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911.
Downward fluxes of microbial assemblages associated with sinking particles sampled in sediment traps deployed at nominal depths of 1000 m (trap A), 3000 m (trap B) and 4700 m (trap C) were measured between October 1995 and August 1998 on the Porcupine Abyssal Plain (PAP, NE Atlantic). The goal of the study was to provide detailed information on the microbial contributions to the particulate organic carbon and DNA fluxes. Bacterial fluxes associated with settling particles in the PAP area were generally low and significantly lower than bacterial fluxes reported from the same area during 1989–90. Marked seasonal pulses in the microbial assemblages were observed in all years that were associated with particle flux maxima in April–June. No significant differences were found in microbial fluxes between 1000 and 4700 m depth, but both the bacterial biomass flux and the frequency of dividing bacteria increased with depth, suggesting that organic matter turnover and conversion into bacterial biomass increased in the deeper traps. The structure of microbial assemblages displayed clear changes with increasing depth; the ratios of bacteria to both flagellates and cyanobacteria increased up to 4-fold between 1000 and 4700 m, showing a marked increase in bacterial dominance in the deeper layers of the water column. A parallel increase of the bacterial contribution to particulate organic carbon (POC) and DNA fluxes was observed. Total microbial contribution to the POC flux in the PAP area was about 2%, whereas the contribution of cyanobacteria was negligible. Fluxes of microbial assemblages were significantly correlated with DNA fluxes and on average the bacteria accounted for 5% of DNA fluxes. Data reported here confirm that the “rain” of particulate bacterial DNA may represent an important source of nucleotides for deep-sea bacteria, but also suggests that a much larger pool of detrital DNA is potentially available to deep-sea micro-organisms.  相似文献   
912.
913.
We construct a one-dimensional ecosystem model (nitrate, ammonium, phytoplankton, zooplnakton and detritus) with simple physics and biology in order to focus on the structural relations and intrinsic properties of the food web that characterizes the biological regime in the central equatorial Pacific at 140°W. When possible, data collected during the EgPac and other cruises were used to calibrate model parameters for two simulations that differ in the limiting nutrient, i.e. nitrogen or iron. Both simulations show annual results in good agreement with the data, but phytoplankton biomass and primary production show a more pronounced annual variability when iron is used as the limiting nutrient. This more realistically reproduces the variability of biological production and illustrates the greater coupling between vertical physical processes and biological production when the limiting nutrient is iron rather than nitrogen. The iron simulation also illustrates how iron supply controls primary production variability, how grazingbalances primary production and controls phytoplankton biomass, and how both iron supply and grazingcontrol primary production. These results suggest that it is not possible to capture primary production variability in the central equatorial Pacific with biological models using nitrogen as the limiting nutrient. Other indirect results of this modeling study were: (1) partitioning of export production between dissolved and particulate matter is almost equal, suggesting that the importance of DOC export may have been previously overestimated; (2) lateral export of live biomass has to be taken into account in order to balance the nitrogen budget on the equator at 140°W; and (3) preferential uptake of ammonium (i.e. nitrate uptake inhibition by ammonium) associated with high regeneration of nitrogen (low f ratio as a consequence of the food web structure imposed by iron limitation) largely accounts for the surface build-up of upwelled nitrate.  相似文献   
914.
Variation in the sensitivity to stress of Macoma balthica was measured in several French and Dutch estuaries. For adult and juvenile Macoma balthica exposed to copper under conditions of starvation, differences in mortality rate, condition, glycogen, burrowing rate and copper content were assessed. No significant differences were observed between adults and juveniles; the influence of treatment and origin was always evident. Animals from the most southern estuaries, Loire and Gironde, near to the species's southern limit of distribution, showed, in the field, the strongest deviations for the ecophysiological traits measured, and were in the experiments the most sensitive to stress.  相似文献   
915.
Catalytic cathodic stripping voltammetry (CSV) preceded by adsorptive collection of complexes of 1-nitroso-2-napthol (NN) can be used to determine iron in seawater. It is shown here that iron(II) is effectively masked in the presence of 2,2-dipyridyl (Dp) so that iron(III) is measured selectively. The concentration of iron(II) is then calculated as the difference between the concentrations of reactive iron (FeR) in the absence and presence of 2 μM Dp, FeR being defined as that which was complexed by 20 μM NN at pH 6.9 in the presence of 1.8 mM H2O2 and 5 ppm sodium dodecyl sulphate. A 30 min reaction time was allowed for Dp to react with iron(II) in seawater prior to the determination of reactive iron(III) using the same conditions as used for FeR. Detection limits of 0.08 nM, 0.077 nM and 0.12 nM were obtained for FeR, iron(III) and iron(II), respectively, using a 60 s deposition time.The method was utilised to determine the redox speciation of iron in the northern North Sea. Concentrations of FeR ranged between 0.8 and 3.5 nM with nutrient-like depth profiles. Iron(II) was found to be present at concentrations up to 1.2 nM, the highest concentrations occurring in the upper 20 m of the water column.  相似文献   
916.
Flow slides may affect the stability of dikes. A flow slide is an instability of a submerged slope caused by liquefaction of loose, (medium) fine sand. Whether a flow slide will occur depends on the properties of the sand, which are a function of its density, and the geometry of the slope, as determined by wave and current induced scour and sedimentation. The influence of sand properties and the geometry parameters on the risk of flow slides are discussed. The application of a flow slide prediction method to an example and a risk analysis is briefly discussed.  相似文献   
917.
Current specification of the ocean wave environment for the design of offshore platforms does not adequately describe the directional nature of a real seaway. The strong wave frequency dependent nature of the directional behavior of observed seas is often over-simplified for design. A general formulation encompassing a wide range of directional sea models is presented. Parameter values used in some of the more popular directional sea models are examined. Approximate expressions for the two frequency dependent parameters in a modified Longuet-Higgins cosine wave spreading model are presented. A general procedure which allows an engineer to estimate parameters for alternate wave spreading models is discussed. To illustrate this procedure an empirically based modified cosine spreading model is used as the basis to estimate frequency dependent parameters for circular normal and wrapped Gaussian wave spreading models. A comparison of the contours of the various directional sea models and the prediction of the root-mean-square velocity distribution is presented.  相似文献   
918.
Cd, Pb and Cu concentrations, dissolved and total, have been determined in the German Bight. In 1975, 1977, 1978, 1980 and 1983 water samples were collected at 215 sampling stations. In the 1983 mission samples were also analysed for Ni and Co. The samples were filtered and acidified immediately after collection. Analysis was performed by voltammetry.The analytical data indicate that a net portion of the Cd, the Weser being a distinct source for it, will cross the estuarine zone. Open sea Cd levels in the German Bight are about a factor 10 higher than North Atlantic values. Pb, being transported mainly with the suspended particulate phase (up to 99.5%), is better eliminated by sedimentation in the estuaries. Ni and Cu occur mainly in the dissolved phase. Pollution by Co is distinct in the Elbe, but the output is rapidly diluted to background contents below 10 ng kg−1. An interpretation of the results from the different years yields a rather steady state situation in the German Bight.  相似文献   
919.
A theoretical assessment is made of mean wave drift forces on groups of vertical circular cylinders, such as the columns of a floating offshore platform. A complete analytical solution is obtained for two cylinders extending from seabed to free surface, and a long wave approximation is found to provide reliable predictions of the drift force in line with the waves at low frequencies. For moderate separation between the two cylinders, this force is found to tend at low frequencies to a value four times the force on an isolated cylinder.A numerical method is employed to study two surface piercing cylinders truncated below the free surface, and an arrangement of four vertical cylinders characteristic of a floating offshore platform. The mean vertical drift force is found to be reasonably well approximated, over the frequency range of practical interest, by the force on an individual cylinder considered in isolation multiplied by the number of cylinders in the group. Interaction effects, however, have a profound influence on the total horizontal drift force. At low frequencies this force is found to tend to the force on an isolated cylinder multiplied by the squate of the number of cylinders in the group.  相似文献   
920.
Experimental investigation is made on the boundary layers of the transformation zone (i.e. the region between the last symmetrical wave profile depth and the breaking point) of plunging breakers propagating on a smooth beach with 1/12 uniform slope. Using a laser anemometer, the particle velocities are measured at four verticals along the transformation zone for three different steepnesses of waves within the plunging breaker range. The boundary layer flow in the transformation zone is found mostly of turbulent character and vertical distribution of particle velocities does not seem to conform to the classical law of the wall distribution given for steady-flow boundary layers. The results show that free-stream particle velocities, in the boundary layer of the breaker under the crest phase, increase considerably as the wave progresses towards the breaking point. The boundary layer thickness, defined as the velocity-affected region, remains constant throughout the transformation zone but it decreases with increasing deep-water wave steepness for the particular beach slope tested.  相似文献   
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