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. Graham a , M. A. Eaves a , J. G. Farmer a , J. Dobson b and A. E. Fallick c a Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, U.K. b 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. 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 ( 13 C and  15 N) 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  13 C values of 23·6‰ and C/H ratios of 3·9 0·3 for lower estuary sediments did suggest a change to a more marine-influenced system.  15 N 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:  13 C;  15 N; 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 pro- cesses, 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). 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 sedi- ments, 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 con- trolling factor for suspended particulate matter trans- port 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 0272–7714/01/030375+06 $35.00/0  2001 Academic Press