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1.
Extensive artificial waterways have replaced natural wetlands and created new estuarine habitats on the southern Queensland coast, Australia. Economically important fish species found in adjacent natural wetlands of mangrove, saltmarsh and seagrass also occur in the artificial waterways. Stable isotope analyses (δ13C, δ15N) were used to test whether the relative importance of basal sources of energy varied for foodwebs found in artificial (canals and tidal lakes) and natural waterways. None of the fish species differed in their isotope values between artificial waterways. In contrast, isotopic signatures of snub-nosed garfish (Arrhamphus sclerolepis; Hemiramphidae) varied greatly between natural and artificial waterways, having highly enriched δ13C values (−10.5‰) in natural wetlands, demonstrating reliance on seagrass (−11.4‰), and significantly less enriched values (−19.0‰) in artificial waterways, consistent with either local algal sources (−19.8 to −20.4‰) or a mixture of seagrass and other less enriched autotrophs from adjacent natural wetlands. Isotopic signatures of sand whiting (Sillago ciliata; Sillaginidae) were also significantly more enriched in natural (−18.2‰) than artificial (−21.0‰) habitats, but means were not far enough apart to distinguish between different sources of nutrition. δ13C values of yellowfin bream (Acanthopagrus australis; Sparidae) did not differ between artificial and natural habitats (about −20‰ in both). δ15N values of fish varied among habitats only for A. sclerolepis, which in artificial waterways had values enriched by 2‰ over those in natural waterways. This was consistent with a shift from seagrass (relatively depleted δ15N) as a source in natural habitat to algal sources (relatively enriched δ15N) in artificial habitats. This study provides some of the first evidence that at least some fish species rely on different autotrophs in artificial waterways than in adjacent natural wetlands.  相似文献   

2.
A common first step in conservation planning and resource management is to identify and classify habitat types, and this has led to a proliferation of habitat classification systems. Ideally, classifications should be scientifically and conceptually rigorous, with broad applicability across spatial and temporal scales. Successful systems will also be flexible and adaptable, with a framework and supporting lexicon accessible to users from a variety of disciplines and locations. A new, continental-scale classification system for coastal and marine habitats—the Coastal and Marine Ecological Classification Standard (CMECS)—is currently being developed for North America by NatureServe and the National Oceanic and Atmospheric Administration (NOAA). CMECS is a nested, hierarchical framework that applies a uniform set of rules and terminology across multiple habitat scales using a combination of oceanographic (e.g. salinity, temperature), physiographic (e.g. depth, substratum), and biological (e.g. community type) criteria. Estuaries are arguably the most difficult marine environments to classify due to large spatio-temporal variability resulting in rapidly shifting benthic and water column conditions. We simultaneously collected data at eleven subtidal sites in the Columbia River Estuary (CRE) in fall 2004 to evaluate whether the estuarine component of CMECS could adequately classify habitats across several scales for representative sites within the estuary spanning a range of conditions. Using outputs from an acoustic Doppler current profiler (ADCP), CTD (conductivity, temperature, depth) sensor, and PONAR (benthic dredge) we concluded that the CMECS hierarchy provided a spatially explicit framework in which to integrate multiple parameters to define macro-habitats at the 100 m2 to >1000 m2 scales, or across several tiers of the CMECS system. The classification's strengths lie in its nested, hierarchical structure and in the development of a standardized, yet flexible classification lexicon. The application of the CMECS to other estuaries in North America should therefore identify similar habitat types at similar scales as we identified in the CRE. We also suggest that the CMECS could be improved by refining classification thresholds to better reflect ecological processes, by direct integration of temporal variability, and by more explicitly linking physical and biological processes with habitat patterns.  相似文献   

3.
The influence of prolonged mouth closure on the population dynamics of the caridian shrimp, Palaemon peringueyi and the estuarine isopod, Exosphaeroma hylocoetes, in the littoral zone of temporarily open/closed Kasouga Estuary located on the south-eastern coastline of southern Africa was assessed monthly over the period October 2007 to September 2008. Prolonged mouth closure of the estuary contributed to hypersaline conditions (psu > 35) prevailing throughout the estuary for the last four months of the study. The high salinities coincided with a decrease in the areal extent (up to 80%) of the submerged macrophytes, mainly Ruppia maritima, within the littoral zone of the estuary. Total abundance and biomass values of the shrimp and isopod over the period of investigation ranged from 0 to 14.6 ind m−2, from 0 to 13.3 mg dwt m−2, from 12 to 1540 ind m−2 and from 0.1 to 2.16 mg dwt m−2, respectively. Maximum values of both the shrimp and isopod were recorded in the upper reaches of the estuary in close association with R. maritima. Over the course of the investigation, both the abundance and biomass values of the shrimp decreased significantly (P < 0.05 in both cases) which could be related to reduced habitat availability, R. maritima, that acts as a refuge against fish predation. Additionally, the decrease in abundance and biomass values could be attributed to reduced recruitment opportunities for the shrimp and the cessation of reproduction in the estuarine isopod. The establishment of a link to the marine environment following an overtopping event in September 2008 contributed to a decrease in salinity within the system although no recruitment of either the isopod or shrimp was recorded.  相似文献   

4.
Habitat variability is one of the factors influencing species richness within estuarine systems, and a loss of habitat can result in a restructuring of the estuarine ichthyofaunal assemblage, particularly if these conditions persist over long time periods. The potential effects of the loss of extensive submerged macrophyte beds (Ruppia cirrhosa and Potamogeton pectinatus) on an estuarine fish assemblage were investigated through an analysis of a long-term seine net catch dataset from the temporarily open/closed East Kleinemonde Estuary, South Africa. Catch data for a 12-year period, encompassing six years of macrophyte presence and six years of macrophyte senescence, indicated that the loss of this habitat did not influence species richness but changes in the relative abundance of certain species were evident. A shift in dominance from vegetation-associated species to those associated with sandy environments (e.g. members of the family Mugilidae) was observed. However, species wholly dependent on macrophytes such as the critically endangered estuarine pipefish Syngnathus watermeyeri were only recorded during years when macrophyte beds were present, while vegetation-associated species such as the sparid Rhabdosargus holubi persisted at lower levels of relative abundance. The reduced abundance of all vegetation-associated fish species during years of macrophyte senescence was probably reflective of declining food resources resulting from the loss of macrophyte beds and/or increased vulnerability to predation. Submerged beds of aquatic plants are therefore important habitats within temporarily open/closed estuaries, South Africa’s dominant estuary type.  相似文献   

5.
This study provides one of the first quantitative assessments of a fish community within a pre-operational deep-water port system. Between September 2006 and September 2007 a total of 4 559 fish, representing 47 species and 27 different families, were caught within the Port of Ngqura, Africa's deepest port, in 1 713 h of shore-angling (2.3 fish angler?1 h?1) and 239 h of boat-based angling (2.8 fish angler?1 h?1). Catches included species representative of both estuarine and shore fisheries. The majority (83.0%) of species caught were marine and most were less than size-at-50% maturity (71.4%). The most abundantly caught species were dusky kob Argyrosomus japonicus (25.5%), elf Pomatomus saltatrix (24.9%), garrick Lichia amia (17.7%) and dusky sharks Carcharhinus obscurus (10.7%). Fish assemblages differed significantly (p < 0.05) between three habitats within the port: Dolosse, Quay Wall and Sandy Shore. The Dolosse habitat supported the greatest abundance (3.1 fish angler?1 h?1) and diversity (43) of fish species. The Port of Ngqura is thus functioning as an important habitat for both juvenile and adult fish. The diversity and abundance of fish suggests the potential use of ports and other artificial structures for enhancing local fisheries in South Africa.  相似文献   

6.
Terebralia palustris (Linnaeus, 1767), the giant mangrove whelk, is a prominent component of mangrove ecosystem malacofauna in the Indo-West-Pacific region. On the east coast of Africa, the range of T. palustris extends from Kenya to the south-eastern region of South Africa. Here we confirm reports that, in the past three decades, South African populations of T. palustris have declined. Terebralia palustris was recorded from only two estuarine mangrove forests during a survey conducted in 2013/2014, compared with six estuaries in the mid-20th century. At Kosi Bay, the average density was 186 ind. m-2 (SD 106) in comparison to only 7.1 ind. m-2 (SD 2.5) at Durban Bay. Additionally, the average shell height of individuals at Durban Bay (60.3 mm; SD 13.6) was larger than at Kosi Bay (31.84 mm; SD 16.09), indicating the absence of juvenile snails at Durban. In South Africa there have been no previous ecological assessments for T. palustris and, aside from one detailed survey in the early 1960s, most of the occurrence records are vague and incomplete. The ultimate causes for the decline of this charismatic gastropod in South Africa are unknown, although habitat loss and degradation are suspected. Further research is urgently required, particularly into aspects concerning its disappearance at the limit of its global distribution.  相似文献   

7.
近30年来莱州湾滨海湿地景观格局变化特征研究   总被引：1，自引：0，他引：1  
基于莱州湾滨海湿地特征以及野外实地观测数据,利用landsat遥感影像手段,揭示1990-2018年莱州湾滨海湿地景观格局变化特征,并结合景观指数分析莱州湾滨海湿地异质性。结果表明,1990-2018年间,莱州湾滨海湿地呈现人工滨海湿地面积增加而天然滨海湿地面积减少的趋势。截至2018年莱州湾滨海湿地面积总计1954 km2,其中天然滨海湿地面积约811 km2,人工滨海湿地面积约1143 km2,分别占42%和58%,滨海湿地向建设用地转化的趋势明显。斑块数量(NP)、景观形状指数(LSI)等景观指数值逐年增加,表明莱州湾景观格局变化明显且破碎化程度加重。莱州湾滨海湿地不同岸段之间景观格局变化的驱动机制不完全一样,造成其破碎化程度加重的自然驱动力有河口冲淤、降水量变化等;人工驱动力主要为城市建设、港口码头、集约用海区建设、养殖业和制盐业的开发利用等。以期对莱州湾滨海湿地生态系统退化原因、合理利用和优化土地空间布局、采取有效措施保护及恢复滨海湿地提供一定的技术支持。  相似文献   

8.
The Berriasian-Valanginian Springhill Formation of the Austral Basin of southern South America comprises fluvial to marine deposits. In order to interpret depositional systems and unravel the stratigraphic architecture of this unit in the southern region of the basin (Tierra del Fuego Province, Argentina), 500 m of cores combined with well-log data from 41 wells were studied. Facies associations corresponding to fluvial (A1-A6), estuarine (B1-B5) and open-marine (C1-C4) depositional environments were identified. These facies associations succeed each other vertically across the entire study area (6800 km2) forming a ∼120-m-thick transgressive succession. This unit filled a north-south-oriented valley system, developed in the underlying Jurassic volcanic complex.Lowstand fluvial deposits of the first stage of the valley-system fill occur in downdip segments of the system above a sequence boundary (SB). These fluvial deposits are overlain by coastal-plain and tide-dominated estuarine strata across an initial transgressive surface (ITS). In the northern sector the earliest valley infill is characterized by a transgressive fluvial succession, overlying a merged SB/ITS that is probably time-equivalent of marginal-marine deposits of the southern sector. The fluvial strata in the north are overlain by wave-dominated estuarine deposits. A drastic change to open-marine conditions is marked by a marine flooding surface, with local evidence of marine erosion (FS-RS). Open-marine strata are thin (<10 m) and dominated by lower-shoreface and offshore-transition deposits. They are capped by a younger flooding surface (FS), which represents the onset to offshore conditions across the study area due to a continuous long-term transgression that persisted until the Barremian.Although the interpreted depositional systems and stratigraphic architecture of the Springhill Formation resemble transgressive incised-valley-fill successions, the greater thickness and larger size of the Springhill valleys suggest inherited rift topography rather than valley development during a relative sea-level fall.  相似文献   

9.
This study investigated whether the current lateral boundary for estuaries in South Africa, i.e. the estuarine functional zone (EFZ), includes all estuarine habitats. The EFZ covers 173 930 ha in 304 estuaries/outlets nationally. Field surveys and analysis of available aerial images showed that 82 (12 956.70 ha) of these estuaries (26%) had estuarine habitats occurring outside of this boundary. As a result of mapping scale, the National Vegetation Map does not represent habitats that are associated with small estuaries (approximately 50% of South Africa's estuaries). For estuaries in the Western and Eastern Cape provinces, most habitats have been lost due to urban development, whereas in subtropical areas (northern Eastern Cape and KwaZulu-Natal), cultivation has removed estuarine habitat. Although delineation of boundaries can be complicated by landcover changes, the estuarine lateral boundary in Cape estuaries could be identified based on sediment characteristics (moisture content, organic content, electrical conductivity), groundwater characteristics (salinity, conductivity and depth) and plant species. The delineation of the EFZ needs to be consistent, inclusive of all estuarine physical and biological processes, and cost-effective to identify so that it can protect estuarine habitats.  相似文献   

10.
Orangemouth weakfish is a highly important commercial and game fish species in the Tropical Eastern Pacific. However, studies assessing changes in its trophic ecology relevant to size, habitat (estuarine or marine), sex, and season are non-existent. We assessed its feeding habits and trophic position (TP) according to the aforementioned factors. Its dietary breadth was composed of 33 types of prey. The feeding habits and isotopic values (δ15N and δ13C) registered differences between sizes and habitat, indicating ontogenetic changes in habitat as well as TP, as small organisms (TP 3.1) inhabited the estuarine area, feeding on benthic and demersal organisms, whilst large individuals (TP 4.1) were found in the marine environment, preying on nektonic fauna. No differences were found according to sex or season. The isotopic niche showed that the different sizes of this species are generalist consumers with a clear separation between them and between habitats, as estuarine juvenile and preadult organisms have a larger isotopic niche than adult marine organisms, being an opportunist tertiary estuarine consumers during their early stages, and a top marine predator during the adult phase.  相似文献   

11.
The Mfolozi estuary, located on the east coast of South Africa, was historically directly linked to the adjacent St Lucia estuarine system, the largest estuarine system in Africa and a World Heritage Site. The Mfolozi used to be the main feeder system to maintain lake levels in St Lucia, but increased siltation from sugar cane farming in the Mfolozi floodplain led to artificial separation of the two systems in 1950. Reduced freshwater inflow due to drought conditions caused the St Lucia mouth to remain closed from June 2002 to present, coinciding with low lake levels and hypersaline conditions, except for a brief period during 2007 after the St Lucia mouth breached. These conditions led to disruption of larval recruitment into the system and major changes in biotic communities. Due to the importance of the St Lucia – Mfolozi System link, a study was initiated in 2007 on the fish community of the Mfolozi system, which was sampled using seine and gill nets. The 48 species recorded were dominated by juveniles of marine spawners, particularly Leiognathus equula and Valamugil cunnesius and the estuarine spawners Ambassis dussumieri and Ambassis natalensis. Estuarine dependent marine spawning species formed 68% of both the species numbers and CPUE, an indication of the regional importance of the Mfolozi estuary as an alternate refuge for juvenile marine fish during periods when the St Lucia system remained closed. Post-larval recruits of marine spawning species were particularly abundant, but low zoobenthic densities caused a rapid decline in numbers of benthic feeders shortly after their recruitment into the system. The importance of the Mfolozi estuary in maintaining marine brood stocks of estuarine dependent marine fish is discussed with particular reference to estuarine degradation and the ecological integrity of the St Lucia system.  相似文献   

12.
The Early Cretaceous South Atlantic Magmatic Province (SAMP), which includes the Paraná-Etendeka LIP, produced about 8 million km3 of tholeiitic basalt and diabase over an area of 4 million km2. Huge pre-salt oil reserves, discovered in 2007 by Petrobras in non-marine carbonates, are estimated at more than 45 billion barrels. Here we show the close causal relationship of the southward increasing width of the wedge-shaped South Atlantic rift with the similarly southward increase in igneous activity, in the thicknesses of non-marine carbonate and salt, and in the size of oil reserves, all controlled mainly by South America’s early clockwise rotation away from Africa about a pole in its northeast. Large diabase dike swarms transversal to the rift witness to South America’s rotation that opened in its wake the southward widening South Atlantic rift. Westward increasing pressure on the Equatorial margin by South America’s clockwise rotation forced open the Benue trough and created pre-late-Aptian folds in the Demerara Plateau and in Brazil’s Solimões (Upper Amazonas) basin. Prerift and synrift volcanic activity increases southward, culminating in the Parana-Etendeka LIP and in the offshore volcanic SDRSs that continue southward to the Cape Basin. Berriasian-Valanginian rift sediments deposited from about 145 Ma, 10 Ma before the flood basalts of the Parana-Etendeka LIP. The largest transversal dike swarm continued in the proto-Walvis Ridge that separated the central South Atlantic endorheic rift basin from the sea in the south; erosion and leaching of basalts supplied Ca, Mg, and SiO2 to the endorheic basin for the deposition of non-marine carbonates and authigenic clays. Basalt flows intercalated with carbonates nearly until salt deposition about 113 Ma. Hypogenic leaching of carbonates by mantle-derived CO2 created optimal reservoirs. Supergiant oil deposits occur where the widest endorheic basin and the volcanic province overlap.  相似文献   

13.
The annual total and organic mercury bioaccumulation pattern of Scrobicularia plana and Hediste diversicolor was assessed to evaluate the potential mercury transfer from contaminated sediments to estuarine food webs. S. plana was found to accumulate more total and organic mercury than H. diversicolor, up to 0.79 mg kg−1 and 0.15 mg kg−1 (wet weight) respectively, with a maximum annual uptake of 0.21 mg kg−1 y−1, while for methylmercury the annual accumulation was similar between species and never exceeded 0.045 mg kg−1 y−1. The higher organic mercury fraction in H. diversicolor is related to the omnivorous diet of this species. Both species increase methylmercury exposure by burrowing activities and uptake in anoxic, methylmercury rich sediment layers. Integration with the annual biological production of each species revealed mercury incorporation rates that reached 28 μg m−2 y−1, and to extract as much as 11.5 g Hg y−1 (of which 95% associated with S. plana) in the 0.4 km2 of the most contaminated area, that can be transferred to higher trophic levels. S. plana is therefore an essential vector in the mercury biomagnification processes, through uptake from contaminated sediments and, by predation, to transfer it to economically important and exploited estuarine species.  相似文献   

14.
Estimates of the tropospheric lapse rate γ and analysis of its relation to the surface temperature T s in the annual cycle and interannual variability have been made using the global monthly mean data of the NCEP/NCAR reanalysis (1948–2001). The tropospheric lapse rate γ is about 6.1 K/km in the Northern Hemisphere (NH) as a whole and over the ocean and about 6.2 K/km over the continents. The value of γ decreases from 6.5 K/km at low latitudes to 4.5 K/km at polar latitudes. The values of dγ/dT s, the parameter of sensitivity of γ to the variation of T s for the NH in the interannual variability, are found to be about 0.04 km?1 (0.041 km?1 for the NH as a whole, 0.042 km?1 over the ocean, and 0.038 km?1 over the continents). This corresponds to an increase in γ of approximately 0.7% when the surface temperature of the NH is increased by 1 K. Estimates of dγ/dT s vary from about 0.05 km?1 in the subtropics to 0.10 km?1 at polar latitudes. When dγ/dT s is positive, the surface and tropospheric warming means a temperature decrease above a certain critical level H cr. The height of the level H cr with constant temperature, which is defined by the inverse value (dγ/dT s)?1, is about 25 km for the NH as a whole, i.e., above the tropopause. In the subtropics, H cr is about 20 km. At polar latitudes, H cr decreases to about 10 km. Positive values of dγ/dT s characterize a positive climatic feedback through the lapse rate and indicate a general decrease in the static stability of the troposphere during global warming. Along with a general tendency of γ to increase with rising T s, there are regional regimes with the opposite tendency, mainly over the ocean. The negative correlation of γ with T s is found over the oceanic tropics and midlatitudes, in particular, over the oceanic belt around Antarctica.  相似文献   

15.
Abstract

Numerous large sediment slides and slumps have been discovered and surveyed on the continental margins of Northwest Africa, Southwest Africa, Brazil (Amazon Cone), the Mediterranean, the Gulf of Mexico, and North America over the past 10 years. The mass movements are of two primary types: (1) translational slides, and (2) rotational slumps. Translational slides are characterized by a slide scar and a downslope zone of debris flows, after traveling in some areas for several hundreds of kilometers on slopes of less than 0.5°. Rotational slumps are bounded by steep scarps, but they do not involve large‐scale translation of sediments, although seismic records indicate disturbance in the down‐dropped block. Many of the slides and slumps have occurred in water depths greater than 2000 m on initial slopes of less than 1.5°. The largest slide so far discovered is off Spanish Sahara; in this case, the slide scar is 18,000 km2 in area, at least 600 km3 in volume of translated sediments. No apparent consistent relationship has yet been observed between the presence of the slides and the sedimentary environment in which they occurred. The slides off Southwest Africa and Spanish Sahara occurred in pelagic sediments rich in planktonic organic matter. In contrast, the slides off North America, Senegal‐Mauritania, and Brazil (Amazon Cone) occurred in sediments containing a high percentage of terrigenous material from nearby landmasses. Large sediment slides have also occurred in pelagic sediments on isolated oceanic rises such as the Madeira Rise (East‐Central Atlantic) and the Ontong‐Java Plateau (Pacific), where sedimentation rates are less than 2 cm/1000 years. The failure mechanism of the slides initiated near the shelf edge can probably be explained by sediment overloading during low glacio‐eustatic sea level, which allowed rivers to debouch sediments directly onto the outer shelf or upper slope. Possible mechanisms of failure of the deepwater slides and slumps include earthquakes, undercutting of the slope by bottom currents, and changes in porewater pressures induced as a direct or indirect result of glacio‐eustatic changes in sea level.  相似文献   

16.
River plumes are the regions where the most intense river-sea-land interaction occurs, and they are characterized by complex material transport and biogeochemical processes. However, due to their highly dynamic nature, global river plume areas have not yet been determined for use in synthetic studies of global oceanography. Based on global climatological monthly averaged salinity data from the NOAA World Ocean Atlas 2009 (WOA09), and monthly averaged salinity contour maps of the East and South China Seas from the Chinese Marine Atlas, we extract the monthly plume areas of major global rivers using a geographic information system (GIS) technique. Only areas with salinities that are three salinity units lower than the average salinity in each ocean are counted. This conservative estimate shows that the minimum and maximum monthly values of the total plume area of the world’s 19 largest rivers are 1.72 × 10 6 km 2 in May and 5.38 × 10 6 km 2 in August. The annual mean area of these river plumes (3.72 × 10 6 km 2 ) takes up approximately 14.2% of the total continental shelves area worldwide (26.15 × 10 6 km 2 ). This paper also presents river plume areas for different oceans and latitude zones, and analyzes seasonal variations of the plume areas and their relationships with river discharge. These statistics describing the major global river plume areas can now provide the basic data for the various flux calculations in the marginal seas, and therefore will be of useful for many oceanographic studies.  相似文献   

17.
The scale of landscape pattern formation of an ecological community may provide clues as to the processes influencing its spatial and temporal dynamics. We conducted an examination of the spatial organization of an annual seagrass (Halophila decipiens) in an open ocean setting at two spatial scales and growing seasons to identify the relative influence of external (hurricanes) versus internal (clonal growth) factors. Visual surveys of seagrass cover were conducted over 2 years within three replicate 1 km2 study areas each separated by ∼25 km in an inshore–offshore transect along the southwest coast of Florida at depths between ∼10 and 30 m. A towed video sled allowed observations of seagrass cover of 1 m2 areas approximately every 6 m over thousands of meters of evenly spaced transects within the study areas (coarse scale). The towed video revealed that 17.5% of the seafloor was disturbed irrespective of location or sample time. Randomly selected 10 × 10 m quadrats within the larger, 1 km2 study areas were completely surveyed for seagrass cover by divers at 0.625 m2 resolution (fine scale). The coarse-scale observations were tested using both conventional geostatistics and an application of a time-series technique (Runs test) for scale of seagrass cover contiguity. Fine-scale observations were examined using conventional geostatistics and a least squares approach (cumulative logistic).  相似文献   

18.
Twenty-one juvenile Cape stumpnose Rhabdosargus holubi (140–190 mm fork length) were tagged with internal acoustic transmitters in the lower, middle and upper reaches of the Kowie Estuary, South Africa. The movements of each fish were continually monitored from October 2014 to February 2015 using 22 stationary data-logging acoustic receivers situated at approximately equidistant intervals along the length of the estuary (21 km). Juvenile R. holubi spent the greatest proportion of time within the estuarine environment (83%), with the sea (16%) and riverine (1%) environments used to a much lesser extent. Within the estuarine environment, tagged individuals showed high levels of residency and fidelity to their capture and release sites; however, the degree of residency was dependent on the position of the release site, with batches in the upper and lower reaches exhibiting different space-use patterns. When larger juvenile R. holubi migrated back to the sea for the next phase of their life cycle, they generally did not return to the estuarine environment, thus indicating a permanent ontogenetic shift in habitat use with the onset of sexual maturity. This contribution to our understanding of the ecology of a ubiquitous estuarine fish further highlights the importance of estuarine habitats as nursery areas that require effective management.  相似文献   

19.
Spartina alterniflora Loisel., widely recognised as an aggressive invader of estuaries and salt marshes around the world, was discovered growing in the temporarily open/closed Great Brak Estuary on the southern Cape coast of South Africa in 2004. This is the first record of this invasive plant in Africa as well as its first occurrence in an estuary that closes to the sea. Plant traits and sediment characteristics were measured in 2009 and 2011 and found to be comparable to those reported elsewhere. Prior to the 2011 sampling, S. alterniflora stands had been flooded for almost eight months. As a result, sediment redox potential (?268 + 4 mV) was significantly lower in 2011. Sediments were mostly clay in 2009 (71 ± 0.01%) compared to a predominance of sand in 2011 (40 ± 0.02%). These differences were related to the artificial breaching of the estuary one month prior to sampling in March 2011. The grass currently occupies 1.1 ha in the salt marsh, sandflat and mudflat habitats of the estuary where its cover is expanding at a rate of 0.162 ha y?1. Individual stands numbered about 12 in 2006, but have increased to 24 in 2011. These stands are expanding laterally at 0.9 m y?1 although the long period of inundation during 2010 reduced this to 0.6 m y?1. Expansion is due to vegetative spread as an analysis of the sediment seed bank showed no S. alterniflora seeds and very few salt marsh seeds (1 132 seeds m?2). If left unchecked, S. alterniflora has the potential to replace 42.9 ha or 41% of the total estuary habitat in the Great Brak Estuary, but also has the potential to invade other estuaries in South Africa, especially those with extensive intertidal habitat and containing S. maritima (19 estuaries in total). This study illustrates the adaptive potential of this invasive marsh plant and indicates the possibility of invasion in seasonally closed estuaries in other locations around the world.  相似文献   

20.
Possible changes in the climate characteristics of the Northern Hemisphere in the 21st century are estimated using a climate model (developed at the Obukhov Institute of Atmospheric Physics (OIAP), Russian Academy of Sciences) under different scenarios of variations in the atmospheric contents of greenhouse gases and aerosols, including those formed at the OIAP on the basis of SRES emission scenarios (group I) and scenarios (group II) developed at the Moscow Power Engineering Institute (MPEI). Over the 21st century, the global annual mean warming at the surface amounts to 1.2?C2.6°C under scenarios I and 0.9?C1.2°C under scenarios II. For all scenarios II, starting from the 2060s, a decrease is observed in the rate of increase in the global mean annual near-surface air temperature. The spatial structures of variations in the mean annual near-surface air temperature in the 21st century, which have been obtained for both groups of scenarios (with smaller absolute values for scenarios II), are similar. Under scenarios I, within the extratropical latitudes, the mean annual surface air temperature increases by 3?C7°C in North America and by 3?C5°C in Eurasia in the 21st century. Under scenarios II, the near-surface air temperature increases by 2?C4°C in North America and by 2?C3°C in Eurasia. An increase in the total amount of precipitation by the end of the 21st century is noted for both groups of scenarios; the most significant increase in the precipitation rate is noted for the land of the Northern Hemisphere. By the late 21st century, the total area of the near-surface permafrost soils of the land of the Northern Hemisphere decreases to 3.9?C9.5 106 km2 for scenarios I and 9.7?C11.0 × 106 km2 for scenarios II. The decrease in the area of near-surface permafrost soils by 2091?C2100 (as compared to 2001?C2010) amounts to approximately 65% for scenarios I and 40% for scenarios II. By the end of the 21st century, in regions of eastern Siberia, in which near-surface permafrost soils are preserved, the characteristic depths of seasonal thawing amount to 0.5?C2.5 m for scenarios I and 1?C2 m for scenarios II. In western Siberia, the depth of seasonal thawing amounts to 1?C2 m under both scenarios I and II.  相似文献   

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