Organic matter characterization in a tropical estuarine

Estuarine, Coastal and Shelf Science 84 (2009) 617–624

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Organic matter characterization in a tropical estuarine-mangrove ecosystem of India: Preliminary assessment by using stable isotopes and lignin phenols M. Bala Krishna Prasad a, *, A.L. Ramanathan b a b

Earth System Science Interdisciplinary Center, University of Maryland, 5825 University Research CT (Ste. 4001), College Park, MD 20740-3823, USA School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110067, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 May 2009 Accepted 29 July 2009 Available online 5 August 2009

In order to characterize the sources and fate of organic matter (OM) in the Pichavaram estuarinemangrove ecosystem (east coast of India), stable isotope (d13C and d15N) ratios and molecular lignin analyses were conducted in plant litter, benthic algae, sediment, particulate matter and in a variety of benthic invertebrate species. The d13C signature of plant litter ranges from 29.75& to 27.64& suggesting that mangrove trees follow the C3 photosynthetic pathway. Sedimentary d13C signature (28.92& to 25.34&) demonstrates the greater influence of plant litter organic matter on sedimentary organic matter. Suspended particulate organic pool was influenced by terrestrial source and also seems to be influenced by the marine phytoplankton. Enriched signature of d15N in surface sediments (4.66– 8.01&; avg. 6.69&) suggesting the influence of anthropogenic nitrogen from agricultural fields and human settlements. Spatial chemical variability in availability of nitrogen and plant associated microbial interactions demonstrate variability in d15N signature in mangrove plant litter. Two (lower and higher) trophic levels of invertebrates were identified with and observed >4& gradient in d13C signal between these two trophic groups. The observed d13C values suggest that the lower level invertebrates feed on phytoplankton and higher level organisms have a mixed source of diet, phytoplankton, sediment and particulate organic matter. Lignin phenol analyses explain that the benthic surface layer was almost free of lignin. The ratio between syringyl phenols to vanillyl phenols (S/V) is 1.14–1.32 (avg. 1.23) and cinnamyl phenols to vanillyl phenols (C/V) is 0.17–0.31 (avg. 0.24), demonstrate non-woody angiosperm tissues was the major sources of lignin to this ecosystem, while aldehyde to acid ratios (Ad/Al) describe diagenetic nature of sediment and is moderately to less degraded. A two-end-member mixing model indicate that the terrigenous OM was dominant in the estuarine zones, while in the mangrove zone terrigenous supply accounts for 60% and marine input accounts for 40%. Ó 2009 Elsevier Ltd. All rights reserved.

Keywords: mangrove stable isotopes lignin phenols organic carbon Pichavaram India

1. Introduction Mangrove forests are highly productive ecosystems and they fringe about 60–75% of the tropical coast line (Clough, 1998). It has been proposed that the mangrove ecosystems play an important role in the carbon balance of coastal ecosystems by exporting substantial amount of terrestrial carbon (11%) into the ocean and 15% of total carbon accumulating in modern marine sediments (Jennerjahn and Ittekkot, 2002). Likewise, Dittmar et al. (2006) quantified that mangroves contribute approximately 10% of terrestrial dissolved organic carbon (DOC) exported to the global

* Corresponding author. E-mail addresses: [email protected], [email protected] (M. Bala Krishna Prasad). 0272-7714/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2009.07.029

ocean, despite their small area relative to other habitats. The export of these large amount of organic matter has a recognizable effect on the food webs of coastal waters (e.g. Hedges et al., 1988; Alongi, 1990; Dittmar and Lara, 2001; Bouillon et al., 2008). In the recent decades, the coastal ecosystems are heavily impacted by the natural climate change and by the anthropogenic activities affecting the coastal food web structure resulting in widespread economic consequences (Alongi, 1990; Trott and Alongi, 2000; Chong et al., 2001; Alongi, 2008). Further, adjacent ocean also supplies considerable amount of nutrients to the coastal ecosystems and regulates the nutrient dynamics (Kemp et al., 2005; Delgadillo-Hinojosa et al., 2008). Hence, characterization of organic matter (OM) sources and defining the processes that affect its dynamics in intertidal estuarine-mangrove ecosystems may provide better constraints on the mechanisms that underlie fate and preservation of OM in the coastal sediments.

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M. Bala Krishna Prasad, A.L. Ramanathan / Estuarine, Coastal and Shelf Science 84 (2009) 617–624

Chemical tracers (stable isotopes and lignins) have been applied to identify the source and fate of organic matter in coastal environments (e.g. Meyers-Schulte and Hedges, 1986; Lobbes et al., 2000; Kuramoto and Minagawa, 2001; Graham et al., 2001; ˜ i, 2003; Tremblay et al., 2007; Loh et al., 2008). Gordon and Gon Stable isotopes (d13C and d15N) have been used not only to infer OM sources and cycling but also to characterize the food web structure in the coastal ecosystems (Saino and Hattori, 1980; Kwak and Zedler, 1997; Fredriksen, 2003; Cole et al., 2004; McCallister et al., 2004). However, the relative contributions of multiple sources to OM pools and trophic structure can be difficult to ascertain because of overlap in the isotopic signatures of different components (Cloern et al., 2002). Therefore, the simultaneous use of duel isotopic tracers may help to overcome some of these limitations (Bauer et al., 2002). Both d13C and d15N isotopes have been used, with different degrees of success, for the identification of OM sources and trophic relationships. Natural abundance of carbon forms has the potential to provide substantial resolution in discerning the relative source origin (Cherrier et al., 1999). Besides, fractionation of 13C is limited to about 1% per trophic level, and it can also be used to identify ultimate sources (Fry and Sherr, 1984). Conversely, the 15N content of consumers is typically enriched by 3–4& relative to their prey and results can be used to construct trophic relationships in ecosystem where feeding relationships are unknown (Hobson and Welch, 1992; Vander Zanden and Rasmussen, 1999). Further, in recent times, d15N levels in the coastal ecosystems are elevated to 8& which is mainly because of discharges of high concentrations of 15N effluents from agricultural lands, aquaculture ponds and domestic settlements (Cifuentes et al., 1988; Savage, 2005; Mutchler et al., 2007; Lepoint et al., 2008). The accumulation of enriched 15N fraction in sediments is causing a stepwise enrichment of 15N within the food chains (Kwak and Zedler, 1997; Cole et al., 2004). Thus, combined measurements of d13C and d15N isotopes can provide information on both OM sources and trophic structure. Besides stable isotopes as a chemical tracer, to date lignin is the only established molecular tracer for terrigenous organic matter in the coastal and ocean waters. It is an unambiguous tracer for vascular plants and can even distinguish vegetation types. The CuO oxidation of lignin yields a suite of compounds (Hedges and Ertel, 1982) that include vanillyl phenols [vanillin (VAL), acetovanillone (VON) and vanillic acid (VAD)], syringyl phenols [syringaldehyde (SAL); acetosyringone (SON) and syringic acid (SAD)] and cinnamyl phenols [p-coumaric acid (CAD) and ferulic acid (FAD)] and also p-hydroxy phenols [p-hydroxybenzaldehyde (PAL), p-hydroxyacetophenone (PON) and p-hydroxybenzoic acid (PAD)]. Thus, the alkaline CuO oxidation is the best analytical method for molecular level analyses of lignin that has been widely used in the last decades to trace the transport and fate of terrigenous organic matter in the coastal ecosystems (Dittmar et al., 2001; Loh et al., ˜ i, 2003; Tremblay et al., 2007; Loh et al., 2002; Gordon and Gon 2008). Benner et al. (1990) reported that lignin-derived phenols were leached from mangrove leaves during early diagenesis. However, lignin concentration has decreased within just a few days due to photolytic degradation, making the quantification of terrigenous organic matter in the coastal systems difficult (Opsahl and Benner, 1998). The objective of the present study is to characterize the sources of OM in the Pichavaram estuarine-mangrove ecosystem of India by using stable isotopes (d13C and d15N) and lignin phenols. Isotope and lignin analyses are conducted on mangrove plant litter, sediment, particulate matter, benthic plankton and some invertebrates (only isotope analysis) to characterize the nature of OM and trophic structure in this ecosystem.

2. Materials and methods 2.1. Study area The research area, the Pichavaram estuarine-mangrove 0 0 ecosystem (Lat 1125 N and Long 79 47 E), is a dynamic intertidal environment lying in between the Vellar and Coleroon estuaries on the east coast of India (Fig. 1). The mangrove covers an area of about 1100 ha, consists of 51 islets, of which 50% is covered by forest, 40% by waterways and the remaining filled by sand-flats and mud-flats (Kathiresan, 2000). The southern part near the Coleroon estuary is predominantly dominated by mangroves, while the northern part near the Vellar estuary is dominated by mud-flats. Tides in this ecosystem are semi-diurnal and vary in amplitude from 0.15 to 1 m. Tidal ocean water enters through a direct connection with the sea at the Chinnavoikal mouth and estuarine waters enter through the two adjacent river systems. Geomorphology of this ecosystem is mostly covered by flood plain, sedimentary plain and beach sand. Alluvium is dominant in the western part, whereas fluvial marine and beach sand dominates in the eastern part (Ramanathan, 1997). The average dissolved inorganic nitrogen and ortho-phosphate concentrations in this mangrove waters are 157 mM and 61 mM respectively (Prasad et al., 2006) and mangrove soils are composed of fine sand (45%), with a mixture of clay (25%), silt (15%) and coarse sand (15%) (Prasad and Ramanathan, 2008). Sediments contain 24.37 mg/g of total carbon (TC), 6.62 mg/g of total nitrogen (TN), 0.41 mg/g of total sulfur (TS) and 16.23 mg/g of organic carbon (OC) (Prasad and Ramanathan, 2008). Mangrove vegetation is primarily dominated by Avicennia marina followed by Avicennia officinalis, Rhizophora apiculata, Rhizophora mucronata and Exocecaria agallocha. Major invertebrates in this ecosystem include Penaeus indicus, Heteromastus similes, Cerithidea fluviatilis, Pythia plicata, Cassidula nucleus, Crassostrea madrasensis, Assiminea nitida, Halmyrapseudes killaiyensis, Ligia exotica, Metapenaeus sps. etc. (Kathiresan, 2000). The climate is sub-humid and precipitation to evapotranspiration ratio (P/Etp) ranges between 0.5 and 0.75 with maximum

Fig. 1. Map of the Pichavaram estuarine-mangrove ecosystem, east coast of India.


precipitation (75–90%) during the north-east monsoons (Selvam, 2003) and an average annual rainfall is 146.4 cm (Prasad, 2005). 2.2. Sample collection and preparation Mangrove plant leaves, sediments and benthic invertebrates were collected from various locations across the mangrove system (Fig. 1) in the first week of April 2004 and were stored in an ice box on board. Fresh leaves from two mangrove species (Avicennia marina and Rhizophora apiculata) were collected from the tidal mangrove forest and were ground to powder for chemical analyses after drying at 60 C for 24 h. Suspended particulate organic matter (SPOM) was collected in March 2005 from the mangrove waterways by filtration of large volumes (>5 l) of water samples through the pre-combusted Whatman GF-C glass-fibre filters (pore size 1 mm). The particulate matter was collected along the mangrove creeks far from the local discharge points of the anthropogenic particulate sources so as to avoid cross-contamination. In addition, water samples for particulate matter were also collected during low tide from the mangrove channel to minimize the influence from material of marine origin. The collected sediment samples were crushed and sieved through a 250 mm mesh to remove large plant debris and coarse sand. The crushed sediment and particulate matter were first air dried at room temperature for 24 h and also dried in an electric oven at 60 C. Benthic macro algae samples were also collected in March 2005 from the plant root zone and care was taken while collecting the samples without mixing plant organic matter with the algal sample. Benthic algal samples were ground to powder for chemical analyses after drying at 60 C for 24 h. After transportation to the field laboratory, all benthic invertebrates, Penaeus indicus, Heteromastus similes, Crassostera madrasensis, Cerithidea fluviatilis, Pythia plicata, Cassidula nucleus and Ligia exotica, were washed and removed outer calcareous shell materials. Then the invertebrate samples were dried at 60 C for at least 24 h. Then the samples were ground to fine power and divided into subsamples for chemical analyses. All the collected samples were subsampled for chemical analyses. Sub-samples for d13C analysis were washed with dilute HCl to remove possible carbohydrates and were re-dried. Subsamples for d15N analysis did not receive this treatment as this has been reported to affect d15N values (Bunn et al., 1995; Pinnegar and Polunin, 1999). 2.3. Elemental and stable isotope analyses Elemental carbon to nitrogen ratios (C/N) of the collected environmental samples were analysed by combusting pre-weighed samples in a Carlo Erba NA 1500 Series II, similar to the method described by Nieuwenhuize et al. (1994). Samples for stable isotope analyses were similarly combusted in the Elemental Analyser, coupled to a Finnigan Delta Plus isotope ratio mass spectrophotometer via a Finnigan ConFoll open split interface. The relative abundance of the heavy and light stable isotopes of C and N are expressed as d13C and d15N values, that is in relative conventional standards, PDB limestone for carbon and atmospheric N2 for nitrogen. d13C and d15N values were calculated according to the following formula:

dXðin &Þ ¼

h

i

Rsample =Rstandard 1 1000

13

15

13

12

15

14

where X ¼ C or N and R ¼ C/ C or N/ N. Precision on IAEA – N1 (SRM – [email protected]&) and USGS – 24 (SRM – [email protected]&) were better than 0.15&. Weight percent concentrations were standardized via acetanilide (71.09% C and 10.38% N) reproducible to better than 0.8% for C and 0.2% for N at w0.5 mg.

619

2.4. Lignin analysis Lignin was characterized by disrupting its macromolecular structure by alkaline cupric oxide (CuO) oxidation (Hedges and Ertel, 1982, slightly modified by Lobbes et al., 1999). Eleven phenolic monomers produced by this method were determined by a highperformance liquid chromatography (HPLC). Briefly, the analysis was performed on a Merck-Hitachi HPLC system with diode array detection (DAD) using a reverse phase column (Lichrosphere 100 RP 18, 5 mm particle diameter, 250 mm length, 4 mm inner diameter) and a multi-step solvent gradient system for separation. The phenols were identified by their retention times and UVabsorption spectra between 230 and 370 nm, recorded continuously by the DAD detector. The extinction at 280 nm was used for quantification. External standards of the 11 phenols (Fluka, Switzerland; Aldrich, USA; Sigma, USA) were used for calibration. The detection limit for the individual phenols ranged from 15 to 40 pmol (p ¼ 0.05). All samples were analyzed in duplicate, including CuO oxidation, extraction and quantification. The coefficient of variation (CV) was 15% on average, which agrees well with data reported in the literature (see Lobbes et al., 1999, and references therein). The CV of the HPLC quantification was 4&. The d13C values of lower trophic level benthic invertebrates in this ecosystem are depleted (27.08& to 23.29&) and close to sedimentary carbon signature (28.43& to 24.37&) which explain that the sedimentary organic matter appears to be a principal diet for these species. Most of the gastropod invertebrates (higher trophic level species) are filter feeders and their carbon signal is close to benthic phytoplankton, indicating that these species feed directly on the benthic matter and also depend upon SPOM. The observed variability in d13C of the lower trophic level benthic invertebrates, in comparison with sedimentary d13C data indicate that a marked selectivity for pelagic food sources. Besides,

the d13C levels of the high trophic level invertebrates ranged in between the benthic algae and sediment and SPOM pools, this explains a mixed carbon sources. In fact, benthic algae and phytoplankton are expected to exhibit enriched d13C levels because of a more 13C depleted in the DIC pool (dissolved inorganic carbon) in and near the mangrove creeks, where bacterial respiration of 13C depleted vascular plant detritus will result in a dilution of DIC pool with isotopically light CO2 (e.g. Marguillier et al., 1997). Thus, it is possible that the spatial differences in organic matter pool (mangrove and terrestrial carbon) could be very important in the intertidal mangrove zone yet be fully replaced by algal sources towards the marine end, in which case we would also expect to find a progressive gradient in d13C towards the marine end. In order to study this phenomenon, samples have to be collected from all sections of the ecosystem. Due to the practical and logistical difficulties, we restricted our sampling strategy within the mangrove zone for invertebrate samples. However, this study will give an idea about carbon flow from lower to higher trophic level. In all samples, the d15N signature is well above 4.5& (Table 1) and there is a small difference (˜0.5&) in d15N between the two trophic groups. In general, NO3 from domestic activities has been found to have significantly higher d15N values than other NO3 sources (Heaton, 1986; Macko and Ostrom, 1994), and thus ecosystem d15N signature has been found to increase with the degree of urbanization of the watershed (e.g. McClelland et al., 1997; McClelland and Valiela, 1998; Fry, 1999). In this ecosystem, enrichment of d15N has been observed with the degree of changes in land use pattern, agricultural and aquacultural influences (Ramesh, 2003; Subramanian, 2004) and we did not observe significant difference in d15N between the two trophic groups. Marıń-Guirao et al. (2008) found enriched d15N values in invertebrates in the Mediterranean coastal lagoon which is polluted from urban waste waters. Anthropogenic inputs of nitrogen have been recognized to significantly increase the d15N in coastal ecosystems (e.g. McClelland and Valiela, 1998; Fry et al., 2003; Vizzini et al., 2005; Piola et al., 2006), even on small spatial scales (Vizzini and Mazzola, 2006). 4.2. Lignin phenols in mangrove leaves, sediment and benthic algae Upon cupric oxide (CuO) oxidation, lignin yields a suite of phenolic acids, aldehydes and ketones (Hedges et al., 1982) and these oxidation products can be used to trace the contributions of

622


vascular plant derived carbon to OM in coastal sediments (Miltner and Emeis, 2000; Dittmar and Lara, 2001; Loh et al., 2002; Gordon ˜ i, 2003; Loh et al., 2008). In Pichavaram, the fresh and Gon mangrove litter has a lignin yield of Xlignin z 4.3&, which is low compared to other tropical mangrove plants in Bahamas (Benner et al., 1990) and in Furo do Meio, Brazil (Dittmar and Lara, 2001). Most notably, the benthic surface layer is very poor in lignin (on carbon basis), the organic matter there is essentially free of lignin (Table 2) and OM supply from benthic systems is nearly minimum. According to Hedges and Parker (1976) and Miltner and Emeis (2001), high ratios of syringyl phenols to vanillyl phenols (S/V) indicate a higher abundance of angiosperms and high ratios of cinnamyl phenols to vanillyl phenols (C/V) are indicative of non˜ i et al. (2000) also concluded that S/V and woody plant tissues. Gon C/V ratios more than 0.4 and 0.15 respectively imply the dominance of non-woody angiosperm tissues. The S/V ratios in this mangrove sediments ranged from 1.14 to 1.32 and C/V ratios 0.17–0.31 indicate the predominance of non-woody angiosperm tissues (Fig. 3). The source of these tissues is likely to be leaf tissues, as leaves are considered to be the non-woody tissues of a plant (Hedges and Mann, 1979). The sediment S/V values (1.26) are close to plant litter S/V vales (1.24), this suggests that a similar diagenetic reactivity for syringyl and vanillyl phenols in the mangrove sediment (Dittmar and Lara, 2001). The C/V ratio is also indicative of the degree of sedimentary organic matter diagenesis and exhibited greater difference between leaf litter and sediment than d13C. This explains that the reactive part of cinnamyl phenols had already been released or modified. The cinnamyl phenols are often ester-bound to lignin (Kirk et al., 1980) but can also associate with carbohydrates (Hartley, 1973). Sediment has low levels of p-coumaric acid (CAD) than the plant litter. This may be due to diagenetic depletion of CAD in sediments and also established that this component is loosely bound to the lignin polymer or associated with chemical components less recalcitrant than lignin (Dittmar and Lara, 2001). The ratio of p-hydroxyacetophenone to p-hydroxyl phenols (PON/P) in sediment (0.059) is close to Rhizophora apiculata (0.062), indicating that the reactivity of p-hydroxyacetophenone is similar to total p-hydroxyl phenols during decomposition. It supports a unique source (lignin) for all p-hydroxyl phenols. Hence, P/(V þ S) can be used as an indicator for demethylation. Lignin decay is generally associated with oxidation of propyl side chain,

Angiosperm wood

1.4

S/V

1.0 0.8

Angiosperms leaves

0.6 0.4 0.2 Gymnosperm wood

4.3. Quantitative account of sedimentary organic matter A complete understanding of organic matter cycling in this estuarine-mangrove ecosystem may be gained by the quantitative evaluation of the marine and terrigenous composition of the sedimentary organic matter. Geochemical investigations have historically employed a two-end-member mixing model between marine phytoplankton and isotopically depleted vascular plant debris to quantify the terrigenous component of OM in this mangrove environment as follows:

%OCTerr ¼

d13 Csample d13 Cmarine 13

d Cmarine

.

d13 Criverine

where d13Csample is the measured isotopic composition at each station, d13Cmarine is a published value for marine POM close to the mouth of the Cauvery River (Ramanathan et al., 1993) and d13Criverine is the average composition of Cauvery River suspended matter (Ramanathan et al., 1993). The fraction of terrigenous and marine OM deposited within the three zones of estuarine-mangrove ecosystem is shown in Fig. 4. The fraction of terrigenous OM is higher in the Vellar and Coleroon estuarine zones than the mangrove zone; this is mainly because of influence of distributaries of Cauvery River (Vellar and Coleroon). The OM deposited within the mangrove zone is nearly 60% from terrigenous and 40% from algal sources. In two estuarine regions, it is calculated to be 100% terrigenous supply because of overwhelming contribution of isotopically depleted C3 plant debris.

1.6

1.2

demethylation of the methoxyl groups or both. However, in Pichavaram neither P/(V þ S) values nor the acid to aldehyde ratios (Ad/Al) are higher compared with those of plant leaf litter. This explains that aromatic ring cleavage is the principal mechanism for the lignin degradation in this ecosystem. Benner et al. (1991) also observed a similar mechanism during early diagenesis of Spartina alternifolia lignin in a reducing salt-marsh ecosystem. In general, low Ad/Al ratios indicate little diagenesis in the Pichavaram surface sediments. However, comparatively higher Ad/Al ratio (1.32) is observed in sediment collected from Avicennia zone which explains that sediment in this zone is degraded. In fact, high Ad/Al ratios result from an oxidation of the propyl chain of lignin units, typical for lignin decomposition under oxic conditions (Hedges and Ertel, 1982). Thus, the rationale for these contrasting results might be that Rhizophora forest develops under the influence of marine water whereas Avicennia develops under the influence of freshwater. This difference induces development of redox conditions in sediments. In Pichavaram, seaward sediments are the site for anoxic conditions and OM decomposition mainly mediated by sulfate reducers, as highlighted by high sulfur concentration (Alongi et al., 2005). Conversely, on the landward side, mangrove sediments are oxygenated and OM degradation is mediated by metal reducers (Alongi et al., 2005). This supports the view that different processes of lignin decomposition occur within the sediments under various mangrove forests, as a result of their location in the swamp and respective influence and frequency of fresh and sea water input.

Gymnosperm needles

5. Summary and conclusion

0.0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

C/V Fig. 3. Syringyl/vanillyl phenol ratio vs. cinnamyl/vanillyl phenol ratio. Typical ranges for woody and non-woody tissues of both angiosperm and gymnosperm vegetation is indicated. Solid circles indicate R. apiculata; open circles indicate A. marina and solid inverted triangles indicate sediments.

By using parameters such as d13C, d15N, C/N, lignin, S/V, C/V, Ad/ Al, PON/P, and P/(V þ S), the sources and fate of OM and trophic structure in the Pichavaram estuarine-mangrove ecosystem have been elucidated. The d13C signature and C/N ratio of sediment explains that plant litter supplies substantial fraction of OM to the


References

1.6 Terrigenous Marine 1.2

Fraction OM (%)

623

0.8

0.4

0.0

Velar

Mangrove

Coleroon

Fig. 4. Relative fraction of terrigenous organic matter (OM) deposited within the Pichavaram ecosystem calculated by a two-end-member mixing model.

sedimentary organic pool. Where as in case of SPOM, d13C is little enriched and close to the marine phytoplankton range and it seems to be influenced by multiple OM sources of plant litter, terrigenous matter and marine phytoplankton. The d15N signature of sediment and SPOM depicts the influence of mangrove detritus and anthropogenic impacts on N dynamics. Based on the distribution of d13C, the benthic invertebrates are classified as lower and higher trophic level species. The lower level invertebrates seem to depend upon the phytoplankton and higher level organisms have a mixed source of carbon. Lignin phenol assessment explains that the Pichavaram mangrove litter yields very low Xlignin (z4.3&) compared to other tropical mangroves. The benthic surface layer is almost free of lignin and thus it is not considered as an important carbon source in this ecosystem. The S/V and C/V ratios of sediment demonstrate that the non-woody angiosperm tissues are the major sources of lignin to this ecosystem. The Ad/Al ratios describe that the sedimentary OM is moderately degraded. A two-end-member mixing model is employed to quantify OM contributions from terrigenous and marine sources to this mangrove sedimentary organic pool. In the estuarine zones, terrigenous OM supply is dominant and calculated to be 100%. While in the mangrove zone 60% of OM is supplied from the terrigenous supply and rest 40% is from the marine input. With some independent measurements of sediment burial rates, vertical distribution of lignin and isotope parameters and oceanic isotopic signature, the results will permit estimates to be made of the local rate of burial of terrigenous organic carbon and the proportion of marine organic carbon that is being removed from the biosphere, that is locked up in the mangrove sediments. Acknowledgements Financial support was granted by the International Foundation of Sciences (Sweden) project ‘Carbon Biogeochemistry of the Pichavaram Mangroves’. We are grateful to Dr. Adina Paytan (Stanford University, USA) and Dr. Thorsten Dittmar (Florida State University, USA) for stable isotope and lignin phenol analyses. We are thankful to Prof. L. Kannan (Vice-Chancellor, Thiruvalluvar University, India) and his research group for identifying invertebrates collected in this survey. We are also thankful to Dr. S. Chidambaram (Dept. of Earth Sciences, Annamalai University, India) for his help during sampling and for his hospitality during our stay in Chidambaram. We are grateful to the reviewers for their comments which greatly improved and strengthened this manuscript.

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