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1.
The gastropod Terebralia palustris often dominates the surface of muddy to sandy substrates of intertidal mudflats and mangrove forests, where they clearly destabilize the sediment. In the present study, it was investigated whether and to what extent the behaviour of juvenile and adult snails differs among habitats (mudflat vs. mangrove stand) in a Sonneratia alba mangal at Gazi Bay, Kenya. For this purpose we: (1) examined their distribution along three land–sea transects; and (2) applied stable isotope analysis to determine the feeding patterns of different-sized snails from the mangrove and mudflat habitats. Additionally, we investigated if these gastropods exert an impact on microphytobenthic (diatom) biomass, and whether this is size-dependent. The latter objective was met by either enclosing or excluding different-sized snails from experimental cages on the intertidal mudflat and the subsequent assessment of a change in pigment concentration of the sediment surface. In agreement with several previous studies conducted in other mangroves and geographical locations, a spatial segregation was demonstrated between juveniles (more common on the mudflat) and adults (more common in the mangrove forest). On the intertidal mudflat juveniles avoided sediment patches characterized by highly saline water in intertidal pools and a high mud content, while adults tended to dwell on substrates covered by a high amount of leaf litter. Stable carbon isotope analysis of the foot tissue of snails sampled from the S. alba stand and the mudflat indicated a transition in food source when a shell length of 51 mm is reached. Considering the δ13C value of juveniles, it seems they might be selecting for microphytobenthos, which might explain their preference for the mudflat. The diet of size classes found in both habitats did not differ significantly, although juveniles inhabiting the mangrove forest were slightly more depleted in 13C compared to those residing on the mudflat. Assuming juveniles feed on benthic microalgae and considering the lower microalgal biomass inside the mangrove forest, this may be a consequence of a higher contribution of other, more 13C depleted organic carbon sources, like phytoplankton, to their diet. Experimental results indicate a negative, but insignificant, impact on benthic diatom biomass by juveniles (due to grazing) and adults (due to physical disturbance). This finding seems to be in agreement with the results of the stable carbon isotope analysis, strongly suggesting the selective feeding of juvenile T. palustris on benthic diatoms.  相似文献   

2.
In situ degradation rates of feeder roots, pneumatophores, and leaves of the New Zealand mangrove Avicennia marina (Forsk) Vierh. var. resinifera (Forst. f) Bakh. are described. At the sediment‐water interface, leaves degrade faster than roots, which degrade faster than pneumatophores, whereas roots and pneumatophores immersed in sediment degrade at the same rate. However, roots and pneumatophores degrade faster at the sediment‐water interface than in sediment. Added nitrogen accelerates feeder root and pneumatophore degradation in sediment; added phosphorus has little influence on pneumatophore degradation but inhibits root degradation in sediment.  相似文献   

3.
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   

4.
Terebralia palustris is a common mud-whelk present at a particularly high density in all Indo-West Pacific mangroves. Young snails feed on nothing but mud while larger specimens are able to feed on fallen leaves too. In Kenya (Mida Creek) under the canopy, competition for mangrove leaves can be very high due to the high density of Sesarmidae crabs. On open exposed muddy platforms, no Sesarmidae occur but the leaf density is very low because the leaves are only randomly present as they are deposited and removed twice a day by the tide. However, the snail density is always very high, raising the question as to whether the snails use a special searching strategy to optimize their resource finding rather than a purely random movement. By analyzing the snails' movements on a uniform area at different levels and comparing them with simulated random paths, we could show that the snails' movements are not purely random. The distribution of different size classes of T. palustris in Mida Creek was known to be quite odd: the same simulation approach suggests that the zonation asymmetry could reasonably be due to the stochastic recruitment of juveniles in space and time and maintained by a substantial long-lasting spatial inertia.  相似文献   

5.
The snail, Littoraria scabra, is a dominant grazer on tropical mangrove trees, and may play an important role in the food web dynamics of these ecosystems. Its daily vertical migration to avoid tidal submersion results in exposure to varying food types and abundances. A comprehensive diet analysis – gut contents, fatty acid profiles, and stable isotopes (δ15N and δ13C) – was conducted on snails migrating along mangrove trees and snails maintained in non-tidal mesocosms at Nananu-i-ra, Fiji Islands. In addition, fatty acid profiles and stable isotope signatures were obtained from surface scrapings of mangrove roots, trunks, branches, and leaves. Results from this multi-technique study indicate that L. scabra is mainly a generalist herbivore, which easily shifts diets depending on food availability, and which also has the ability to ingest and assimilate zooplankton. Ingestion of greater quantities of diverse foods (i.e., microalgae, foliose/corticated macrophytes, filamentous algae, mangrove tissues, zooplankton) takes place in the bottom areas of mangrove trees (roots and trunks) during low tides, while top areas (branches and leaves) provide limited food resources for snails feeding during high tides. However, snails preferentially assimilate microalgae and bacteria, regardless of their feeding habitat (different areas within mangrove trees and non-tidal mesocosms). The daily vertical movements of this snail result in variable feeding times, ingestion of different food types and amounts, and different assimilations. These findings also suggest that organic matter derived from mangrove tissues may not be readily transferred to higher trophic levels through this grazing pathway.  相似文献   

6.
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   

7.
The speciation of sedimentary sulfur (pyrite, acid volatile sulfides (AVS), S0, H2S, and sulfate) was analyzed in surface sediments recovered at different water depths from the northwestern margin of the Black Sea. Additionally, dissolved and dithionite-extractable iron were quantified, and the sulfur isotope ratios in pyrite were measured. Sulfur and iron cycling in surface sediments of the northwestern part of the Black Sea is largely influenced by (1) organic matter supply to the sediment, (2) availability of reactive iron compounds and (3) oxygen concentrations in the bottom waters. Biologically active, accumulating sediments just in front of the river deltas were characterized by high AVS contents and a fast depletion of sulfate concentration with depth, most likely due to high sulfate reduction rates (SRR). The δ34S values of pyrite in these sediments were relatively heavy (−8‰ to −21‰ vs. V-CDT). On the central shelf, where benthic mineralization rates are lower, re-oxidation processes may become more important and result in pyrite extremely depleted in δ34S (−39‰ to −46‰ vs. V-CDT). A high variability in δ34S values of pyrite in sediments from the shelf-edge (−6‰ to −46‰ vs. V-CDT) reflects characteristic fluctuations in the oxygen concentrations of bottom waters or varying sediment accumulation rates. During periods of oxic conditions or low sediment accumulation rates, re-oxidation processes became important resulting in low AVS concentrations and light δ34S values. Anoxic conditions in the bottom waters overlying shelf-edge sediments or periods of high accumulation rates are reflected in enhanced AVS contents and heavier sulfur isotope values. The sulfur and iron contents and the light and uniform pyrite isotopic composition (−37‰ to −39‰ vs. V-CDT) of sediments in the permanently anoxic deep sea (1494 m water depth) reflect the formation of pyrite in the upper part of the sulfidic water column and the anoxic surface sediment. The present study demonstrates that pyrite, which is extremely depleted in 34S, can be found in the Black Sea surface sediments that are positioned both above and below the chemocline, despite differences in biogeochemical and microbial controlling factors.  相似文献   

8.
The distribution and abundance of viable and non-viable (so-called resting eggs) embryos of the calanoid copepod Tortanus forcipatus were determined in the laboratory by the enumeration of nauplii that emerge from sediments collected in Victoria Harbor (Hong Kong). Sediment cores sliced down to a depth of 37 cm showed the highest number of viable resting eggs near the surface layer (0–5 cm). The number of viable eggs sharply decreased with sediment depth, particularly at the inner harbor stations, although diapause eggs remained viable as deep as 25 cm. 210Pb analyses of the sediments indicated that the mean egg age was 4.9 years. The egg mortality of T. forcipatus in the sediments was 0.135 year−1, or 78.22% annual egg survival, calculated by regressing ln (egg density) from sediment age. The range of horizontal distribution of viable resting eggs was 24.25 × 103–58.90 × 103 m−2, with a mean value of 36.8 × 103 m−2 over all stations. The accumulation of viable resting eggs that can persist for an extended period of time provided evidence for the existence of an egg bank of T. forcipatus in the sediments of Victoria Harbor.  相似文献   

9.
A C25 highly branched isoprenoid (HBI) monoene hydrocarbon, designated IP25, has been proposed previously to originate from diatoms living in Arctic sea ice, while the presence of IP25 in sediments has been suggested to be a proxy for the occurrence of former Arctic sea ice. Here, we show that the 13C isotopic composition of IP25 in sea ice, in sediment trap material collected under sea ice, and in high latitude northern sediments, is distinctive (isotopically ‘heavy’) and distinguishable from that of organic matter of planktonic or terrigenous origin. Mean δ13C values for IP25 were − 22.3 ± 0.4‰ (sea ice), − 19.6 ± 1.1‰ (sediment traps) and − 19.3 ± 2.3‰ (sediments). These measurements, therefore, support further the proposed use of IP25 as an Arctic sea ice proxy.  相似文献   

10.
The trophic position of Calanus finmarchicus in the Trondheim Fjord in 2004 was determined through stable isotope analyses. Wild specimens were sampled monthly in the fjord and δ13C and δ15N signatures of the developmental stages from CIII to adults were measured. There were statistically significant differences in the δ13C and δ15N signatures of three identified groups: overwintered parental generation, developing new generation and new generation preparing for overwintering. C. finmarchicus individuals raised in a laboratory on a pure algal diet (Dunaliella tertiolecta and Isochrysis galbana) provided stable isotope signatures for purely herbivorous copepods. With these signatures as comparison, the trophic position of C. finmarchicus in the Trondheim Fjord in 2004 was determined as trophic level 2.4, thus indicating omnivory under natural conditions. Additionally, our data suggest that seasonal differences in the δ13C signatures of C. finmarchicus are due to the varying lipid content of the different developmental stages.  相似文献   

11.
Field measurements on leaf removal by populations of sesarmid crabs at different locations in the Bangrong mangrove forest, Phuket, Thailand, indicated that crabs on average can remove 87% of the daily leaf litter fall by ingestion or burial. The removal rate is correlated positively with the number of crab burrows and negatively with tidal inundation time. The results from the field were supplemented with observations on the behavior of Neoepisesarma versicolor in laboratory microcosms and a mangrove mesocosm. N. versicolor feeds primarily at night and total time spent feeding was up to an order of magnitude higher in the artificial microcosms than under simulated in situ conditions in the mesocosm. Most of the time during both day and night was spent resting near the entrance or inside burrows. N. versicolor mainly feeds on mangrove leaves and scraps of food material from the sediment surface. This is supported by examinations of stomach content, which showed that 62% is composed of higher plant material and 38% of detritus and mineral particles from the sediment. The nutritive value of leaves and detritus is insufficient to maintain crab growth. Sesarmid crabs may instead obtain the needed nutrients by occasional consumption of nitrogen-rich animal tissues, such as carcasses of fish and crustaceans, as indicated by the presence of animal remains in the stomach and the willingness of crabs to consume fish meat. Laboratory experiments on leaf consumption and leaf preferences of N. versicolor indicate that they preferentially feed on brown leaves, if available, followed by green and yellow leaves. If all species of sesarmid crabs in the Bangrong mangrove forest consume leaves at the same rate as N. versicolor, they could potentially ingest 52% of the total litter fall.  相似文献   

12.
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   

13.
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   

14.
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   

15.
In this study we test the hypothesized negative relationship between seagrass status and porewater hydrogen sulfide (H2S) levels, through a comparative analysis within a range of seven Posidonia oceanica meadows growing over carbonate sediments in the NW Mediterranean Sea around Mallorca Island. The studied meadows range from meadows growing on sediments with very low sulfide porewater concentrations (4.6 μM) to those growing over higher sulfide conditions (33.5 μM). Organic matter content, sulfate reduction rates and sulfide porewater concentrations in the sediments were determined concurrently with the assessment of demographic plant dynamics (specific mortality and net population growth rates). Sulfide porewater concentration increased with increasing organic matter content in the sediment, while net population growth decreased significantly with low increases of sulfide concentrations. Our results confirm the previously suspected vulnerability of seagrass meadows growing on carbonate sediments to increased sulfide levels. An excess of 10 μmols H2S L−1 porewater is identified to already conduce P. oceanica meadows to decline, which this study identifies, particularly, as strongly sensitive to sulfides. The results reported here suggest that even moderate increases in organic carbon inputs may lead to enhancement of dissolved sulfides and may be an important factor for seagrass status in these iron-depleted carbonate sediments from the Mediterranean Sea.  相似文献   

16.
Knowledge of patterns of spatial variability of vegetative development, epiphyte load and nutrient availability in seagrass meadows is essential for the adequate design of research and environmental monitoring programmes. Differences in shoot size, epiphyte load and nutrient content of leaves and epiphytes of the Mediterranean endemic seagrass Posidonia oceanica at spatial scales ranging from metres to hundreds of metres are evaluated using a hierarchical nested sampling design. The size and epiphyte load of P. oceanica shoots and the nitrogen and phosphorus content of leaves and epiphytes were different in most of the spatial scales considered. Sampling efforts concentrated at the metre scale incorporated most of the variability in size, epiphyte load and nutrient content of the leaves and epiphytes of P. oceanica shoots. Epiphyte load showed no correlation with nutrient content in the epiphytes or in the leaves. However, epiphyte load and shoot size were negatively correlated, which suggests that light penetration in the canopy may be a main determinant of epiphyte load.  相似文献   

17.
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   

18.
The recovery capacity of shallow Posidonia oceanica meadows degraded by beach replenishment eighteen years before was assessed in two impacted meadows and compared with other two undisturbed localities. Inside each locality, we selected randomly three sites separated by 500–1000 m. At site level we study the vitality of P. oceanica meadow assessing the vegetative growth, leaf characteristics, and non-structural carbohydrates of the plants. Additionally, at locality level, silt-clay fraction, organic matter, pH and light intensity incident on the sea bottom were measured to evaluate the environmental conditions. Covering of P. oceanica was significantly lower at the impacted localities while amount of dead “matte” was higher. Leaf production of horizontal rhizomes (14.6 ± 1.11 vs 19.47 ± 1.45 leaves y−1), net total rhizomes recruitment (2.33 ± 0.17 vs 4.3 ± 0.33 branches y−1) and starch concentration (43.625 ± 0.67 vs 54.45 ± 0.74 mg per g of rhizome) at impacted meadows were significantly lower than controls. Leaf features, epiphytes biomass, colonization, elongation and horizontal and vertical rhizome production did not show significant differences. Sediments at impacted localities contained higher silt-clay fraction and higher organic matter load while pH was lower. Light intensity on the sea bottom measured at all localities was over the minimum light requirements estimated for P. oceanica. Our results show that the press impact produced by beach replenishment was enduring in the time slowing natural recovery by 45%. This impact may be related with changes in the sediment features.  相似文献   

19.
The stable carbon isotope composition of particulate organic carbon (POC) from plankton, sediment trap material and surface sediments from the Atlantic sector of the Southern Ocean were determined. Despite low and constant water temperatures, large variations in the δ13C values of plankton were measured. 13C enrichments of up to 10‰ coincided with a change in the diatom assemblage and a two-fold increase in primary production. Increased CO2 consumption as a result of rapid carbon fixation may result in diffusion limitation reducing the magnitude of the isotope fractionation. The δ13C values of plankton from sea-ice cores display a relationship with the chlorophyll a content. High ‘ice-algae’ biomass, in combination with a limited exchange with the surrounding seawater, results in values of about − 18 to − 20‰. It is assumed that these values are related to a reduced CO2 availability in the sea-ice system. In comparison with plankton, sinking krill faeces sampled by traps can be enriched by 2–5‰ in 13C (e.g. central Bransfield Strait). In contrast, the transport of particles in other faeces, diatom aggregates or chains results in minor isotope changes (e.g. Drake Passage, Powell Basin, NW Weddell Sea). A comparison between the δ13C values of sinking matter and those of surface sediments reveals that 13C enrichments of up to 3–4‰ may occur at the sediment-water boundary layer. These isotopic changes are attributed to high benthic respiration rates.  相似文献   

20.
Ecosystem engineers can influence community and ecosystem dynamics by controlling resources, modifying the flow of energy or biomass, or changing physical characteristics of the habitat. Invasive hybrid cordgrass (Spartina alterniflora × Spartina foliosa) is an ecosystem engineer in salt marshes in San Francisco Bay, California, U.S.A. that raises intertidal elevations and may be either increasing C4 plant carbon input into food webs or tying up carbon in a form that is not usable by consumers. A manipulative experiment compared abundance, growth, and stable isotope (δ13C and δ15N) composition of the clam Macoma petalum (=M. balthica) among native marsh, hybrid Spartina, and mudflats in central San Francisco Bay. We found higher densities (individuals m−2) of M. petalum on mudflats compared to either native or hybrid Spartina (p < 0.001). Macoma petalum shell growth was significantly greater in mudflats than in either vegetation type in 2002 (p = 0.005) but not 2003. Differences in shell growth between native and hybrid Spartina were not significant. Stable isotope results showed differences between habitats in δ13C but not δ15N. Carbon signatures of M. petalum placed in Spartina were much more depleted than the isotopic signature of Spartina. Neither native nor hybrid Spartina appears to be a significant carbon source for M. petalum in San Francisco Bay, and we found no evidence that hybrid Spartina contributes carbon to M. petalum beyond what is provided by S. foliosa, despite the hybrid's much greater biomass. Our results show that loss of mudflat habitat, rather than increased input of C4 carbon, is the greatest effect of the invasion of hybrid Spartina on M. petalum.  相似文献   

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