Dynamics of suprabenthos off the Ebro Delta (Catalan Sea: western Mediterranean): Spatial 13 and temporal patterns and relationships with environmental factors

Dynamics of suprabenthos (hyperbenthos) composition and biomass have been simultaneously analyzed at two sites (S1, S2) off the Ebro River Delta (western Mediterranean). The stations, separated by ca. 5 km, differed in terms of depth (S1, 47 m; S2, 61 m), distance to the river mouth (S2 south of S1 and farther from the mouth) and fishing activity (S1 is a fishing ground; S2 is in an area closed to fishing). Peracarids (gammaridean amphipods, mysids, and cumaceans) were dominant among suprabenthic taxa. Seasonality was the main explanation for changes in taxonomic composition, with two seasonal groups indicated by MDS analyses (late summer–autumn, August–September, and November 2003; early summer, June and July samples). Peracarids at both S1 and S2 showed a peak of abundance in early July, with the highest densities reaching 5400 individuals (100 m)⁻² at S2. There was a sharp decrease of density in late July (S1) and August (S2), then an increase in August (S1) and in September (S2), respectively. A secondary peak of abundance occurred in November (S1) and December (S2). There was, therefore, a similar picture in the dynamics of suprabenthic peracarids at both sites, though with a delay of 1 month at the deeper S2. This pattern coincided with changes in river discharge (specifically, a decrease of suprabenthos when influx was below 200 m³ s⁻¹ in the period June–September 2003), and with the formation of a thermal gradient (also in June–September 2003) between S1 and S2 associated with the 15 °C isotherm. In addition, the decrease of suprabenthos in late July–August also coincided with the massive occurrence of mucilaginous aggregates close to the bottom (at between 0 and 4 m above the bottom), in the usual habitat of suprabenthos. C/N ratios in sediments (an indicator of the degree of degradation of organic matter (OM)) increased during this peak abundance of mucilaginous aggregates. The impoverishment of sediments in total organic carbon (TOC) was parallel to the decline of suprabenthos in late July–August, which is consistent with negative or non-significant correlations (evidenced both by Spearman's r and multi-regression models) between suprabenthos densities and TOC. Zooplankton taxa (e.g., copepods, chaetognaths, and fish/crustacean larvae living in the entire water column) showed significant correlations with a number of environmental factors (basically temperature), in contrast to suprabenthos. In conclusion, suprabenthos abundance was influenced by a number of natural factors, both intra-annual (e.g., OM quality, river discharges) and inter-annual (e.g., accumulation of mucilaginous remains). Probably due to their swimming capability, suprabenthos were not influenced by trawling activity. Considering the high P/B exhibited by suprabenthic crustaceans in comparison to infauna, this compartment likely has an important role in regulating food webs in those communities submitted to high fishing impact.     

Mucus Aggregates in the Adriatic Sea: An Overview of Stages and Occurrences

Abstract. The wide range of macroscopic mucus aggregates in the Adriatic Sea is documented. These structures arc considered to be initially produced by diatoms. A differentiation into five states or stages: macroflocs, stringers, clouds, creamy surface layers, and gelatinous surface layers is proposed. This classification is based not only on size and shape, but also takes relative position in the water, stability, behavior, and effect on benthos into consideration. An interrelationship between these aggregates - as different manifestations or phases of the same phenomenon - is postulated.
Macroflocs and stringers arc more common. Larger aggregates (clouds, creamy layers, gelatinous layers) arc less frequent. The general term "marine snow" is inadequate to describe the entire range of aggregate sizes and shapes, while the Italian designation "mare sporco" (dirty sea) is misleading. Two massive occurrences of dense clouds coupled with surface accumulations were recorded in the Adriatic in 1988 and 1989. The effect of the various stages on the shallow coastal ecosystem of the Northern Adriatic Sea is discussed.     

The aerodynamic behaviour of volcanic aggregates

A large proportion of solid material transported within the atmosphere during volcanic eruptions consists of particles less than 500 m in diameter. The majority of these particles become incorporated into a wide range of aggregate types, the aerodynamic behaviour of which has not been determined by either direct observation or in the laboratory. In the absence of such data, theoretical models of fallout from volcanic plumes make necessarily crude assumptions about aggregate densities and fall velocities. Larger volcanic ejecta often consists of pumice of lower than bulk density. Experimental data are presented for the fall velocities of porous aggregates and single particles, determined in systems analogous to that of ejecta falling from a volcanic plume. It is demonstrated that the fall of aggregates may be modelled in identical fashion to single particles by using a reduced aggregate density dependent on the porosity, and a size corresponding to an enclosing sphere. Particles incorporated into aggregates attain a substantially higher fall velocity than single particles. This is due to the larger physical dimensions of the aggregate, which overcomes the effect of lower aggregate density. Additionally, the internal porosity of the aggregate allows some flow of fluid through the aggregate and this results in a small increase in fall velocity. The increase in fall velocity of particles incorporated into aggregates, rather than falling individually, results in the enhanced removal of fine material from volcanic plumes.     

Stochastic modelling of erosion and deposition in cohesive soils

A new stochastic method of detachment rate estimation was used in erosion modelling. This method was based on calculating the probability of driving forces exceeding resistance forces in the interaction of oscillating flow and structured soil. Knowledge of the probability density functions for flow velocity, soil cohesion, aggregate size and soil integrity makes it possible to calculate theoretically the erosion rate of cohesive soil for any combination of these stochastic variables. The proposed theory explains the variability in relationships between rate of detachment and flow velocity. With flow velocity, detachment rate increases more rapidly for more integrated soil with higher cohesion and larger aggregates. This theory also shows the great difference between soil erosion type for relatively high and relatively low flow velocities, and explains rather high errors, even with detailed models, in the calculation of low soil erosion rate. Copyright © 2004 John Wiley & Sons, Ltd.     

冻融交替对砒砂岩与沙复配土壤团粒结构的影响

为研究冻融交替对砒砂岩与沙复配土壤团粒结构的影响,采用室内模拟冻融实验方法,研究了不同冻融周期和黏粒含量对复配土壤团粒结构的影响.结果表明:冻融周期、黏粒含量及二者的交互作用对复配土壤各级团粒结构组成均具有显著或极显著的影响.复配土壤黏粒含量越高,团粒结构的稳定性越高;在冻融作用下,随着复配土壤中沙比例的增大,大团粒结构含量总体呈降低趋势,降低速率先减小后增大,且砒砂岩与沙比例为1:1时大团粒结构降低的速率最小.结果说明冻融交替作用降低了土壤团粒结构的稳定性,将较大颗粒团粒结构崩解破碎成小粒级颗粒,更易于砒砂岩和沙复配成土.     

Variability of shallow overland flow velocity and soil aggregate transport observed with digital videography

Field experiments at Tiramoana station 30 km north of Christchurch, New Zealand using an erosion plot 16·5 m long, 0·6 m wide, and with a slope of 14–14·5° on rendzina soil aimed to measure the variability of flow velocity and of soil aggregates transport rate in shallow overland flow. Discharge/cross‐section area ratio was used to estimate mean velocity, and high‐speed digital video camera and image analysis provided information about flow and sediment transport variability. Six flow runs with 0·5–3·0 L s^?1 discharges were supercritical with Froude numbers close to or more than 1. Mean flow velocity followed Poiseuille law, float numbers were more than 1·5 and hydraulic resistance was an inverse proportional function of the Reynolds number, which is typical for laminar flows. Hence actual velocity varied through time significantly and the power spectrum was of ‘red‐noise’, which is typical for turbulent flow. Sediment transport rates had even higher variability, and soil aggregates transport was a compound Poisson process. Copyright © 2008 John Wiley & Sons, Ltd.     

表土结皮形成影响因素述评

西北干旱半干旱地区,表土结皮普遍发育,其形成受诸多因素影响。土壤质地是影响表土结皮形成的最重要因素。此外,团聚体稳定性、表面粗糙度、矿物质和交换性离子浓度、降雨历时、降雨强度、前期含水量、湿润速度等因素同样对结皮的形成具有重要影响。结皮的形成可以简要归纳为两个主要过程,一是团聚体的分散,二是分散后的粘粒阻塞土壤孔隙。因此团聚体稳定性和粘粒含量是影响表土结皮形成的关键。研究发现,土壤中粘粒含量为20%～30%时,极易形成结皮,当粘粒含量超过40%时,团聚体变得较为稳定,结皮形成受到一定抑制。此外,土壤中如果富含蒙脱石矿物,或者土壤溶液具有较低的交换性钠离子和电解质浓度,则表土结皮易于形成。前期含水量和湿润速度对结皮形成也同样具有重要影响。试验表明,粘粒含量较高的试样在较低的前期含水量情况下更容易形成结皮,因为稳定性较高的团聚体在干燥条件下更容易因迅速湿润而崩解。湿润速度对表土结皮的影响作用随着粘粒含量的增加而增大,但在土壤结构不稳定条件下,其影响作用受到很大限制。影响结皮形成的各因素并不是孤立的,它们之间互相作用、互相影响,这给定量研究和分析结皮形成敏感性带来很大难度,如何对各影响因子的临界值进行确定以及定量研...     

土石混合体渗透性现场试坑试验研究

针对唐钢司家营铁矿Ⅲ采场露天转井下开采回填废石设计研究工程要求,在该矿区Ⅲ采场附近进行了大尺度现场渗透试验。开挖3个大尺寸试坑,在各试坑中分层回填不同粒径范围及厚度的废石（土）进行试坑注水试验;测定回填土石混合体的近似渗透系数,测试土石混合体的粒径组成,并计算出各坑土石混合体的平均粒径和非均匀度。试验研究发现,回填土石混合体的渗透系数分别随平均粒径与非均匀度的增大而增大。降低回填废石（土）的平均粒径和非均匀度可以有效降低回填废石层的渗透性。     

Formulae for the statistical interrelationships between several orbital parameters of particles in the Oort Cloud and other orbiting aggregates

Assuming the motion of particles in an orbiting aggregate (e.g., the Oort Cloud is unperturbed Keplerian, the mean joint density of distance and speed depends only upon the densities of eccentricity and semi-major axis length. We derive a formula for the mean joint density of distance and speed in terms of these densities. Also provided are formulae which, given an observed mean joint density of distance and speed, permit the computation of the corresponding semi-major axis length and eccentricity densities. The results of this paper permit one to derive the structure of an orbiting aggregate given a minimum of information.Box 58421 Houston, Texas 77058