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1.
The behavior of dissolved and particulate iron and manganese and dissolved silicon has been studied as a function of chlorinity in the Peconic River estuary, New York. This study sought to identify important geochemical processes in a relatively pristine estuary facing increasing anthropogenic impact.Dissolved iron behaved in the classical non-conservative manner exhibiting removal of nearly 80% at very low chlorinities, while particulate iron increased by a corresponding amount over the same chlorinity range. Dissolved manganese was enriched by up to 200% over its predicted concentration at low and intermediate chlorinities by desorption from suspended particulates and by a probable benthic flux. Dissolved silicon was enriched by up to 100% at low and intermediate chlorinities also from a probable benthic flux. These fluxes were estimated to be 5 μg cm?2 day?1 for dissolved manganese and 70 μg cm?2 day?1 for dissolved silicon.The quantity of both particulate iron and manganese increased at high chlorinities due to an influx of suspended inorganic particulates. In the intermediate to high chlorinity region, oxidation of sediment-derived manganese is believed to contribute to the observed increase in particulate manganese.Total iron was essentially conservative throughout most of the estuary, while total manganese was non-conservative presumably due to extensive remobilization of dissolved manganese from the sediments.  相似文献   

2.
226Ra and 228Ra have non-conservative excess concentrations in the mixing zones of the Pee Dee River-Winyah Bay estuary, the Yangtze River estuary, and the Delaware Bay estuary. Laboratory experiments, using Pee Dee River sediment, indicate desorption of 226Ra to increase with increasing salinities up to 20‰. In Winyah Bay desorption from river-borne sediments could contribute almost all of the increases for both isotopes. Desorption adds only a portion of the excess 228Ra measured in the Yangtse River and adjacent Shelf waters and Delaware Bay. In the Yangtze River the mixing zone extends over a considerable portion of the Continental Shelf where 228Ra is added to the water column by diffusion from bottom sediments, while 226Ra concentrations decrease from dilution. Diffusion of 228Ra from bottom sediments in Delaware Bay primarily occurs in the upper part of the bay (< 22‰ water) where fine grained sediments predominate. A diffusive flux for 228Ra of 0·33 dpm cm−2 year was determined for Delaware Bay.  相似文献   

3.
In this study, we examine the distribution and carbon stable isotope signature of dissolved organic carbon (DOC) and humic substances (HS) along a salinity gradient in the Altamaha and Satilla River estuaries. The maximum DOC concentrations in the Altamaha and Satilla were 10 and 29 mg C l−1, respectively, though concentrations were similar at the mouth of both estuaries. There was a decrease in HS content of DOC from 50 to 80% at the head of the estuaries to 10% at salinities higher than 30‰. The δ13C DOC varied between −25.5 and −19‰ and between −27 and −21‰ in the Altamaha and Satilla estuaries, respectively. The tendency towards more depleted δ13C DOC in the Satilla, especially in the lower salinity portion of this estuary, suggests greater terrestrial inputs in the Satilla than in the Altamaha. Seasonal fluctuations were observed in the form of increased (two to three times) range in DOC concentration, heavier δ13C DOC and increased proportion of estuarine–marine-derived DOC (average enrichment of δ13C DOC from +1 to +2) during low river flow (July–October). The δ13C HS in both rivers showed a similar trend, but was consistently more depleted than DOC, with an average range from −28 to −24.5‰. This suggests that HS have larger proportions of terrestrial components (a maximum of >60% at the mouth of the estuary) than DOC. The less depleted δ13C values of DOC in comparison with HS indicate a different source for the non-humic (non-HS) component of DOC (range in δ13C non-HS, −22 to −16‰). That source could either be the decomposition of detrital material derived from saltmarsh environments or microalgal-derived DOC of estuarine or marine origins.  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.
Water samples from the Lena River and stratified Laptev Sea (northeastern Siberia) have been analyzed to determine their stable oxygen isotope composition (18O/16O). Measurements at the Lena River reference station give a δ18O value of −18.9‰ in both surface and bottom waters. In the brackish water surface plume, a nearly perfect correlation is found between δ18O and chlorinity
δO=−18.9+0.7C1−(n=15; r=0.999)
A few values lie distinctly below this correlation; they all correspond to surface samples collected in the semi-enclosed Buorkhaya Gulf, and they most likely reveal the occurrence of ‘old’ water masses. Some of the δ18O values in the deep waters collected in the same zone also fall below the surface-plume correlation line.Dissolved silicate concentrations exhibit a large variability. However, when they are related to the different water masses identified using oxygen isotope data, a more coherent picture is obtained. Concentrations in the surface plume decrease more or less regularly from 50 to 72 μmol in the Lena River, to 7 μmol at the ‘marine’ end-member (Cl− = 14 g l−1). Dissolved silicate results in the Buorkhaya Gulf are quite distinct, with a clear deficiency in the surface waters, and an excess in the deep waters.These δ18O and dissolved silicate variations are discussed in relation to the hydrology and the biological productivity of the investigated area.  相似文献   

6.
Behaviour of industrially added boron in the well mixed Ambika river estuary of South Gujarat, India has been studied along with that of calcium and magnesium. The ratios of Ca : Cl and Mg : Cl at varying chlorinities were always within the normal ranges found in river water at one end and seawater at the other end. B : Cl exceeded the river water end limit at the ebb tide at an upstream station. Percentage addition or removal of boron, calcium and magnesium was calculated using the concept of the theoretical dilution line. The external input raised the boron concentration by 263% in the vicinity of the industrial discharge. A correlation coefficient of ?0·82 between chlorinity and percentage addition or removal of boron supports the theory of boron incorporation in sediments in amounts proportional to salinity. Inverse relationship between percentage addition or removal of boron and suspended solids was observed. Behaviour of calcium and magnesium was in line with the replacement of Ca2+ ions occupying the majority of ion-exchange sites in riverine clays by Mg2+, Na+ and K+ during their first encounter with seawater.  相似文献   

7.
The macro-tidal Keum River Estuary located in the eastern Yellow Sea has been suffering siltation and morphological change since 1994. To understand the effects of the large-scale coastal developments on the sedimentation processes in the estuary, hydrodynamic and sedimentary data collected from 1985 to 2002 were analyzed and numerical experiments of hydrodynamics were performed. The sedimentation rate in the estuary increased by a factor of 1.9, from 3.5 × 106 to 6.7 × 106 m3 y−1, after the construction of a dam in the upper reaches of the estuary in 1994. Large part of the estuary is veneered by the muddy sediments noticeably, which were rarely found before dam construction. Since then, siltation has concentrated in the upper estuary rather than the lower. The upstream transport and accumulation of fine-grained sediments is due to: (1) the change to flood-dominance in the main channel, i.e. the relative intensification of flood current and the flood-directed residual current; and (2) the decrease in transport capacity in the upper estuary, i.e. the marked decrease in current velocity, which was induced by dam construction. The former has resulted in the ebb-dominance of the Gaeya channel, a distributary in the north of the main channel. The tidal pumping of fine sediments was reinforced not by the freshwater/saltwater interaction but by the residual tidal circulation. The sediment fluxes observed in 2001–2002 demonstrate year-round net inflow both at the entrance of the jetties and at the Gaeya channel, which implies that the sediments delivered by the Keum River are entirely confined to the estuary, incapable of escaping to the sea. The net inward transport of fine sediments may accumulate pollutants adsorbed to or absorbed in the sediment grains in the estuary, thus deteriorating the benthic environment gradually and the water quality eventually.  相似文献   

8.
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   

9.
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   

10.
The Loire estuary has been surveyed from 1982 to 1985 by 13 isochronous longitudinal profiles realized at low tide. Nutrient (SiO2, NO3−, NH4+, PO3−4, particulate organic carbon or POC) patterns are very variable depending on the season, the estuarine section [river, upper-inner estuary, upstream of the fresh-water-saline-water interphase FSI, the lower-inner estuary characterized by the high turbidity zone (HTZ), the outer estuary] and the river discharge. Biological processes are dominant. In the eutrophied River Loire (summer pigment > 100 μg l−1), the high algal productivity (algal POC > 3 mg l−1) results in severe depletion of SiO2, PO43−, NO3−. The enormous biomass (55 000 ton algal POC/year) is degraded in the HTZ where bacterial activity is intense. As a result, there is generally a regeneration of dissolved SiO2 and PO43−, a marked NH4+ maximum, while NO3− is conservative or depleted when the HTZ is nearly anoxic. Other processes can be considered including pollution from fertilizer plans (PO43−, NH4+) and from a hydrothermal power plant (NH4+). In the less turbid outer estuary, nutrients are generally conservative. Major variations of concentrations are observed in the lowest chlorinity section (Cl− < 1 g kg−) and also upstream the FSI, defined here as a 100% increase in Cl−. Nutrient inputs to the ocean are not significantly modified for SiO2 and NO2−, but are increased by 70% and 180% for PO43− and NH4+ and depleted by 60% for POC. Odd hydrological events, especially some floods, may perturbate or even mask the usual seasonal pattern observed in profiles.  相似文献   

11.
The adsorption of trace metals (Pb, Zn, Cu, Cd and Hg) on calcite, kaolinite and bentonite has been studied in Krka river water of various salinities (S = 3, 20 and 38‰).Model experiments were performed at pH 8 in natural water and in UV-irradiated water samples to determine the importance of natural soluble organic matter. Cold vapour atomic absorption spectrometry was used for the Hg determination, and a differential pulse anodic stripping voltammetry (DPASV) method for determination of other trace metals, using either a hanging mercury drop electrode or a mercury-coated glassy carbon working electrode. The following results have been obtained on calcite:
1. (1) a powerful absorption of Pb and Zn was observed for both natural and UV-irradiated waters at S=20‰;
2. (2) a strong adsorption of Hg was observed in natural water, and significantly lower adsorption in UV-irradiated water at all three salinities, particularly at low salinity;
3. (3) Cd adsorption was of medium intensity in both waters at S=20‰;
4. (4) Cu was practically not adsorbable in both waters at S=20‰ if the total Cu ion concentration was lower than the copper complexing capacity value;
5. (5) Cu adsorption was of a medium intensity at S=38‰. It has not influenced by the EDTA model ligand.
Trace metals Pb, Zn and Cu are adsorbable on kaolinite and bentonite at S=38‰, whereas Cd is practically not adsorbed. The Krka River is a calcareous river, which supplies the sea with calcite and aluminosilicates. In the light of our model experiments, we suggest that the self-purification of the Krka River is remarkable in respect to Pb, Zn and Hg; this occurs to a lesser extent for Cu, and is negligible in the case of Cd.  相似文献   

12.
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   

13.
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   

14.
Phytoplankton community composition, productivity and biomass characteristics of the mesohaline lower Neuse River estuary were assessed monthly from May 1988 to February 1990. An incubation method which considered water-column mixing and variable light exposure was used to determine phytoplankton primary productivity. The summer productivity peaks in this shallow estuary were stimulated by increases in irradiance and temperature. However, dissolved inorganic nitrogen loading was the major factor controlling ultimate yearly production. Dynamic, unpredictable rainfall events determined magnitudes of seasonal production pulses through nitrogen loading, and helped determine phytoplankton species composition. Dinoflagellates occasionally bloomed but were otherwise present in moderate numbers; rainfall events produced large pulses of cryptomonads, and dry seasons and subsequent higher salinity led to dominance by small centric diatoms. Daily production was strongly correlated (r = 0·82) with nitrate concentration and inversely correlated (r = −0·73) with salinity, while nitrate and salinity were inversely correlated (r = −0·71), emphasizing the importance of freshwater input as a nutrient-loading source to the lower estuary. During 1989 mean daily areal phytoplankton production was 938 mgC m−2, mean chlorophyll a was 11·8 mg m−3, and mean phytoplankton density was 1·56 × 103 cells ml−1. Estimated 1989 annual areal phytoplankton production for the lower estuary was 343 gC m−2.  相似文献   

15.
The C/N and stable C and N isotope ratios (δ13C, δ15N) of sedimentary and suspended particulate matter were determined in the Schelde Estuary. Suspended matter was divided into 2 to 5 size fractions by centrifugation. Four major pools of organic matter were recognized: riverine, estuarine, marine and terrestrial materials. Terrestrial organic matter (δ13C≈−26‰, δ15N≈3.5‰, C/N≈21) is important for the sedimentary pool, but suspended matter is dominated by the marine (δ13C≈−18‰, δ15N≈9‰, C/N≈8), riverine (δ13C≈−30‰, δ15N≈9‰, C/N≈7.5) and estuarine (δ13C≈−29‰, δ15N≈15‰, C/N≈8) end-members. In the upper estuary, the suspended matter size fractions vary systematically in their carbon and nitrogen biogeochemistry, with the small particles having low C/N ratios, depleted δ13C and enriched δ15N values relative to large particles. Moreover, sedimentary and suspended matter differ significantly in terms of C/N ratios (17 vs. 8.9), δ13C (−26.3 vs. −28.9‰) and δ15N (+6.9 vs. 12.0‰). In the lower estuary, suspended matter fractions are similar and sedimentary and suspended organic matter differ only in terms of δ13C (−23.5 vs. −20.1‰). Our data indicate that autochthonous organic matter contributes significantly to the total suspended matter and that the suspended organic matter composition cannot be explained in terms of conservative mixing of riverine and terrestrial sources on the one hand and marine sources on the other hand.  相似文献   

16.
A program of long-term observation of suspended solids (TSS), particulate organic carbon (POC) and cadmium transported into the Gironde estuary (France) by its major tributaries has been carried out between 1990 and 1999. This decade included contrasting hydrologic cycles and appears representative of a much longer period (1959–1999). The Garonne and the Dordogne river systems are the main tributaries of the Gironde estuary and derive their waters from drainage basins with different geological, industrial and agricultural features. To better understand their respective contributions, they have been observed separately and compared. Water and TSS fluxes of the Garonne River show greater temporal variations and discharge is more related to the hydrology of the drainage basin (e.g. wet/dry years, local flood events etc.). As POC and particulate Cd concentrations in suspended matter are much less variable than turbidity, their fluxes are mainly controlled by the TSS transport. A major part of annual fluxes of TSS and associated pollutants may occur within few flood days (depending on various parameters, e.g. intensity, duration, season, etc.), and also the succession of dry and wet years has an important influence on annual fluxes. The presented data allow calculating fluvial inputs into the Gironde as the sum of fluxes transported by its major tributaries, the Garonne and the Dordogne river systems. Mean annual fluxes into the Gironde observed in 1990–1999 are about 34×109 m3 year−1 for river water, 3.24×106 t year−1 for suspended solids (TSS) and 9.88×109 mol year−1 for particulate organic carbon (POC). Generally, these fluxes are dominated by the contributions of the Garonne River. However, in dry years, the mean contribution of the Dordogne river system (including Dronne and Isle rivers) to the POC input into the estuary exceeded that of the Garonne. This reflects significant differences in vegetation and soil due to natural properties and land management of the basins. Mean Cd fluxes into the estuary are about 110×103 mol year−1 of which 19.6×103 mol year−1 are transported in the dissolved and 90.8×103 mol year−1 in the particulate phases, respectively. In 1991 (dry year), the net (dissolved) Cd flux towards the ocean exceeded the gross fluvial input of total Cd, suggesting the release of Cd from an important stock in the maximum turbidity zone (MTZ) or the fluid mud of the Gironde estuary.  相似文献   

17.
Radionuclides (i.e., 7Be and 210Pb) can be used to trace particle and sediment dynamics and to quantify coastal oceanic processes with time scales ranging from a few days to a hundred years. Here, we study the settling dynamics of suspended particles and the implication by sedimentary heavy metals in the Wenjiao/Wenchang River and Wanquan River estuaries through the measurement of the particulate 7Be and 210Pb nuclides. Activity in the particulate phase had a range of 2.1–54.5 and 4.6–67.9 Bq kg−1 for 7Be and excess 210Pb (210Pbxs), respectively, in the Wenjiao/Wenchang River estuary. In the Wanquan River estuary, activity is in the range of 1.2–43.5 Bq kg−1 for 7Be and 6.2–194.5 Bq kg−1 for 210Pbxs. At the same time, activity in the dissolved phase had a range of 0.46–1.26 and 0.30–1.17 Bq m−3 for 7Be and 210Pb, respectively, in the Wenjiao/Wenchang River estuary; ranges of 0.10–2.31 and 0.09–1.87 Bq m−3 for 7Be and 210Pb, respectively, were observed in the Wanquan River estuary. The distribution coefficients (Kd) for the two nuclides decreased within increased in suspended particle matters (SPM) concentration and/or salinity in Wanquan River estuary. The residence times of particulate 7Be and 210Pbxs had ranges of 0.4–1.6 and 1.65–5.15 days, respectively, in the Wenjiao/Wenchang River estuary; and ranges of 0.02–3.2 and 0.61–4.44 days, respectively in the Wanquan River estuary. All residence times for the two nuclides increased in the seaward direction. In the Wenjiao/Wenchang River estuary, we found that 11.8–21.0% of Cu, 3.0–9.0% of Zn and 43.2–69.9% for Cd is removed from the water column and deposited into the estuary, and 24.2–34.8% for Cu, 7.2–23.8% for Zn, and 70.0–82.5% for Cd in the Wanquan River estuary, respectively.  相似文献   

18.
The nearshore shelf of the Beaufort Sea is defined by extreme physical and biological gradients that have a distinctive influence on its productivity and trophic structure. Massive freshwater discharge from the Mackenzie River, along with numerous smaller rivers and streams elsewhere along the coast, produce an environment that is decidedly estuarine in character, especially in late spring and summer. Consequently, the Beaufort coast provides a critical habitat for several species of amphidromous fishes, some of which are essential to the subsistence lifestyle of arctic native populations. Because of its low in situ productivity, allochthonous inputs of organic carbon, identifiable on the basis of isotopic composition, are important to the functioning of this arctic estuarine system. Coastal erosion and river discharge are largely responsible for introducing high concentrations of suspended sediment from upland regions into the nearshore zone. The depletion in the 13C content of invertebrate and vertebrate consumers, which drops about 4–5‰ eastward along the eastern Alaskan Beaufort Sea coast, may reflect the assimilation of this terrestrial organic matter into local food webs. In addition, the large range in 13C values of fauna collected in the eastern Beaufort (nearly 8‰) compared to the same species in the northeastern Chukchi (3‰), indicate a lower efficiency of carbon transfer between trophic levels in the eastern Beaufort. The wider spread in stable isotope values in the eastern Beaufort may also reflect a decoupling between benthic and pelagic components. Isotopic tracer studies of amphidromous fishes in the Simpson Island barrier island lagoon revealed that terrestrial (peat) carbon may contribute as much as 30–50% of their total dietary requirements. On the eastern Alaska Beaufort Sea coast, the δ13C values of arctic cod collected in semi-enclosed lagoons were more depleted, by 3–4‰, compared to fish collected in the coastal Beaufort Sea. Calculations from isotopic mixing equations indicate cod from lagoons may derive 70% of their carbon from terrestrial sources. The δ15N values of lagoon fish were also 4‰ lower than coastal specimens, reflective of the lower δ15N values of terrestrially derived nitrogen (0–1.5‰ compared to 5–7‰ for phytoplankton). The role of terrestrial carbon in arctic estuarine food webs is especially important in view of the current warming trend in the arctic environment and the role of advective processes that transport carbon along the nearshore shelf. Biogeochemical studies of the arctic coastal estuarine environment may provide more insights into the function of these biologically complex ecosystems.  相似文献   

19.
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   

20.
The distributions of dissolved organic carbon (DOC), Ba, U, and a suite of naturally occurring radionuclides in the U/Th decay series (222Rn, 223,224,226,228Ra) were studied during high- and low-discharge conditions in the Loxahatchee River estuary, Florida to examine the role of submarine groundwater discharge in estuarine transport. The fresh water endmember of this still relatively pristine estuary may reflect not only river-borne constituents, but also those advected during active groundwater/surface water (hyporheic) exchange. During both discharge conditions, Ba concentrations indicated slight non-conservative mixing. Such Ba excesses could be attributed either to submarine groundwater discharge or particle desorption processes. Estuarine dissolved organic carbon concentrations were highest at salinities closest to zero. Uranium distributions were lowest in the fresh water sites and mixed mostly conservatively with an increase in salinity. Suspended particulate matter (SPM) concentrations were generally lowest (< 5 mg L− 1) close to zero salinity and increased several-fold ( 18 mg L− 1; low discharge) toward the seaward endmember, which may be attributed to dynamic resuspension of bottom sediments within Jupiter Inlet.Surface water-column 222Rn activities were most elevated (> 28 dpm L− 1) at the freshwater endmember of the estuary and appear to identify regions of the river most influenced by the discharge of fresh groundwater. Activities of four naturally occurring isotopes of Ra (223,224,226,228Ra) in this estuary and select adjacent shallow groundwater wells yield mean estuarine water-mass transit times of less than 1 day; these values are in close agreement to those calculated by tidal prism and tidal frequency. Submarine groundwater discharge rates to the Loxahatchee River estuary were calculated using a tidal prism approach, an excess 226Ra mass balance, and an electromagnetic seepage meter. Average SGD rates ranged from 1.0 to 3.8 × 105 m3 d− 1 (20–74 L m− 2 d− 1), depending on river-discharge stage. Such calculated SGD estimates, which must include both a recirculated as well as fresh water component, are in close agreement with results obtained from a first-order watershed mass balance. Average submarine groundwater discharge rates yield NH4+ and PO4− 3 flux estimates to the Loxahatchee River estuary that range from 62.7 to 1063.1 and 69.2 to 378.5 μmol m− 2 d− 1, respectively, depending on river stage. SGD-derived nutrient flux rates are compared to yearly computed riverine total N and total P load estimates.  相似文献   

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