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1.
The Late Cretaceous to Early Tertiary sequence of the Vøring and Møre Basins from the Norwegian Sea has been examined with respect to mineralogy based on 319 cutting samples from five wells. A clear relationship between mineralogy and well log data is demonstrated. A significant change with respect to velocity, porosity and density occurs within the depth interval corresponding to 80–90 °C. At shallow depths/temperatures (<2.0 km/70 °C), compaction is mainly mechanical and the physical properties are similar to what has been measured by experimental compaction of mudstones. At greater depths, however, the log derived velocities and densities are higher than those produced by experimental compaction indicating significant chemical compaction. XRD analyses show a progressive alteration of smectite to illite (S–I) within this depth/temperature interval which results in the release of significant amounts of silica into solution. Detrital silt and fine-grained quartz showed no secondary quartz overgrowths. These grains are isolated within a clay matrix and surrounded by clay minerals, thus limiting the available surface area and pore space for quartz overgrowths. Chemical analyses (XRF) indicate that silica is conserved within this depth interval, and the amount released from S–I alteration was locally precipitated. Field emission gun-scanning electron microscopy (FEG-SEM) and cathode luminescence (CL) identified authigenic micro-crystalline quartz cement within the clay matrix at temperatures above ∼85 °C. This is accompanied by an increase in velocity and density indicating that the S–I reaction and the precipitation of authigenic quartz caused a significant change in the rock stiffness.  相似文献   

2.
Late Cretaceous mudstones from two wells located in the northern North Sea and the Norwegian Sea have been examined with respect to quartz cement. Two different types of quartz cement (Type 1 and Type 2) have been identified using SEM/EDS/CL-analysis of drill-bit cuttings at depths 2370–2670 m (80–85 °C). Type 1 appears as relatively large aggregates (30–100 μm) of depth/temperature related crypto- or microcrystalline to macrocrystalline irregular quartz cement formed by local re-crystallization of biogenic silica. The CL-responses of Type 1 quartz cement give a clear indication of an authigenic origin. Type 2 quartz cement represents relatively high amounts of extremely fine-grained micro-sized (1–3 μm) crystals embedded as discrete, short chains or small clusters/nests within the illitized clay matrix. The CL-responses of micro-quartz crystals indicate an authigenic origin. The micro-quartz is most probably sourced from silica released during the smectite to illite dissolution–precipitation reaction. The petrographic evidence indicates that most of the silica released by the smectite to illite reaction has not been exported out of the mudstones. The silica released produce a subtle inter-connected micro-quartz network interlocked with aggregates of micro-quartz and authigenic clay crystals. This micro-quartz cementation process causes a significant and sharp change in the mudstone stiffness at the onset of the chemical compaction regime. This is indicated by an abrupt increase in well log velocity (Vp) and change in seismic facies close to 2500 m (80/85 °C).  相似文献   

3.
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4.
A 43 cm long E271 sediment core collected near the East Pacific Rise(EPR) at 13°N were studied to investigate the origin of smectite for understanding better the geochemical behavior of hydrothermal material after deposition.E271 sediments are typical metalliferous sediments. After removal of organic matter, carbonate, biogenic opal,and Fe-Mn oxide by a series of chemical procedures, clay minerals(2 μm) were investigated by X-ray diffraction,chemical analysis and Si isotope analysis. Due to the influence of seafloor hydrothermal activity and close to continent, the sources of clay minerals are complex. Illite, chlorite and kaolinite are suggested to be transported from either North or Central America by rivers or winds, but smectite is authigenic. It is enriched in iron, and its contents are highest in clay minerals. Data show that smectite is most likely formed by the reaction of hydrothermal Fe-oxyhydroxide with silica and seawater in metalliferous sediments. The Si that participates in this reaction may be derived from siliceous microfossils(diatoms or radiolarians), hydrothermal fluids, or detrital mineral phases. And their δ30 Si values are higher than those of authigenic smectites, which implies that a Si isotope fractionation occurs during the formation because of the selective absorption of light Si isotopes onto Feoxyhydroxides. Sm/Fe mass ratios(a proxy for overall REE/Fe ratio) in E271 clay minerals are lower than those in metalliferous sediments, as well as distal hydrothermal plume particles and terrigenous clay minerals. This result suggests that some REE are lost during the smectite formation, perhaps because their large ionic radii of REE scavenged by Fe-oxyhydroxides preclude substitution in either tetrahedral or octahedral lattice sites of this mineral structure, which decreases the value of metalliferous sediments as a potential resource for REE.  相似文献   

5.
Abstract

The present study aims to assess whether the smectite‐rich Cochin and Mangalore clays, which were deposited in a marine medium and subsequently uplifted, exhibit consistency limits response typical of expanding lattice or nonexpanding (fixed) lattice‐type clays on artificially changing the chemical environment. The chemical and engineering behaviors of Cochin and Mangalore marine clays are also compared with those of the smectite‐rich Ariake Bay marine clay from Japan. Although Cochin, Mangalore, and Ariake clays contain comparable amounts of smectite (32–45%), Ariake clay exhibits lower consistency limits and much higher ranges of liquidity indices than the Indian marine clays. The lower consistency limits of the Ariake clay are attributed to the absence of well‐developed, long‐range, interparticle forces associated with the clay. Also, Ariake clay exhibits a significantly large (48–714 times) decrease in undrained strength on remolding in comparison to Cochin and Mangalore clays (sensitivity ranges between 1 and 4). A preponderance of long‐range, interparticle forces reflected in the high consistency limits of Cochin and Mangalore clays (wL range from 75 to 180%) combined with low natural water contents yield low liquidity indices (typically <1) and high, remolded, undrained strengths and are considered to be responsible for the low sensitivity of the Indian marine clays.  相似文献   

6.
黏土矿物研究在示踪物源、源区风化历史乃至洋流变迁能力等方面具有重要的作用,尽管目前对南海表层沉积物黏土矿物宏观分布特征已获得了较系统的认识,但在受珠江和海南岛物源共同影响的南海西北部其高分辨率的黏土矿物学工作还未开展。本文对南海西北部表层沉积物中的黏土矿物组成、结晶学特征进行了分析,讨论了研究区黏土矿物的物质来源。结果表明,南海西北部表层沉积物的黏土矿物组成以伊利石为主（平均45%）,蒙脱石（平均27%）与高岭石（平均21%）次之,绿泥石含量最少（平均6%）。在区域空间分布上,蒙脱石含量呈现西南高东北低的特征,伊利石含量则与之相反;高岭石含量表现为近岸高离岸低,而绿泥石含量呈现离岸高近岸低的特征。总体上,研究区表层沉积物黏土矿物组成与现代珠江沉积物组成差异较大,而与珠江口2.5 ka前的沉积物黏土矿物组成比较接近,并且结合研究区岩心揭露的全新世以来极低的沉积速率（4 cm/ka）以及上部很薄（小于0.6 m）的泥质沉积,推测该区表层沉积物可能并非是现代沉积的结果,更可能主要来自全新世末期古珠江输送的沉积物。除古珠江作为研究区表层沉积物中蒙脱石的一个主要来源外,海南岛北部河流也贡献了少量蒙脱石到其邻近的东南海域。运用高岭石/伊利石比值和伊利石化学指数等进一步对比分析表明,珠江是研究区沉积物中伊利石、高岭石和绿泥石的主要物源,其次台湾和海南岛也可能分别提供了少量富含伊利石/绿泥石和高岭石的沉积物。本工作对于南海西北部沉积物源-汇过程和相应的古环境研究有重要的参考意义。  相似文献   

7.
Ketzin, in the Northeast German Basin (NEGB), is the site for pilot injection of CO2 (CO2SINK project) into a saline aquifer (the Upper Triassic Stuttgart Formation) situated at a depth of about 630–700 m. This paper reports the baseline characterization of the reservoir formation based on new core material and well-logs obtained from one injection well and two observations wells, drilled at a distance from 50 m to 100 m from each other. The reservoir is lithologically heterogeneous and made up by fluvial sandstones and siltstones interbedded with mudstones showing remarkable differences in porosity. The thickest sandstone units are associated with channel sandstone, whose thickness varies over short lateral distances. In-depth petrographic, mineralogical, mineral-chemical, and whole-rock geochemical analysis were performed focusing on the sandstone intervals, which display the best reservoir properties for CO2 injection. The dominantly fine-grained and well to moderately-well sorted, immature sandstones classify as feldspathic litharenites and lithic arkoses. Quartz (22–43 wt.%), plagioclase (19–32 wt.%), and K-feldspar (5–13 wt.%) predominate mineralogically. Muscovite plus illite and mixed-layer minerals are omnipresent (4–13 wt.%). Quartz, feldspar, as well as meta-sedimentary and volcanic rock fragments comprise the most abundant detrital components, which often are rimmed by thin, early diagenetic coatings of ferric oxides, and locally of clay minerals. Feldspar grains may be unaltered and optically clear, partially to completely dissolved, partially altered to sheet silicates (mainly illite), or albitized. Analcime and anhydrite constitute the most widespread, often spatially associated pore-filling cement minerals. Authigenic dolomite, barite, and coelestine is minor. The percentage of cements ranges in total from about 5 vol.% to 32 vol.%. Except of samples intensely cemented by anhydrite and analcime, total porosities of the sandstones range from 13% to 26%. The fraction of intergranular porosity varies between 12% and 21%. About 1–5% porosity has been generated by dissolution of detrital plagioclase, K-feldspar, and volcanic rock fragments. The comparatively large modal abundance of feldspars, micas, chlorite, clay minerals, Fe–Ti-oxides, and analcime account for the richness in Ti, Al, Fe, Mg, Na, and K, and the paucity in Si, of the Stuttgart sandstones relative to mature sandstones. Altogether, these sandstones are comparatively rich in minerals that may potentially react with the injected CO2.  相似文献   

8.
This study aims to determine whether the principle that “divalent cation predominance in the pore water precludes quick clay development” applies to the Ariake Bay sediments. The chemical and geotechnical properties of an Ariake clay sediment are presented, and sensitivity is discussed with a focus on pore-water salinity and cation composition. In recent years, reduction of pore-water salinity has occurred due to permeation of river water through the sediments because of overpumping of groundwater. Sodium remains the dominant pore-water cation in an upper zone, whereas divalent cations are dominant in the deeper zone. Divalent cation domination in the deeper zone is ascribed to Ca release from nearby cement-stabilized sediments and to Mg increase in response to a change in river water quality. The upper zone's sensitivity ranged from 15 to 77, and the remolded strength was mostly <0.5 kPa, such that quick clay was present over much of its depth. In contrast, the deeper zone's sensitivity was <40, and its remolded strength exceeded 0.5 kPa; quick clay was not present despite the <2 g/L salinity. The absence of quick clay is ascribed to the high remolded strength caused by the pore-water divalent to monovalent cation ratio being greater than 0.25.  相似文献   

9.
The objective of this study was to investigate the phosphorus distribution in the estuarine sediments of the Daliao river, intensively affected by municipal effluent and agricultural activity for about 50 years. Surface sediment samples were taken at 35 sites in the estuarine area and phosphorus species and contents of total P, Al, Fe, and Ca in the sediments were measured. Results showed that the content of total P in the sediments ranged from 230 to 841 mg kg−1, with an average of 549 mg kg−1. Ca–bound P, residual P, Al–bound P, reductant–soluble P, Fe–bound P, and soluble and loosely bound P were averagely 44.5, 21.6, 13.6, 11.7, 8.9 and 0.2% of total P, respectively. With the gradual increase of total P content, Al–bound P, reductant–soluble P, and Fe–bound P generally increased, while the rest species of P did not. This might indicate that anthropogenic P is bound to Al and Fe oxides. Regression analysis showed that Al–bound P and sum of Fe–bound P and reductant–soluble P were correlated to the contents of total Al and Fe, respectively. On the other hand, Ca–bound P was not correlated to the content of total Ca in the sediment, probably suggesting that Ca–P was mainly from authigenic marine origin. Whereas the content of total P in the estuarine sediments of the Daliao river was within the range of total P content for Chinese and worldwide river estuaries as well as coastal sediments, non-calcium apatite phosphorus content in the estuarine sediments of the Daliao river was relatively higher, indicating higher release risk and bioavailability of P in the sediment. On the other hand, the molar ratio of total Fe to total P was 16–34 in the estuarine sediments of the Daliao river, suggesting that iron oxides/hydroxides in the sediments might be able to sequester more phosphorus. Therefore, the accumulation or release of P in/from the estuarine sediments might be dependent on the external loading of P and the estuarine eutrophication may be sustained by the internal bioavailable P pools following the decrease of the external P loading.  相似文献   

10.
Fine fractions (<2 μm) from smectite-rich flood-plain sediments from the Elbe and Weser rivers in Germany were treated in the laboratory with synthetic sea salt solutions at different concentrations and for different periods of time to document sheet delamination of smectite particles at the fluvial/marine interface and thus simulate changes in clay minerals at the mouth of a river. The project consisted of two steps: (1) Pilot tests were made in 0.5% salt solutions (brackish environment) showing a nearly 2-min delay of the clearing of the clayey suspensions, when the suspension was redispersed 15 and 30 min later after the first measurement. This was due to an obviously increased number of fine laminae from disintegrated smectite particles. (2) X-ray diffraction studies (XRD) of the clayey material from suspensions in 0.0, 0.5, 1.0, and 3.0% salt solutions after standing 10 min, and one and three weeks were made to document alteration reactions of smectite. An aliquot was pipetted periodically from each series, and oriented clay-mineral specimens were analyzed by XRD in air-dried, glycol- and glycerin-solvated states. The peak intensities in the XRD patterns of the clay minerals smectite, random-ordered mixed-layer illite/smectite (I–S), kaolinite, chlorite, and illite did not change in the salt-free series during the experiments. In contrast the peak intensities of smectite and I–S mineral in the air-dried and glycerin-solvated states after suspension in saline solutions decreased significantly. The d-values of the smectitic materials in the air-dried state also decreased after contact with the saline solutions, and in the glycerin-solvated state the full-width-at-half-maximum (FWHM) of the 001 smectite reflection increased by about 20%, indicating a reduction in grain size.  相似文献   

11.
The organic carbon of 280–320 m deep Laurentian Trough sediments at landward and seaward sites (13–24 mgN/g) consisted of carbohydrates (15–22%), hydrolysable amino acids (7–13%), lipids (1–5%), labile proteins (0.3–1%) and a non-characterized fraction (62–74%). Amino acids, proteins and uncharacterized compounds accounted for 21–43, 0.9–4 and 51–78%, respectively, of total nitrogen (1.2–2.2 mgN/g). A clear reactivity trend (pheopigments ? lipids > proteins > amino acids ≈ nitrogen > carbon > carbohydrates) was deduced from the concentration decreases between settling particles and surficial sediments. This was confirmed by one-year inventories in the top cm, burial rates at 35 cm depth, and one-G model calculations. Lipids were a dominant substrate near the sediment-water interface whereas carbohydrates and amino acids constituted the principal energy sources deeper in the sediment. In the porewaters, DOC levels were low (2–6 mg/l) in the top 4 cm, indicating rapid removal (i.e. consumption, irrigation, diffusion), and increased with depth (8–12 mg/l), reflecting the buildup of refractory products. There were also clear compositional changes of DOC with depth. Geographical differences in water column fluxes were recorded in the sediments. The organic contents and ratios were higher at the landward site due to higher rates of sedimentation, bioturbation and terrestrial and total organic inputs. At the seaward station, the lower rates of these processes and stronger marine influence resulted in lower ratios and a more complete decay of organic matter within the top 35 cm sediments.  相似文献   

12.
采用X射线衍射(XRD)分析技术分析了长江水下三角洲地区65个表层沉积物样品的粘土矿物。结果表明,长江水下三角洲粘土矿物主要由伊利石、绿泥石、高岭石和蒙皂石构成;长江水下三角洲亚环境粘土矿物组成则不同,三角洲前缘亚环境中高岭石和绿泥石的平均含量高于前三角洲的含量,而前三角洲亚环境中伊利石和蒙皂石的平均含量高于三角洲前缘的含量。研究区粘土矿物主要来自长江流域,属于长江源,其分布主要受沉积动力环境和粘土本身性质的制约。  相似文献   

13.
Three cores of marine sediments from the Shenhu area in the northern part of the South China Sea were analyzed by XANES analysis for sulfur speciation. The area has been investigated for the presence of hydrocarbons and potential gas hydrate formations. Cored samples of site 4B showed a specific profile of sulfur speciation with sharp and frequent variations in relative contents of sulfate and sulfide, which differed greatly from the profiles obtained for the sediments taken at sites 5B and 6A. The upper part of core 4B (of 0–95 cm) was soft and rich in pore water, containing mainly coarse silt sand. The lower part of the core (i.e., depth > 95 cm) was relatively dryer and darker in color, and dominated by silts and clay resembling sediments from mud volcanoes. The sulfur speciation results revealed that sulfate makes up almost 100 percent of all sulfur species in the upper part of the core 4B, which indicates strong oxidizing conditions, whereas the lower part of the same core has high relative contents of sulfide, sometimes close to 100% S2−. In the lower part of the core, the relative content of sulfide and sulfate changes rapidly and frequently, indicating rapid changes of oxidizing and reducing conditions. On the other hand, the vertical profiles of sulfur species for the cores from sites 5B and 6A are relatively consistent with lower sulfide contents indicative of stable and weaker reducing conditions. We hypothesize that the frequent and sharp variations in the ratios of sulfide to sulfate at site 4B may indicate some intermittent eruption of methane with clay from petroleum reservoirs underneath the sea floor over a relatively short period of time.  相似文献   

14.
Matrix bound phosphine (MBP) is commonly considered an important phosphorus form in the natural environment. This paper presents an investigation of the spatial distribution of MBP and its relationship with the environmental factors in the Changjiang Estuary and its adjacent shelf areas (29.5–32°N, 122–124°E). A total of 32 surface sediment samples were collected from April 24 to 30, 2008. MBP was detected in all samples from the Changjiang Estuary and its adjacent shelf surface sediments (0.1–29.7 ng kg−1). The concentration of MBP decreased from the Changjiang Estuary to the East China Sea shelf. Environmental factors such as organic matter, type of sediments, and redox conditions were found to affect the concentration and distribution of MBP in the marine sediments. Although the MBP levels were low in the marine sediments, the marine biogeochemical cycles of MBP in sediments were established and the information presented here will contribute to the understanding of the biogeochemical cycle of phosphorus.  相似文献   

15.
Quartz and feldspar extracts, obtained by chemical dissolution of the clay minerals, provide insight into the inorganic flocculation and mineralogical influences upon the texture of fine-grained glaciomarine deposits. Glacial comminution in higher latitudes explains the relatively high contents of quartz and feldspars. Quartz and feldspar are better sorted and were apparently less influenced by flocculation than were phyllosilicates, except for particles less than approximately 16 μm, which are poorly sorted due to their inclusion in flocs. The 16-μm boundary is suggested to be useful for approximately separating fines that are usually involved in flocculation and coarse silt and larger grains that are less influenced.  相似文献   

16.
It is generally assumed that estuarine mixing is continuous for metals from terrestrial sources, gradually decreasing towards the open ocean endmember. Here we show that, chemical reactivity, determined by ion exchange method, and molecular weight distributions, obtained using cross-flow ultrafiltration, of dissolved Cd, Cu, and Ni in the surface waters of the Gulf of Mexico varied systematically across the estuarine mixing zone of the Mississippi River. Most size or chemical affinity fractions of dissolved metals (<0.4 μm) were linearly related to salinity (10.8–36.6), suggesting that the distribution of these elements was mainly controlled by continuous mixing processes. Dissolved concentrations across the salinity gradient ranged for Cd: 87–187 pM; Cu: 1.4–18.3 nM; and Ni: 2.6–18.8 nM, with highest values near the Mississippi river mouth, and lowest concentrations in a warm core ring in the Gulf of Mexico. Dissolved Cd was mostly present as a truly dissolved (<10 kDa, 97 ± 1%) and cationic fraction (Chelex-100 extractable, 94 ± 4%). A novel observation across the estuarine mixing zone was that colloidal metal concentrations were identical to either inert (for Cu, Ni) or AMPG-labile anionic (Cu, Cd) fractions. The difference in behavior between Cu and the other two metals might indicate differences in the biopolymeric nature of the metal–organic chelates. In particular, the anionic-organic Cd fractions accounted for just 3 ± 1%, on average. However, for Cu, it was 24 ± 4%, and for Ni, it was 9 ± 6%. The fractions of the total dissolved metal fractions that were “inert” averaged 31 ± 10% for Cu and 29 ± 12% for Ni. Small but noticeable amounts (6 ± 3%) of dissolved inert Cd fractions were also present. Apparent non-local transport processes, likely associated with cross-shelf sediment resuspension processes, could have been responsible for the relatively high concentrations of ‘inert’ and ‘anionic’ metal fractions in high salinity coastal waters, and accounting for the persistence of metals bound to humic substances in the Gulf of Mexico.  相似文献   

17.
The range expansion patterns of Spartina alterniflora and the roles which sexual reproduction and asexual propagation play in range expansion were investigated at the Chongming Dongtan nature reserve in the Yangtze Estuary, China. Two range expansion patterns of S. alterniflora at its advancing fronts could be found (1) S. alterniflora–mudflat front (S–M) and (2) S. alterniflora–Scirpus mariqueter–mudflat front (S–S–M). One feature revealed by this study was that a flush of seedling recruitment and establishment in spring was a crucial way for S. alterniflora to colonize new habitats and achieve a fast rate of range expansion. The mean number of seedlings recruited at the S–M front was much higher than that at the S–S–M front. Once established, the survivorship of seedlings was high, both at the S–M and S–S–M fronts. The established seedlings formed new tussocks quickly by vegetative tillering and growth of rhizomes and these finally merged into dense meadows. The mean distance of range expansion of S. alterniflora, after one growing season at the S–M front, was 25.4 ± 3.1 m yr−1 and 2.7 ± 0.5 m yr−1 at the S–S–M front. Sexual reproduction by seedlings and asexual propagation by tillering and growth of rhizomes were the two main means by which S. alterniflora could maintain a fast rate of range expansion on the salt marshes of the Yangtze Estuary. The colonization behaviors of S. alterniflora on advancing fronts differed as a reaction to various external and internal factors. The impact of abiotic and biotic factors governing the range expansion of S. alterniflora and its implications for the spatial structure of tidal wetlands are discussed.  相似文献   

18.
基于409个表层沉积物样品黏土组成的分析测试结果,结合周边河流沉积物黏土矿物组成数据,研究了渤海—北黄海底质沉积物中黏土矿物的分布特征、来源和输运趋势。渤海—北黄海底质沉积物中黏土矿物组分质量分数从高到低依次为伊利石(70%)、高岭石(12%)、绿泥石(11%)和蒙皂石(7%)。根据沉积物黏土矿物的组成和分布特征,可以把研究区分为2个区和6个亚区。I区物质来源主要为黄河,分布在黄河口外,向东北延伸至渤海海盆、东南向经莱州湾再向东北延伸至北黄海南部海域;II区物质来源主要为潍河、六股河、滦河和辽河等,还包括海岸侵蚀物质,主要分布在辽东湾、渤海湾北部和北黄海北部等海域。黏土矿物的输运和分布除了受到物源的影响外,主要受渤海环流的控制。  相似文献   

19.
The tissue concentrations and contents of calcium, copper and zinc were determined at different stages in the moult cycle of the shore crab Carcinus maenas. The crabs were dissected into the following tissues: the midgut gland, the gill, the ‘rest tissue’ (the latter defined as the left over tissue, i.e. muscles and exoskeleton), and the haemolymph, and analysed as such. One group of crabs was maintained in clean seawater. In these crabs, calcium was lost from haemolymph during premoult and apparently stored in the midgut gland. At ecdysis approximately 90% of the whole body calcium content was lost with the exuviae while the haemolymph calcium content doubled. During early postmoult calcium stored in the midgut gland was used in combination with calcium absorbed from the surrounding seawater for a rapid calcification of the new exoskeleton. Both the copper and zinc concentrations declined in the ‘rest tissue’ compartment during premoult. At ecdysis the haemolymph copper and zinc contents increased 35 and 86%, respectively, from premoult to the newly moult stage. At the same time, however, the copper and zinc haemolymph concentrations decreased. During postmoult, copper (25%) and zinc (45%) were lost from haemolymph and mainly excreted but a minor fraction (5–10%) was stored in the midgut gland.A second group of crabs was maintained in copper-contaminated seawater (0.1 mgl−1) during ecdysis and postmoult. Following exposure of the animals to raised ambient levels of copper, the calcium and zinc contents of the midgut gland were reduced while the copper contents of the midgut gland and most other tissues were increased in comparison to crabs held in clean seawater.  相似文献   

20.
Fish communities of the Ria Formosa coastal lagoon (south Portugal) were sampled on a monthly basis with a beach seine at 4 sites, during two different time periods: 1980–1986 and 2001–2002. Community indices, species ranking and multivariate analysis were used in order to identify changes in the fish community between the two time periods. A total of 153,511 fish representing 57 taxa were recorded. Although species composition was very similar for both sampling periods, multivariate analysis performed on annual species abundance in number and weight revealed differences in fish community structure between the two periods. Structural changes in fish community were related mostly to a sharp decrease in the abundance of Mugilidae from 1980–1986 to 2001–2002. These changes were probably associated to a decrease in organic matter contents and nutrients concentrations due to improvements in sewage treatment and better water circulation inside the lagoon. The changes in fish community structure are more evident in the inner areas of the lagoon than near the inlet. The association between changes in sewage patterns and changes in the ichthyofaunal community structure reinforces the importance of fish communities as a biological indicator of human induced changes in marine systems.  相似文献   

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