Evaluation of seasonal nutrient dynamics in the oosterschelde (SW Netherlands)

A large set of data obtained from several monitoring programmes covering more than 3 years of registration of environmental parameters in the Oosterschelde estuary were used to evaluate seasonal trends in dissolved nutritive compounds. The deviations from conservative behaviour were calculated. It is concluded that specific estuarine processes rule the distributions of nitrogen compounds within the estuary and that these processes are largely dependent on the input of organic material from the sea. Levels of silica are almost completely governed by fresh water discharges throughout the year, except for the Keeten-Krammer area in spring. Phosphorus concentrations are steady throughout the year with minor influences of exchange with sediment in spring and summer and an influence of the North Sea in spring.     

Environmental characteristics and distribution of macrobenthos in a mudflat of the west coast of Korea (Yellow Sea)

The spatio-temporal changes of environmental factors and the distribution patterns of macrobenthos were investigated on an intertidal mudflat near Panweol on the west coast of Korea. Temperatures of the surface sediments and salinities of the intertitial water were highest in summer and lowest in winter, while water content of the sediment was lowest in summer and highest in winter. Hourly variations of sediment temperature, water content and interstitial salinity during the daytime depended largely upon both the timing of ebb tide and daylight intensity.Temperature, water content and interstitial salinity showed a marked variation with sediment depth. Temperatures decreased sharply from the surface to 10 or 15 cm depth, and below this depth they were nearly constant, except in summer. Water content and interstitial salinity underwent a rapid decline in this upper layer and began to rise from this depth. From the high tide mark toward the main tidal channel, water content increased but salinity decreased. The sand fraction increased with the decrease of clay content toward the main tidal channel. This tidal flat could be divided into two zones: the “upper intertidal zone” and the “lower intertidal zone”, based on the spatial differences of the water content, interstitial salinity and grain-size composition in the surface sediment.There were three distinct zones of macrobenthos. The upper intertidal zone was dominated by burrowing deposit feeders, such as Ilyoplax dentimerosa, Helice tridens sheni, Cleistostoma dilatatum and Perinereis vancaurica tetradentata. The middle intertidal zone was characterized by other burrowing deposit feeders, such as Ilyoplax pingi, Macrophthalmus japonicus and Periserrula leucophryna. And the lower intertidal zone was dominated by suspension feeders represented by Laonome tridentata and Potamocorbula amurensis. Each zone was named after these characteristic species.Although the species composition of the upper intertidal community was different from that of the middle intertidal, the way of living and the feeding behaviour of the species occurring in these two zones were very similar. But the lower intertidal zone showed a characteristic community structure differing not only in species composition but also in ways of living and feeding of the species. Some sets of environmental factors are considered to explain the zonation. Water content of the sediment and interstitial salinity will be the main factors causing the observed zonation.     

Oxygen profiles and organic carbon fluxes in Laurentian Trough sediments

Sediment trap samples and undisturbed cores of bottom sediment were obtained during spring and summer from a 350-m-deep site in the Laurentian Trough, Gulf of St. Lawrence, and used to determine the flux of organic carbon to the sediment surface, and oxygen and organic carbon concentration profiles close to the sediment-water interface. Oxygen uptake rates by the sediments, calculated from micro-electrode profiles and a one-dimensional molecular diffusion transport model, can only account for 20% of the organic carbon that is mineralized in the sediment. Mechanisms other than molecular diffusion must dominate the transport of oxygen across the sediment-water interface in these deep coastal sediments.     

Suspended-sediment transport in the Dollard estuary

For an analysis of suspended-sediment transport in the Dollard estuary, water samples were taken and current velocities and directions measured on the tidal flats and in the channels. Measurements carried out during fair-weather conditions in summer were compared with measurements carried out during a heavy gale in winter. During the onset of the flood, high suspended-sediment concentrations occurred as a result of resuspension of the top layer of the bottom sediment. Measurements carried out in the channels pointed to sediment transport from the estuary seaward, but inward transport over the flats during fair weather apparently compensates for the outflow during ebb and storms: the result is a net deposition in the order of 1 to 2 mm (up to 8 mm) per year, as previously observed in this area.     

Rates of sedimentation on the Cape Verde Rise

The hypothesis that the Cape Verde Rise, situated on the lee-side of the Canaries current and being an area of low current velocities, would be a sediment trap, was tested by determining Holocene sedimentation rates on the Rise as well as in the adjacent ocean. The rates were found to vary between 1.9 and 2.6 cm per 1000 years and were not higher on the Rise than in the deeper ocean around it, indicating that under the present conditions of sediment supply, current pattern and bottom configuration the Cape Verde Rise is not a sediment trap. During the Holocene, sedimentation has been rather uniform, consisting of finegrained mineral particles and (organic) carbonate, but during the Würm period also turbidites, large amounts of volcanic minerals and fine quartz of probably eolian origin were deposited. The data on the Würm and Eemian deposits are not sufficient to compare sedimentation rates on the Rise with those in the adjacent ocean. The sedimentation rates found for the Holocene agree well with those found by others in this area.     

Tidal asymmetry and estuarine morphology

Estuarine morphology is to a large extent determined by the residual sediment transport pattern. However, the inverse statement is also true. Residual sediment transport depends on differences in magnitude and duration between ebb and flood tidal currents. Such differences (“tidal asymmetry”) are produced by the distortion of the tidal wave propagating on the coastal shelf and entering bays and estuaries. In this study the relationship between tidal asymmetry and estuarine morphology is investigated. Based on theoretical considerations some general principles are derived and compared with field observations.     

The effects of bioturbation on sediment transport on an intertidal mudflat

Laboratory flume studies were conducted to determine the relative importance of various biological and physical factors controlling sediment erosion threshold and transport rate on an intertidal mudflat in the Bay of Fundy. Studies were conducted in a July period of maximum solar exposure. The upper and mid-intertidal stations of the flat were dominated by silt-clay sediments, while the lower intertidal was dominated by very fine sand. The tube-dwelling amphipod Corophium volutator was the most abundant infaunal species with densities exceeding ∼1300 ind·m⁻² based on counts of burrow openings. Sediment-penetrometry and water-content measurements indicated no change in unconsolidated shear strength and porosity, respectively, along the intertidal transect. Despite the apparently cohesive nature of the sediment, erosion occurred as small ripples. Critical shear velocities (u_crit) for erosion determined with intact cores in a laboratory flume were relatively consistent between stations and sampling dates (mean = 2.1 cm·s⁻¹ ± 0.2 SD), with no relationship to Corophium density, sediment chlorophyll a, or physical variables. Field-treatment of sediment with formalin did not cause an obvious change in u_crit as determined by flume experiments. Corophium seemed to have little effect on erosion thresholds because incipient motion could be observed between tube burrows, beyond the local influence of the amphipod. In contrast to erosion thresholds, sediment-erosion rates measured with bedload traps were negatively correlated with density of small Corophium, probably due to binding of sediment into burrows and the ambient sediment microfabric, all of which reduce the availability of sediment for transport. Adult amphipods, which occurred at low density probably due to territorial/competitive interactions, had no obvious effect on erosion rate since only a small proportion of the sediment surface was impacted by their bioturbation. Although a portion of the amphipod population exerts a stabilizing influence on sediment-erosion rates, concurrent studies at the site indicate that Corophium seasonally reduces the sediment-erosion threshold by grazing on microflora which would otherwise inhibit the initiation of grain motion. Due to this decoupling of erosion rate and threshold, it is necessary to measure both processes in assessing the effects of benthic biota on sediment transport.     

The role of epibenthic predators in structuring the marine invertebrate community of a British coastal salt marsh

The marine fauna of salt marshes are subjected to predation by birds, tidally feeding flatfish, crabs, prawns and small gobiid fish. The role of these epibenthic predators in structuring the community was investigated using cages to exclude predators. A range of designs of cages and partial cages was employed to control for artefacts due to caging, and sufficient cages were employed so that each cage was only sampled once to prevent the compounding of disturbance due to predation and sampling. Two mesh sizes were employed, a fine mesh excluding epibenthic predators and a coarse mesh allowing access by small crabs, prawns and gobiid fish but excluding birds and larger fish. The exclusion was maintained for 2 years. The presence of any experimental structure had a significant effect on the sedimentary regime within the cage. Epibentic predator exclusion let to an increase in infaunal predator density, but had no significant effect on the infaunal deposit feeders. There was some evidence that predators limit the surface deposit feeding gastropood Hydrobia ulvae during the winter. The gastropod Littorina littorea responded positively to the presence of any caging structure; this may be the result of changes in the availability of food, as the sides of a cage support a diatom flora which this species can exploit. The lack of a response from the infaunal deposit feeders is attributed to their horizontal mobility within the sediment. The possible interactions between epibenthic and infaunal predators are discussed.     

Primary production and import of particulate organic matter on a salt marsh in the Netherlands

The macrophyte production and the transport of particulate organic matter between marsh and adjacent estuary have been investigated for a 0.3 km² salt marsh along the Oosterschelde estuary, The Netherlands.The primary production of macrophytes, measured with Smalley's (1959) method was 837 to 1030 g dry organic matter·m⁻²·y⁻¹. Measurements of amounts of particulate organic matter transported through one of the main tidal creeks in the salt marsh resulted in a net import. On an average 31% of the material brought in by the flood settled in the marsh. The majority of this material is smaller than 63 μm. On the other hand large floating material is exported during storm tides, although the quantity seems to be smaller than that of the suspended material imported.The differences between various marshes with regard to export and import of organic matter are explained in terms of marsh level, primary production, turbulent diffusion, sinking and resuspension of particulate matter and biotic transformations.     

Transport,deposition and in situ decay of seagrasses in a tropical mudflat area (Banc D'Arguin,Mauritania)

Extensive seagrass beds cover large areas of the mudflats and subtidal parts of the Banc d'Arguin (Mauritania, West Africa). The fate of seagrass biomass (mainly Zostera noltii and Cymodocea nodosa) was investigated during a 1-month expedition to the area (May 1988). Only minor amounts of leaf material were found to be transported by water currents at that time. Subtidal detritus depositions were not found in the area. Though conspicuous accumulations of leaf material were present on some parts of the shore, the amounts washed ashore per flood tide formed only a minor proportion of the daily seagrass production. Thus, transport of leaf litter away from the seagrass beds of the Banc d'Arguin appears insignificant. The major part of senescent leaves probably remains trapped within the seagrass beds and will decompose in situ. The observation that a large part of the particulate carbon present in the surface sediment of seagrass beds was derived from above-ground biomass (55% and 37% in Zostera and Cymodocea beds, respectively), is in agreement with this conclusion.Using the litterbag technique, the rate of leaf decomposition in a seagrass bed was investigated. The time required for 50% weight loss of Zostera and Cymodocea leaf litter in the intertidal zone of the seagrass bed was 158 and 50 days, respectively. In the subtidal zone these figures were 49 and 37 days.     

Seasonal variations and spatial gradients of nitrogenous nutrients in waters of Southeastern Vancouver Island,British Columbia,Canada

The study was conducted to begin testing the hypothesis that supplies of inorganic nitrogen are adequate to support rates of phytoplankton production observed in Saanich Inlet, a fjord in Vancouver Island, British Columbia, Canada. To test this hypothesis, rates of nitrogen transport into and within the euphotic zone by advection and diffusion are needed. Computation of these rates requires knowledge of spatial gradients of inorganic nitrogen, which were determined from spatial distributions of NO₃-N, NO₂-N, and NH₄-N measured during spring, summer and fall months of 1977, 1983 and 1984 in water of the southeastern coast of Vancouver Island. Of the nitrogenous nutrients, vertical gradients of NO₃-N were largest, usually exceeding 0.75 μM-m⁻¹ in the upper 10 m of the water column during late spring and summer months. During these same months, horizontal gradients were small, varying between 1.5 and 2.0 μM-km⁻¹. A model (Eppley et al., 1979) was used to estimate the relative importance of horizontal and vertical diffusion to nutrient supply in the euphotic zone and results indicated that vertical diffusion would dominate. Thus, it is likely that vertical advection and diffusion driven by winds, internal waves, and tidal currents were important sources of nutrients to phytoplankton in Saanich Inlet during late spring and summer months.     

Physical and biological aspects of shell accumulation on a modern macrotidal flat,Inchon, Korea

Shell beds are comparatively rare on large tidal flats near Inchon, Korea, where barrier islands, well-developed intertidal drainage channels, and landward salt marshes also are absent. Even though landward transport of non-indigenous shells is discernible, most shell accumulations evidently originate through comparatively rapid deposition of sediments coupled with gradual mortality within local populations; individual valves typically experience only minor lateral displacement. Apparent cohort size classes remain distinguishable within some assemblages. Old shells predominate among gastropod assemblages whereas new shells predominate among bivalve assemblages, implying greater rates of productivity among bivalves than among gastropods.     

Discussion on 2-D Math Model of Sediment in Gravel Dominantly Riverbed

The present paper gives a discussion about transport coefficient of suspended sediment, coefficient of saturation recovery, riverbed sediment grading, deposition/scour thickness and normalization of the sediment composition in gravel dominantly riverbed. The math model above mentioned has been tested in two rivers with different riverbed sediment composition and a rather good agreement has been obtained.     

On selective tidal transport in the migration of North Sea Plaice (pleuronectes platessa) and other flatfish species

Evidence is given for selective tidal transport in North Sea plaice and other flatfish species. Data collected during midwater plankton surveys in two Dutch Wadden Sea inlets in early spring in the years 1970 to 1977 show that plaice, sole, flounder and dab demonstrate pelagic swimming behaviour in that period. Plaice, sole and flounder show a clear selective tidal transport mechanism to re-enter the Wadden Sea after staying in the open sea in winter whereas dab tends to leave the Wadden Sea. Data on by-catches of semi-pelagic pair trawl fishery for gadoids in the Dutch coastal area show that small quantities of plaice may be caught off the bottom throughout the whole year whereas sole moves to higher levels in two distinct periods only. The first period lower left diagram) demonstrate a strong preference for northgoing currents. This is also the direction towards the areas from which the plaice used had been transported to the laboratory and therefore this preference for northgoing tides might be interpreted as a clear case of homing to the Silverpits and Borkum areas.Also the rate of dispersion of the cluster of displaced tagged plaice in general (Fig. 18, lower right diagram) is much faster in the first months after release than that of the local plaice. Thus displaced plaice are more strongly motivated to migrate than non-displaced fish. This may well have been the case too in the plaice equiped with the transponding acoustic tags. This together with the release in an area with strong currents and narrow tidal ellipses means that the plaice should be considered to have been in an exceptional situation, extremely suited for travelling at great speeds in a fixed direction. Fig. 18 also shows the average surface tidal current velocities, derived from the 13 maps with hourly data on water circulation in the Stroomatlas Nederland (1963). It is obvious that the Lowestoft plaice, mostly released nearer to the coast within the release area, encountered the highest average current velocities observed in the North Sea. It cannot be proven that these circumstances explain completely the vast speed of the Lowestoft plaice but it certainly demonstrates that there is a bias in these experiments which could easily affect the results. When drawing conclusions in respect to the “normal” picture of plaice migration this bias should not be ignored.In the common sole selective tidal transport is observed in juveniles when entering the Wadden Sea in spring. In adult soles this mechanism is a common feature in March to May during the spawning migration but no evidence for the use of this mechanism is available during other periods of the year. The sole thus is an example of a flatfish species using selective tidal transport in a restricted period and for special purposes only.In plaice the use of selective tidal transport in the Wadden Sea inlets has been shown for juveniles. In adult plaice the need for using the mechanism might be much less although plaice have been seen swimming at the surface in spring (van den Ende, 1849) during day time quite in contrast with their normal behaviour. In spring nights, among soles swimming at the surface, plaice too have been observed (together with some dabs and turbots) but in much lower numbers than the sole (de Veen, 1967).On the whole there is not much evidence for a strong continuous use of selective tidal migration in plaice. No serious drops in the catch rates during specific tidal phases in the bottom trawl fisheries have been recorded by the results of the UK 81 otter trawl fishery. The coincides with the re-entering of soles in the Wadden Sea and with the well-known phenomenon of soles swimming at the surface. During this period the soles use selective tidal transport to reach the spawning grounds near the continental coasts.Data on catch per unit effort of a Dutch ottertrawler in 1958 to 1960, constantly fishing in one restricted area near Terschellingerbank lightvessel, reveal significant differences between day and night catch rates and a change from a preference for flood tide in the first half of the year to a preference for ebb tide in the second half.The available evidence for selective tidal transport in plaice is discussed in relation to the patterns of migration in 4 plaice sub-stocks studied by means of tagging experiments, and in relation to results of displacement experiments with mature and immature plaice. The tagging and displacement experiments present a picture of migration in which the directional swimming is a weak component superimposed on largely random movements and the speeds of travelling are low. The plaice react to displacement by trying to return to their original grounds and this tendency to home lasts for a full year.This picture about selective tidal transport contrasts with results obtained by the Lowestoft Laboratory. Plaice equiped with transponding acoustic tags did show largely directed and fast tidal migration before and after spawning. Arguments are given that these plaice were, however, in an exceptional position and may have shown homing behaviour. Notwithstanding this reservation to the Lowestoft experiments, these results and the present ones are explained if it is assumed that apparently only a fraction of the population uses the mechanism of selective tidal migration, thus saving energy and achieving quick transport when necessary. It is likely that in plaice as in sole the mechanism will be used in special cases only, probably for short periods, in certain areas.     

影响山西夏季降水的水汽输送研究

利用山西省58个代表站近50年的夏季降水资料和NCAR/NCEP逐月再分析资料,分析影响山西夏季降水的水汽输送特征,应用SVD方法等探讨影响山西夏季降水异常的水汽源地和水汽输送关键区。结果表明：山西夏季降水与热带西太平洋—南海的东风气流,热带印度洋越赤道气流以及南海—华南的水汽输送带有关,同时也受到中纬度西风带中水汽输送影响。夏季水汽输送与山西夏季降水具有显著的遥相关关系,山西夏季降水异常的水汽异常来源包括副高南侧的热带西太平洋—南海的偏东风水汽输送异常,和印度洋—阿拉伯海地区越赤道气流的偏西风水汽输送异常,山西夏季降水的水汽输送“关键区”位于云贵高原中部到四川盆地东部的矩形区域。由整层水汽输送通量标准化处理建立的水汽输送源指数和水汽输送指数,可以表征水汽输送异常对于山西夏季降水的影响。研究结果可以为山西汛期降水异常分析和汛期预测提供参考依据。     

Splash-saltation transport under wind-driven rain

Soil detachment and transport by wind- driven rain differs from that by rainless wind and from that by windless rainfall. This study deals with field measurements of particle movement during periods in which rain and wind coincide. The effect of rainfall on the movement of soil particles by wind is analyzed using a sediment sensor called SALTIPHONE. Some results are presented of field experiments at two sites in the Netherlands: a cutover peat soil and a beach sand. Results show an increase of particle movement at the sensor height during rainfall. This can be attributed to the combined action of saltation and rain-induced uplift of soil particles and subsequent transport by wind.     

Animal traces on a tidal flat in Hangzhou Bay,China

Despite prevailing low salinity (S ranging from 1 to 10) of the Hangzhou Bay, its extensive (up to 2 km wide) flats along the northern coast have a species-rich macrobenthic fauna of crustaceans, bivalves, gastropods and polychaetes. These produce a variety of crawling, feeding, dwelling and resting traces, some comparable to those observed on the flats of Korea and Taiwan. The characteristic starlike feeding traces of the bivalve Tellina (Moerella) iridescens are described here for the first time. Sediment reworking appears to be largely confined to the surface layer. The burrow-inhabiting crabs are also mainly surface sediment feeders. These Chinese flats lack organisms reworking deeper sediment layers such as lugworms or callianassid crustaceans. As compared to the Wadden Sea, sedimentation rate is higher and subsurface sediment reworking less intense, resulting in relatively few bioturbation structures in vertical sections of the Hangzhou tidal flat sediments.     

CFD-DEM Simulation to Study the Filtration Characteristic of the Fine Particle in the Filter Media

The gas-solid two-phase flow characteristics of fine particles in the filter media were simulated based on CFD-DEM (Discrete Element Method) in this study, and the particle group, particle size distribution, and the rebound effect between particles, particle and fiber were taken into account the calculations. Additionally, the movement characteristics of the particle group and the deposition form of the fine particles in the fibrous filtration were analyzed. The results indicate that it is convenient and feasible to simulate the filtration process of the filter media as well as the deposition process and form of the fine particle on the media surface using CFD-DEM, and the simulation results are consistent with the experimental observations in the literature. The surface filtration has larger contribution to the filtration process, and most of the particles are captured by the media surface. Meanwhile, parts of smaller particles that enter into the media are captured by the depth filtration. A larger number of particles are collected by the particle-particle capture mechanism. The filtration efficiencies of the filter media are varied with the particle systems, and for the model of the filter media in this study, the filtration efficiency of the multi-particle system is about 20% higher than that of the single-particle one.     

不同年代退耕地上2种积沙仪风沙流特征比较

在不同年代退耕地上安装2种不同类型的积沙仪,通过风沙流结构特征的比较研究,分析2种积沙仪收集风沙流的异同点,为干旱荒漠区不同风沙环境条件下风沙流研究筛选出理想的积沙仪.研究结果表明:(1)2号积沙仪的年输沙量明显高于1号积沙仪,风沙流越大,差异越显著,且与退耕地距沙源的远近关系密切,距沙源越近,差异越显著;(2)2种积...     

Suspended matter and bottom deposits of the Orinoco delta: Turbidity,mineralogy and elementary composition

A three-layer stratification was found in the Boca Grande estuary: a layer of fluid mud near the bottom, a layer of low salinity and low turbidity at the surface, and a saltwedge of high salinity and low turbidity in between. The position of the fluid mud layer is related to the nodal point where the movement of the bottom water changes from a resultant landward to a resultant seaward direction. It is not clear to what extent the formation of the fluid mud is influenced by the dredging of the navigation channel and to spoiling practices.The data on sediment supply from the Orinoco river and from the Amazon along the north coast of South America indicate that about twice as much sediment is supplied by the Amazon to the Orinoco delta as by the Orinoco river. The clay mineral and elementary composition of the Orinoco delta sediments supports this.In the Orinoco delta iron- and aluminiumhydroxydes are adsorbed onto clay minerals and other particles as coatings and form chlorite-like structures with swelling minerals. This prevents swelling and makes that the swelling minerals are not recognized as such with normal X-ray diffraction procedures when the hydroxydes are not removed. It is possible that through adsorption of hydroxydes on degraded illites also some swelling minerals are formed, resulting in relatively low illite crystal lattice, as was pointed out by Quigley (1969), so that the clays start to expand. On removal of the iron and alumina with a sodium-citrate-dithionite buffer, they are replaced by water and sodium and subsequently, after treatment with CaCl₂, by Ca. The illites then collapse when brought under vacuum.