Pages 22 - 29
Keywords 13C, 15N, Corg:N ratio, Isotopic composition, Organic matter, Tropical wetland
F. Virginia Pérez-Castillo a M. Catalina Alfaro-De la Torre a* Rebeca Y. Pérez-Rodriguez b and Francisco A. Comín Sebastián c
a Laboratorio de Elementos Traza, Facultad de Ciencias Químicas, Universidad Autónoma de San Luis Potosí, C. P. 78210, San Luis Potosí, México
b Instituto de Geología, Facultad de Ingeniería, Universidad Autónoma de San Luis Potosí, C. P. 78280, San Luis Potosí, México
c Instituto Pirenaico de Ecología-CSIC, C. P. 50059 Zaragoza, Spain
* Corresponding author: firstname.lastname@example.org
In this study, we estimated the depth profiles of nutrients in the natural wetland “Ciénega Tamasopo” (Central Mexico) located in the Neotropic region. The concentration profiles of Corg, NT, Corg/NT ratios and isotopic composition (δ13C, δ15N) were determined in sediment cores collected at five sites throughout the wetland to estimate the contribution of nutrient sources to the sediments through analysis of profile shape. The results showed a recent enrichment in organic matter in the upper sediment layers at three sites (S1 to S3). Changes in the Corg/NT ratios with the sediments depth suggested that organic matter is autochthonous at the pristine sites (upper part of the wetland) characterized by an abundant coverage of vegetation. The isotopic (δ13C, δ15N) profiles (-35 to -25.8 ‰ for δ13C and 0.7 to 4.1 ‰ for δ15N) supported these conclusions and pointed to the entrance of allochthonous materials through local perturbations as a cause of changes in wetland productivity at the lower part. Analysis of isotopic composition can be used to evaluate trace productivity changes in tropical wetlands exposed to anthropogenic perturbations in this area.
.- Elemental and isotopic Corg and NT depth-profiles are useful to trace OM in the “Ciénega Tamasopo” wetlands.
.- Corg/NT ratio with δ13C or δ15N highlights changes in the nutrient sources in the wetlands.
.- Nutrients from external sources may alter OM deposition by increasing productivity
In order to identify organic matter sources in sediments from aquatic systems such as wetlands, as well as to trace past productivity, to assess the impact of humans on local ecosystems and changes in the supply of nutrients to lakes, some studies have analyzed changes in the compositions of Corg, NT, Corg/NT ratios, and carbon and nitrogen isotopes (δ13C and δ15N) -. Corg/NT ratios are widely used to distinguish the proportions of algal and land-plant organic matter , . δ13C allows for the identification of carbon sources and photosynthetic pathways in plants (e.g. C3 and C4 plant differentiation) ,  and to determine enhanced algal productivity . In freshwater systems, organic matter from algal production has a Corg/NT ratio from 4 to 10 for cellulose poor and protein rich organic matter, 10 to 20 from aquatic/terrestrial sources and >20 from terrestrial sources (cellulose poor and protein rich organic matter) , . The δ13C values of organic matter can range from −9 to – 30 ‰  and can differentiate between C3 (− 24 to − 30‰) and C4 plants (− 9 to – 15 ‰) in different environments (e.g. aquatic, terrestrial) -, -. The δ15N values of organic matter from sediments can be used to differentiate between algal (aquatic) and land plant sources of organic matter and as evidence of nitrogen fixation and enhanced algal productivity. Environmental changes can affect the δ15N in the sediments. As an example, Meyer  reported a 2 ‰ shift toward higher δ15N values in Nevada Lake caused by water level variations and changes in the organic matter sources.
Brenner et al.  found isotopic signatures of δ13C (-27.2 to -22.9 ‰) and δ15N (-2.3 to 0.5 ‰) in sediments from a shallow lake dominated by macrophytes in Florida (USA), attributing the origin of the organic matter to the aquatic plants. δ15N values in that lake correlated with an increased P concentration from human activities and forest clearance. Chang et al.  recorded isotopic signatures of δ13C (-33.6 to -27.4 ‰) and δ15N (-4.8 to 8.6 ‰) in the surficial sediment (~5 cm depth) of a Florida subtropical wetland attributed to changes in the water chemistry and wetland hydrology influencing the organic matter content as a result of modifications in the hydrophyte vegetation structure.
Nitrogen inputs from anthropogenic sources (fertilizers from agriculture, sewage, manures, etc.) can negatively affect the water quality of wetland ecosystems. This impact can be traced by measuring δ15N as an indicator of the availability of nitrogen for biota . The N-isotopic composition of settling organic detritus thus varies, depending on the extent of nitrogen utilization by organisms: low 15N content indicates low relative utilization and high 15N content indicates high utilization .
This study reports the vertical patterns with depth of Corg, NT, P, Corg/NT ratios and carbon and nitrogen isotopes (δ13C and δ15N) in sediments of a natural wetland (Ciénega Tamasopo, Central Mexico). This wetland supplies water for agriculture, livestock, and towns and is impacted by the residues. The entrance of nutrients should favor increased productivity; thus, organic matter contributions to the wetland sediments could originate from internal sources (e.g. phytoplankton, aquatic macrophytes) and external sources (e.g. sewage, sediments from farmlands and agriculture). The aim is to trace changes in organic matter deposition and the impact of local human perturbations on the wetland. We consider that the perturbations caused by local inhabitants can be estimated from the variability of nutrient input to the wetland sediments, assuming an increased contribution of nutrients to the wetland from the surrounding agriculture and towns.
2. Materials and methods
2.1 Study site
Ciénega Tamasopo is a freshwater marsh (1364 ha) located in the Neotropic in Central Mexico . Water inputs come from rainwater and springs in the upper part of the basin (twelve have been characterized). The average rainfall is 1500 mm/a with intensive precipitation from July to September. There is a main stream flowing from the upper (North) part that collects water from secondary streams and finally this stream forms the “El Trigo” river (see Figure 1). The main stream is not directly in contact with land runoff or sewage particularly at the upper part of the wetland, which has dense plant coverage. The wetland supplies water for 15 towns (~ 250 inhabitants per town) located along its margin and for sugar cane agriculture and livestock. Nearby houses do not have drainage systems and use latrines (wet or dry). The wetland is shallow (0.3 - 1.2 m depth).
During the dry season (May to June), the water depths decrease by 0.2-0.3 m Precipitation causes a significant increase in the water column level and floods the sugar cane fields and some livestock lands. The Ciénega Tamasopo wetland is surrounded by forest, mainly in the Northern part, and agricultural fields with sugarcane, towns and, to a lesser degree, livestock ranches mainly located to the South. Wetland vegetation is abundant and covers ~68 % of its surface. It is dominated by hydrophyte plants, water lilies (Nymphaea sp.), water lettuces (Pistia stratiotes), waterweeds (Elodea sp.), water hyacinth (Eichhornia crassipes), saw grass (Cladium sp.), and cattail (Typha domingensis).
Soils in the northern and highest part of the wetland range from the rendzina type (forest dominated) to litosol in the southern part of the watershed. The soils in the wetland are vertisols. From field observation at the time of sampling, the surficial sediments (~5 cm) are comprised of (dark) unconsolidated and fine-sized grain, while the deepest sediments (>10 cm) are more compacted. The sediments are mostly anoxic and there is limited oxygen availability in the underlying water (DO 0-1 mg/L; September 2012); the sulfide concentrations in porewater ranges from 2.2 to 300 μM at the sediment-water interface (0-3 cm depth), except at site S2, where the sulfides were detected in deeper sediments (>5 cm).
2.2 Sediment sampling and analyses
Some anthropogenic and major economic activities (agriculture and cattle livestock) affect the natural resources in the basin due to the extraction of water, the infiltration of wastes from agriculture, animals and wastewater, and the loss of biological diversity (flora and fauna). For this study, we have taken sediment cores at five sites in the wetland (S1, S2, S3, S4 and S5). Sites S1 and S2 are located in the (Northern) upper part of the wetland considered as the more pristine part. Site S3 may be affected by the construction of a channel that was initiated in 1995 (~12 km long and at least 1 m depth) to dry this part of the wetland and use the land for agriculture. The authorities stopped this work and the construction of the channel was not completed. Plants now cover the site affected by the construction. Site S3 was considered because it is located in the vicinity of this area. The shallowest sites at S4 (0.3 m) and S5 (0.7 m) have been the most perturbed for anthropogenic actions, since S4 is close to a secondary stream where people have cut the vegetation to access the wetland and drain water to the sugarcane fields at the right margin of the wetland (East side). At the left margin (West side), the most important influence appears to be from water extracted to supply rural towns.
Two sediment cores of ~20 cm in length were collected (gravity sampler; Wildco 2404-A14) at each of the five sampling sites (S1 to S5, Figure 1) in September 2012 in zones not covered by plants. Cores at sites S1 to S3 and S5 were collected in the main stream. Cores at site S4 were collected in a secondary stream because the main stream was densely covered by vegetation making access difficult. In situ, sediments were sliced at 0.5, 1 and 2 cm intervals for 0-5, 6-10 and >10 cm depth, respectively. Samples were preserved at 4 °C and then oven dried (60 °C, 12 h) in the laboratory. Dry sediments were acid digested to determine the total phosphorus using a colorimetric method  (molybdenum blue method; SD ± 0.2 %). Dry sub-samples (0.5 - 1.0 g) of homogenized sediment were treated with 0.1 N H2SO4 (at 60 °C, 1 h) to dissolve carbonates. The slurry was filtered, and the solid was dried (60 °C, 12 h) for C analysis. Corg (%), NT (%), δ13C (‰) and δ15N (‰) were determined at the Environmental Isotope Laboratory (University of Arizona) with a continuous flow Isotope Ratio Mass Spectrometer (IRMS; Finnigan Delta PlusXL) coupled to an Elemental Analyzer (Costech); samples were combusted in the elemental analyzer. Instrument calibration was based on acetanilide for elemental concentration (SD ± 0.1 %), NBS-22 and USGS-24 for δ13C, and IAEA-N-1 and IAEA-N-2 for δ15N. Precision was better than ± 0.1 ‰ for δ13C and ± 0.2 ‰ for δ15N (1σ) and was based on repeated internal standards. The delta values (parts per thousand; ‰) were in relation to the standard (Equation 1).
where Rx and Rs are the ratios of heavy and light isotopes (e.g., 13C/12C and 15N/14N) in the sample (Rx) and the standard (Rs).
A statistical analysis of multiple comparisons of means (Tukey test; GraphPad inStat Software Inc. v 3.06, 2003) was applied to identify significant differences in the contributions of Corg, NT, δ13C and δ15N between sites (S1 to S5; each core is an experimental unit) and at different sediment depths (0-5 cm, 6-10 cm and >10 cm). This criterion was established by the variability in the pattern of vertical profiles at different depths. The data were normalized with ANOVA during the Tukey test.
The vertical patterns of the Corg (%), NT (%), P (%) and isotopic δ13C (‰) and δ15N (‰) determined from the sediment cores from the “Ciénega Tamasopo” wetland are shown in Figure 2. From the analysis of these patterns, it is possible to highlight changes in the proxies when comparing the upper part (0-5 cm), the middle part (6-10 cm) and the lower part (>10 cm) of the sediment cores by site and between sites.
Corg in the upper part of the sediment cores (0-5 cm; Figure 2) shows concentrations at sites S1 to S3 (23.3 ± 1.44, 25.5 ± 2.23 and 30.4 ± 5.95 %, respectively) higher than the concentrations at sites S4 and S5 (15.5 ± 9.81 and 16.5 ± 4.11 %, respectively). A slight enrichment is observed at the sediment – water interface at sites S1 and S2 and especially at S3 (0-3 cm depth). The sediments in the middle part of the cores (6-10 cm) showed low variability at S1 (22.4-25 %) and S3 (24.2 %-25.1 %) and a decreasing concentration at S2 (from 27.4 to 23.4 %) and S4 (33.3 to 14.8 % with a minimum of 11.3 %). At the bottom of the sediment cores (>10 cm), the organic C concentrations were higher than those at the upper part of sediment cores, particularly at S4 (26.5 ± 15.7 %) and to a certain extent at S2 (26.9 ± 1.9 %) and S5 (20.1 ± 3.9 %).
NT shows the highest variability at S3 (1.1-2.9 %) in the upper sediments (0-5 cm) and low variability in the sediments in the middle part of the cores at S1 (1.3-1.5 %), S2 (1.6-1.8 %), S3 (1.6-1.7 % with a peak of 2.6 % at 9 cm) and S4 (0.6-1.2 %). Low variability was observed in the lower part of the cores (>10 cm) at S1 (1.4-1.6 %), S2 (1.6-2.0 %), S3 (1.3-1.9 %) and S4 (1.0-2.3 %). At S5, the NT concentrations (1.4 ± 0.2 %) were not significantly different to those in the upper sediments (0.5 cm). The highest P concentrations occurred in upper layer of the sediment (0-5 cm) at all sites (except at S3), ranging from 0.2 to 0.8 % (0-3 cm). At S3 the highest P concentrations (0.5-1.2 %) were observed in the middle part of the sediment core (6-10 cm). While, at the bottom of the cores, P was detected only at S3-S5, and the concentrations were similar to the concentrations found in the middle part of the sediment cores.
δ13C (‰) values did not show substantial changes at sites S1 to S3 (-28.4±0.2, -28.5±0.1, -28.6±0.3, respectively) contrasting with those at S4 (-27.7±1.4) and S5 (-25.8±1.3). At sites S4 and S5, δ13C varied as follows: in the upper sediments at S4 (-26.4 to -30.6 ‰) and at S5 (-26.6 to -25.2 ‰); in the middle part of the cores, at S4 values increased from -27.7 to -25.8 ‰; and at S5 the δ13C increased from -24.2 to -26.1 ‰. Finally, from the bottom up to 10 cm, δ13C varied at S4 from -29.0 to -26.7 ‰ and at S5 from -29.4 to -23-3 ‰. δ15N (‰). The values did not show substantial changes at sites S1-S3 (3.69±0.20, 3.47±0.17, 3.54±0.24, respectively) with respect to S4 (1.92±0.73) and S5 (2.36±0.36). The δ15N values from 0 to 5 cm, showed low variability at S1 (3.6-4.1 ‰), S2 (3.6-3.8 ‰) ‰), and S3 (3.7 to 3.0 ‰) compared with S4 (from 1.8 to 3.6 ‰) and S5 (from 2.5 to 3.5 ‰). In the middle part of the cores, δ15N values found at S4 (from 0.7 to 1.8 ‰, a peak of 2.2 ‰ at 8 cm) showed a sharp change between 8 and 10 cm. A similar pattern was observed at S5 (from 1.3 to 2.6 ‰). The δ15N values were lower at S4-S5 with respect to S1-S3. At the bottom (>10 cm), values for the δ15N at S4 (1.6 to 0.78 ‰) increased to 1.8 ‰. At S5, the δ15N value varied from 1.3 to 1.7 ‰. δ15N values in sediment >10 cm were lower at S4 and S5 with respect to S1-S3.
show the most significant differences (p < 0.001) in organic matter content and isotope signature. For the upper part of the sediment cores (0-5 cm) the most significant differences are observed between sites S2 - S3 vs. S4 - S5 (Corg, NT, δ13C and δ15N), S1 vs. S5 (δ13C and δ15N), S1 vs. S3 (organic C) and S1 vs. S4 (for δ15N). In the middle section (6-10 cm), significant differences are observed between S2 - S3 vs. S4 - S5 and S1 vs. S5 for δ13C and δ15N. In the deepest sediments (>10 cm) the significant differences are observed between sites S1 - S3 vs. S4 - S5 for δ15N.
sediments. To trace the contributions of the various organic matter sources, first, we compared the vertical patterns of Corg, NT, P, Corg/NT, δ13C and δ15N between sites to highlight changes or similarities in the nutrient content and isotopes in the sediments (Figure 2 and Figure 3; Table S1). Then, by comparing the δ13C, δ15N, and Corg/NT ratios measured in the sediments with values from the literature (e.g., terrestrial vegetation, emergent macrophytes, plankton, sewage, etc.), we intended to distinguish the main contributions of organic matter to the sediments.
The results of Tukey tests (Table S1 in Supplementary Information) support the similarity in the organic matter sources at S1-S3 and differences with S4-S5 in the upper part of the sediment cores. Less significant differences were found in the middle and lower parts of the sediment cores but still suggest similar organic matter sources at S1- S3 compared with S4-S5. There is no information on sedimentation rates in Ciénega Tamasopo and we assume that the differences in the nutrient profiles between sites S1-S3 vs S4-S5 and in the profiles at each site reveal the differences in the sources of organic matter deposited in the sediments. The statistical analysis showed significant differences between the sediments in the upper part of the cores (0-5 cm depth) compared to the lower part (>10 cm depth) for δ15N at S4 (p<0.01) and S5 (p<0.001), suggesting that the contribution of organic matter to the sediment column comes from different sources at these two sites; these differences were not observed at sites S1-S2.
sediments at S1-S3 suggest that an increasing productivity in these sites could occur even if the source of the nutrients has not changed due to the very low variability in the C and N isotopes profiles. In contrast, at sites S4 and S5, two possibilities arise: 1) there is a contribution of organic matter from a different source (e.g. from allochthonous materials) because the δ13C values are higher and the δ15N values are lower than the values at S1-S3; or 2) there is low mineralization of the organic matter as the δ15N values increase at both sites in the upper sediments. Low variability of δ13C and δ15N values and different pattern profiles between S1-S3 vs. S4-S5 for these isotopes suggest differences in the productivity or in the sources of organic matter deposited in the sediments.
Several processes explain the variability in the organic matter concentrations in the sediments. Enhanced aquatic productivity as a consequence of changes in the nutrient supply (from internal or external sources) is one of the considered explanations. Consequently, Corg concentration increases  and this could be the case at S1-S3. In wetlands, the residues of plants provide organic matter and the nutrients are recycled through the mineralization of those residues. In this case, the increase in organic matter arises from internal sources. In addition, high Corg content in the sediment is due to low mineralization of the organic matter during sedimentation because the microorganisms preferentially consume N from the organic matter, increasing the Corg/NT ratio , -. Corg/NT ratios are used to distinguish the origin of the sedimentary organic matter because those ratios generally survive sinking and sedimentation ; Corg/NT ratios from 4 to 10 are associated with organic matter from algal production, from 10 to 20 with aquatic/terrestrial organic matter and > 20 with terrestrial origins . Figure 3 shows the Corg/NT ratios estimated from the sediments and their relationship with δ13C or δ15N. There is low variability in the Corg/NT ratio at S1-S2 in contrast with the higher variability observed at S3-S4. In the upper part of the sediments (0-5 cm) the Corg/NT decreased to the surface (organic C and NT increased) at S1 (from 20 to 15.6) and S3 (from 23 to 15.6). Low variability was observed at S2 (~15.6; Corg, P and NT increased). At S4, the Corg/NT ratios decreased (Corg, NT and P decreased) from 20 to 13.8 and increased to 27.5 at the sediment surface. Finally, at S5, Corg/NT varied from 13.8 to 18.8.
The Corg/NT ratios suggest aquatic plants as the main source of organic matter at sites S1 – S3 and S5, the contribution of external sources at S4 and periods with increased productivity at S3 (0-10 cm), S4 (2-4.5 cm) and S5 (0-5 cm). Brenner et al.  proposed Corg/NT values for submerged and floating-leaved aquatic macrophytes (14 - 22) and for emergent vegetation (~24), and Corg/NT values for river phytoplankton (5.2–14.6) were proposed by Cunha et al. . An increase in the Corg/NT ratio is associated with changes in N concentrations due to organic matter mineralization  in which the transformation of nitrogen is dependent on the redox conditions of the sediments  or enhanced productivity . As described in Methods, sediments are mostly reduced with limited dissolved oxygen (0-1 mg/L) in the overlying water. This condition should limit nitrification processes. Variations of stable isotopes of C and N in the sediments can help to elucidate these processes.
the wetland could occur at S1, S3 and S4 (2 - 5 cm depth) where the most important P concentrations were also detected and the Corg/NT decreased. Two possibilities could explain the higher Corg/NT ratios at S4 (0-1.5 cm): low mineralization of the organic matter or recent contributions from external sources (Corg/NT ratios > 20). The vertical patterns of the Corg/NT ratios from depths of 6-10 cm at S3 (11.3-20.4) and S4 (>20) suggest changes in productivity or the contribution of external sources. At S1, S2, S5, there is low variability in the Corg/NT ratios, suggesting that the contribution of organic matter is from the same source (values varied from 15 to 19). In the lower part of the cores (>10 cm), there is a high variability in Corg/NT (from 14.4 – 26) at S4 and this suggests a contribution of organic matter from terrestrial sources (C/N >20 for terrestrial sources; , ); as indicated in Materials and Methods, this site is affected by the extraction of water for the agriculture.
The δ15N or δ13C against Corg/NT in the sediments helps to distinguish between the aquatic or terrestrial origins of the organic matter , . The plots δ13C or δ15N vs. Corg/NT (Fig. 3f-j) show that C and N contributions at S1 and S2 have the same origin, which is related to internal recycling of nutrients by the aquatic macrophytes; δ13C or δ15N indicate that the C and N sources remained the same. In contrast, from S3 onwards there is variability in the content and/or sources of N and organic C. At site S3, δ13C is constant with depth, organic C and NT increase in the recent sediments while δ15N decreases. An external contribution with nutrients is possible at this site because it is close to a deforested area with intensive sugar cane agriculture and cattle breeding. In 1995 a drain was constructed near to this site and could disturb the sediments at the shoreline, suspending the particles and/or favoring the entrance of allochtonnous matter. These perturbations affect the productivity of the wetland (δ15N decreased also the Corg/NT ratio and Corg increased; 0-5 cm depth). Routh et al.  explained internal changes in productivity through C/N ratios, δ13C and δ15N in the sediments as follows: the C/N ratio remains constant, while the values of δ13C and δ15N vary during periods of increased productivity. Low productivity periods are characterized by relatively constant compositions of δ13C and δ15N, and variable values of C/N. The latter could partially explain the productivity in the upper sediments at S3 due to the low variability of δ13C; however, this study considers that the decreasing content of δ15N and marked increase in NT concentration in the upper sediments are related to external sources.
Sites S4 and S5 show differences; both C and N isotopic signatures showed changes in the sediment cores. The Corg/NT at S4 is not constant, while this ratio showed low variability with depth at S5. At these sites, it is more complex to explain the sources of the organic matter and the process affecting the productivity in this part of the wetland. The most significant findings are related to the marked decreases in concentrations of δ13C and increases in δ15N (0-10 cm), suggesting the entrance of substances enriched in 15N (e.g., sewage, fertilizers, etc.) or changes in productivity. Increased mineralization of the organic matter due to the shallow conditions at these sites is possible since the organic C profiles show decreasing concentrations (0-10 cm). However, the reducing conditions in the sediments could prevent nitrification and thus N enrichment at these sites although NT sees slight decreases (0-10 cm).
Variations in the δ15N values have been used to explain paleoenvironmental reconstructions, even if the multiple processes involved in the N biogeochemical cycles complicate the interpretation . Application of δ15N values to distinguish organic matter sources is founded on the difference between the 15N/14N ratios of the inorganic nitrogen pools available to plants in water. Nitrate is the most common form of dissolved inorganic nitrogen (DIN) used by not-N2 fixing algae, whereas land plants receive N2 from the atmospheric N2 fixers in soil . Thus, δ15N is related to the N source and helps to investigate the source of organic matter based on the biogeochemical process affecting the δ15N records in the sediments. Increased concentrations of δ15N at the surface compared to deeper layers of soils from undisturbed forest ecosystems are related to high nitrification rates, which under humid conditions correlate with loss of N. In contrast, low 15N abundance indicates N limitation and a low nitrification rate . As explained previously, nitrification is likely to be limited due to the reducing conditions of the sediments.
External nitrate loading from agricultural runoff and sewage as nitrate derived from human and animal wastes is enriched in 15N. Denitrification in anoxic basins will considerably enrich the residual DIN in 15N. Both will increase δ15N. An increase in abundance of N-fixing cyanobacteria, which directly fix atmospheric N2 (δ15Nair = 0 ‰), would decrease δ15N in the organic matter . The results observed at sites S4 and S5 are probably more related to the contribution of external sources enriched in 15N or denitrification processes promoted by the reduced conditions in the sediments that lead to loss of N.
There is a low variability in δ13C with depth at stations S1-S3, contrasting with S4 and S5 (Figure 2). The statistical analysis also revealed differences between sites (Table S1). Lower values were found in the sediments at S1-S3 (~ -28 ‰) and in the upper part of the core at S4 (~ -30 ‰), and the highest values were found at S4 (~ -26 ‰; 5-7 cm) and S5 (-23.3 ‰; 16 cm), suggesting differences in the organic matter contributions to the sediments between S1-S3 vs. S4-S5. The dominant vegetation in the wetland is comprised of the C3 plants Typha domingensis, Cladium sp., and Nymphaea sp., and the δ13C values of these are -24, -28 and -25.2 ‰ (root), respectively , . River phytoplankton has more negative values than aquatic plants (-35 ‰ to -25 ‰) . The δ13C values correspond to the preferential C source taken by aquatic plants and phytoplankton or the available source. For example, organic matter from phytoplankton and watershed C3 plants in lakes can be indistinguishable if they use an identically dissolved C source, but δ13C is different if the organic matter sources are C4 plants , .
Based on the above and due to the lack of measured δ13C in the organic matter sources possibly contributing to the sediments, in this study we compared our results with the values from the literature for the aquatic plants in the wetland and terrestrial plants. The values considered are δ13C for aquatic C3 plants (-32 to -24 ‰) and for terrestrial C4 plants (-17.0 to -9.0 ‰) , ; sewage (δ13C -23(± 2.5) ‰)  and phytoplankton (-25 ‰ to -35 ‰)  to distinguish the main sources contributing to the organic matter deposited in the sediments in the wetland (Figure 4).
to those reported for aquatic macrophytes (C3 plants). Sugarcane plants (C4; δ13C -10.5 ‰, δ15N 4.4 ‰) do not explain the organic matter sources in the sediment of the wetland, suggesting that the sugarcane agriculture may contribute through fertilizers entering the wetland due to land runoff (δ15N of synthetic fertilizers -3 to 3 ‰) ; δ15N is 5 ‰ for phytoplankton . This is reasonable because the sugarcane production is processed far from the agricultural land. In addition, the results obtained were similar to the δ13C values in sediment from wetlands in Florida . However, other researchers attribute the origin of the organic matter in the sediment to the aquatic vegetation and suggest that changes in wetland hydrology and water chemistry affect the structure of the hydrophyte vegetation .
The results of this study suggest that the spatial variability in the organic matter content is described mainly by the internal recycling of nutrients at S1-S3 with some impact from external sources (terrestrial plants, fertilizers from agriculture) that can affect the upper sediments at S3, S4 and S5 as can be deduced from the variability in δ13C and δ15N. Anthropogenic perturbations cause changes in the contribution of the OM at S3 (0-5 cm) due to increased productivity, as shown by the δ15N, organic C and Nitrogen concentrations. Therefore, the main contributions of organic matter in the wetland come from the abundant vegetation covering 68 % of its area. The preservation of the wetland implies regulation of water extraction from the area, improvement of the irrigation systems, to consider an alternative crop to the sugar cane or re-evaluate the land use through provision of ecosystem services. Surely, such actions can affect the current socioeconomic organization of villagers and could complicate the preservation of the wetland.
This work was carried out with financial support from CONACYT project 90228 and the PROMEP-RED for the UASLP-CA-37. FVPC was awarded a graduate fellowship from CONACYT No 290674. The authors gratefully for the support from the Tamasopo municipality (Eladio Ruiz Sánchez), the Ejido Cabezas, and the field work support of Claudio M. Padilla González, Isidro Montes Ávila, Fortunato and Virgilio Landaverde.
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