Picoplankton structure in clear and turbid eutrophic shallow lakes: A seasonal study

The relative abundance of the different picoplankton components (eukaryotic picophytoplankton (Peuk), picocyanobacteria (Pcy) and bacterioplankton), and their relationships with the lake conditions were studied in three types of shallow lakes from the Pampa Plain (Argentina) that differ in their optical properties: clear-vegetated, phytoplankton-turbid and inorganic-turbid. All the selected lakes, but one, are characterized by their different alternative steady state (clear-vegetated and phytoplankton-turbid water phases) following the model proposed by Scheffer et al. (1993).Autotrophic and heterotrophic picoplankton abundances were analyzed seasonally in relation to environmental variables. All the lakes presented high concentrations of total nitrogen (TN) (>229 μg L⁻¹), total phosphorus (TP) (>46 μg L⁻¹) and dissolved organic carbon (DOC) (>13.7 mg L⁻¹). Clear-vegetated lakes were characterized by vertical diffuse PAR (photosynthetic active radiation) attenuation coefficient (kd_PAR) lower than 11 m⁻¹, whereas inorganic-turbid lake always showed values higher than 21.1 m⁻¹. The euphotic zone depth (Z_1%) was wider in clear-vegetated lakes (40–140 cm) and thinner in the inorganic-turbid (10–20 cm). The phytoplankton-turbid lakes presented a wide range in the values of these variables (kd_PAR: 5.2–35.8 m⁻¹; Z_1%: 10–90 cm). Phytoplankton chlorophyll-a (Chl-a) strongly differed, ranging from 1.6 to 334.6 μg L⁻¹. Picophytoplankton was mainly represented by phycocianine-rich (PC-rich) Pcy in all cases, dominating over Peuk algae. The total and relative abundances of eukaryotic picophytoplankton, Pcy and bacterioplankton, as well as the size structure of the phytoplankton community differed among the water bodies. In general, clear-vegetated water bodies exhibited similar abiotic characteristics, picophytoplankton/bacterioplankton ratios, and phytoplankton size structure. Contrarily, no clear trend was identified for the group of turbid lakes. The contrasting results obtained for the importance of the picoplankton components in phytoplankton-turbid shallow lakes evidence that the availability of the energetical and nutrient resources cannot be solely considered to predict their relative importance in this type of shallow lake.     

Determination of parameters of low-frequency internal waves in the coastal zone of a fringing sea using field measurements and basing on the nonlinear theory

A method is presented to determine the quadratic nonlinearity parameter and amplitude of low-frequency internal gravity waves in the coastal zone of a fringing sea, based upon their propagation rate dependence on local value of pycnocline vertical displacement produced by the waves. To test the method, the internal wave field observations in the coastal zone of the Sea of Japan are used. The testing results show that the internal wave parameters calculated using the proposed method and the experimental data are in a good agreement with those calculated from theoretical formulas.     

Influence of the plasma zonal E × B drift on the electron concentration in the low-latitude ionospheric F region at the minimum of solar activity near the spring equinox

The three-dimensional nonstationary theoretical model of the concentrations and temperatures of electrons and ions in the ionospheric F region and plasmasphere at low and middle latitudes is used to study variations in the concentration NmF2 and height hmF2 of the ionospheric F2 layer under the action of the plasma zonal drift in the direction geomagnetic west-geomagnetic east perpendicularly to the electric E and geomagnetic B fields. The calculated and measured values of NmF2 and hmF2 for 16 ionospheric sounding stations during the quiet geomagnetic period on March 28–29, 1964 at low solar activity are compared. This comparison made it possible to correct the input parameters of the model: [O] from the NRLMSISE-00 model and the meridional component of the neutral wind velocity from the HWW90 model. It is shown that the nighttime NmF2 values decrease up to twice at low solar activity in the low-latitude ionosphere, and the hmF2 values change by up to 16 km, if the plasma zonal E×B drift is not taken into account. Under the daytime conditions, the influence of the plasma zonal E×B drift on NmF2 can be neglected.     

Effects of water extraction in a vulnerable phreatic aquifer: Consequences for groundwater contamination by pesticides, Sint-Jansteen area, The Netherlands

Pesticides are a potential threat to the quality of extracted groundwater when the water-supply area is used for agricultural activities. This problem is discussed for the water-supply area of Sint-Jansteen, The Netherlands, where measured pesticide concentrations in the extracted water regularly exceed EU limits (0.1 μg/L). Groundwater samples taken from the aquifer within the water-supply area show low contamination, but samples taken from the extracted water occasionally contain pesticides, making the water inadequate for drinking-water purposes. The more intense contamination of the extracted water is caused by the change in the natural groundwater flow pattern near the extraction wells. In this area, pesticide use cannot be avoided easily, and an approach is given to differentiate pesticide use in the area according to expected travel time toward the wells and the chemical characteristics of the pesticides. A groundwater flow model for the area is developed and the effects of groundwater extraction on the natural flow pattern are evaluated. Using particle tracking, the travel-time zones are determined. Combining these results and the degradation behavior of certain pesticides led to an optimal scheme to integrate agricultural activities and groundwater extraction in the area. This is illustrated for five different types of pesticides (atrazine, simazine, bentazone, MCPA, and mecoprop). Received, October 1998/Revised, July 1999, September 1999/Accepted, November 1999     

A comparative study of the modelling of cement hydration and cement-rock laboratory experiments

The use of cement and concrete as fracture grouting or as tunnel seals in a geological disposal facility for radioactive wastes creates potential issues concerning chemical reactivity. From a long-term safety perspective, it is desirable to be able model these interactions and changes quantitatively. The ‘Long-term Cement Studies’ (LCS) project was formulated with an emphasis on in situ field experiments with more realistic boundary conditions and longer time scales compared with former experiments. As part of the project programme, a modelling inter-comparison has been conducted, involving the modelling of two experiments describing cement hydration on one hand and cement-rock reaction on the other, with teams representing the NDA (UK), Posiva (Finland), and JAEA (Japan).This modelling exercise showed that the dominant reaction pathways in the two experiments are fairly well understood and are consistent between the different modelling teams, although significant differences existed amongst the precise parameterisation (e.g. reactive surface areas, dependences of rate upon pH, types of secondary minerals), and in some instances, processes (e.g. partition of alkali elements between solids and liquid during cement hydration; kinetic models of cement hydration). It was not conclusive if certain processes such as surface complexation (preferred by some modellers, but not by others) played a role in the cement-rock experiment or not. These processes appear to be more relevant at early times in the experiment and the evolution at longer timescales was not affected. The observed permeability profile with time could not be matched. The fact that no secondary minerals could be observed and that the precipitated mass calculated during the simulations is minor might suggest that the permeability reduction does not have a chemical origin, although a small amount of precipitates at pore throats could have a large impact on permeability.The modelling exercises showed that there is an interest in keeping the numerical models as simple as possible and trying to obtain a reasonable fit with a minimum of processes, minerals and parameters. However, up-scaling processes and model parameterisation to the timescales appropriate to repository safety assessment are of considerable concern. Future modelling exercises of this type should focus on a suitable natural or industrial analogue that might aid assessing mineral-fluid reactions at these longer timescales.     

Specific composition of native silver from the Rogovik Au–Ag deposit,Northeastern Russia

The first data on native silver from the Rogovik Au–Ag deposit in northeastern Russia are presented. The deposit is situated in central part of the Okhotsk–Chukchi Volcanic Belt (OCVB) in the territory of the Omsukchan Trough, unique in its silver resources. Native silver in the studied ore makes up finely dispersed inclusions no larger than 50 μm in size, which are hosted in quartz; fills microfractures and interstices in association with küstelite, electrum, acanthite, silver sulfosalts and selenides, argyrodite, and pyrite. It has been shown that the chemical composition of native silver, along with its typomorphic features, is a stable indication of the various stages of deposit formation and types of mineralization: gold–silver (Au–Ag), silver–base metal (Ag–Pb), and gold–silver–base metal (Au–Ag–Pb). The specificity of native silver is expressed in the amount of trace elements and their concentrations. In Au–Ag ore, the following trace elements have been established in native silver (wt %): up to 2.72 S, up to 1.86 Au, up to 1.70 Hg, up to 1.75 Sb, and up to 1.01 Se. Native silver in Ag–Pb ore is characterized by the absence of Au, high Hg concentrations (up to 12.62 wt %), and an increase in Sb, Se, and S contents; the appearance of Te, Cu, Zn, and Fe is notable. All previously established trace elements—Hg, Au, Sb, Se, Te, Cu, Zn, Fe, and S—are contained in native silver of Au–Ag–Pb ore. In addition, Pb appears, and silver and gold amalgams are widespread, as well as up to 24.61 wt % Hg and 11.02 wt % Au. Comparison of trace element concentrations in native silver at the Rogovik deposit with the literature data, based on their solubility in solid silver, shows that the content of chalcogenides (S, Se, Te) exceeds saturated concentrations. Possible mechanisms by which elevated concentrations of these elements are achieved in native silver are discussed. It is suggested that the appearance of silver amalgams, which is unusual for Au–Ag mineralization not only in the Omsukchan Trough, but also in OCVB as a whole, is caused by superposition of the younger Dogda–Erikit Hg-bearing belt on the older Ag-bearing Omsukchan Trough. In practice, the results can be used to determine the general line of prospecting and geological exploration at objects of this type.     

A subsidiary fast-diffusing substitution mechanism of Al in forsterite investigated using diffusion experiments under controlled thermodynamic conditions

Diffusion of Al in synthetic forsterite was studied at atmospheric pressure from 1100 to 1500 °C in air along [100] with activities of SiO₂, MgO and Al₂O₃ (a_SiO2, a_MgO and a_Al2O3) buffered. At low a_SiO2, the buffer was forsterite + spinel + periclase (fo + sp + per) at all temperatures, while at high a_SiO2 and subsolidus conditions a variety of three-phase assemblages containing forsterite and two other phases from spinel, cordierite, protoenstatite or sapphirine were used at 1100–1350 °C. Experiments at high a_SiO2 and 1400 °C used forsterite + protoenstatite + melt (fo + en + melt), and at 1500 °C, fo + melt. The resulting diffusion profiles were analysed by LA–ICP–MS in scanning mode. Diffusion profiles in the high a_SiO2 experiments were generally several hundred microns in length, but diffusion at low a_SiO2 was three orders of magnitude slower than in high a_SiO2 experiments carried out at the same temperature, producing short profiles only a few microns in length and close to the spatial resolution of the analytical method. Interface concentrations of Al in the forsterite, obtained by extrapolating the diffusion profiles to the crystal/buffer interface, were only a fraction of those expected at equilibrium, and varied among the differing buffer assemblages according to (a_Al2O3)^1/2 and (a_SiO2)^3/4, pointing to the substitution of Al in forsterite by an octahedral-site, vacancy-coupled (OSVC) component with the stoichiometry Al _4/3 ³⁺ vac_2/3SiO₄, whereas the main substitution expected from previous equilibrium studies would be the coupled substitution of 2 Al for Mg + Si, giving the stoichiometry MgAl₂O₄. It is proposed that this latter substitution is not seen on the length scales of the present experiments because it requires replacement of Si by Al on tetrahedral sites, and is accordingly rate-limited by the slow diffusivity of Si. Instead, diffusion of Al by the OSVC mechanism is relatively fast, and at high a_SiO2, even faster than Fe–Mg interdiffusion.     

Is There a Link between Arctic Sea Ice Loss and Increasing Frequency of Extremely Cold Winters in Eurasia and North America? Synthesis of Current Research

Studies dealing with impact of the Arctic warming and related sea ice decline on the Northern Hemisphere atmospheric circulation are considered. The causes of occurrence of extremely cold winters over the mid-latitude continents observed in the recent decades against the warming background are discussed. Several conceptions are outlined which explain potential reasons for occurrence of this phenomenon. The paper discusses impacts of the Arctic sea ice loss on the large-scale atmospheric circulation, oscillations of planetary waves. It also discusses issues related to sea ice changes in the Barents and Kara seas and their link to the frequency of extremely cold winters observed in Eurasia and North America, the contribution of internal atmospheric variability to the increasing frequency of cold weather, and the role of the Atlantic Multidecadal Oscillation in the Arctic sea ice reduction.     

Numerical modelling of stream–aquifer interaction: Quantifying the impact of transient streambed permeability and aquifer heterogeneity

Stream–aquifer interaction plays a vital role in the water cycle, and a proper study of this interaction is needed for understanding groundwater recharge, contaminants migration, and for managing surface water and groundwater resources. A model‐based investigation of a field experiment in a riparian zone of the Schwarzbach river, a tributary of the Rhine River in Germany, was conducted to understand stream–aquifer interaction under alternative gaining and losing streamflow conditions. An equivalent streambed permeability, estimated by inverting aquifer responses to flood waves, shows that streambed permeability increased during infiltration of stream water to aquifer and decreased during exfiltration. Aquifer permeability realizations generated by multiple‐point geostatistics exhibit a high degree of heterogeneity and anisotropy. A coupled surface water groundwater flow model was developed incorporating the time‐varying streambed permeability and heterogeneous aquifer permeability realizations. The model was able to reproduce varying pressure heads at two observation wells near the stream over a period of 55 days. A Monte Carlo analysis was also carried out to simulate groundwater flow, its age distribution, and the release of a hypothetical wastewater plume into the aquifer from the stream. Results of this uncertainty analysis suggest (a) stream–aquifer exchange flux during the infiltration periods was constrained by aquifer permeability; (b) during exfiltration, this flux was constrained by the reduced streambed permeability; (c) the effect of temporally variable streambed permeability and aquifer heterogeneity were found important to improve the accurate capture of the uncertainty; and (d) probabilistic infiltration paths in the aquifer reveal that such pathways and the associated prediction of the extent of the contaminant plume are highly dependent on aquifer heterogeneity.