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1.
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   

2.
The concentrations of total carbonate (Ct), sulphate, sulphide, thiols and oxygen, the ratio between the stable sulphur isotopes 34S and 32S in sulphate and sulphide, and the density (used to calculate salinity) were determined on samples from the water column of Framvaren, a superanoxic fjord in southern Norway. From a depth of 18m (the oxic-anoxic boundary) the initial sulphate concentration, ([SO4]init), as calculated from salinity, is significantly higher than the sum of the measured sulphur species. This is attributed to a loss of sulphur from the water column. The amount of total carbonate produced, corrected for the initial concentration (Ct - 2.4 Sal/35) is found to be proportional to the amount of sulphate consumed, ([SO4]init - [SO4]), according to the following relation Ct- 2.4 Sal/35 = 1.84 ([SO4]init - [SO4]). Isotopic fractionation caused by bacterial sulphate reduction in the anoxic part of the water column produces sulphide with a δ34S 40‰ lower than the δ34S for sulphate at corresponding depths. The isotopic fractionation also results in δ34S value for the remaining sulphate at depths below 80 m being considerably higher than the mean value for ocean water, which is close to + 20‰. The δ34S values for sulphate at depths between 10 and 50 m were lower than + 20‰ which indicates oxidation of sulphide, which follows upon diffusion of sulphide from deeper parts of the water column and inflow of oxygenated seawater over the sill into the anoxic water of the fjord. A conclusive scenario of the Framvaren sulphur chemistry is presented.  相似文献   

3.
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   

4.
This study examined the relationship between carbon isotopic composition of sinking organic matter (OM) and the biological, physical and chemical properties of the surface ocean in the Cariaco Basin. The 13C/12C ratio of OM (δ13Corg) in sinking particles was determined on sediment trap samples from four depths collected from 1996 to 1999 as part of the CArbon Retention In A Colored Ocean time series. Water column properties, including temperature, productivity, chlorophyll and concentration of dissolved CO2, were concurrently measured on monthly cruises. The δ13Corg varied from a high of –17.7‰ to a low of –22.6‰ during the study period. The variation of the δ13Corg throughout seasonal cycles was directly proportional to the strength of upwelling and was negatively correlated with temperature (r2=0.64). During the 1996–1997 upwelling event, the strongest during the study period, the δ13Corg increased by 4.4‰ whereas during the 1998–1999 upwelling event, the weakest during the study period, the δ13Corg only increased by 3.3‰. Contrary to most previous studies, we observed a negative relationship (r2=0.53) between [CO2 aq] and the estimated isotopic fractionation factor (εp). However, there was no correlation between εp and the calculated growth rates indicating that there was non-diffusive uptake of carbon into phytoplankton cells. It thus appears that [CO2 aq] does not control the δ13Corg in the water column of the study site. The best explanation for the isotopic enrichment observed is a carbon concentrating mechanism (CCM) in phytoplankton. The existence of a CCM in phytoplankton has major implications for the interpretation of the δ13Corg in the Cariaco Basin.  相似文献   

5.
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   

6.
The Arabian Sea is subject to intense seasonality resulting from biannual monsoons, which lead to associated large particulate fluxes and an abundance of organic carbon, a potential food source at the seafloor for benthic detritivores. We used the stable isotopes of carbon and nitrogen alongside lipid analyses to examine potential food sources (particulate and sedimentary organic matter, POM and SOM respectively) in order to determine trophic linkages for the twelve most abundant megafaunal species (Pontocaris sp., Solenocera sp., Munidopsis aff. scobina, Actinoscyphia sp., Actinauge sp., Echinoptilum sp., Pennatula aff. grandis, Astropecten sp. Amphiura sp. Ophiura euryplax, Phormosoma placenta and Hyalinoecia sp.) at the Pakistan Margin between 140 and 1400 m water depth. This transect spans a steep gradient in oxygen concentrations and POM flux. Ranges of δ13C and δ15N values were narrow in POM and SOM (4‰ and 2‰ for δ13C and δ15N, respectively) with little evidence of temporal variability. Labile lipid compounds in SOM originating from phytoplankton did exhibit seasonal change in their concentrations at the shallowest sites, 140 and 300 m. Benthic megafauna had broad ranges in δ13C and δ15N (>10‰ and >8‰ for δ13C and δ15N, respectively) suggesting they occupy several trophic levels and utilize a variety of food sources. There is evidence for feeding niche separation between and within trophic groups. Lipid biomarkers in animal tissues indicate a mixture of food sources originating from both phytoplankton (C20:5(n-3) and C22:6(n-3)) and invertebrate prey (C20:1 and C22:1). Biomarkers originating from phytodetritus are conserved through trophic transfer to the predator/scavengers. Six species (Pontocaris sp., Solenocera sp., Actinoscyphia sp., Echinoptilum sp., Amphiura sp. and Hyalinoecia sp.) showed a significant biochemical response to the seasonal supply of food and probably adapt their trophic strategy to low food availability. Biotransformation of assimilated lipids by megafauna is evident from polyunsaturated fatty acid distributions, for example, Echinoptilum sp. converts C20:5(n-3) to C24:6(n-3).  相似文献   

7.
A model is presented for flux evaluation of trace metals in situ. For the practically feasible simplified model, mere estimation of metals in various dissolved states (e.g. Cr(III) and Cr(VI)) and in particulate phase, in a single vertical profile at least, together with temperature, salinity, O2, titration alkalinity (TA), Ca2+, NO3−, HPO42− and H4SiO4 parameters enable evaluation of the amount of metal removed in situ, to a first approximation. From the evaluated net metal removed (MR), estimation could be made for removal constants, i.e. removal times (t), residence times (τ) and removal rate constants (ψ), not only for the average ocean but also for the oxygen-minimum zone. Besides, MR can be used to calculate t, τ and ψ of various species of the metal concerned, that are not yet available. The present model evaluates t, τ and ψ for total chromium from a single station, which are consistent with the reported values, and shows that chromium is removed with fast-settling particulate matter. This study also reveals that t, τ and ψ values of 0.001 years 7 years and 0.150 y−1, respectively, for Cr(III) in the oxygen-minimum zone and 0.006 y, 211 y and 0.005 y−1 in the marine environment, respectively.  相似文献   

8.
A quasi-permanent transverse asymmetry in the thermohaline properties of the Ria de Pontevedra (northwestern Spain) was measured from October 1997 to October 1998 during 23 cruises. On average, the density increased form north to south. This asymmetry was maintained by salinity during the wet season, and by salinity and temperature during the dry season. The density difference between coasts (Δσt = σtsouth − σtnorth) and the river discharge were positively correlated (correlation coefficient, approximately 0.6) throughout the year. The density difference was negatively correlated with the upwelling index during the wet season (correlation coefficient, approximately −0.45) and positively correlated during the dry season (correlation coefficient, approximately 0.55).  相似文献   

9.
New sour pools have recently found in the Lower Triassic Feixianguan Fm carbonate reservoirs in the East Sichuan Basin in China with H2S up to 17.4% by volume. A recent blowout from a well drilled into this formation killed hundreds of people as a result of the percentage concentrations of H2S. In order to assess the origin of fatal H2S as well as the cause of petroleum alteration, H2S concentrations and the isotopes, δ34S and δ13C have been collected and measured in gas samples from reservoirs. Anhydrite, pyrite and elemental sulphur δ34S values have been measured for comparison. The high concentrations of H2S gas are found to occur at depths >3000 m (temperature now at 100 °C) in evaporated platform facies oolitic dolomite or limestone that contains anhydrite nodule occurrence within the reservoirs. Where H2S concentrations are greater than 10% its δ34S values lie between +12.0 and +13.2‰ CDT. This is within the range of anhydrite δ34S values found within the Feixianguan Fm (+11.0 to +21.7‰; average 15.5±3.5‰ CDT). Thus H2S must have been generated by thermochemical sulphate reduction (TSR) locally within the reservoirs. Burial history analysis and fluid inclusion data reveal that the temperature at which TSR occurred was greater than about 130–140 °C, suggesting that the present depth-temperature minimum is an artifact of post-TSR uplift. Both methane and ethane were actively involved in TSR since the petroleum became almost totally dry (no alkanes except methane) and methane δ13C values become significantly heavier as TSR proceeded. Methane δ13C difference thus reflects the extent of TSR. While it is tempting to use a present-day depth control (>3000 m) to predict the distribution of H2S in the Feixianguan Fm, this is an invalid approach since TSR occurred when the formation was buried some 1000–2000 m deeper than it is at present. The likelihood of differential uplift across the basin means that it is important to develop a basinal understanding of the thermal history of the Feixianguan Fm so that it is possible to determine which parts of the basin have been hotter than 130–140 °C.  相似文献   

10.
The Upper Gulf of California is the shallow (depth <30 m), tidal area at the head of the Gulf of California. It is an inverse estuary, due to the high evaporation rate (E1·1 m year−1) and almost nil freshwater input from rainfall and the Colorado River. Historical and recent hydrographic data show that the area is almost vertically well-mixed throughout the year, that the horizontal distribution of properties follows the bathymetry, and that the hydrography has a strong annual modulation. As in other negative estuaries, the year-round salinity increase toward the head causes the density to do likewise, despite the seasonally reversing temperature gradient. The pressure gradient thus formed leads to water-mass formation and gravity currents (speed 0·1 ms−1), both in winter and in summer. In winter, the high salinity water sinks beyond 200 m, while in summer it only reaches a depth of 20–30 m. The gravity currents appear to be modulated by the fortnightly tidal cycle, with events in neap tides. This phenomenon causes the presence, at least during neap tides, of slight stratification (Δσt≈−0·2).  相似文献   

11.
Displaying “calculated minus observed” data for precise titrations of seawater with strong acid permits direct evaluation of important parameters and detection of systematic errors.At least two data sets from the GEOSECS (Geochemical Ocean Sections) program fit an equilibrium model (which includes carbonate, borate, sulfate, silicate, fluoride, and phosphate) within the most stringent experimental error, less than 2 μmol kg−1. The effect of various parameters on the fit of calculated to observed values depends strongly on pH. Although standard potential E0, total alkalinity At, total carbonate Ct, and first acidity constant of carbon dioxide pK1 are nearly independent, and can be determined for each data set, other parameters are strongly correlated. Within such groups, all but one parameter must be determined from data other than the titration curve.Adding an acid-base pair to the theoretical model (e.g. Cx=20 μmol kg−1, pKx=6.2) produces a deviation approaching 20 μmol kg−1 at constant Ct; however, adjustment of Ct by about −18 μmol kg−1 to produce a good fit leaves only ± 1.5 μmol kg−1 residual deviation from the reference values. Thus, at current standards of precision, an unidentified weak acid cannot be distinguished from carbonate purely on the basis of the titration curve shape.There are few full sets of numerical data published, and most show larger systematic errors (3–12 μmol l−1) than the above; one well-defined source is experiments performed in unsealed vessels. Total carbonate can be explicitly obtained as a function of pH by a rearrangement of the titration curve equation; this can reveal a systematic decrease in Ct in the pH range 5–6, as a result of CO2 gas loss from the titration vessel. Attempts to compensate for this by adjustment of At, Ct, or pK1 produce deviations which mimic those produced by an additional acid-base pair.Changing from the free H+ scale (for which [HSO4−] and [HF] are explicit terms in the alkalinity) to the seawater scale (SWS) (where those terms are part of a constant factor multiplying [H+]) requires modification of the titration curve equation as well as adjustment of acidity constants. Even with this change, however, omission of pH-dependent terms in [HSO4−] and [HF] produces small systematic errors at low pH.Shifts in liquid junction potential also introduce small systematic errors, but are significant only at pH <3. High-pH errors due to response of the glass electrode to Na+ as well as H+ can be adequately compensated to pH 9.5 by a linear selectivity expression.  相似文献   

12.
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   

13.
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   

14.
The apparent solubility product K′sp of calcite in seawater was measured as a function of temperature, salinity, and pressure using potentiometric saturometry techniques. The temperature effect was hardly discernible experimentally. The value of K′sp at 25°C was 4.59·10−7 mole2/(kg seawater)2 at 35‰S, 5.34·10−7 at 43‰S, and 3.24·10−7 at 27‰S. The apparent partial molal volume was found to be −34.4 cm3 at 25°C and −42.3 cm3 at 2°C from a linear fit of log(K′sp P/K′sp 1). These results were used in conjunction with field data to calculate the degree of saturation in the oceans and showed undersaturation at shallower depths than previously reported.  相似文献   

15.
The acute lethality and sublethal heavy metal contamination potential to marine organisms of a waste brine solution from a potash mining operation were determined. The brine was acutely lethal to marine organisms of the Bay of Fundy, Canada, at concentrations between 47 and 55‰ salinity (96-h LC50 values: threespine stickleback, Gasterosteus aculeatus, 47·7‰; polychaete worm, Nephtys incisa, 52·5‰; blue mussel, Mytilus edulis, lethal to whole animal and reduced ciliary activity of gills above 55‰).The brine diluted to sublethal concentrations (≤40‰) contained manganese (≤ 49·5 ×), copper and zinc (≤ 5·2 ×), lead (≤ 2·6 ×) and cadmium (≤ 0·7 ×) in decreasing order of enrichment relative to control seawater. Copper was accumulated in mussels exposed to these solutions over 44 days to the greatest degree (4 ·4 ×), followed by manganese and lead. Tissue lead concentration reached a steady value within 44 days and cadmium was not accumulated.  相似文献   

16.
The stable carbon isotopic composition of particulate organic matter in the ocean, δ13CPOC, shows characteristic spatial variations with high values in low latitudes and low values in high latitudes. The lowest δ13CPOC values (−32‰ to −35‰) have been reported in the Southern Ocean, whereas in arctic and subarctic regions δ13CPOC values do not drop below −27‰. This interhemispheric asymmetry is still unexplained. Global gradients in δ13CPOC are much greater than in δ13CDIC, suggesting that variations in isotopic fractionation during organic matter production are primarily responsible for the observed range in δ13CPOC. Understanding the factors that control isotope variability is a prerequisite when applying δ13CPOC to the study of marine carbon biogeochemistry. The present model study attempts to reproduce the δ13CPOC distribution pattern in the ocean. The three-dimensional (3D) Hamburg Model of the Oceanic Carbon Cycle version 3.1 (HAMOCC3.1) was combined with two different parametrizations of the biological fractionation of stable carbon isotopes. In the first parametrization, it is assumed that the isotopic fractionation between CO2 in seawater and the organic material produced by algae, P, is a function of the ambient CO2 concentration. The two parameters of this function are derived from observations and are not based on an assumption of any specific mechanism. Thus, this parametrization is purely empirical. The second parametrization is based on fractionation models for microalgae. It is supported by several laboratory experiments. Here the fractionation, P, depends on the CO2 concentration in seawater and on the (instantaneous) growth rates, μi, of the phytoplankton. In the Atlantic Ocean, where most field data are available, both parametrizations reproduce the latitudinal variability of the mean δ13CPOC distribution. The interhemispheric asymmetry of δ13CPOC can mostly be attributed to the interhemispheric asymmetry of CO2 concentration in the water. However, the strong seasonal variations of δ13CPOC as reported by several authors, can only be explained by a growth rate-dependent fractionation, which reflects variations in the cellular carbon demand.  相似文献   

17.
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   

18.
Population structure and distribution of Terebralia palustris were compared with the environmental parameters within microhabitats in a monospecific stand of Avicennia marina in southern Mozambique. Stable carbon and nitrogen isotope analyses of T. palustris and potential food sources (leaves, pneumatophore epiphytes, and surface sediments) were examined to establish the feeding preferences of T. palustris. Stable isotope signatures of individuals of different size classes and from different microhabitats were compared with local food sources. Samples of surface sediments 2.5–10 m apart showed some variation (−21.2‰ to −23.0‰) in δ13C, probably due to different contributions from seagrasses, microalgae and mangrove leaves, while δ15N values varied between 8.7‰ and 15.8‰, indicating that there is a very high variability within a small-scale microcosm. Stable isotope signatures differed significantly between the T. palustris size classes and between individuals of the same size class, collected in different microhabitats. Results also suggested that smaller individuals feed on sediment, selecting mainly benthic microalgae, while larger individuals feed on sediment, epiphytes and mangrove leaves. Correlations were found between environmental parameters and gastropod population structure and distribution vs. the feeding preferences of individuals of different size classes and in different microhabitats. While organic content and the abundance of leaves were parameters that correlated best with the total density of gastropods (>85%), the abundance of pneumatophores and leaves, as well as grain size, correlated better with the gastropod size distribution (>65%). Young individuals (height < 3 cm) occur predominantly in microhabitats characterized by a low density of leaf litter and pneumatophores, reduced organic matter and larger grain size, these being characteristic of lower intertidal open areas that favour benthic microalgal growth. With increasing shell height, T. palustris individuals start occupying microhabitats nearer the mangrove trees characterized by large densities of pneumatophores and litter, as well as sediments of smaller grain size, leading to higher organic matter availability in the sediment.  相似文献   

19.
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   

20.
Midsummer (1 August) population estimates of about 2 million O-group plaice (Pleuronectes platessa L.) were derived for sandy bays around the Firth of Forth in 1979–1980. This is an order of magnitude less than similar estimates made for the Clyde Sea Area in 1973–1974. Autumn population estimates of 0·4–1·0 million fish were comparable to estimates by the Ministry of Agriculture, Fisheries and Food for the area between the Scottish border and Flamborough Head (2·3 million for 1970 and 1973) which represented 4·8% (1973) to 5·3% (1970) of the total number of O-group fish on the English east coast.Largo Bay was the most important nursery area holding 25% of the total population. It is particularly well situated to receive newly metamorphosed plaice carried in water currents along the north side of the Forth from the spawning ground off Fife Ness. Plaice in the Forth are mainly distributed on fine to medium sandy beaches (186–480 μm), the mean number per haul in midsummer (D) being correlated with the median diameter (m.d. in μm) of the low water sediments by the equation: D=−45·7666+0·2327 m.d. (n=11,r=0·68,P<0·02 but>0·01).The shallow inshore water in sandy bays in the outer Firth was well mixed and more marine than estuarine (27·7–35·0‰). The correlation coefficient between fish density and water temperature was low, while that with salinity (S‰) was: D=6·1618+0·2238S (n=23,r=0·62,P<0·005).Regression analysis demonstrated that the relationship between the instantaneous mortality rate (Z) and the initial population density (Dp) was: Z×100=0·7480+0·0546dp (n=12,r=0·87,P<0·001).The mean mortality rate for the O-group plaice in the Forth nursery areas was 53% month−1.  相似文献   

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