Sources of organic matter in seagrass-colonized sediments: A stable isotope study of the silt and clay fraction from Posidonia oceanica meadows in the western Mediterranean

Organic Geochemistry Organic Geochemistry 36 (2005) 949–961 www.elsevier.com/locate/orggeochem

Sources of organic matter in seagrass-colonized sediments: A stable isotope study of the silt and clay fraction from Posidonia oceanica meadows in the western Mediterranean S. Papadimitriou b

a,*

, H. Kennedy a, D.P. Kennedy b, C.M. Duarte b, N. Marba´

b

a School of Ocean Sciences, University of Wales – Bangor, Menai Bridge, Anglesey LL59 5AB, UK IMEDEA (CSIC-UIB), Institut Mediterra`ni dÕ Estudis Avanc¸ats, C/Miquel Marque`s 21, 07190-Esporles, Mallorca (Illes Balears), Spain

Received 1 March 2004; accepted 7 December 2004 (returned to author for revision 16 June 2004) Available online 31 March 2005

Abstract The origin of sedimentary organic matter in 22 sandy beds of Posidonia oceanica (L.) Delile on the coast of the Iberian Peninsula and the Balearic Islands in the northwestern Mediterranean Sea was investigated using natural abundance stable isotope measurements of carbon and nitrogen (d13C and d15N) in fine (<63 lm grain size) surface sediments and in the local primary sources of detrital organic matter, seston, above- and below-ground seagrass tissues and bulk epiphytes. The d13C measurements provided the greatest power for resolving primary sources of the fine sedimentary organic matter in these shallow coastal ecosystems. Their use in an end member, isotopic mass balance approach showed that sestonic particles were as important as seagrass-derived detrital material as a source of sedimentary organic carbon in the P. oceanica meadows. The d15N measurements for the sediments exhibited patterns of isotopic depletion on a regional scale relative to the local primary sources of organic matter, which could be associated with occurrence of microbial fixation of molecular nitrogen (N2), indicating potential contribution of this process to the pool of sedimentary nitrogen in the P. oceanica meadows. Ó 2005 Elsevier Ltd. All rights reserved.

1. Introduction Over the last two decades, seagrass ecosystems have received considerable attention as diverse habitats which rank amongst the most productive in the marine environment (Pergent et al., 1997; Duarte and Chiscano,

*

Corresponding author. Tel.: +44 1248 38 8116. E-mail address: [email protected] (S. Papadimitriou).

1999; Cebria´n and Duarte, 2001) but have experienced increasing rates of deterioration and loss worldwide (Duarte, 2002). These submerged, rooted macrophytes enhance biogeochemical and physical processes in their immediate environment. Seagrasses can play a key role not only in producing, but also in actively trapping and retaining, suspended particles as a result of attenuation of hydrodynamic conditions within their canopies (Duarte et al., 1999; Gacia and Duarte, 2001) and development of a dense rhizosphere (e.g., Duarte et al., 1998). As a consequence, seagrass stands influence

0146-6380/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2004.12.002

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sedimentation primarily by reducing erosion within their meadows (Gacia and Duarte, 2001) and affect the composition of local sediments through supply of in situ produced biogenic material (Duarte et al., 1999; De Falco et al., 2003). Seagrass meadows have been linked to storage of significant amounts of detrital organic carbon in local sediments from both seagrass production and that of sestonic particles trapped from the water column (Fry et al., 1977; Romero et al., 1994; Cebria´n and Duarte, 1995; Duarte et al., 1999; Gacia et al., 2002; Kennedy et al., 2004). They are also strongly linked to microbial fixation of molecular nitrogen (N2; Welsh, 2000; Miyajima et al., 2001) in substantial amounts on a global scale (Capone and Carpenter, 1982). The sources of organic matter deposited in coastal environments have often been elucidated using measurements of the stable isotopic composition of its carbon (d13C) and nitrogen (d15N; Fry et al., 1977; Sweeney and Kaplan, 1980; Cifuentes et al., 1988; Thornton and McManus, 1994; McClelland and Valiela, 1998; Kennedy et al., 2004). The use of stable isotopes can be helpful in resolving the provenance of detrital organic matter when its sources have distinct isotopic composition from each other on both temporal and spatial scales, and the isotopic effect of secondary biological processes (i.e., degradation and respiration of primary organic matter) is inconsequential in magnitude. Seagrass meadows host a variety of primary producers (i.e., seagrasses, planktonic, epiphytic and benthic algae), support a rich community of secondary producers (Dauby, 1989; Lepoint et al., 2000; Vizzini et al., 2002) and supply a substantial fraction of the organic matter deposited locally (Fry et al., 1977; Gacia et al., 2002). The d13C and d15N values of the biomass of aquatic primary producers reflect the isotopic discrimination against the heavy isotopes, 13C and 15N, respectively, during photosynthesis, as well as the isotopic composition of the dissolved inorganic pool of these elements (Fogel and Cifuentes, 1993). Maximum discrimination occurs when the inorganic ion supply exceeds demand by the plants, resulting in biomass which is depleted in the heavy isotope relative to the inorganic ion pool. Minimum discrimination occurs when the inorganic ion availability becomes limiting and the isotopic composition of the biomass is equivalent to that of the dissolved inorganic ion pool. Aquatic primary producers rely primarily on the C3 cycle of carbon assimilation, in which the active inorganic carbon substrate that is assimilated into biomass is carbon dioxide (CO2). This is derived directly from the dissolved carbon dioxide, CO2(aq), or from extra- or intra-cellular conversion to CO2 of the bicarbonate ion ðHCO
3 Þ, both available in the external dissolved inorganic carbon (DIC) pool. The overall carbon isotope discrimination in aquatic primary producers depends, therefore, on the extent that CO2 supply limits carbon

assimilation. Seagrasses are typically the most 13C-enriched primary producers in coastal environments and the rates of carbon supply and demand are closely balanced, so they exhibit less carbon isotope discrimination (Andrews and Abel, 1979; Benedict et al., 1980). The dissolved inorganic carbon (DIC) pool in open seawater is large (2000 lM) and isotopically relatively homogeneous (0 ± 1&), and the d13C of the aquatic primary producers in coastal ecosystems can reflect mostly photosynthetic isotope discrimination patterns across species. The dissolved inorganic nitrogen (DIN) pool is less than the DIC pool by two or more orders of magnitude. The d15N of primary producers reflects that of the DIN only when its availability becomes limiting either seasonally during periods of maximum growth, or systematically as in nutrient-poor, oligotrophic environments when the contribution from N2 fixation is insignificant. In areas of complex terrestrial, riverine and estuarine ecosystems, the application of stable isotopes as tracers of organic matter sources can be ineffective owing to prohibitively large, unpredictable and overlapping isotopic ranges of the various components of the living biomass and inorganic ion inputs to such systems in space and time (Cloern et al., 2002; Lehmann et al., 2004). Here, we assess the relative importance of seagrass tissues as sources of sedimentary organic matter in shallow beds of the seagrass Posidonia oceanica (L.) Delile from the western Mediterranean (Spain) using a large scale geographical distribution of the d13C values of local surface sediments, seagrass tissues, their epiphytes and sestonic particles. We further use the corresponding d13N measurements to assess possible contributions from the meadows to N2 fixation. This seagrass species is endemic in the Mediterranean, forms extensive meadows down to 35 m water depth and long-term (up to millennia) deposits of belowground tissues in structures known as mattes, resulting in burial of substantial amounts of seagrass-derived organic carbon and nitrogen (Romero et al., 1994; Mateo et al., 1997; and references therein). A detailed study of a P. oceanica meadow in NE Spain, showed, however, that seagrass- and seston-derived organic matter contributed equally to the local detrital organic carbon pool, the latter being a major source of the nitrogen requirement of the plants (Gacia et al., 2002). Posidonia oceanica meadows have also been identified to be a potentially major contributor to N2 fixation in the Mediterranean Sea (Bethoux and Copin-Montegut, 1986). The results from this study will help clarify the role of P. oceanica meadows in the regional budget of carbon and nitrogen suggested by the existing evidence for the Mediterranean Sea. The measurements were obtained during a wider programme investigating the response of European seagrasses to changing environmental conditions.

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2. Study sites

3. Methods

Sampling was conducted at 22 P. oceanica meadows from different sites along the Mediterranean coast of mainland Spain (hereafter, Iberian Peninsula) and the Balearic Islands of Mallorca, Cabrera and Formentera (Fig. 1). Balearic meadows grow in biogenic carbonate sediments, while Iberian meadows grow in mixed biogenic carbonate and terrigenous sediments. In this study, all meadows were specifically selected for their remoteness from the direct influence of the major riverine outflows of mainland Spain into the western Mediterranean, the Ebro and the Tordera Rivers, in order to minimize contribution from land-derived detrital material to the composition of local sediments. The meadows are at depths ranging from 3 to 17 m, overlain by highly transparent oligotrophic waters (>15 m Secchi depth throughout; N. Marba´, unpublished data). All the sites are close to conurbations, except Xilxes, Torre de la Sal and the sites on Cabrera and Formentera islands, while LÕ Arenal and Campomanes are also close to seasonally discharging streams. The sites were visited on a number of occasions between July and October 2001, except for Cala Jonquet, Port Lligat, Cala Giverola and Fanals, which were visited in July and August 2002. The samples therefore represent a spatially wide and temporally narrow scale.

At each site, the top 2 cm of sediment from within the seagrass meadows, the youngest leaf of P. oceanica shoots (which is still free of epiphytes and, therefore, best reflects the isotopic composition of carbon and nitrogen derived from photosynthesis) as well as seagrass roots and rhizomes, epiphytic material from older leaves, sestonic particles and shoot density data were collected as described in Kennedy et al. (2004). The sediment samples were obtained from 10–20 cm long cores (4.3 cm internal diameter). Samples of seagrass tissue (youngest leaf, root, and young rhizome) consisted of pooled material collected from 10 different shoots within a meadow. Sestonic samples were obtained from 4 to 10 L of seawater. Isotopic analyses were conducted as described by Kennedy et al. (2004) on a single sample of seston and pooled seagrass tissue, and on up to six samples of epiphytic material collected from an equivalent number of seagrass shoots. The sediment samples were obtained from three replicate cores taken from within each meadow. The samples were dried in a fan oven and separated into three grain size fractions with a mechanical dry sieve after manual removal of pebbles and large debris, with cut off grain sizes being 500, 250 and 63 lm. The silt and clay fraction of the sediments (i.e., <63 lm; hereafter, referred to as fine fraction) was

Fig. 1. Location of Posidonia oceanica meadows in the northwestern Mediterranean Sea on the Iberian coast (a) and the Balearic Islands of Mallorca (b), Cabrera (c) and Formentera (d).

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ground and homogenized in a Fritsch pulverizer. A few mg of the ground material were weighed into precombusted (500 °C for 3 h) silver pans and were acidified with 2 M HCl at 40 °C to eliminate carbonate material. Acid-treated sediment samples were analyzed for organic carbon (Corg) and total nitrogen (NT) on a Europa Scientific ROBOPREP C/N Analyzer, while their isotopic composition was determined on the CO2 and N2 gases from acid-treated samples, which had been sealed under vacuum in quartz tubes (Kennedy and Kennedy, 1998) and combusted at 962 °C for 3 h. The gases were collected by vacuum distillation and their isotopic composition was measured using a PDZ-EUROPA GEO 20/20 and a SIRA VG II mass spectrometer for d13C and d15N, respectively. The isotopic measurements are reported in the d notation relative to the standards Vienna Pee Dee Belemnite for carbon and air for nitrogen, i.e., dsample = 1000 [(Rsample/Rstandard)
1], where R = 13C/12C, or R = 15N/14N. The accuracy of isotopic measurements was 0.1& for carbon and 0.2& for nitrogen based on internal standard (L -alanine), while the precision based on 1r of available replicate samples ranged from 0.1& to 0.7&.

4. Results 4.1. Sediments The sediments at all sites were predominantly coarse to medium sands, with the fine fraction contributing up to 15–20% to the bulk sediment dry weight but being mostly less than 5% (Fig. 2a). Seagrass shoot densities fell in the range of 150–800 m
2, with two exceptions at La Fossa on the Iberian coast and at Colonia St. Jordi on Mallorca, where shoot densities were highest at 980 ± 71 and 1551 ± 454 m
2, respectively (Fig. 2a). Overall, the majority of the sites from the Iberian Peninsula exhibited shoot densities of less than 500 m
2, while that in the meadows on the Balearic Islands exceeded 500 m
2. The Corg and NT content of the fine sediment fraction (Fig. 2b and c) ranged over one order of magnitude from 235 to 3802 ± 433 lmol C g
1 (0.3–4.6% dry weight) and from 21 to 229 ± 47 lmol N g
1 (0.03–0.32% dry weight), respectively, and did not correlate significantly with shoot density. The isotopic composition of the sedimentary organic carbon (d13Csed) exhibited a 6& range across all sites from
15.8& to
21.5&, and co-varied with Corg, such that the Corg was more 13C-enriched (i.e., had a less negative isotopic composition) in organic-rich than in organic-poor fine sediments (Fig. 2b). Exceptions to this central positive trend were four sites from the Balearic Islands, with three offset above and one below the trend. Excluding these four sites, the correlation between the remaining d13Csed and Corg was significant (r = 0.770, p < 0.001), indicating a link between organic

carbon content of the fine fraction and its isotopic composition (i.e., source) in the majority of the meadows. The d13Csed also correlated positively and significantly with shoot density (r = 0.653, p = 0.003), except for the two sites with the highest shoot density. The isotopic composition of sedimentary nitrogen (d15Nsed) exhibited a narrow 3& range across all sites, from 1.6& to 4.5& (Fig. 2c), but had no systematic relation with any of the sediment and seagrass properties described above. Based on the d15Nsed data, the sites can be separated in two distinct groups (Fig. 2(c)), one with a d15Nsed > 3&, which includes all sites from the Iberian Peninsula and the four sites along the southern coast of Mallorca Island (Fig. 1), with an overall mean (±1r)d15Nsed = 3.9 ± 0.5 (n = 13), and a separate group with a d15Nsed < 3&, which is formed by the sites from the Balearic Islands of Formentera and Cabrera and on the northern coast of Mallorca, with an overall mean (±1r)d15Nsed = 2.1 ± 0.4 (n = 9). 4.2. Primary sources of organic matter The measurements of d13C and d15N in new leaves of P. oceanica, below-ground tissues, epiphytes and sestonic particles (Table 1) form part of a concurrent study assessing stable isotopes as indicators of environmental change. The geographical separation of sites seen on the basis of d15Nsed is evident in the d 15N of the primary sources of sedimentary organic matter, i.e., marked by a tendency for isotopically depleted d15N values in the meadows around the Balearic islands of Cabrera and Formentera. This regional isotopic depletion was discernible in terms of spatial variability (i.e., 1r in Table 1) and amounted to 2–3& on average relative to their counterparts on Mallorca Island and the Iberian coast. Within each region, all the primary organic matter sources have almost identical or overlapping d15N values (Table 1). In contrast, there was no systematic geographical distribution discernible in the d13C values of the primary organic matter sources (Table 1), which averaged
22.1 ± 1.7&,
17.8 ± 1.8&,
12.1 ± 0.9& and
12.6 ± 1.2& for sestonic particles, epiphytes, below-ground tissues and new leaves, respectively, in the entire study area. The d13C values are distinct amongst the primary sources, with a 4–6& difference between epiphytes and either seston or seagrass tissues on average (Table 1), reflecting their varying extent of carbon isotope discrimination.

5. Discussion 5.1. Assessment of the origin of sedimentary organic matter using d13C and d15N as tracers Identification of organic matter sources in the detrital organic pool of local sediments requires that they have

S. Papadimitriou et al. / Organic Geochemistry 36 (2005) 949–961

953

20

(a)

Iberian coast Balearic Islands

%Fines (< 63 µm)

15

10

5

0 0

500

1000

1500

2000

-2

Shoot density (m ) -14

(b) -15

δ15 N NO

3

δ Csed (‰)

-16 -17

13

-18 -19 -20

Iberian coast Balearic Islands

-21 -22 0

1000

2000

3000

4000

-1

5000

Corg (µmol g ) 6

(c) 5

15

δ Nsed (‰)

4

W. Mediterranean

3 2 1

Iberian coast Balearic Islands

0 0

50

100

150

200 -1

250

300

NT (µmol g ) Fig. 2. Measurements for the fine fraction (<63 lm grain size) of sediments in Posidonia oceanica meadows on the Iberian coast (filled symbols) and the Balearic Islands (open symbols) in the northwestern Mediterranean Sea: (a) percent contribution (% Fines) to the dry weight of bulk sediment vs. seagrass shoot density, (b) d13C vs. concentration of organic carbon in the fine fraction (d13Csed and Corg, respectively), and (c) d15N vs. concentration of total nitrogen in fine fraction (d15Nsed and NT, respectively). Solid line on the y-axis indicates the mean d15N of nitrate in the deep western Mediterranean waters (1r = 0.5&; taken from Pantoja et al., 2002).

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S. Papadimitriou et al. / Organic Geochemistry 36 (2005) 949–961

Table 1 The d13C and d15N (mean ± 1r) of Posidonia oceanica new leaves, root, rhizome, and epiphytic material, and sestonic particles from a number (n) of seagrass meadows on the Iberian coast and the Balearic Islands in north-western Mediterranean Sea d13C

d15N

Iberian coast (n = 9) Leaf Root and Rhizome Epiphytes Seston


11.8 ± 0.8
11.7 ± 1.1
18.1 ± 1.7
22.1 ± 1.1

5.1 ± 0.5 5.5 ± 0.1 5.0 ± 0.5 5.5 ± 1.3

Balearic Islands Mallorca (n = 6) Leaf Root and Rhizome Epiphytes Seston


12.7 ± 0.6
12.0 ± 0.8
17.9 ± 0.8
22.0 ± 2.1

4.0 ± 0.9 5.0 ± 1.7 3.9 ± 1.2 5.7 ± 1.1

Cabrera and Formentera (n = 7) Leaf
13.7 ± 1.4 Root and Rhizome
12.8 ± 0.7 Epiphytes
17.4 ± 2.6 Seston
22.3 ± 2.1

2.5 ± 0.8 2.9 ± 0.9 2.4 ± 0.4 3.5 ± 1.4

signatures which are distinct from each other. The source resolution offered by the d15N measurements in seagrass tissues, bulk epiphytes and sestonic particles, assumed to represent mostly phytoplankton, is clearly diminished in the study area, with indistinguishable d15N values amongst all the three primary sources of sedimentary organic matter but with a uniform regional isotopic depletion of up to 3& in the Balearic Islands of Formentera and Cabrera (Table 1). Their d13C values offer a suitable separation of signatures amongst the primary sources (Table 1) and are comparable to those previously reported for P. oceanica ecosystems in the north-western Mediterranean (Dauby, 1989; Lepoint et al., 2000; Gacia et al., 2002; Vizzini et al., 2002; Vizzini et al., 2003). Overall, sestonic particles can provide the most 13C-depleted detrital organic matter to the underlying sediments and seagrass tissues the most isotopically enriched, with epiphytes falling in between these two extremes. A prerequisite for the application of d13C values as a tracer is that the measurements on the sources are representative of the material actually deposited. Posidonia oceanica meadows produce large detrital deposits, consisting mainly of leaf sheaths, roots and rhizomes, which turn over very slowly (Romero et al., 1992; Romero et al., 1994; Pergent et al., 1994; Pergent et al., 1997; Cebria´n and Duarte, 2001). Although a large part of the leaf production is exported outside the meadow, some detrital leaf blade material can be expected to enter the organic matter pool in the surface sediments (e.g., <5% of the total detrital stock; Cebria´n and Duarte, 2001). The d13C

of above- and below-ground seagrass tissues may differ by up to 2&, while the d13C of above-ground tissues can also vary seasonally and with leaf age by up to 3& (Anderson and Fourqurean, 2003; Fourqurean and Schrlau, 2003; Vizzini et al., 2003). Our results are consistent with previously reported isotopic differences amongst seagrass tissue types, as they indicate a non-systematic tendency for isotopically enriched below-ground tissues of P. oceanica by up to 1.5& at some of the present sites on the Balearic Islands. The above differences, however, are equivalent to the spatial variability of the d13C in above- and below-ground tissues (Table 1) and are, more importantly, small relative to the difference in d13C values amongst the three organic matter sources considered here (i.e., up to 10&; Table 1). Under productive meso- and eutrophic conditions the application of d13C as a tracer can be confounded by large seasonal and regional variations of up to 10& in sestonic particles resulting from both the seasonal variation in photosynthetic isotopic fractionation and from inputs of isotopically diverse, land-derived organic matter (Cifuentes et al., 1988; Hollander and McKenzie, 1991; Cloern et al., 2002; Lehmann et al., 2004). In contrast, the low productivity and riverine input characteristic of the Mediterranean results in smaller seasonal changes in the number of sources and their isotopic composition. Available seasonal isotopic data reported for the oligotrophic surface waters over P. oceanica meadows in Spain (Gacia et al., 2002) and Corsica (Lepoint et al., 2000) in the north-western Mediterranean indicate a seasonal d13C range of 2–3& for sestonic particles and of up to 4& for epiphytes (Lepoint et al., 2000). Assuming that they are representative of the western Mediterranean, these ranges are within the source separation offered by the d13C of the primary sources of organic matter at our study sites. Meaningful comparisons between sedimentary organic matter and its putative primary sources can be drawn if the d13C in the original organic materials does not alter during pre- and post-depositional decomposition, or, if it does, the offset is such that it does not distort or eliminate the original inter-source differences. An isotopic depletion of about 2& has been demonstrated experimentally to occur in the residual organic matter during the decomposition of phytoplankton and seagrass leaves, but not rhizomes, over timescales of up to 1 year (Lehmann et al., 2002; Fourqurean and Schrlau, 2003). Occurrence of a diagenetic isotopic shift of this magnitude (i.e., up to 2&) is again small by comparison with the spatial variability and inter-source differences in this study. Overall, the d13C difference amongst individual parts of seagrass plants and the isotopic shift associated with organic matter decomposition can be in the opposite direction from each other and within ±2& of the isotopic composition of live tissue. Following the above

S. Papadimitriou et al. / Organic Geochemistry 36 (2005) 949–961

considerations, the relative contribution of the primary organic matter sources to that in the fine sediment fraction can be assessed using the available d13C measurements. For seagrass tissues, the average value for new leaves, root and rhizome was used where root and rhizome measurements were available. Comparison between the d13Csed and the d13C of each of the sources at each site indicates that, in the majority of the studied meadows, the organic matter of the fine sediment fraction was isotopically more depleted than seagrass tissue and less depleted than sestonic particles, but clustered around the 1:1 line with the d13C of the epiphytic material at each site (Fig. 3). The broad implication of this comparison is that the fine sedimentary organic matter was predominantly either of epiphytic origin, or of a mixed origin, resulting from a combination of seagrass and sestonic material with or without epiphytes. A better constraint on the origin of the sedimentary organic matter in the Spanish meadows of P. oceanica can be placed by the following isotopic mass balance (Dauby, 1989): d13 Csed ¼

n X

fi d13 Csource
i ;

ð1Þ

i¼1

where fi and d13Csource
i represent the fractional contribution and isotopic composition of the ith potential source, respectively. In the case of three sources, n = 3 and d13Csed = fseston13 d Cseston + fepiphytesd13Cepiphytes + fseagrassd13Cseagrass, with f1 + f2 + f3 = 1. This system of equations is indeterminate with respect to the unknowns, f1, f2 and f3. However, a range of possible values can be calculated for each fi to satisfy the constraint 0 6 fi 6 1. By setting each fi to its limits of 0 and 1, a number of sites (n = 12) were identified, mainly from the Iberian Peninsula, where the d13Csed could not be reproduced without the contribution of sestonic organic matter (i.e., fseston > 0; Fig. 4a). In half of these sites (n = 6), this contribution was calculated to be important, i.e., organic matter of sestonic origin represented greater than 40– 50% of the sedimentary organic matter pool. An fseston = 0.57 can be inferred from similar calculations previously reported (Gacia et al., 2002) for one of the P. oceanica meadows on the Iberian coast (Fanals; Fig. 1), which compares well with our current estimated range of fseston of 0.51–0.86 at this site. In the remaining sites (n = 10), mainly from the Balearic Islands and with a d13Csed >
18&, seagrass tissue was both an essential source (i.e., fseagrass > 0) and an important component (i.e., fseagrass > 0.4–0.5) of the sedimentary organic matter pool (Fig. 4b). It is noted that, at these sites, the seagrass shoot density exceeded 400 m
2 and the organic carbon content of the fine sediment fraction exhibited values that included the highest measured in the study (Fig. 2b). In contrast, at the sites where the fractional

955

contribution of sestonic particles was calculated to be important (Fig. 4a), the seagrass shoot density was lower than 400 m
2 and the fine sediment fraction was comparatively less rich in organic carbon (Fig. 2b). This implies that the input of seagrass-derived organic matter to the sediments becomes predominant at relatively high shoot densities and can be linked to elevated organic carbon content in the sediments due to the high content of carbon-rich organic compounds in seagrass tissues (i.e., cellulose). In this light, the correlation between d13Csed and Corg (Fig. 2b) can be viewed as a mixing line, with seston and seagrass as two essential end-members. The calculations presented in Fig. 4(a) indicate that seston is an important source of organic matter for seagrass-colonized sediments, as previously demonstrated for one of the study sites (Gacia et al., 2002). This result is still surprising considering the profuse production of detritus by P. oceanica meadows (Cebria´n and Duarte, 2001), the prominent mats these meadows develop, and the oligotrophic nature of the Mediterranean planktonic community. However, recent findings have highlighted the capacity of seagrasses, and P. oceanica in particular, to trap and facilitate retention of particles from the water column (Gacia et al., 2002; Agawin and Duarte, 2002). The model could not resolve the role of epiphytic material per se, as it assigned a large range of possible values to its fractional contribution, which included zero. The implication that epiphytic material is not a necessary component of sedimentary organic carbon reflects more the limitation of the single tracer (d13C) model used here than in-situ conditions. Epiphytes contribute up to 50% of the total primary production of seagrass meadows (Hemminga and Duarte, 2000), of which a substantial component is consumed by herbivores (40%; Cebria´n et al., 1997). A fraction of the epiphytic production can be expected to enter the organic matter pool of the local surface sediment, as deemed to occur in tropical seagrass meadows (Kennedy et al., 2004). Clearly, an additional suitable tracer is required to resolve whether or not epiphytes make a significant contribution to the pool of sedimentary organic matter. 5.2. Assessment of N2 fixation using d15N The d15N measurements for the sedimentary organic matter and its primary sources indicated a geographical zonation manifest in the distinctly 15N-depleted values in the Balearic Islands of Formentera and Cabrera in both reservoirs (Fig. 2c; Table 1). The narrow, overlapping ranges of d15N for sestonic, epiphytic and seagrass material in the meadows on the Iberian coast (Fig. 5a) allows integration of all the primary sources into an overall mean for this region of d15NPS = +5.3 ± 0.8& (n = 31). Similar integration of the comparatively wider but also overlapping d15N ranges for Mallorca (Fig.

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-10

(a)

Seston, Iberian coast Seston, Balearic Islands 1:1

-15

-20

-25

-25

-20

-15

-10

13

δ Cseston (b)

Epiphytes, Iberian coast Epiphytes, Balearic Islands 1:1

-15

13

δ Csed

-10

-20

-25 -25

-20

-15

-10

13

δ Cepiphytes -10

(c)

Seagrass, Iberian coast Seagrass, Balearic Islands 1:1

-15

-20

-25 -25

-20

13

-15

δ Cseagrass

-10

Fig. 3. d13C of organic matter in fine sediment fraction (d13Csed) vs. d13C of primary organic matter sources: (a) sestonic particles (d13Cseston), (b) epiphytic material (d13Cepiphytes), and (c) seagrass tissues (d13Cseagrass), in Posidonia oceanica meadows on the Iberian Peninsula (filled symbols) and the Balearic Islands (open symbols) in the northeastern Mediterranean Sea. The straight lines represent 1:1 relationship (solid line). All values are means (except for those for sestonic particles), with error bars indicating 1r.

S. Papadimitriou et al. / Organic Geochemistry 36 (2005) 949–961

fseston

1.0

957

(a)

0.5

0.0

fseagrass

-22 -21 1.0 (b)

-20

-19

-20

-19

-18

-17

-16

-15

-18

-17

-16

-15

0.5

0.0 -22

-21

13

δ Csed Fig. 4. Fractional contribution of (a) organic matter of sestonic origin (fseston) and (b) organic matter of seagrass origin (fseagrass) to the organic matter pool in the fine fraction (<63 lm grain size) of sediments from Posidonia oceanica meadows in the Spanish Mediterranean coast. The values of the fractional contribution are plotted against the carbon isotopic composition of the sedimentary organic matter in the meadows (d13Csed). All illustrated values are plotted as ranges computed from solving Eq. (1; see text for details), except for a single site on the Iberian coast (+ = Xilxes), at which the sedimentary organic matter in the meadow was predicted to be exclusively of sestonic origin (i.e., fsegrass = fepiphytes = 0).

5b), as well as Formentera and Cabrera (Fig. 5c), yields a mean regional d15NPS of +4.6 ± 1.4& (n = 24) and +2.8 ± 1.1& (n = 19), respectively. The expression of biological fractionation of nitrogen isotopes during uptake by primary producers should be restricted by the relatively limiting DIN pool, i.e., dissolved nitrate þ ðNO
3 Þ and ammonium ðNH4 Þ, in the oligotrophic western Mediterranean waters (<10 lM; Pantoja et al., 2002; Lepoint et al., 2002; Lucea et al., 2003); its complete consumption will lead to convergence of the d15NPS with that of the DIN pool consumed during primary production (Fogel and Cifuentes, 1993). On this basis, the overlap of d15NPS on the Balearic Islands with the d15N of NO
3 in the deep western Mediterranean waters (shown in Fig. 5(b) and (c); d15 NNO3 ¼ þ3:4  0:5‰; Pantoja et al., 2002) implies reliance on deep water NO
3 in this region. However, the tendency for isotopic enrichment in the primary producers on Mallorca relative to deep water d15 NNO
3 (Fig. 5b) suggests that an isotopically enriched DIN pool should con-

tribute to the nitrogen requirements of the local primary production. A similar conclusion can be drawn from the distinct positive isotopic offset of the primary producers on the Iberian coast relative to those on the Balearic Islands and the deep water d15 NNO
3 (Fig. 5a). This pool of isotopically enriched DIN could derive from local sources, such as remineralized NHþ 4 . Clearly, the resolution of the nitrogen isotope dynamics during primary production in the study area requires a more detailed study. Comparison of d15N measurements for the sediments with those in the primary sources of detrital organic matter shows that the sedimentary nitrogen pool was systematically 15N-depleted by 1–2& at the Iberian sites (Fig. 5a), averaging +3.7 ± 0.5& (n = 9). The analysis of d13Csed values from these sites indicated a predominantly sestonic origin for the sedimentary organic matter (Fig. 4(a)), with a small contribution from seagrass tissues and, potentially, from epiphytes. The deviation of the measured d15Nsed from the expected narrow range

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S. Papadimitriou et al. / Organic Geochemistry 36 (2005) 949–961 8

(a)

7

seagrass tissues

seston

6 5

epiphytes

4 3 2

Sediments, Iberian coast Primary sources, Iberian coast Deep water W. Mediterranean

1 0 -25 8

-20

-15

-10

(b)

7

seagrass tissues

seston

6

15 δ N

5 4

epiphytes

3 2

Sediments, Mallorca Primary Sources, Mallorca Deep Water W. Mediterranean

1 0 -25 8

-20

(c)

7 6

-15

-10

Primary sources, Cabrera & Formentera Sediments, Cabrera & Formentera Deep Water W. Mediterranean

5

seston

4 3

epiphytes

2

seagrass tissues

1 0 -25

-20

-15

-10

δ 13C Fig. 5. d15N vs. d13C of organic matter for fine fraction of sediments from P. oceanica meadows on (a) the Iberian Peninsula (filled circles), (b) Mallorca (open circles), and (c) Cabrera and Formentera, Balearic Islands (open circles). The filled squares linked by the dashed line indicate the d15N and d13C of organic matter in sestonic particles, bulk epiphytes, and seagrass tissues (youngest leaf, roots, and rhizomes). All values are means, with error bars indicating 1r. Solid line on the y-axis indicates the mean d15N of nitrate in the deep western Mediterranean waters (1r = 0.5&; taken from Pantoja et al., 2002).

S. Papadimitriou et al. / Organic Geochemistry 36 (2005) 949–961

of the regional d15NPS (Fig. 5(a)) indicates an additional influence necessary for the isotopic balance of the sedimentary nitrogen pool. The discernible negative isotopic shift in the sediments from the Iberian meadows could be a post-depositional diagenetic effect (Cifuentes et al., 1988; Lehmann et al., 2002). The post-depositional isotopic excursion may be expected, however, to be predominantly in the opposite direction, i.e., positive, as a result of loss of the isotopically depleted component of deposited organic nitrogen during decomposition in surface sediments overlain by oxygenated waters (e.g., Thornton and McManus, 1994; Sacks and Repeta, 1999). Furthermore, it was not evident for the sediments from the Balearic meadows, where there was an overall good correspondence of d15Nsed with that of seagrass tissues and epiphytes (Fig. 5b and c), consistent with the origin indicated by the d13Csed (Fig. 4b). A post-depositional d15Nsed excursion relative to autochthonous organic matter may not be universal, unidirectional, or instantaneous in all depositional environments and, thus, cannot be ruled out on the basis of the above regional evidence as the agent of the negative isotopic shift in the sediments of the Iberian meadows. The offset between d15Nsed and d15NPS observed for the Iberian meadows can be the result of seasonal variability. Depending on sedimentation rate, the d15Nsed measurements integrate one or more seasonal cycles of autochthonous organic matter production and deposition, and the current d15NPS values, collected between July and October, may not be representative of its integrated annual isotopic composition. The d15Nsed in the Iberian sites is comparable with the average d15 NNO
3 of the deep western Mediterranean waters (Fig. 5a), which would also suggest reliance on this DIN source of the primary producers over whole seasonal cycles in these localities. A seasonal d15NPS variability, therefore, of up to 2&, as has been reported for primary producers in a P. oceanica meadow in a similar coastal environment in the northwestern Mediterranean (Lepoint et al., 2000), can explain the observed offset. Alternatively, we hypothesize that the d15Nsed offset in the Iberian meadows may have resulted from the mixing of isotopically enriched nitrogen from the primary sources with isotopically depleted nitrogen fixed in the sediments. Microbial N2 fixation produces organic nitrogen with an average d15NNF =
2.6 ± 1.3& (Sacks and Repeta, 1999). Heterotrophic N2 fixation by sulfate reducers in seagrass beds has been demonstrated to occur in the rhizosphere (Welsh, 2000; Miyajima et al., 2001). The d15Nsed measurements from these sites can then be used to estimate the contribution of N2 fixation to the pool of sedimentary nitrogen by using the following isotopic mass balance: d15 Nsed ¼ d15 NPS fPS þ d15 NNF fNF

ð2Þ

959

with d15NNF the isotopic composition of fixed nitrogen, as before, d15NPS the regional mean isotopic composition of the primary sources, as before, and fNF, fPS their fractional contribution to the sedimentary nitrogen pool. Substitution of fPS = 1
fNF into Eq. (2) allows a unique solution per meadow with respect to fNF. The results of this calculation show fNF from 0.12 to 0.27, i.e., 10–30% N2 fixation-derived sedimentary nitrogen. The calculated fNF can be combined with the fractional contribution of the fine fraction to the bulk sediment (Fig. 2a) and its measured nitrogen content (Fig. 2c) to yield an estimate of the nitrogen pool in the bulk sediment attributable to N2 fixation in the order of 0.1–1 lmol g
1. Using the rates reported in Miyajima et al. (2001) for bulk sandy sediments (i.e., 1.23 nmol N g
1 h
1 in September for 12h daylight conditions), a realistic time frame of less than 45 days can be calculated for the buildup of a nitrogen pool of this size by N2 fixation. It is noted that the above estimates are minimum values that reflect the standing stock of the sedimentary nitrogen pool, because loss by remineralization has not been taken into account. The good correspondence in d15N between primary producers and sediments in the Balearic meadows (Fig. 5b and c) does not allow assessment of a possible role of sedimentary N2 fixation in this region. This process, however, is implied through their similarity to the deep water d15 NNO
3 of the western Mediterranean Sea (Fig. 5b and c), which is isotopically depleted by comparison with the global oceanic average value as a result of N2 fixation (Pantoja et al., 2002). Together with the negative isotopic shift apparent in the sedimentary environment of the P. oceanica ecosystem along the Iberian coast (Fig. 5(a)), the current d15N data may serve as isotopic evidence for N2 fixation associated with the meadows of this seagrass species in the north-western Mediterranean. This corroborates the proposition of Bethoux and Copin-Montegut (1986) that this process in the indigenous seagrass beds can balance a substantial amount (31%) of the whole-basin nitrogen budget. The present isotopic evidence for the potential occurrence of N2 fixation in P. oceanica meadows should be viewed with caution because it is indirect and not unique to this process in view of the contributing factors of diagenetic alteration and seasonality. Validation from more detailed studies of the nitrogen isotope dynamics in the water column and sediments is clearly required for the meadows of this seagrass species in the Mediterranean Sea.

Acknowledgements The research was part of the M & Ms project (Monitoring and Managing European Seagrass Beds), funded by the European Commission (contract

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