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1.
Invasions by non-indigenous species (NIS) have been suggested to alter local, regional and global biota on unprecedented scales. To manage NIS, it is pivotal to identify whether a species is introduced or native, but even today the geographical origin of thousands of species worldwide remain uncertain. Most of these ‘cryptogenic species’ are inconspicuous and rare, but in a few instances, they can also be abundant and conspicuous species, with large impacts on community structure. The identification of cryptogenic species, and summarizing information on their most likely origin, is an important task in invasion biology, and can highlight the need for research and management. Here, we document that the gastropod Batillaria australis in the Swan River estuary (Perth, Western Australia) is a conspicuous species of uncertain origin. A literature review combined with new survey data revealed that all evidence point to a recent human-mediated transfer; for example, it is absent from the fossil record, was first collected in 1954, has a low parasite diversity, has increased its population size dramatically in recent times, is separated by >3000 km from conspecifics, has no long-distance dispersal mechanisms, and existing ocean currents run against a natural range extension. Surprisingly, despite political and scientific focus on NIS hardly any ecological data have been published on this species from Western Australia. We show that B. australis is highly abundant in both seagrass beds (424 ± 29 ind m−2) and on unvegetated sand flats (92 ± 22 ind m−2) being orders of magnitudes more abundant than any native gastropod in the Swan River. Experiments showed that high resistance to predation and environmental stress potentially explains its success. From our survey data, we calculated that >3.6 billion invasive snails today occupy the Swan River. This large snail populations support other organisms; for example, almost 1 billion macroalgae are found attached to living B. australis and >100 million hermit crabs occupy its empty shells. Given Battilaria’s high abundance, wide distribution, large size, persistent shells that support other organisms and bioturbating behavior, it seems inescapable that this potential invader has impacted the ecosystem functioning of the Swan River. We argue that the search for abundant species of uncertain origin should continue, and that these species generally should be treated with the same interest as high status invaders to mitigate impacts in already invaded systems and to avoid secondary spread into neighboring ecosystems.  相似文献   

2.
There have been comparatively few large-scale studies on spatial genetic structure of bathyal seafloor fauna, despite the importance of these data to the successful management of the world's oceans. We use a comparative analysis of mitochondrial DNA from five bathyal (200–3500 m) species of brittle-stars (Ophiuroidea) to assess phylogeographic structure along an extensive (8000 km) longitudinal gradient at temperate latitudes (28–56°S) from south-west Australia (113°E) to seamounts east of New Zealand (175°W). We found no evidence of a genetic discontinuity between Australia and New Zealand, either across the temperate Tasman Sea or across the Southern Ocean between the South Tasman Rise and the Macquarie Ridge. However, there were latitudinal phylogeographical breaks between tropical, temperate and polar regions; longitudinal breaks across the eastern Indian Ocean; and a bathymetric break at approximately 1700 m. Although there was limited regional structure in the frequency of haplotype distributions within the major clades, and no clade appeared to be strictly panmictic, the regional structure in general was not concordant with a simple isolation-by-distance model. Demographic structure varied with three clades having a simplified haplotype network, low effective population sizes and no evidence of significant population expansion, and two clades having a high diversity of haplotypes, relatively high effective population sizes and signs of recent population expansion. These results are discussed with respect to putative dispersal strategies. We hypothesise that the ‘brooding’ species produce both brooded young and pelagic larvae, allowing for both the maintenance of local populations and long-distance dispersal.  相似文献   

3.
Transplant trials of the seagrass Posidonia australis were carried out after loss of seagrasses following eutrophication and increased turbidity in two marine inlets on the south coast of Western Australia. A pilot study in Oyster Harbour measured survival and growth in situ for 4 years. Long-term survival rates were high (96–98%), providing plants were anchored into the sediment. All unanchored plants were lost in the first winter. Following the success of the pilot study, a more comprehensive program began 3 years later with over 500 transplant units collected from either actively growing edges of nearby patches (plagiotropic growth form) or within established meadows (orthotropic growth form). Transplant units from edges expanded at a faster rate compared to units from mid-meadow but increases in shoot numbers were similar. Growth rates in the first 2.5 years averaged 10–20 cm yr−1 horizontal rhizome extension, depending on the source of the transplant units, and 4–12 shoots per initial shoot yr−1, depending on the initial shoot number of the transplant unit. After 5 years, shoot numbers of individual transplants were similar to shoot densities recorded for natural meadows, >500 shoots m−2. Approximately, 10% of transplants from mid-meadow flowered in the first year, whereas transplants from edges flowered only after 5 years.  相似文献   

4.
The Dongsha Basin, circling Dongsha Island that is amid the northern margin of the South China Sea, is characterized by thin (∼0.5 km) Cenozoic sediments veneering on thick (up to 5 km) Mesozoic strata. Recently, several geophysical and geological surveys, including multiple channel reflection seismic, sub-bottom profiling and benthic dredging, have been conducted on the slope southwest to the Dongsha Island, where the water depth varies from 400 m to 2000 m. A novel discovery is numerous submarine mud volcanoes of various sizes over there, typically 50–200 m high and 0.5–5 km wide. Geophysical profiles document their unusual features, e.g., roughly undulating seafloor, high-amplitude seabed reflectivity, foggy hyperbolic diffractions up to 50 m in water column above seabed, and internal reflection chaos and wipe-out down to 2–3 km level or deeper below the seabed. Benthic dredging from the mud volcanoes gives abundant faunas of high diversity, e.g., scleractinian (stony coral), gorgonian, black coral, thiophil tubeworm, glass sponge, bryozoan etc., indicating booming chemosynthetic community, among which the Lophelia pertusa-like coral and the Euretidae-like glass sponges are the first reports in the South China Sea. Concomitantly with them, there are also abundant authigenic carbonate nodules and slabs, raw, brecciated and breccias with bio-clasts congregation. Besides, there coexist massive mudflows and allogenic coarse-grained quartz, feldspar and tourmaline most likely brought out by mud volcanism. Geochemical analysis of the bottom water samples give dissolved methane concentration up to 4 times higher than the background average. These results lend comprehensive evidences for the ongoing and historical mud volcanism. The escaping methane gas is inferred to source mainly from the Mesozoic strata. Occupying a large province of the deep water slope, ca. 1000 km2 or more, the mud volcanoes is prospective for gas hydrate and natural gas for the Dongsha Basin.  相似文献   

5.
The biomass of reproductive litter from Posidonia oceanica deposited on a 3.5 km stretch of beach in the north-western Mediterranean, as a consequence of a storm in May 2004, was quantified. The damage caused by this storm to the meadow from which fruits originated was evaluated in terms of loss of seed production. Intermediate fruits (i.e., developing fruits) were the most important reproductive component, followed by immature and damaged fruits. No fully mature fruits were found. No significant differences in the average number of fruits and biomass accumulated were detected among beach sections hundreds of metres apart. Extrapolation of the results at four beach sections indicated that about 1 million fruits were deposited on the entire (3.5 km) beach. This was equivalent to the seed production potential of about 313,217 inflorescences, or a flowered area of 1500 m2. The organic input to the beach was 224 kg ash-free dry weight (AFDM). These results suggest that storms may provide an unpredictable source of seed mortality in P. oceanica. The reproductive material produced by storms, however, may constitute an important source of allochthonous organic matter to the beach.  相似文献   

6.
New data support our previously published propagule dispersal hypothesis and show that propagules of some benthic foraminiferal species can survive for two years before growth commences. Following exposure to simulated shallow-water conditions, shallow-water species of benthic foraminifera appeared and grew in large numbers (commonly >100 ind/12 ml sediment) in the <32 µm-size sediment fraction collected from 320 m water depth in the Skagerrak basin (North Sea). None of the shallow-water species that grew abundantly (Planorbulina mediterranensis, Morulaeplecta bulbosa, Bolivina pseudoplicata, Cuneata arctica, Eggerelloides scaber, Gavelinopsis praegeri) seem to grow or reproduce at or in the vicinity of the sampling site. Consequently, they must have been transported there as <32 µm-sized individuals. Their sudden appearance when exposed to shallow-water conditions suggests that they had been transported to the sampling site as propagules and that they could survive in the sediments until conditions became suitable for growth and, for some, reproduction. The lack of agglutination on the proloculi of the agglutinated taxa that appeared in the growth-chambers may enhance their passive transport via currents and, thereby, dispersal. Of all the indigenous foraminiferal species that occur at the sampling site, only Textularia earlandi and Bolivinellina pseudopunctata continued to grow and reproduce when transferred from bathyal (320 m) to simulated shallow-water (0 m) conditions. The former is considered a highly opportunistic species. According to the literature, most of the morphospecies which grew in the experiments are cosmopolitan. Our results indicate substantial inter-specific differences in dispersal potential and support previous suggestions that among free-living species, some serial forms have the potential for long-distance dispersal. Still, oceanographic, physical and ecological boundaries and barriers constrain the distribution of most species. In addition to benthic foraminifera, Gromia spp. (rhizarian protists related to the foraminifera) grew in >60% of the experimental growth-chambers.  相似文献   

7.
Larval transport from distant populations is essential for maintenance and renewal of populations in patchy and disturbed ecosystems such as deep-sea hydrothermal vents. We use quasi-geostrophic modeling to consider the potential for long-distance dispersal of hydrothermal vent larvae in mesoscale eddies interacting with the northern East Pacific Rise. Modeled eddy dynamics were similar to the observed propagation dynamics of Tehuantepec eddies, including their ability to cross the ridge. Simulated surface anticyclones were associated with coherent cyclones in the deep layer with relatively strong current velocities that could significantly increase the dispersal potential of passive particles. Eddy interactions with ridge topography further enhanced tracer dispersal along the ridge axis through shearing and elongation of the eddy core. Simulations suggest that the passage of an eddy would result in local loss from the vent field and aggregate transport with potential enhancement of dispersal between vent fields separated by up to 270 km. Based on the latitude at which most Tehuantepec eddies cross the ridge, eddy-induced flows would enhance connectivity between the 13°N, 11°N, and 9°N vent fields along the East Pacific Rise asymmetrically with higher transport from northern vent fields to southern vent fields.  相似文献   

8.
The Alpine Foreland Basin is a minor oil and moderate gas province in central Europe. In the Austrian part of the Alpine Foreland Basin, oil and minor thermal gas are thought to be predominantly sourced from Lower Oligocene horizons (Schöneck and Eggerding formations). The source rocks are immature where the oil fields are located and enter the oil window at ca. 4 km depth beneath the Alpine nappes indicating long-distance lateral migration. Most important reservoirs are Upper Cretaceous and Eocene basal sandstones.Stable carbon isotope and biomarker ratios of oils from different reservoirs indicate compositional trends in W-E direction which reflect differences in source, depositional environment (facies), and maturity of potential source rocks. Thermal maturity parameters from oils of different fields are only in the western part consistent with northward displacement of immature oils by subsequently generated oils. In the eastern part of the basin different migration pathways must be assumed. The trend in S/(S + R) isomerisation of ααα-C29 steranes versus the αββ (20R)/ααα (20R) C29 steranes ratio from oil samples can be explained by differences in thermal maturation without involving long-distance migration. The results argue for hydrocarbon migration through highly permeable carrier beds or open faults rather than relatively short migration distances from the source. The lateral distance of oil fields to the position of mature source rocks beneath the Alpine nappes in the south suggests minimum migration distances between less than 20 km and more than 50 km.Biomarker compositions of the oils suggest Oligocene shaly to marly successions (i.e. Schoeneck, Dynow, and Eggerding formations) as potential source rocks, taking into account their immature character. Best matches are obtained between the oils and units a/b (marly shale) and c (black shale) of the “normal” Schöneck Formation, as well as with the so-called “Oberhofen Facies”. Results from open system pyrolysis-gas chromatography of potential source rocks indicate slightly higher sulphur content of the resulting pyrolysate from unit b. The enhanced dibenzothiophene/phenanthrene ratios of oils from the western part of the basin would be consistent with a higher contribution of unit b to hydrocarbon expulsion in this area. Differences in the relative contribution of sedimentary units to oil generation are inherited from thickness variations of respective units in the overthrusted sediments. The observed trend towards lighter δ13C values of hydrocarbon fractions from oil fields in a W-E direction are consistent with lower δ13C values of organic matter in unit c.  相似文献   

9.
TAIGER project deep-penetration seismic reflection profiles acquired in the northeastern South China Sea (SCS) provide a detailed view of the crustal structure of a very wide rifted continental margin. These profiles document a failed rift zone proximal to the shelf, a zone of thicker crust 150 km from the shelf, and gradually thinning crust toward the COB, spanning a total distance of 250–300 km. Such an expanse of extended continental crust is not unique but it is uncommon for continental margins. We use the high-quality images from this data set to identify the styles of upper and lower crustal structure and how they have thinned in response to extension and, in turn, what rheological variations are predicted that allow for protracted crustal extension. Upper crustal thinning is greatest at the failed rift (βuc ≈ 7.5) but is limited farther seaward (βuc ≈ 1–2). We interpret that the lower crust has discordantly thinned from an original 15–17 km to possibly less than 2–3 km thick beneath the central thick crust zone and more distal areas. This extreme lower crustal thinning indicates that it acted as a weak layer allowing decoupling between the upper crust and the mantle lithosphere. The observed upper crustal thickness variations and implied rheology (lower crustal flow) are consistent with large-scale boudinage of continental crust during protracted extension.  相似文献   

10.
Amala Mahadevan   《Ocean Modelling》2006,14(3-4):222-240
Through a suite of three-dimensional, high-resolution numerical modeling experiments, we examine the role of nonhydrostatic effects on O(1 km) submesoscale processes at ocean fronts, with particular focus on the vertical velocity field. Several differences between nonhydrostatic and hydrostatic models are pointed out using a framework that enables precise comparison, but it is difficult to identify categorical differences between the model solutions at the grid resolutions afforded. The instantaneous vertical velocity structure is sensitive to the model choice and, even more so, to grid resolution, but the average vertical flux is similar in both hydrostatic and nonhydrostatic cases.When a frontal region with horizontal density gradients is perturbed by wind, a profusion of submesoscale, O(1 km), secondary circulation features develops in the upper 50 m. Narrow, elongated cells of intense up- and down-welling are found to occur close to the surface, overlying broader regions of weaker up- and down-welling associated with the mesoscale meanders of the baroclinically unstable front. The submesoscale down-welling is considerably stronger than up-welling and is concentrated in 1–2 km width filaments within which velocities can attain magnitudes as high as 200 m day−1. The submesoscale features are found to be robust at horizontal grid resolutions varying between 1 and 0.25 km and exist even in the hydrostatic model. Submesoscale circulation is difficult to observe or resolve in coarser resolution circulation models, but is likely to play a significant role in the exchange of energy and properties between the surface ocean and thermocline. Possible mechanisms for the generation of these features are investigated in a follow-on paper.  相似文献   

11.
Three time-series sediment traps were deployed in the Japan Trench at 40°26′N, 144°28′E, from October 1994 to May 1995. The depths were approximately 1, 4.2 and 6.8 km and the water depth was 7150 m. There were large mass fluxes in spring at 1 and 4.2 km depths, whereas increased fluxes appeared from 27 December 1994 to 29 January 1995, at 4.2 and 6.8 km depths. The 1994 Sanriku-Oki earthquake (Mw=7.7) occurred on 28 December 1994, at 40°27′N, 143°43′E, adjacent to the study site. Distinct increases in non-biogenic material were observed at both 4.2 and 6.8 km just after the earthquake; the material seems to have originated from the surface sediments, though differing Mn/Al of particulate materials at the two depths imply a difference in their source areas. Analysis indicates that the main part of the increased particulate fluxes at 6.8 km depth derived from the sediment on the eastern slope of the Japan Trench.  相似文献   

12.
The fugacity of CO2 and abundance of chlorophyll a (Chla) were determined in two long transects from the Polar Front to the Antarctic Continent in austral summer, December 1995–January 1996. Large undersaturations of CO2 in the surface water were observed coinciding with high Chla content. In the major hydrographic regions the mean air–sea fluxes were found to range from −3 to +7 mmol m−2 d−1 making these regions act as a sink as well as a source for CO2. In the total 40-d period, the summation of the several strong source and sink regions revealed an overall modest net source of 0.3 mmol m−2 d−1, this based on the Wanninkhof (J. Geophys. Res. 97 (1992) 7373) quadratic relationship at in situ windspeed. A simple budget approach was used to quantify the role of phytoplankton blooms in the inorganic carbonate system of the Antarctic seas in a time frame spanning several weeks. The major controlling physical factors such as air–sea flux, Ekman pumping and upwelling are included. Net community production varies between −9 and +7 mmol m−2 d−1, because of the large oscillations in the dominance of autotrophic (CO2 fixation) versus heterotrophic (CO2 respiration) activity. Here the mixed layer depth is the major controlling factor. When integrated over time the gross influx and efflux of CO2 from air to sea is large, but the net residual air/sea exchange is a modest efflux from sea to atmosphere.  相似文献   

13.
Top seals and faults represent key risks to trap integrity and therefore preservation of hydrocarbons in the frontier Ceduna Sub-basin, offshore Southern Australia. Due to a paucity of well data in the basin, to provide constraint to the stratigraphic distribution of the prospective Cretaceous deltaic and marine sequences, stratigraphic forward modelling was utilised to create facies, grain size and Vshale volumes. These modelled Vshale volumes were subsequently used to investigate the structural control(s) on potential hydrocarbon leakage and migration within key stratigraphic sequences in the sub-basin.A set of coarse (20 km horizontal resolution), large scale (1100 × 600 km) stratigraphic forward models simulated the deposition of Late Jurassic to Tertiary stratigraphic sequences in the sub-basin with an initial 1 Ma interval. Smaller (80 × 60 km), finer scale (0.5 km horizontal resolution, 200 ka interval), models focussing on the Tiger and Hammerhead Supersequences over the Trim 3D seismic survey were used to investigate fault seal and top seal frameworks, using shale gouge ratio and silt and shale thicknesses from Vshale volume. Four stratigraphic forward models were produced to match a range of estimates of Vshale derived from the Gnarlyknots-1A well, the only well penetrating the central Ceduna Sub-basin. These stratigraphies were in turn integrated into a geological model interpreted from the Trim 3D seismic survey creating a geocellular model to test potential migration and trapping scenarios for potential hydrocarbons generated in the sub-basin.Fault and top seal models from the most likely scenario suggest (i) restricted potential for structural trapping near the base of the Tiger Supersequence, (ii) the possible presence of a regional migration pathway associated with sandy shoreface deposits at the transition between the Tiger and Hammerhead Supersequences, and (iii) the association of intraformational top seals and increasing fault seal potential in the deltaic sediments of the Hammerhead Supersequence feasibly resulting in a series of stacked structural traps.  相似文献   

14.
MPAS-Ocean is used to simulate an idealized, density-driven overflow using the dynamics of overflow mixing and entrainment (DOME) setup. Numerical simulations are carried out using three of the vertical coordinate types available in MPAS-Ocean, including z-star with partial bottom cells, z-star with full cells, and sigma coordinates. The results are first benchmarked against other models, including the MITgcm’s z-coordinate model and HIM’s isopycnal coordinate model, which are used to set the base case used for this work. A full parameter study is presented that looks at how sensitive overflow simulations are to vertical grid type, resolution, and viscosity. Horizontal resolutions with 50 km grid cells are under-resolved and produce poor results, regardless of other parameter settings. Vertical grids ranging in thickness from 15 m to 120 m were tested. A horizontal resolution of 10 km and a vertical resolution of 60 m are sufficient to resolve the mesoscale dynamics of the DOME configuration, which mimics real-world overflow parameters. Mixing and final buoyancy are least sensitive to horizontal viscosity, but strongly sensitive to vertical viscosity. This suggests that vertical viscosity could be adjusted in overflow water formation regions to influence mixing and product water characteristics. Lastly, the study shows that sigma coordinates produce much less mixing than z-type coordinates, resulting in heavier plumes that go further down slope. Sigma coordinates are less sensitive to changes in resolution but as sensitive to vertical viscosity compared to z-coordinates.  相似文献   

15.
High-resolution underway temperature and conductivity measurements collected by R/V Knorr during winter and spring 2003 are used to characterize errors associated with spatial aliasing in the northern and central Adriatic Sea. During winter, 99th percentile temperature, salinity and density errors were 0.62 °C, 0.25 and 0.12 kg/m3 (0.25 °C, 0.10 and 0.05 kg/m3) for sampling at 10 km (5 km) horizontal resolution, respectively. The corresponding values in spring were 1.31 °C, 0.50 and 0.40 kg/m3 (0.93 °C, 0.25 and 0.22 kg/m3) for the 10 km (5 km) sample spacing, respectively. The largest errors were associated with energetic regions over the shallow, western Adriatic, in front of the Po River mouth and off the tip of the Istrian peninsula. The deeper eastern basin exhibited smaller errors. The variability of errors in time and space reflected the variability of small-scale density features, characterized by wavelengths as small as 2 km in winter and 1 km in spring and being more pronounced in the western and northern parts of the Adriatic. As these results indicate that errors associated with undersampling can be considerable, they should be taken into account while planning future CTD measurements in the region.  相似文献   

16.
An ocean acoustic tomography system covering the region of 800×1000 km with the spatial resolution of eddy-resolving scales has been designed on the basis of computer experiments using the hydrographic data collected in the Sea of Japan. The optimum number of acoustic sources required for 20 acoustic receivers was estimated as 13 by changing the source number. The spatial resolution for the optimum system was 41 km smaller than the dominant size of meso-scale eddies in the Sea of Japan. The effect of travel-time errors on tomographic maps is also quantified.  相似文献   

17.
This study compared seasonal growth, development and reproduction of the invasive brown macroalga Sargassum muticum in habitats with different wave exposure on the Irish west coast. Three field sites with different degrees of wave exposure were chosen for monthly observations to reflect different habitats that were characteristic of the Irish west coast. Growth and receptacle development differed considerably between sites. Growth and receptacle development was lower at the most sheltered site. Here, S. muticum showed signs of early fragmentation in April/May during the two years of investigation (2007 and 2008), whilst the population at an exposed site developed normally and plants grew to a maximum average length of 163 cm by July, with the onset of fragmentation in August. Sargassum muticum in a tide pool exhibited a similar seasonal growth cycle as plants at the exposed open shore site. Overall growth however was stunted, with plants reaching a maximum length of only 30–40 cm in July. Receptacle development was also inhibited at the sheltered site, with a maximum of only 10% of plants found to be fertile during spring and summer 2008, while plants at the exposed site and the tide pool exhibited 100% plant fertility by August. An extensive occurrence of the native epiphyte Pylaiella littoralis on S. muticum was noticed during field sampling at the sheltered study site which may have contributed to inhibited development of S. muticum observed in this area.  相似文献   

18.
Evidence suggests that patterns of benthic community structure are functionally linked to estuarine processes and physical characteristics of the benthos. To assess these linkages for coarse-sediment shorelines, we used a spatially nested sampling design to quantify patterns of distribution and abundance of both macroinfauna and macroepibiota. We examined replicate beach segments within a site (∼1 km), sites within areas of relatively uniform salinity and temperature (∼10 km), and areas (∼100 km) in the two major basins of Puget Sound, Washington. Because slight variations in physical characteristics of a beach can lead to significant alterations in biota, we minimized confounding physical influences by working only in the predominant shoreline habitat type in Puget Sound, a mixture of sand, pebbles and cobbles. Species richness decreased steadily from north to south along gradients of declining wave energy, increasing temperature and decreasing salinity. A few taxa were confined to the South Basin, but many more were found in the North. Most of the variability in population abundance was captured at the smaller spatial scales. Physical conditions tend to become increasingly different with distance among sites. Communities became more different from north to south as species intolerant of more estuarine conditions dropped out. There was significant spatial autocorrelation among populations on neighboring beach segments for 73 of the 172 species sampled. Populations of these benthic species may be connected via dispersal on scales of at least km in Puget Sound. Our results strengthen prior conclusions about the strong linkages between the biota and physical patterns and processes in estuaries. It is important for monitoring and impact-detection studies to account for natural variation of physical gradients across the sampling scales used. Nested, replicated sampling designs can facilitate the detection of environmental change at spatial scales ranging from global (e.g., warming or El Niño), to regional (e.g., estuary-wide changes in salinity patterns), to local (e.g., from development at a site).  相似文献   

19.
Spatial gradients in biomass and community composition have important consequences for ecosystem structure and function. In this study, small-scale inshore-offshore (1-10 km) and vertical (1-50 m) patterns of microphytoplankton biomass and community composition are described, and the environmental controls of microphytoplankton biomass are evaluated in a coastal ecosystem of the Southern California Bight (SCB). During a two-year period, persistent inshore-offshore gradients in phytoplankton biomass and occasional inshore-offshore gradients in community composition, coincident with regional precipitation, were found, although the strength of the gradients varied between sampling periods. The chlorophyll a maximum was generally present between 15 and 45 m, the cell abundance maximum occurred in surface waters, and there was little evidence of vertical gradients in community composition. Variability in chlorophyll a concentrations was linked to variability in environmental parameters only after some rain and upwelling events. This study demonstrates that inshore-offshore patterns in phytoplankton biomass previously documented at large spatial scales (100-700 km) in the SCB can also persist at smaller scales (1-10 km), although the mechanisms for the gradients are likely to be different at the different spatial scales. The results provide a baseline data set that can be used to focus monitoring and management efforts in the SCB. In particular, this work shows that a limited number of sampling stations are sufficient for phytoplankton monitoring in Santa Monica Bay.  相似文献   

20.
Drilling/coring activities onboard JOIDES Resolution for hydrate resource estimation have confirmed gas hydrate in the continental slope of Krishna-Godavari (KG) basin, Bay of Bengal and the expedition recovered fracture filled gas hydrate at the site NGHP-01-10. In this paper we analyze high resolution multi-channel seismic (MCS), high resolution sparker (HRS), bathymetry, and sub-bottom profiler data in the vicinity of site NGHP-01-10 to understand the fault system and thermal regime. We interpreted the large-scale fault system (>5 km) predominantly oriented in NNW-SSE direction near NGHP-01-10 site, which plays an important role in gas hydrate formation and its distribution. The increase in interval velocity from the baseline velocity of 1600 m/s to 1750–1800 m/s within the gas hydrate stability zone (GHSZ) is considered as a proxy for the gas hydrate occurrence, whereas the drop in interval velocity to 1400 m/s suggest the presence of free gas below the GHSZ. The analysis of interval velocity suggests that the high concentration of gas hydrate occurs close to the large-scale fault system. We conclude that the gas hydrate concentration near site NGHP-01-10, and likely in the entire KG Basin, is controlled primarily by the faults and therefore has high spatial variability.We also estimated the heat flow and geothermal gradient (GTG) in the vicinity of NGHP-01-10 site using depth and temperature of the seafloor and the BSR. We observed an abnormal GTG increase from 38 °C/km to 45 °C/km at the top of the mound, which remarkably agrees with the measured temperature gradient at the mound (NGHP-01-10) and away from the mound (NGHP-01-03). We analyze various geological scenarios such as topography, salinity, thermal non-equilibrium of BSR and fluid/gas advection along the fault system to explain the observed increase in GTG. The geophysical data along with the coring results suggest that the fluid advection along the fault system is the primary mechanism that explains the increase in GTG. The approximate advective fluid flux estimated based on the thermal measurement is of the order of few tenths of mm/yr (0.37–0.6 mm/yr).  相似文献   

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