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1.
Organic carbon (C) and total nitrogen (N) contents and corresponding isotope ratios were determined in surficial sediment (0–3 cm) at 94 stations ranging from 21 to 1995 m water depth off Tokachi, Hokkaido, Japan, to elucidate the distribution and source of sedimentary organic matter. Suspended particulate organic matter (POM) in the seawater and suspended POM and sediment in the Tokachi River were also examined. δ13C, δ15N and C / N ratios of the samples in the Tokachi River suggest that the spring snowmelt is an important process for the transport of terrestrial organic matter to the coastal waters. δ13C values of suspended POM in the surface seawater were higher in May and November than in August, while δ15N values of the POM were higher in May and August than in November. These changes are attributed to seasonal changes in phytoplankton growth rate and nitrate availability. δ13C and δ15N values in the sediments off Tokachi were lowest near the Tokachi River mouth, and increased offshore to constant values that persisted from 134 to 1995 m water depth. The spatial variation in C / N ratios in the sediment mirrored those of δ13C and δ15N. Comparison of δ13C, δ15N and C / N ratios in the sediments off Tokachi with those in the Tokachi River and seawater indicates that about half of the organic matter in the sediment was of terrestrial origin near the Tokachi River mouth, and the sedimentary organic matter from 134 to 1995 m water depth was of marine origin. The organic C content in the sediment was high near the Tokachi River mouth, and also around 1000 m water depth. The C content was significantly correlated with silt plus clay content, with different regression lines for those stations shallower and deeper than 134 m, owing to several stations of higher C content with the elevated C / N ratio on the inner shelf. These results suggest that transport and deposition of organic-rich fine sediment particles by hydrodynamic processes were major factors controlling C content off Tokachi. In addition, the supply of a fraction of terrestrial organic matter with high C / N probably also affected C content on the inner shelf.  相似文献   

2.
In order to characterize the sources and fate of organic matter (OM) in the Pichavaram estuarine-mangrove ecosystem (east coast of India), stable isotope (δ13C and δ15N) ratios and molecular lignin analyses were conducted in plant litter, benthic algae, sediment, particulate matter and in a variety of benthic invertebrate species. The δ13C signature of plant litter ranges from −29.75‰ to −27.64‰ suggesting that mangrove trees follow the C3 photosynthetic pathway. Sedimentary δ13C 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 δ15N 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 δ15N signature in mangrove plant litter. Two (lower and higher) trophic levels of invertebrates were identified with and observed >4‰ gradient in δ13C signal between these two trophic groups. The observed δ13C 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%.  相似文献   

3.
Carbon and nitrogen stable isotope ratios of particulate organic matter (POM) were studied approximately weekly during spring and summer 2003 and 2004 in the Gulf of Trieste (northern Adriatic Sea) in order to track the temporal variations and differences between two years. In parallel, particulate organic carbon (POC) and particulate nitrogen (PN), phytoplankton biomass (chlorophyll a), and N and P nutrients were monitored. All studied parameters, especially N and P nutrients and chlorophyll a, showed higher concentrations and larger variability in spring 2004. As a consequence the macroaggregates were produced in late spring 2004. The C and N isotope composition of POM was not directly linked to phytoplankton biomass dynamics. The δ13CPOC values covaried with temperature. In 2004, δ13CPOC variations followed the δ15NPN values as well as the δ13CDIC values which were probably more dependent on the photosynthetic use of 12C. Variations in δ15NPOM values were most probably the consequence of variations in N nutrient sources used in phytoplankton assimilation. The significant correlation between δ15NPN values and nitrate concentrations in 2004 implies intense nitrate assimilation in the presence of higher nitrate concentration. This suggests nitrate as the key nutrient in the »new primary production«, later producing macroaggregates with a mean δ13C and δ15N values of − 19‰ and 5‰, respectively. A low fractionation factor ε, < 1‰, lower than that reported in other marine and lacustrine systems, was found probably to be a consequence of distinct phytoplankton species, i.e. several classes of autotrophic nanoflagellates, and specific growth conditions present in the Gulf of Trieste. The tentative use of C isotope composition of POM revealed a higher contribution of allochthonous organic matter in 2004 compared to 2003 due to higher riverine inflow.  相似文献   

4.
The 87Sr, 13C, and 15N isotopic signatures of organic matter in sediments from the continental shelf facing the Orinoco Delta were measured to determine the contribution of sediments transported from the Amazon River by the coastal Guayana current and the sediments transported by the Orinoco River. Box core samples between 60 and 300 m water depth collected along 4 transects located eastwards to the Orinoco Delta were analyzed. Carbon and nitrogen concentrations decreased with depth under water on the shelf, and were strongly correlated indicating homogeneity of organic matter composition. Phosphorus content was also associated to organic matter in most samples, but some of them revealed deposition of P-enriched sediments. The 87Sr/86Sr ratios showed a strong continental signature averaging 0.7117, therefore, limiting the possible carbon and nitrogen sources associated with these sediments to C3 trees, C4 grasses, or freshwater phytoplankton. The δ13C values were relatively high averaging −21‰, above values reported for sediments on the Amapá shelf and the Amazon River in Brazil. Average δ13C values did not differ significantly among transects. High δ13C values point to the influence of organic matter transported from the C4-plants dominated savannas in the northern fringe of the Orinoco River. δ15N values were positive and averaged 5‰, being within the range of values measured in the Marajo island (Amazon River) and the estuary of the Pará River. The δ15N values differed significantly among transects (4.9–5.2‰), lowest values corresponding to the northernmost transect near the coast of Trinidad, and the highest values corresponding to the transect located at the southernmost position.  相似文献   

5.
The contribution of detritus from seagrass and other primary producers to faunal production in unvegetated nearshore areas was examined primarily using stable isotopes. Fish, macroinvertebrates, meiofauna and primary producers (seagrasses, macroalgae, seston and benthic microalgae) were sampled from sites in south-western Australia. All samples were analysed for δ13C and δ15N values and fish gut contents were determined. δ13C values for seagrasses in the region were high compared to other macrophytes, ranging from 49.9 to −8.2‰ compared to −19.8 to −12.6‰ for macroalgae. The δ15N values ranged between 4.0 and 7.7‰ for the red, brown and green algae, and between 3.2 and 5.9‰ for seagrasses. Seston and benthic microalgae samples had a mean δ13C value of −12.8 and −14.0‰, respectively, and their δ15N values were comparable to the macroalgae. All invertebrate fauna had mean δ13C values considerably lower than seagrasses. However, individual samples harpacticoid copepods and polychaetes had a value as high as −11.7‰. δ15N values for consumers were higher than those of the primary producers, except for copepods and amphipods. The δ13C values for fish had a relatively small range, between −16.6 and −13.1‰, and the δ15N values of fish were elevated compared to the invertebrates and primary producers, ranging mostly between 10.0 and 12.6‰. Mixing model analysis based on δ13C values indicated that seagrass ranked low as a likely carbon source for all invertebrates other than harpacticoid copepods at a single site and some samples of polychaetes. The δ13C values for fish were similar to those of a combination of harpacticoid and calanoid copepods, amphipods and polychaetes. The consumption of harpacticoid copepods by some fish species indicates that Amphibolis and Posidonia species in south-western Australia can contribute to the food web of unvegetated nearshore areas as detritus, but brown algae is likely to make a greater contribution. At least for the time of year that was sampled, the flow of detrital seagrass material into the foodweb may be mediated by specific detrivores, in this case harpactacoid copepods, rather than by all detritivores.  相似文献   

6.
Extensive artificial waterways have replaced natural wetlands and created new estuarine habitats on the southern Queensland coast, Australia. Economically important fish species found in adjacent natural wetlands of mangrove, saltmarsh and seagrass also occur in the artificial waterways. Stable isotope analyses (δ13C, δ15N) were used to test whether the relative importance of basal sources of energy varied for foodwebs found in artificial (canals and tidal lakes) and natural waterways. None of the fish species differed in their isotope values between artificial waterways. In contrast, isotopic signatures of snub-nosed garfish (Arrhamphus sclerolepis; Hemiramphidae) varied greatly between natural and artificial waterways, having highly enriched δ13C values (−10.5‰) in natural wetlands, demonstrating reliance on seagrass (−11.4‰), and significantly less enriched values (−19.0‰) in artificial waterways, consistent with either local algal sources (−19.8 to −20.4‰) or a mixture of seagrass and other less enriched autotrophs from adjacent natural wetlands. Isotopic signatures of sand whiting (Sillago ciliata; Sillaginidae) were also significantly more enriched in natural (−18.2‰) than artificial (−21.0‰) habitats, but means were not far enough apart to distinguish between different sources of nutrition. δ13C values of yellowfin bream (Acanthopagrus australis; Sparidae) did not differ between artificial and natural habitats (about −20‰ in both). δ15N values of fish varied among habitats only for A. sclerolepis, which in artificial waterways had values enriched by 2‰ over those in natural waterways. This was consistent with a shift from seagrass (relatively depleted δ15N) as a source in natural habitat to algal sources (relatively enriched δ15N) in artificial habitats. This study provides some of the first evidence that at least some fish species rely on different autotrophs in artificial waterways than in adjacent natural wetlands.  相似文献   

7.
In Fiordland, New Zealand, large volumes of organic matter are deposited into the marine environment from pristine forested catchments. Analyses of δ15N, δ13C and δ34S were employed to determine whether these inputs were contributing to marine food webs via assimilation by common macroinvertebrates inhabiting the inner reaches of the fjords. Terrestrially derived organic matter (TOM) had values of δ15N, δ13C and δ34S that were distinct from other carbon source pools, providing sufficient power to quantify the contribution of TOM to the benthic food web. Isotopic values among macroinvertebrates varied significantly, with consistently low values of δ15N, δ13C and δ34S for the abundant deposit feeders Echinocardium cordatum (Echinodermata) and Pectinaria australis (Annelida), indicating assimilation of TOM. High concentrations of bacterial fatty acid biomarkers in E. cordatum, and values of δ13C of these biomarkers similar to TOM (−27 to −30‰) confirmed that TOM is indirectly assimilated by these sea urchins via heterotrophic bacteria. TOM was also found to enter the infaunal food web via chemoautotrophic bacteria that live symbiotically within Solemya parkinsonii (Bivalvia). Echinocardium cordatum, Pectinaria australis and S. parkinsonii comprised up to 33.5% of the biomass of the macroinfaunal community, and thus represent strong pathways for movement of organic matter from the forested catchments into the benthic food web. This demonstration of connectivity among adjacent marine and terrestrial habitats has important implications for coastal land management, and highlights the importance of intact coastal forests to marine ecosystem function.  相似文献   

8.
Carbon and nitrogen isotopic composition was used to identify the main sources of carbon and describe the main trophic pathways in Deluge Inlet, a near-pristine mangrove estuary in tropical north Queensland, Australia. Producers' δ13C varied from −28.9‰ for mangroves to −18.6‰ for seagrass. Animals were also well separated in δ13C (−25.4‰ to −16.3‰ for invertebrates and −25.2‰ to −17.2‰ for fish), suggesting considerable differences in ultimate sources of carbon, from a substantial reliance on mangrove carbon to an almost exclusive reliance on seagrass. In general, invertebrates had lower δ15N than fish, indicating lower trophic levels. Among fish, δ15N values reflected well the assumed trophic levels, as species from lower trophic levels had lower δ15N than species from higher trophic levels. Trophic levels and trophic length were estimated based on δ15N of invertebrate primary consumers (6.1‰), with results suggesting a food web with four trophic levels. There was also evidence of a high level of diet overlap between fish species, as indicated by similarities in δ13C for fish species of higher trophic levels. Stable isotope data was also useful to construct a general model for this food web, where five main trophic pathways were identified: one based on both mangrove and microphytobenthos, one on plankton, two on both microphytobenthos and seagrass, and one based mainly on seagrass. This model again suggested the presence of four trophic levels, in agreement with the value calculated based on the difference in δ15N between invertebrate primary consumers and top piscivores.  相似文献   

9.
The vertical flux of particulate matter at 330 m depth in San Lázaro Basin off southern Baja California ranged from 63 to 587 mg m−2 d−1 between August and November 1996. Organic carbon contents were between 5.6 and 14.8%, yielding flux rates of 9–40 mgC m−2 d−1. In December 1997 and January 1998, at the height of the strong El Niño event, the respective fluxes (47–202 mg m−2 d−1 and 3–8 mgC m−2 d−1) were comparable. The February–June 1998 records, however, revealed sharply reduced mass (1–6 mg m−2 d−1) and organic carbon (0.2–0.8 mgC m−2 d−1) fluxes. The organics collected in 1996 were predominantly autochthonous (δ13C=−22‰; C/N=8). The variations in δ15N (8.3–11.0‰) suggest an alternation of new and regenerated production, possibly associated with fluctuations in the intensity of deep mixing during that autumn. The relatively high organic matter fluxes in December 1997 appear to be associated with regenerated production. The average composition from February to June 1998 (δ13C=−23.6‰; 15N=11.7‰; C/N=10.5) indicates degraded material of marine origin. The maximum δ15N value found (14‰) suggests that deeper, denitrified waters were brought to the surface and possibly advected laterally. Regime changes in the waters of the basin occur at 6–10 week intervals, evidenced by concurrent shifts in most of the measured parameters, including fecal pellet types and metal chemistry. The marine snow-dominated detritus collected showed a shift from a mixed diatom-rich-radiolarian-coccolith assemblage in late 1996 to a coccolith-dominated assemblage, including the contents of fecal pellets, during the 1997–1998 El-Niño period. T–S profiles, plankton analysis and chlorophyll contents of the upper water column indicated that the strong phytoplankton bloom, normally associated with seasonal upwelling along the Pacific coast of Baja, did not occur during the spring of 1998. The persistence of oligotrophic conditions during the 1997–1998 El Niño event favored the dominance of nanoplankton and reduced the vertical flux of particles.  相似文献   

10.
Elemental (TOC, TN, C/N) and stable carbon isotopic (δ13C) compositions and n-alkane (nC16–38) concentrations were measured for Spartina alterniflora, a C4 marsh grass, Typha latifolia, a C3 marsh grass, and three sediment cores collected from middle and upper estuarine sites from the Plum Island salt marshes. Our results indicated that the organic matter preserved in the sediments was highly affected by the marsh plants that dominated the sampling sites. δ13C values of organic matter preserved in the upper fresh water site sediment were more negative (−23.0±0.3‰) as affected by the C3 plants than the values of organic matter preserved in the sediments of middle (−18.9±0.8‰) and mud flat sites (−19.4±0.1‰) as influenced mainly by the C4 marsh plants. The distribution of n-alkanes measured in all sediments showed similar patterns as those determined in the marsh grasses S. alterniflora and T. latifolia, and nC21 to nC33 long-chain n-alkanes were the major compounds determined in all sediment samples. The strong odd-to-even carbon numbered n-alkane predominance was found in all three sediments and nC29 was the most abundant homologue in all samples measured. Both δ13C compositions of organic matter and n-alkane distributions in these sediments indicate that the marsh plants could contribute significant amount of organic matter preserved in Plum Island salt marsh sediments. This suggests that salt marshes play an important role in the cycling of nutrients and organic carbon in the estuary and adjacent coastal waters.  相似文献   

11.
Hydrodynamic processes sort and redistribute organic matter (OM) and minerals on continental margins. Density fractionations were conducted on sediments from diverse margins (Mexico margin, Gulf of Mexico, Mississippi River delta, Eel River margin) to investigate the nature, provenance and age of OM among density fractions. Mass, elemental (C and N), lignin, and surface area distributions, as well as stable carbon and radiocarbon isotopic compositions were measured. The lowest density fractions (< 1.6 g cm− 3) contained the highest organic carbon (OC) (up to 45%) and lignin concentrations (up to 8 mg g− 1) due to abundant woody debris, whereas high density fractions (> 2.5 g cm− 3) were OC-poor (%OC < 0.5) mineral material. Most sediment mass was found in the mesodensity fractions (1.6 to 2.5 g cm− 3) that contained the highest proportion of OC (up to ~ 75%) for each sediment. Stable carbon isotope compositions (δ13C − 25.5‰ to − 22.9‰) show terrigenous OC as a significant component of density isolates from the river-dominated sediments (Gulf of Mexico, Mississippi River, and Eel margin), whereas the Mexico margin, least influenced by riverine input, was dominated by autochthonous marine OC (δ13C ~ − 21.5‰). Radiocarbon compositions of density fractions indicate significant pre-aged OC (Δ14C as low as − 900‰) in river-influenced sediments but not on the Mexico margin (Δ14C > − 200‰). Ratios of vanillic acid to vanillin (Ad/Al)v among lignin oxidation products increase with increasing particle density suggesting variable lignin sources or selective degradation of lignin among the different density fractions.  相似文献   

12.
Dissolved organic carbon (DOC), stable carbon isotopic (δ13C) compositions of DOC and particulate organic carbon (POC), and elemental C/N ratios of POC were measured for samples collected from the lower Mississippi and Atchafalaya rivers and adjacent coastal waters in the northern Gulf of Mexico during the low flow season in June 2000 and high flow season in April 2001. These isotopic and C/N results combined with DOC measurements were used to assess the sources and transport of terrestrial organic matter from the Mississippi and Atchafalaya rivers to the coastal region in the northern Gulf of Mexico. δ13C values of both POC (−23.8‰ to −26.8‰) and DOC (−25.0‰ to −29.0‰) carried by the two rivers were more depleted than the values measured for the samples collected in the offshore waters. Strong seasonal variations in δ13C distributions were observed for both POC and DOC in the surface waters of the region. Fresh water discharge and horizontal mixing played important roles in the distribution and transport of terrestrial POC and DOC offshore. Our results indicate that both POC and DOC exhibited non-conservative behavior during the mixing especially in the mid-salinity range. Based on a simple two end-member mixing model, the comparison of the measured DOC-δ13C with the calculated conservative isotopic mixing curve indicated that there was a significant in situ production of marine-derived DOC in the mid- to high-salinity waters consistent with our in situ chlorophyll-a measurements. Our DOC-δ13C data suggest that a removal of terrestrial DOC mainly occurred in the high-salinity (>25) waters during the mixing. Our study indicates that the mid- to high- (10–30) salinity range was the most dynamic zone for organic carbon transport and cycling in the Mississippi River estuary. Variability in isotopic and elemental compositions along with variability in DOC and POC concentrations suggest that autochthonous production, bacterial utilization, and photo-oxidation could all play important roles in regulating and removing terrestrial DOC in the northern Gulf of Mexico and further study of these individual processes is warranted.  相似文献   

13.
The δ13C and δ15N of particulate organic matter (POM) sampled from the Weddell Sea in 1986 and 1988 ranged from −30.4 to − 16.7%o and from −5.4 to +41.3%o, respectively. These large variations in POM δ13C and δ15N may reflect spatial/temporal changes in the concentrations and isotope abundances of CO2(aq.) and NH4+, respectively. Elevated isotope values were found exclusively in POM in or closely associated with sea ice, which may be the source of the 13C- and 15N-enriched sediments observed in this region.  相似文献   

14.
We used stable C and N isotope ratios of tissues from 29 fish species from a large subtropical lagoon in southern Brazil to examine spatial variability in isotopic composition and vertical trophic structure across freshwater and estuarine habitats. Nitrogen isotope ratios indicated a smooth gradation in trophic positions among species, with most fishes occupying the secondary and tertiary consumer level. Fish assemblages showed a significant shift in their carbon isotopic signatures between freshwater and estuarine sites. Depleted carbon signatures (from −24.7‰ to −17.8‰) were found in freshwater, whereas more enriched signatures (from −19.1‰ to −12.3‰) were obtained within the estuarine zone downstream. Based on our survey of the C3 and C4 plants and isotopic values for phytoplankton and benthic microalgae reported for ecosystems elsewhere, we hypothesized that the observed δ13C differences in the fish assemblage between freshwater and estuarine sites is due to a shift from assimilating organic matter ultimately derived from C3 freshwater marsh vegetation and phytoplankton at the freshwater site (δ13C ranging from −25‰ to −19‰), to C4 salt-marsh (e.g. Spartina) and widgeon grass (Ruppia maritima), benthic microalgae and marine phytoplankton at the estuarine sites (from −18‰ to −12‰). Our results suggested that fish assemblages are generally supported by autochthonous primary production. Freshwater fishes that likely were displaced downstream into the estuary during periods of high freshwater discharge had depleted δ13C values that were characteristic of the upper lagoon. These results suggest that spatial foodweb subsidies can occur within the lagoon.  相似文献   

15.
The reaction pathways of nitrogen and carbon in the Framvaren Fjord (Norway) were studied through stable isotope analysis (δ15N and δ13C) of dissolved inorganic and particulate organic matter (POM). The variations in the isotopic compositions of the various C and N pools within the water column were use to evaluate the historical deposition of material to the sediments. The high δ15N-NH4+ at the O2/H2S interface, as a consequence of microbial uptake between 19 and 25 m, results in extremely depleted δ15N-particulate nitrogen (PN) of approximately 1‰ within the particulate maximum at approximately 19 m. The carbon isotopic distribution of dissolved inorganic carbon (DIC) and particulate organic carbon (POC) within the interface suggests that the distinct microbial flora (Chromatium sp. and Chlorobium sp.) fractionate inorganic carbon to different degrees. The extremely light δ13C-POC within the interface (−31‰) appears to be a result of carbon uptake by Chromatium sp. while δ13C-POC of −12‰ is more indicative of Chlorobium sp. Nitrogen isotopic mass balance calculations suggested that approximately 75% of the material sinking to the sediments was derived from the dense particulate maximum between 19 and 25 m. The sediment distribution of nitrogen isotopes varied from 2‰ at the surface to approximately 6‰ at 30 cm. The nitrogen isotopic variations with depth may be an indicator of the depth or position of the O2/H2S interface in the fjord. Low sediment δ15N indicated that the interface was within the photic zone of the water column, while more enriched values suggested that the interface was lower in the water column potentially allowing for less fractionation during biological incorporation of dissolved inorganic nitrogen. Results indicate that the dense layers of photo-autotrophic bacteria in the upper water column impart unique carbon and nitrogen isotopic signals that help follow processes within the water column and deposition to the sediments.  相似文献   

16.
Fauna species living in seagrass meadows depend on different food sources, with seagrasses often being marginally important for higher trophic levels. To determine the food web of a mixed-species tropical seagrass meadow in Sulawesi, Indonesia, we analyzed the stable isotope (δ13C and δ15N) signatures of primary producers, particulate organic matter (POM) and fauna species. In addition invertebrates, both infauna and macrobenthic, and fish densities were examined to identify the important species in the meadow. The aims of this study were to identify the main food sources of fauna species by comparing isotopic signatures of different primary producers and fauna, and to estimate qualitatively the importance of seagrass material in the food web. Phytoplankton and water column POM were the most depleted primary food sources for δ13C (range −23.1 to −19.6‰), but no fauna species depended only on these sources for carbon. Epiphytes and Sargassum sp. had intermediate δ13C values (−14.2 to −11.9‰). Sea urchins, gastropods and certain fish species were the main species assimilating this material. Seagrasses and sedimentary POM had the least depleted values (−11.5 to −5.7‰). Between the five seagrass species significant differences in δ13C were measured. The small species Halophila ovalis and Halodule uninervis were most depleted, the largest species Enhalus acoroides was least depleted, while Thalassia hemprichii and Cymodocea rotundata had intermediate values. Fourteen fauna species, accounting for 10% of the total fauna density, were shown to assimilate predominantly (>50%) seagrass material, either directly or indirectly by feeding on seagrass consumers. These species ranged from amphipods up to the benthic top predator Taeniura lymma. Besides these species, about half of the 55 fauna species analyzed had δ13C values higher than the least depleted non-seagrass source, indicating they depended at least partly for their food on seagrass material. This study shows that seagrass material is consumed by a large number of fauna species and is important for a large portion of the food web in tropical seagrass meadows.  相似文献   

17.
We determined 15N/14N ratios of total nitrogen in surface sediments and dated sediment cores to reconstruct the history of N-loading of the North Sea. The isotopic N composition in modern surface sediments is equivalent to and reflects the isotopic mixture of oceanic nitrate on the one hand (δ15N = 5‰) and the imprint of river-borne nitrogen input into the SE North Sea (δ15N up to 12‰ in estuaries of the SE North Sea) on the other hand. We compare the results with δ15N records from pre-industrial sediment intervals in cores from the Skagerrak and Kattegat areas, which both constitute significant depositional centres for N in the North Sea and the Baltic Sea/North Sea transition. As expected, isotopically enriched anthropogenic nitrogen was found in the two records from the Kattegat area, which is close to eutrophication sources on land. Enrichment of δ15N in cores from the Skagerrak – the largest sediment sink for nitrogen in the entire North Sea – was not significant and values were similar to those found in sediment layers representing pre-industrial conditions. We interpret this isotopic uniformity as an indication that most riverine reactive nitrogen with its characteristic isotopic signature is removed by denitrification in shallow shallow-water sediments before reaching the main sedimentary basin of the North Sea.  相似文献   

18.
The biogeochemistry of particulate organic matter was studied in the Great Ouse estuary draining to the North Sea embayement known as the Wash from March 1990 to January 1991. Eleven locations were sampled monthly on a 50 km transect across the shallow estuary from the tidal weir to the middle of the Wash. Particulate organic carbon (POC) and total carbohydrate, protein and lipid analyses were combined with the determination of stable carbon isotopes. δ13C often increased from −30‰ in the river to −22‰ in the tidal freshwater reach. The mixing zone between fresh and marine tidal waters displayed only a slight increase in δ13C to −19‰. The change in δ13C values in the freshwater tidal reach demonstrated that mixing of riverborne and marine suspended POC was not the only process affecting the carbon stable isotope composition. Complementary sources, interfering considerably with the two end-member sources, may be identified as autocthonous primary production and resuspension of sediment that may be transported upstream. The respective importance of these sources is subject to seasonal variation. From March to August, high concentrations in carbohydrate and protein through the whole estuary indicate that despite turbidity significant primary production occurred. The proportional importance of the uncharacterized fraction of POC, which is considered as complex organic matter, was high from September to January and low from March to August. During most of the year, the biochemical compositions of particulate organic matter in the turbidity maximum and the rest of the estuary were similar. This contradicted the principle that owing to the long residence times of particles degradation processes largely dominate the production processes within the turbidity maximum. The occurence of significant in situ production in such shallow water estuaries may partially compensate for the degradation of suspended particulate organics, resulting in a complex relationship between the biogeochemical cycling and the fate of nutrients.  相似文献   

19.
The stable isotopes of organic carbon (OC) and nitrogen, contents of OC and nitrogen for roots, bark, leaves, flowers and fruits of various mangrove species from Kisakasaka (Zanzibar) and Bagamoyo (mainland Tanzania) are used to assess (1) if some mangrove species are capable of fixing atmospheric nitrogen, (2) if there are differences between species in the same stand and in different stands and (3) if the mangrove signature is preserved in the sediments. Mean OC stable isotope results of various plant components range from −25.9‰ to −29.1‰ suggesting that mangrove trees in the two stands follow the C3 type of photosynthetic pathway. Mean nitrogen isotope values for various plant components range from −1.5‰ to 3.2‰ suggesting atmospheric nitrogen fixation by mangrove plants, but δ15N values approaching −3‰ that are more negative than typical diazotroph biomass exclude this possibility. Mangrove species thriving further inland are enriched in 13C and 15N relative to those thriving near the shoreline owing to complete utilization of available nutrients.Sediments beneath the mangrove forest are characterized by lower C/N ratio values and enrichment in 13C and 15N relative to plant material owing to mixing of nitrogenous rich material from adjacent area. High concentration of OC is found in bark and roots, while high nitrogen concentration is found in fruits and flowers only.  相似文献   

20.
We examined stable carbon and nitrogen isotopic signatures of 17 fish and 16 invertebrate taxa common to the Newfoundland and Labrador (NL) continental shelf food web. Particular sampling emphasis was placed on Atlantic cod (Gadus morhua) and related prey species (e.g. shrimp, Pandalus borealis, and capelin, Mallotus villosus). We found highly significant (p < 0.0001) differences between near-shore (bays) and offshore (shelf edge) δ15N signatures for cod, ‘other fish’ (pooled) and invertebrates (pooled). In contrast, there were only minor differences in δ13C signatures of ‘other fish’ (p < 0.05) and no difference for cod and invertebrates among the two habitats. We sampled at two times of the year (January and June) and found no systematic effect of season on both δ13C and δ15N in cod, ‘other fish’ and invertebrates. We calculated isotopic fractionation factors for cod from the entire shelf (mixed diet) and for cod with diets composed mainly of capelin or shrimp. These values ranged between 2.2‰ and 3.9‰ for δ15N and −0.4‰ and 0.8‰ for δ13C and, for δ15N, may reflect diet-related differences in bioenergetic status. We discuss potential mechanisms for near-shore versus offshore enrichment of δ15N signatures, and demonstrate the implications of this spatial variation on δ15N-derived trophic position estimates.  相似文献   

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