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291.
Ecological characteristics of Walleye pollock eggs and larvae in the southeastern Bering Sea during the late 1970s 总被引:1,自引:0,他引:1
Kyung-Mi Jung Sukyung Kang Suam Kim Arthur W. Kendall Jr. 《Journal of Oceanography》2006,62(6):859-871
Walleye pollock (Theragra chalcogramma) is an ecologically and economically important groundfish in the eastern Bering Sea. Its population size fluctuates widely,
driving and being driven by changes in other components of the ecosystem. It is becoming apparent that dramatic shifts in
climate occur on a decadal scale, and these “regime shifts” strongly affect the biota. This paper examines quantitative collections
of planktonic eggs and larvae of pollock from the southeastern Bering Sea during 1976–1979. Mortality, advection, and growth
rates were estimated, and compared among the years encompassing the 1970s’ regime shift. These data indicate that pollock
spawning starts in late February over the basin north of Bogoslof Island. Over the shelf, most spawning occurs north of Unimak
Island near the 100 m isobath in early or mid April. Pollock eggs are advected to the northwest from the main spawning area
at 5–10 cm/sec. Larvae are found over the basin north of Bogoslof Island in April, and over the shelf between Unimak Island
and the Priblof Islands in May. Compared to 1977, the spawning period appeared to be later in 1976 (a cold year) and earlier
in 1978 (a warm year) in the study area. At the lower temperatures in 1976, egg duration would be longer and thus egg mortality
would operate over a longer period than in the other years. Mean larval growth appeared to be lower in 1976 than in 1977 and
1979. Estimated egg mortality rate in 1977 was 0.6 in April and 0.3 in early May. 相似文献
292.
Timothy M. Dellapenna Mead A. Allison Gary A. Gill Ronald D. Lehman Kent W. Warnken 《Estuarine, Coastal and Shelf Science》2006,69(3-4):519
To address the relative importance of shrimp trawling on seabed resuspension and bottom characteristics in shallow estuaries, a series of disturbance and monitoring experiments were conducted at a bay bottom mud site (2.5 m depth) in Galveston Bay, Texas in July 1998 and May 1999. Based on pre- and post-trawl sediment profiles of 7Be; pore water dissolved oxygen and sulfide concentration; and bulk sediment properties, it was estimated that the trawl rig, including the net, trawl doors, and “tickler chain,” excavate the seabed to a maximum depth of approximately 1.5 cm, with most areas displaying considerably less disturbance. Water column profile data in the turbid plume left by the trawl in these underconsolidated muds (85–90% porosity; <0.25 kPa undrained shear strength) demonstrate that suspended sediment inventories of up to 85–90 mg/cm2 are produced immediately behind the trawl net; an order of magnitude higher than pre-trawl inventories and comparable to those observed during a 9–10 m/s wind event at the study site. Plume settling and dispersion caused suspended sediment inventories to return to pre-trawl values about 14 min after trawl passage in two separate experiments, indicating particles re-settle primarily as flocs before they can be widely dispersed by local currents. As a result of the passage of the trawl rig across the seabed, shear strength of the sediment surface showed no significant increase, suggesting that bed armoring is not taking place and the trawled areas will not show an increase in critical shear stress. 相似文献
293.
Christopher T. Mills Robert F. Dias Dennis Graham Kevin W. Mandernack 《Marine Chemistry》2006,98(2-4):197-209
Sediment samples ranging from 0.05 to 278 m below sea floor (mbsf) at a Northwest Pacific deep-water (5564 mbsl) site (ODP Leg 191, Site 1179) were analyzed for phospholipid fatty acids (PLFAs). Total PLFA concentrations decreased by a factor of three over the first meter of sediment and then decreased at a slower rate to approximately 30 mbsf. The sharp decrease over the first meter corresponds to the depth of nitrate and Mn(IV) reduction as indicated by pore water chemistry. PLFA-based cell numbers at site 1179 had a similar depth profile as that for Acridine orange direct cell counts previously made on ODP site 1149 sediments which have a similar water depth and lithology. The mole percentage of straight chain saturated PLFAs increases with depth, with a large shift between the 0.95 and 3.95 mbsf samples. PLFA stable carbon isotope ratios were determined for sediments from 0.05 to 4.53 mbsf and showed a general trend toward more depleted δ13C values with depth. Both of these observations may indicate a shift in the bacterial community with depth across the different redox zones inferred from pore water chemistry data. The PLFA 10me16:0, which has been attributed to the bacterial genera Desulfobacter in many marine sediments, showed the greatest isotopic depletion, decreasing from − 20 to − 35‰ over the first meter of sediment. Pore water chemistry suggested that sulfate reduction was absent or minimal over this same sediment interval. However, 10me16:0 has been shown to be produced by recently discovered anaerobic ammonium oxidizing (anammox) bacteria which are known chemoautotrophs. The increasing depletion in δ13C of 10me16:0 with the unusually lower concentration of ammonium and linear decrease of nitrate concentration is consistent with a scenario of anammox bacteria mediating the oxidation of ammonium via nitrite, an intermediate of nitrate reduction. 相似文献
294.
295.
Closely-spaced 3.5 kHz seismic profiles were collected over the north-easterly trending ridge and swale system 50 km east-southeast of Atlantic City, New Jersey. They yield information on the Late Quaternary depositional history of the area, and on the origin of the ridge system. Four of the sub-bottom reflectors identified were sufficiently persistent to warrant investigation and interpretation. These reflectors, which have been cored, lithologically identified, and radiocarbon dated, are stratigraphically higher than the reflectors dealt with by the majority of previous studies. The upper three reflectors are definitely mid- and post-Wisconsin in age and present a record of the most recent glacial cycle. The upper three units associated with the observed reflectors appear to exert a pronounced influence on the bathymetry. The gently corrugated ridge system of Holocene sand is formed over the regionally flat-lying upper unit, an Early Holocene lagoonal silty clay. The characteristically flat, broad depressions of the area are floored by this lagoonal material. Locally, however, marine scour has cut through the silty clay into an underlying unit of unconsolidated fine Pleistocene sand. Several stages of trough development appear to be represented. After penetrating the lagoonal clay, troughs are initially narrow, but when incised through the sand into a lower, Pleistocene, silty-clay unit, the troughs become notably wider. As downcutting is inhibited by the lower clay, the upper clay is undercut as the trough widens in a fashion similar to a desert blowout.
The sub-bottom reflectors indicate that ridge development on the central shelf has involved aggradation as well as erosion. Some ridges seem to have grown by vertical and lateral accretion from small cores. The internal structure of other ridges suggests that they formed by the coalescence of several small ridges. Others appear to have undergone appreciable lateral migration.
The ridges appear to be in a state of continuing adjustment to the hydraulic regime of the deepening post-Pleistocene water column. 相似文献
296.
The natural human female hormones oestrone and 17β-oestradiol have been implicated in the disruption of endocrine systems in some wildlife adjacent to sewage effluents. The sorption behaviour of these two compounds under estuarine conditions was studied by spiking either 2.55 μg of oestrone or 2.65 μg of 17β-oestradiol in kinetic experiments. In equilibrium experiments, 3 ng of oestrone or 3.2 ng of 17β-oestradiol was added in each of the centrifuge tubes. Sorption onto sediment particles was relatively slow, with sorption equilibrium being reached in about 70 and 170 h for oestrone and 17β-oestradiol, respectively. The effects of a variety of environmental parameters on sorption were studied including salinity, sediment concentration (SC), the presence of a third phase, particle size and, also, surfactant concentrations. Results show that although salinity did not induce any statistically significant effect on the sorption of 17β-oestradiol, it did statistically enhance the sorption of oestrone, and a salting constant of 0.3 l mol−1 was derived. The partition coefficient for both compounds decreased with increasing sediment concentration, a phenomenon that has been widely reported and attributed to the presence of colloids (which could enhance dissolved concentrations). In this paper, the true partition coefficients for sediment particles (Kptrue) and colloidal particles (Kctrue) have been calculated, and a Kptrue value of 141 and 102 ml g−1 was obtained for oestrone and 17β-oestradiol, respectively. In addition, Kctrue values for oestrone (222×102 ml g−1) and 17β-oestradiol (135×102 ml g−1) were two orders of magnitude higher than their respective Kptrue values, suggesting that the colloidal particles are significantly stronger sorbents for natural oestrogens than sediment particles. Particles of different sizes were found to have different partition coefficients due to the strong relationships between partition coefficients for the two compounds and particulate organic carbon (POC) contents and specific surface areas (SSAs). The presence of a surfactant was shown to reduce the partition coefficients for the two compounds, although its concentrations being used were higher than those normally found in the natural environment. 相似文献
297.
The growth rate and potential production of bacterioplankton in cold hydrocarbon seeps located along the Louisiana coast were determined using a pulse-labeling technique. Surprisingly, community doubling times are on the order of 1.1 h, which compares to laboratory-grown cultures. We also found that there are differences in growth rates on relatively small geographic scales, suggesting the influence of site-specific geological features (e.g., gas hydrate mounds). Proceeding downslope to deeper waters, methane-oxidizing bacteria appeared to play a more significant role in community productivity. These preliminary experiments indicated, quite unexpectedly, that water column microbes are growing at a more rapid rate than in any other marine system so far studied and that methane may serve as a primary nutrient (carbon) source in these seep-associated microbial assemblages. 相似文献
298.
An extrapolated version of a time-dependent nonreflecting boundary condition is developed for use at artificial boundaries to truncate infinite/semi-infinite domains for transient water wave problems. Test examples include single- and multiple-frequency flap wavemaker problems and free vibrations of a floating circular cylinder. The finite element results compare favorably to analytical solutions. It is shown that with the proposed boundary condition the numerical simulations have little reflections from the artificial boundaries. 相似文献
299.
Quality control in mud coring 总被引:1,自引:0,他引:1
W. R. Parker 《Geo-Marine Letters》1991,11(3-4):132-137
Consideration of factors influencing the interaction between a corer and sediment suggest the likelihood of significant and
anisotropic straining of the particle framework during coring. Direct observations allow the coring process to be discretised,
the relative motion of corer, core and sediment to be visualized and confirm that the development of differences between corer
penetration and core length is not a continuous process. Records suggest the possibility of dislocation of the relative positions
of porewater and particle framework, which may lead to the disequilibration of sorbed species. 相似文献
300.
A densely populated soft-sediment community was experimentally analysed for interspecific competition for space in laboratory experiments. No interspecific competitive interactions leading to a decrease in survivorship could be documented over a five-week period. The four most abundant species all coexisted in laboratory microcosms. The two tube-buiding species, the tanaid Leptochelia dubia and the polychaete Rhynchospio arenincola, could not exclude the mobile bivalve Transennella tantilla or the burrowing amphipod Paraphoxus spinosus. None of the species increases its emigration frequency when other species are present. Paraphoxus is found to be a browsing predator on Phynchospio but inflicts no detectable mortality. The coexistence of these species is permitted by virtue of the fact that none of the species can alter the soft-sediment habitat sufficiently to make the habitat unsuitable for the other species. 相似文献