http://www.idealibrary.com on
A Study of Carbon and Nitrogen Stable Isotope and Elemental Ratios as Potential Indicators of Source and Fate of Organic Matter in Sediments of the Forth Estuary, Scotland M. C. Grahama, M. A. Eavesa, J. G. Farmera, J. Dobsonb and A. E. Fallickc a
Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, U.K. Scottish Environment Protection Agency, Avenue North, Riccarton, Edinburgh EH14 4AP, U.K. c Scottish Universities Research and Reactor Centre, Rankine Avenue, East Kilbride, Glasgow G75 0QF, U.K. b
Received 17 August 1999 and accepted in revised form 24 October 2000 The purpose of this study was to investigate the importance of both natural and anthropogenic sources of organic matter in bottom sediments of the Forth Estuary, east Scotland, U.K. Organic matter from the upper, middle and lower zones of the estuary was characterized using stable isotope (13C and 15N) and elemental (C/H and C/N) ratios. The observed narrow range of isotopic and elemental ratio data, over the entire length of the estuary, was consistent with relatively efficient mixing of sediments, although 13C values of 23·6‰ and C/H ratios of 3·90·3 for lower estuary sediments did suggest a change to a more marine-influenced system. 15N and C/N ratios could not be used, however, to indicate either the source or the fate of sediment organic matter. No specific anthropogenic source could be identified on the basis of elemental and/or stable isotopic information. 2001 Academic Press Keywords: 13C; 15N; C/H; C/N; organic matter; Forth Estuary
Introduction Forth Estuary The Forth Estuary, the inner part of the Firth of Forth, extends over 45 km eastwards from Stirling to the Forth bridges (Figure 1). The estuary is turbid with mean concentrations of suspended particulate matter (SPM) ranging from 50–100 mg l 1 in the lower estuary to >10 000 mg l 1 in the region of maximum turbidity in the upper estuary between Fallin and Alloa (Griffiths, 1987). During the summer months, decay of organic matter in the upper estuary leads to low dissolved oxygen concentrations and significant diminution of water quality (Balls, 1994; Balls et al., 1996). This has been attributed to the high concentrations of SPM in the water column because the high concentration of particles, which increases the surface area for microbiological processes, together with increased water temperatures produce ideal conditions for the microbial breakdown of sediment-derived organic matter. The principal source of SPM is resuspended bed-sediment, which may contain organic matter from anthropogenic as well as from natural sources (Webb & Metcalfe, 1987). 0272–7714/01/030375+06 $35.00/0
Sediment composition at regularly monitored sites has been shown to fluctuate, reflecting both seasonal variability and the highly mobile nature of sediments in the Forth Estuary (Lindsay et al., 1996, 1998). For example, during the summer months there is an accumulation of silts (30–90% dry weight sediment) upstream as a result of tidal pumping of fine sediments, whilst in winter months increased river outflow transports fine sediments downstream (Webb & Metcalfe, 1987). It is the current velocity, rather than wind-induced suspension, that is the principal controlling factor for suspended particulate matter transport in the Forth Estuary (Owen & Balls, 1997). Scouring of the sediment bed in the main channel of the upper estuary results in almost complete removal of free sediment, much of which may be redistributed to four main intertidal areas at the edges of the estuary (Clarke & Elliot, 1998). These dynamic processes within the estuary also influence the distribution of particle-associated organic matter and are likely to play a major role in admixing of organic matter from different sources. Organic matter in estuarine sediments can be derived from natural and anthropogenic sources. The former include autochthonous inputs of planktonic 2001 Academic Press
376 M. C. Graham et al. T 1. C/N, 13Corg and 15N values for relevant natural and pollutant organic materials in estuarine systems Organic material Terrigenous Marine Sewage Spent yeast (brewing) Petroleum Coal
C/N
13Corg (‰)
15N (‰)
Reference
12·17 to 19·50 11·27 12·57 12·51·0
25·4 to 28·0 23·2 26·7 24·83·2 22·82·9 24·81 27·60·05 27 to 30 — 20 to 30
+0·2 to +4·0 +5·9 +2·3 — +3·33·0 — — — +3 to +11 —
Thornton and McManus (1994) Thornton and McManus (1994) Thornton and McManus (1994) Andrews et al. (1987) Lee Van Dover et al. (1992) Andrews et al. (1998) Andrews et al. (1998) Faure (1998) Hoefs (1997) Faure (1998)
6·08 79·09·6 — —
and benthic primary production together with allochthonous inputs, such as terrigenous run-off and marine tidally advected material (Thornton & McManus, 1994). In the Forth Estuary, effluent discharges from sewage treatment works (STWs), petrochemical works, oil refineries, coal-fired power plants, distilleries and yeast manufacturers gave rise to a significant anthropogenic allochthonous organic content of sediments. Since the late 1980s, however, pollutant organic inputs have been substantially reduced (Balls et al., 1996), leading to a general decrease in sediment organic content (McClusky et al., 1993). There is, however, little knowledge about the spatial distribution of organic matter and the relative importance of autochthonous and allochthonous sources in the Forth Estuary.
contributory source. Although the 13Corg signature was a good source indicator, 15N and C/N ratios were found to be unreliable because they were influenced by the degree of diagenetic alteration, as well as the source, of the organic matter. Other studies have demonstrated that the assumption that estuarine organic material is simply a mixture of natural inputs may be incorrect in many instances (Lucotte, 1989; Matson & Brinson, 1989; Andrews et al., 1998). For example, Andrews et al. (1998), in addition to characterizing marine and terrigenous end-members, were able to identify sewage as a major pollutant contributor to sediment organic matter in a poorly flushed, urbanised estuary despite large overlaps in both C/N and 13Corg data sets for natural and pollutant sources. Table 1 summarizes C/N, 13Corg, and 15N values obtained for a range of natural and pollutant organic materials in recent estuarine studies.
Use of stable isotopes and elemental ratios The purpose of this study was to investigate the sources and fate of organic matter in the Forth Estuary using isotopic and elemental ratios to characterise the organic matter in bottom sediments from upper, middle, and lower zones of the estuary. Stable carbon and nitrogen isotope and C/N elemental ratios have been used elsewhere to evaluate the sources and fate of organic matter in estuarine and marine sediments (e.g. Thornton & McManus, 1994; Lee, 1994; Bird & Gro¨ cke, 1997; Andrews et al., 1998). In a study of the Tay Estuary, Thornton and McManus (1994) observed changes in carbon isotopic composition consistent with there being three distinct zones: an upper zone in which natural terrigenous organic matter was the dominant source, a middle zone in which the terrigenous input was diluted with autochthonous organic matter, and an outer zone where allochthonous marine organic matter was also a
Experimental Nine sediment sampling sites were selected from the Scottish Environment Protection Agency (SEPA) bi-monthly water quality sampling stations. Figure 1 shows the location of the sampling sites, STWs and other known effluent discharges. Table 2 shows the distance of each sampling site from the head of navigation at Stirling Bridge. Samples were collected on 5-Jun-97 and 15-Jun-97 from a SEPA survey vessel using a hand-held ‘ Van-veen ’ grab which obtains near-surface sediments (0–10 cm depth). At five stations (1, 2, 4, 5 and 7), three grab samples were collected to assess intra-site variability in sediment composition. Dissolved oxygen concentrations were also obtained from the concurrent water quality survey at each of the sediment sampling locations.
Organic matter in sediments of the Forth Estuary 377
2
Stirling 1 Stirling STW
3
4
Alloa STW
5
6 7 9
8
Forth Road and Rail Bridges
5 km
F 1. Map of the Forth Estuary showing locations of sampling sites (1–9), STWs ( ) and other known effluent discharges ( ).
Wet sediment samples were freeze-dried, ground and stored in plastic containers. Separate subsamples were suspended in 3·5% v/v HCl (12 h) to remove inorganic carbon. Each subsample was again freezedried and ground. A Perkin-Elmer 2400 CHN elemental analyser was used to determine the composition (% N, H and organic C) of sediments. Subsamples were prepared for isotopic analyses by the quartz closed-tube combustion method (Sofer, 1980). Because of the high C/N ratios and concomitant large sample size of sediment required to release adequate N2 for mass spectrometric analysis, carbon and nitrogen isotopic analyses were performed separately. For 13C, preroasted CuO and Cu wire were added to the sediment sample in the quartz combustion tube. Following evacuation and sealing, the tubes were heated to 850 C for 2 h and allowed to cool slowly overnight, to decompose oxides of nitrogen on Cu. The tubes were opened into vacuum, the CO2 cryogenically purified and 13C (‰ vs. PDB) determined on a VG SIRA 10 dual inlet mass spectrometer with a precision of 0·1‰. For nitrogen isotope runs, the combustion tubes also contained CaO which had been preroasted; this reacts with CO2 produced by combustion (Bo¨ hlke et al., 1993). Again, the N2 was cryogenically purified, and 15N (‰ vs. AIR) determined on a VG Optima dual inlet mass spectrometer. The precision on duplicate analyses ranged from 0·1 to 0·8‰; since the analytical technique is capable of 0·2‰ on isotopically homogeneous material, a small degree of intra-sample heterogeneity is implied.
Results Elemental and stable isotopic data for each sampling location are shown in Table 2. An average elemental composition of 4·70·8% organic C, 0·90·2% H, and 0·230·08% N and average C/H and C/N ratios of 5·21·3 and 21·76·6, respectively, were obtained. The corresponding average 13Corg and 15N values were 24·00·2‰ and +5·6 0·6‰, respectively. Intra-site variability in elemental composition was greatest in the upper estuary but inter-site differences in the organic C content of sediments over the entire length of the estuary were clearly small (Figure 2). Sediments with lowest organic C content, however, were located in the lower estuary (sites 8 and 9), for which smaller C/H ratio values (average of 3·90·3 for sites 7–9 compared with an average of 5·71·2 for sites 1–6) were also evident [Figure 3(a)]. Variability of the C/N ratio at and between individual sites was particularly marked in the upper estuary sediments (sites 1–3) whilst relatively constant values (average 19·02·4) were obtained for sites 4–9 [Figure 3(b)]. The sediment stable isotopic signatures at each sampling location are shown in Figure 4. 13C values ranged from 24·4 to23·6‰, with the least negative values being obtained for the lower estuary sites (8–9). There was no significant difference between 15N values in the upper and middle estuary (sites 1–7) and those from the lower estuary (sites 8–9). A pronounced minimum in dissolved oxygen concentration in overlying waters occurred on the
378 M. C. Graham et al. T 2. Elemental and stable isotopic data for Forth Estuary sediments
Sample
Distancea (km) 3 8 11 15 20 28 31 34 42
1 2 3 4 5 6 7 8 9 a
%N (1 SD)
%H (1 SD)
n
% organic C (1 SD)
3 3 1 3 3 1 3 1 1
4·80·5 3·81·0 4·4 4·90·3 5·30·3 6·1 5·20·1 3·1 4·1
0·230·12 0·140·04 0·17 0·250·03 0·300·02 0·25 0·300·03 0·15 0·20
0·90·2 0·60·1 0·7 1·00·1 1·10·05 1·0 1·30·1 0·9 1·0
24·00·1 23·80·1 24·2 24·10·1 24·40·2 24·0 24·00·1 23·6 23·6
13
Corg (‰) (1 SD)
15N (‰) (1 SD) 6·20·6 5·50·8 4·9 5·50·7 5·40·1 4·7 5·50·7 5·7 5·7
Distance from the navigation head at Stirling Bridge.
1.0
10
5
8
4
6
3
4
2
2
1
0
0 5 10 15 20 25 30 35 40 45 Distance (km) from Stirling Bridge
10
0.2 0.0
8 6 C/H
Nitrogen (%)
0.4
Hydrogen (%)
0.6
Organic carbon (%)
(a) 0.8
4 2
F 2. Elemental composition (%N( ), % organic C ( ), %H ( )1 SD) of Forth Estuary sediments.
0
5
10 15 20 25 30 35 40 Distance (km) from Stirling Bridge
45
5
10 15 20 25 30 35 40 Distance (km) from Stirling Bridge
45
50
day of sampling at 5–11 km from Stirling Bridge, encompassing sites 2–3.
(b) 40
Discussion C/N
From the results of this study, a striking feature is the relative uniformity of the isotopic (13C: 24·4 to23·6‰; 15N: 4·7to 6·2‰) and elemental (C: 3·1 to 6·1%; N: 0·14 to 0·30%; H: 0·7 to 1·3%) data over the entire length of the Forth Estuary. Figure 5 contrasts the stable isotope results presented herein with data obtained for the Tay estuary by Thornton and McManus (1994), illustrating the comparatively narrow range of the Forth Estuary data. In contrast with the Tay, there is no evidence, on the basis of 13C and 15N signatures obtained in this study, to support a predominantly terrigenous input to sediment organic mater in the upper Forth estuary. At the turbidity maximum in the upper estuary, indicated by the minimum in dissolved oxygen concentration a small decrease in the organic C content and in the N content of sediments was not accompanied by any significant change in stable
30 20 10
0
F 3. (a) C/H ( ) and (b) C/N ( ) ratios (1 SD) in Forth Estuary sediments.
nitrogen isotopic composition of the sediments. Relatively minor differences (13C and C/H) between the upper/middle and lower estuary are indicative of a greater influence of marine organic matter in lower stretches of the estuary. In agreement with the findings of Thornton and McManus (1994) for the Tay, the lack of any
Organic matter in sediments of the Forth Estuary 379 –22
δ13C
(a)
–24
–26
0
5
10 15 20 25 30 35 40 Distance (km) from Stirling Bridge
45
12 (b)
of correlation between 15N and % N or C/N (R2 =0·003 and 0·108, respectively) also meant that no conclusion could be drawn about the degree of diagenetic alteration of organic matter at any sampling location. In general, the relatively low organic C content suggested no major anthropogenic input to the Forth Estuary. More specifically, there was no localized elevation in sediment organic C content, e.g. close to STWs or industrial discharges. It is proposed that the dynamic processes operating in the main channel (Clarke & Elliot, 1998) result in relatively efficient mixing and redistribution of sediment and that the uniformity of the sediment organic C content and 13C signature precludes evaluation of the importance of any individual pollutant source. Acknowledgements
8
δ15N
We thank Lorna Eades and Terry Donnelly for invaluable technical support with elemental and stable isotopic analyses, respectively. 4
References 0
5
10 15 20 25 30 35 40 Distance (km) from Stirling Bridge
45
F 4. (a) 13Corg ( ) and (b) 15N ( ) signatures (1 SD) of Forth Estuary sediments.
12
δ15N
8
4
0 –27
–26
–25 δ13C
–24
–23
F 5. Comparison of stable isotopic data for sediments from the Forth ( ) (this study1 SD) and Tay ( ) (Thornton & McManus, 1994) estuaries.
correlation between 15N or C/N and 13C (R2 =0·127 and 0·037, respectively) meant that 15N and C/N ratios could not be used to evaluate source or gate of organic matter in the forth. In this study, the absence
Andrews, J. E., Greenway, A. M. & Dennis, P. F. 1998 Combined carbon isotope and C/N ratios as indicators of source and fate of organic matter in a poorly flushed, tropical estuary: Hunts Bay, Kingston Harbour, Jamaica. Estuarine, Coastal and Shelf Science 46, 743–756. Balls, P. W. 1994 Nutrient inputs to estuaries from nine Scottish east coast rivers; influence of estuarine processes on inputs into the North Sea. Estuarine, Coastal and Shelf Science 39, 329–352. Balls, P. W., Brockie, N., Dobson, J. & Johnston, W. 1996 Dissolved oxygen and nitrification in the upper Forth estuary during summer (1982–1992): patterns and trends. Estuarine, Coastal and Shelf Science 42, 117–134. Barth, J. A. C., Veizer, J. & Mayer, B. 1998 Origin of particulate organic carbon in the upper St. Lawrence: isotopic constraints. Earth and Planetary Science Letters 162, 111–121. Bird, M. I. & Gro¨ cke, D. R. 1997 Determination of the abundance and carbon isotope composition of elemental carbon in sediments. Geochimica et Cosomochimica Acta 61, 3413–3423. Bo¨ hlke, J. K., Gwinn, C. J. & Coplen, T. B. 1993 New reference materials for nitrogen-isotope-ratio measurements. Geostandards Newsletter 17, 159–164. Clarke, S. & Elliot, A. J. 1998 Modelling suspended sediment concentrations in the Firth of Forth. Estuarine, Coastal and Shelf Science 47, 235–250. Faure, G. 1998 Principles and Applications of Geochemistry. 2nd Edition, Prentice Hall, NJ, 316 pp. Forth River Purification Board 1986 A second survey of the resuspension of solids in the upper Forth estuary. Report No. ES1/86. Griffiths, A. H. 1987 Water quality of the estuary and Firth of Forth. Proceedings of the Royal Society of Edinburgh 93, 303–314. Hoefs, J. 1997 Stable Isotope Geochemistry. 4th Edition, Springer, Berlin, 46 pp. Lee, C. 1994 Controls on carbon preservation—new perspectives. Chemical Geology 114, 285–288. Lee Van Dover, C., Grassle, J. F., Fry, B., Garritt, R. H. & Starczak, V. R. 1992 Stable isotope evidence of entry of sewagederived material into a deep-sea food web. Nature 360, 153–155.
380 M. C. Graham et al. Lindsay, P., Balls, P. W. & West, J. R. 1996 Influence of tidal range and river discharge on suspended particulate matter fluxes in the Forth estuary (Scotland). Estuarine, Coastal and Shelf Science 42, 63–82. Lindsay, P., Bell, F. G. & Hytiris, N. 1998 Contamination of sediments in the forth estuary, Scotland, In Contaminated Land and Groundwater: Future Directions (Lerner, D. N. & Walton, N. R. G., eds), Geological Society, London, Engineering Geology Special Publications no. 14, pp. 179–187. Lucotte, M. 1989 Organic carbon isotope ratios and implications for the maximum turbidity zone of the St. Lawrence upper estuary. Estuarine, Coastal and Shelf Science 29, 297– 304. Owen, R. E. & Balls, P. W. 1997 Dissolved trace metals in the Tay estuary. Estuarine, Coastal and Shelf Science 44, 421–434. Matson, E. A. & Brinson, M. M. 1989 Stable carbon isotopes and the C:N ratio in the estuaries of the Pamlico and Neuse rivers, North Carolina. Limnology and Oceanography 35, 1290– 1300.
McClusky, D. S., Hull, S. C. & Elliot, M. 1993 Variations in the intertidal and subtidal macrofauna and sediments along a salinity gradient in the upper Forth estuary. Netherlands Journal of Aquatic Ecology 27, 101–109. Sofer, Z. 1980 Preparation of carbon dioxide for stable carbon isotope analysis of petroleum fractions. Analytical Chemistry 52, 1389–1391. Thornton, S. F. & McManus, J. 1994 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 and Shelf Science 38, 219–233. Webb, A. J. & Metcalfe, A. P. 1987 Physical aspects, water movements and modelling studies of the forth estuary, Scotland. Proceedings of the Royal Society of Edinburgh 93B, 259–272. Wu, J. P., Calvert, S. E. & Wong, C. S. 1999 Carbon and nitrogen isotope ratios in sedimenting particulate organic matter at an upwelling site off Vancouver Island. Estuarine, Coastal and Shelf Science 48, 193–203.
A Study of Carbon and Nitrogen Stable Isotope and Elemental Ratios as Potential Indicators of Source and Fate of Organic Matter in Sediments of the Forth Estuary, Scotland M. C. Grahama, M. A. Eavesa, J. G. Farmera, J. Dobsonb and A. E. Fallickc a
Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, U.K. Scottish Environment Protection Agency, Avenue North, Riccarton, Edinburgh EH14 4AP, U.K. c Scottish Universities Research and Reactor Centre, Rankine Avenue, East Kilbride, Glasgow G75 0QF, U.K. b
Received 17 August 1999 and accepted in revised form 24 October 2000 The purpose of this study was to investigate the importance of both natural and anthropogenic sources of organic matter in bottom sediments of the Forth Estuary, east Scotland, U.K. Organic matter from the upper, middle and lower zones of the estuary was characterized using stable isotope (13C and 15N) and elemental (C/H and C/N) ratios. The observed narrow range of isotopic and elemental ratio data, over the entire length of the estuary, was consistent with relatively efficient mixing of sediments, although 13C values of 23·6‰ and C/H ratios of 3·90·3 for lower estuary sediments did suggest a change to a more marine-influenced system. 15N and C/N ratios could not be used, however, to indicate either the source or the fate of sediment organic matter. No specific anthropogenic source could be identified on the basis of elemental and/or stable isotopic information. 2001 Academic Press Keywords: 13C; 15N; C/H; C/N; organic matter; Forth Estuary
Introduction Forth Estuary The Forth Estuary, the inner part of the Firth of Forth, extends over 45 km eastwards from Stirling to the Forth bridges (Figure 1). The estuary is turbid with mean concentrations of suspended particulate matter (SPM) ranging from 50–100 mg l 1 in the lower estuary to >10 000 mg l 1 in the region of maximum turbidity in the upper estuary between Fallin and Alloa (Griffiths, 1987). During the summer months, decay of organic matter in the upper estuary leads to low dissolved oxygen concentrations and significant diminution of water quality (Balls, 1994; Balls et al., 1996). This has been attributed to the high concentrations of SPM in the water column because the high concentration of particles, which increases the surface area for microbiological processes, together with increased water temperatures produce ideal conditions for the microbial breakdown of sediment-derived organic matter. The principal source of SPM is resuspended bed-sediment, which may contain organic matter from anthropogenic as well as from natural sources (Webb & Metcalfe, 1987). 0272–7714/01/030375+06 $35.00/0
Sediment composition at regularly monitored sites has been shown to fluctuate, reflecting both seasonal variability and the highly mobile nature of sediments in the Forth Estuary (Lindsay et al., 1996, 1998). For example, during the summer months there is an accumulation of silts (30–90% dry weight sediment) upstream as a result of tidal pumping of fine sediments, whilst in winter months increased river outflow transports fine sediments downstream (Webb & Metcalfe, 1987). It is the current velocity, rather than wind-induced suspension, that is the principal controlling factor for suspended particulate matter transport in the Forth Estuary (Owen & Balls, 1997). Scouring of the sediment bed in the main channel of the upper estuary results in almost complete removal of free sediment, much of which may be redistributed to four main intertidal areas at the edges of the estuary (Clarke & Elliot, 1998). These dynamic processes within the estuary also influence the distribution of particle-associated organic matter and are likely to play a major role in admixing of organic matter from different sources. Organic matter in estuarine sediments can be derived from natural and anthropogenic sources. The former include autochthonous inputs of planktonic 2001 Academic Press
376 M. C. Graham et al. T 1. C/N, 13Corg and 15N values for relevant natural and pollutant organic materials in estuarine systems Organic material Terrigenous Marine Sewage Spent yeast (brewing) Petroleum Coal
C/N
13Corg (‰)
15N (‰)
Reference
12·17 to 19·50 11·27 12·57 12·51·0
25·4 to 28·0 23·2 26·7 24·83·2 22·82·9 24·81 27·60·05 27 to 30 — 20 to 30
+0·2 to +4·0 +5·9 +2·3 — +3·33·0 — — — +3 to +11 —
Thornton and McManus (1994) Thornton and McManus (1994) Thornton and McManus (1994) Andrews et al. (1987) Lee Van Dover et al. (1992) Andrews et al. (1998) Andrews et al. (1998) Faure (1998) Hoefs (1997) Faure (1998)
6·08 79·09·6 — —
and benthic primary production together with allochthonous inputs, such as terrigenous run-off and marine tidally advected material (Thornton & McManus, 1994). In the Forth Estuary, effluent discharges from sewage treatment works (STWs), petrochemical works, oil refineries, coal-fired power plants, distilleries and yeast manufacturers gave rise to a significant anthropogenic allochthonous organic content of sediments. Since the late 1980s, however, pollutant organic inputs have been substantially reduced (Balls et al., 1996), leading to a general decrease in sediment organic content (McClusky et al., 1993). There is, however, little knowledge about the spatial distribution of organic matter and the relative importance of autochthonous and allochthonous sources in the Forth Estuary.
contributory source. Although the 13Corg signature was a good source indicator, 15N and C/N ratios were found to be unreliable because they were influenced by the degree of diagenetic alteration, as well as the source, of the organic matter. Other studies have demonstrated that the assumption that estuarine organic material is simply a mixture of natural inputs may be incorrect in many instances (Lucotte, 1989; Matson & Brinson, 1989; Andrews et al., 1998). For example, Andrews et al. (1998), in addition to characterizing marine and terrigenous end-members, were able to identify sewage as a major pollutant contributor to sediment organic matter in a poorly flushed, urbanised estuary despite large overlaps in both C/N and 13Corg data sets for natural and pollutant sources. Table 1 summarizes C/N, 13Corg, and 15N values obtained for a range of natural and pollutant organic materials in recent estuarine studies.
Use of stable isotopes and elemental ratios The purpose of this study was to investigate the sources and fate of organic matter in the Forth Estuary using isotopic and elemental ratios to characterise the organic matter in bottom sediments from upper, middle, and lower zones of the estuary. Stable carbon and nitrogen isotope and C/N elemental ratios have been used elsewhere to evaluate the sources and fate of organic matter in estuarine and marine sediments (e.g. Thornton & McManus, 1994; Lee, 1994; Bird & Gro¨ cke, 1997; Andrews et al., 1998). In a study of the Tay Estuary, Thornton and McManus (1994) observed changes in carbon isotopic composition consistent with there being three distinct zones: an upper zone in which natural terrigenous organic matter was the dominant source, a middle zone in which the terrigenous input was diluted with autochthonous organic matter, and an outer zone where allochthonous marine organic matter was also a
Experimental Nine sediment sampling sites were selected from the Scottish Environment Protection Agency (SEPA) bi-monthly water quality sampling stations. Figure 1 shows the location of the sampling sites, STWs and other known effluent discharges. Table 2 shows the distance of each sampling site from the head of navigation at Stirling Bridge. Samples were collected on 5-Jun-97 and 15-Jun-97 from a SEPA survey vessel using a hand-held ‘ Van-veen ’ grab which obtains near-surface sediments (0–10 cm depth). At five stations (1, 2, 4, 5 and 7), three grab samples were collected to assess intra-site variability in sediment composition. Dissolved oxygen concentrations were also obtained from the concurrent water quality survey at each of the sediment sampling locations.
Organic matter in sediments of the Forth Estuary 377
2
Stirling 1 Stirling STW
3
4
Alloa STW
5
6 7 9
8
Forth Road and Rail Bridges
5 km
F 1. Map of the Forth Estuary showing locations of sampling sites (1–9), STWs ( ) and other known effluent discharges ( ).
Wet sediment samples were freeze-dried, ground and stored in plastic containers. Separate subsamples were suspended in 3·5% v/v HCl (12 h) to remove inorganic carbon. Each subsample was again freezedried and ground. A Perkin-Elmer 2400 CHN elemental analyser was used to determine the composition (% N, H and organic C) of sediments. Subsamples were prepared for isotopic analyses by the quartz closed-tube combustion method (Sofer, 1980). Because of the high C/N ratios and concomitant large sample size of sediment required to release adequate N2 for mass spectrometric analysis, carbon and nitrogen isotopic analyses were performed separately. For 13C, preroasted CuO and Cu wire were added to the sediment sample in the quartz combustion tube. Following evacuation and sealing, the tubes were heated to 850 C for 2 h and allowed to cool slowly overnight, to decompose oxides of nitrogen on Cu. The tubes were opened into vacuum, the CO2 cryogenically purified and 13C (‰ vs. PDB) determined on a VG SIRA 10 dual inlet mass spectrometer with a precision of 0·1‰. For nitrogen isotope runs, the combustion tubes also contained CaO which had been preroasted; this reacts with CO2 produced by combustion (Bo¨ hlke et al., 1993). Again, the N2 was cryogenically purified, and 15N (‰ vs. AIR) determined on a VG Optima dual inlet mass spectrometer. The precision on duplicate analyses ranged from 0·1 to 0·8‰; since the analytical technique is capable of 0·2‰ on isotopically homogeneous material, a small degree of intra-sample heterogeneity is implied.
Results Elemental and stable isotopic data for each sampling location are shown in Table 2. An average elemental composition of 4·70·8% organic C, 0·90·2% H, and 0·230·08% N and average C/H and C/N ratios of 5·21·3 and 21·76·6, respectively, were obtained. The corresponding average 13Corg and 15N values were 24·00·2‰ and +5·6 0·6‰, respectively. Intra-site variability in elemental composition was greatest in the upper estuary but inter-site differences in the organic C content of sediments over the entire length of the estuary were clearly small (Figure 2). Sediments with lowest organic C content, however, were located in the lower estuary (sites 8 and 9), for which smaller C/H ratio values (average of 3·90·3 for sites 7–9 compared with an average of 5·71·2 for sites 1–6) were also evident [Figure 3(a)]. Variability of the C/N ratio at and between individual sites was particularly marked in the upper estuary sediments (sites 1–3) whilst relatively constant values (average 19·02·4) were obtained for sites 4–9 [Figure 3(b)]. The sediment stable isotopic signatures at each sampling location are shown in Figure 4. 13C values ranged from 24·4 to23·6‰, with the least negative values being obtained for the lower estuary sites (8–9). There was no significant difference between 15N values in the upper and middle estuary (sites 1–7) and those from the lower estuary (sites 8–9). A pronounced minimum in dissolved oxygen concentration in overlying waters occurred on the
378 M. C. Graham et al. T 2. Elemental and stable isotopic data for Forth Estuary sediments
Sample
Distancea (km) 3 8 11 15 20 28 31 34 42
1 2 3 4 5 6 7 8 9 a
%N (1 SD)
%H (1 SD)
n
% organic C (1 SD)
3 3 1 3 3 1 3 1 1
4·80·5 3·81·0 4·4 4·90·3 5·30·3 6·1 5·20·1 3·1 4·1
0·230·12 0·140·04 0·17 0·250·03 0·300·02 0·25 0·300·03 0·15 0·20
0·90·2 0·60·1 0·7 1·00·1 1·10·05 1·0 1·30·1 0·9 1·0
24·00·1 23·80·1 24·2 24·10·1 24·40·2 24·0 24·00·1 23·6 23·6
13
Corg (‰) (1 SD)
15N (‰) (1 SD) 6·20·6 5·50·8 4·9 5·50·7 5·40·1 4·7 5·50·7 5·7 5·7
Distance from the navigation head at Stirling Bridge.
1.0
10
5
8
4
6
3
4
2
2
1
0
0 5 10 15 20 25 30 35 40 45 Distance (km) from Stirling Bridge
10
0.2 0.0
8 6 C/H
Nitrogen (%)
0.4
Hydrogen (%)
0.6
Organic carbon (%)
(a) 0.8
4 2
F 2. Elemental composition (%N( ), % organic C ( ), %H ( )1 SD) of Forth Estuary sediments.
0
5
10 15 20 25 30 35 40 Distance (km) from Stirling Bridge
45
5
10 15 20 25 30 35 40 Distance (km) from Stirling Bridge
45
50
day of sampling at 5–11 km from Stirling Bridge, encompassing sites 2–3.
(b) 40
Discussion C/N
From the results of this study, a striking feature is the relative uniformity of the isotopic (13C: 24·4 to23·6‰; 15N: 4·7to 6·2‰) and elemental (C: 3·1 to 6·1%; N: 0·14 to 0·30%; H: 0·7 to 1·3%) data over the entire length of the Forth Estuary. Figure 5 contrasts the stable isotope results presented herein with data obtained for the Tay estuary by Thornton and McManus (1994), illustrating the comparatively narrow range of the Forth Estuary data. In contrast with the Tay, there is no evidence, on the basis of 13C and 15N signatures obtained in this study, to support a predominantly terrigenous input to sediment organic mater in the upper Forth estuary. At the turbidity maximum in the upper estuary, indicated by the minimum in dissolved oxygen concentration a small decrease in the organic C content and in the N content of sediments was not accompanied by any significant change in stable
30 20 10
0
F 3. (a) C/H ( ) and (b) C/N ( ) ratios (1 SD) in Forth Estuary sediments.
nitrogen isotopic composition of the sediments. Relatively minor differences (13C and C/H) between the upper/middle and lower estuary are indicative of a greater influence of marine organic matter in lower stretches of the estuary. In agreement with the findings of Thornton and McManus (1994) for the Tay, the lack of any
Organic matter in sediments of the Forth Estuary 379 –22
δ13C
(a)
–24
–26
0
5
10 15 20 25 30 35 40 Distance (km) from Stirling Bridge
45
12 (b)
of correlation between 15N and % N or C/N (R2 =0·003 and 0·108, respectively) also meant that no conclusion could be drawn about the degree of diagenetic alteration of organic matter at any sampling location. In general, the relatively low organic C content suggested no major anthropogenic input to the Forth Estuary. More specifically, there was no localized elevation in sediment organic C content, e.g. close to STWs or industrial discharges. It is proposed that the dynamic processes operating in the main channel (Clarke & Elliot, 1998) result in relatively efficient mixing and redistribution of sediment and that the uniformity of the sediment organic C content and 13C signature precludes evaluation of the importance of any individual pollutant source. Acknowledgements
8
δ15N
We thank Lorna Eades and Terry Donnelly for invaluable technical support with elemental and stable isotopic analyses, respectively. 4
References 0
5
10 15 20 25 30 35 40 Distance (km) from Stirling Bridge
45
F 4. (a) 13Corg ( ) and (b) 15N ( ) signatures (1 SD) of Forth Estuary sediments.
12
δ15N
8
4
0 –27
–26
–25 δ13C
–24
–23
F 5. Comparison of stable isotopic data for sediments from the Forth ( ) (this study1 SD) and Tay ( ) (Thornton & McManus, 1994) estuaries.
correlation between 15N or C/N and 13C (R2 =0·127 and 0·037, respectively) meant that 15N and C/N ratios could not be used to evaluate source or gate of organic matter in the forth. In this study, the absence
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