Large-Eddy Simulations of the Atmospheric Boundary Layer Using a New Subgrid-Scale Model – I. Slightly unstable and neutral cases

Subgrid-scale (SGS) modeling is a long-standing problem and a critical component in the large-eddy simulation (LES) of atmospheric boundary layer. A variety of SGS models with different levels of sophistication have been proposed for different needs, such as Smagorinsky's (1963) eddy viscosity model, Mason and Thomson's (1992) stochastic backscattering model, and Sullivan et al.'s (1994) near surface model. A modified Smagorinsky SGS model has been used in the LES version of Terminal Area Simulation System (TASS-LES). It has successfully simulated the buoyancy-dominated, convective atmospheric boundary layer flows, while simulations of the shear-dominated, slightly unstable, neutral, and stably stratified boundary layer flows are not so good. For the later, we used a simpler version of Sullivan et al.'s subgrid-scale model in which turbulent kinetic energy equation is not included and the model is still the first-order closure. A momentum profile matching approach is adopted in the proposed model. A series of simulations for shear-dominated, slightly unstable and neutral boundary layers are performed using different subgrid-scale models and different grid resolutions. The results are compared with those of Sullivan et al. (1994) and with empirical similarity relations for the surface layer. The simulations with the new SGS model appear to be far more satisfactory than those with the modified Smagorinsky model.     

Using adsorption and sulphide precipitation as the principal removal mechanisms of arsenic from a constructed wetland – a critical review

This review has been undertaken to understand the role of various parameters such as redox potential, microbes, and organic matters on the fate and transport of arsenic in the constructed wetland. A conceptual diagram of arsenic fate and transport in the constructed wetland was developed. Role of various minerals which are produced due to microbial activities are described. The role of these minerals on arsenic adsorption is discussed. It was envisaged that iron sulphide would be the main adsorbent for arsenic in microbe-mediated adsorption process. Beside microbe-mediated arsenic adsorption, roles of various microbes, such as sulphate reducers and methane producers, on arsenic transformation are discussed. Role of various organic carbon sources and electron acceptors on the proliferation of the above mentioned microbes with respect to arsenic transformation was envisaged. Role of dissolved organic matters on arsenic transformation and transport was also discussed in details. To strengthen the review, laboratory studies and modelling of arsenic adsorption and transformation using the Visual Minteq were appended.     

Diet of intertidal bivalves in the Ría de Arosa (NW Spain): evidence from stable C and N isotope analysis

We used natural abundance measurements of stable carbon and nitrogen isotopes and a short-term ¹⁵N-enrichment experiment to explore the importance of river-derived and autochthonous sources to the diet of intertidal suspension-feeding bivalves in the Ría de Arosa, an oceanic-dominated system in northwest Spain. Highly seasonal and intense phytoplankton "bloom" events occur in this estuary. However, relatively low phytoplankton standing crop and productivity are present during much of the year, suggesting that food sources other than phytoplankton must support bivalves during these periods. Bivalves (Cerastoderma edule, Tapes decussatus, Mytilus galloprovincialis) were sampled quarterly at four intertidal stations along the length of the estuary with increasing distance from the Ulla River. Muscle tissue was consistently enriched in ¹³C and ¹⁵N relative to stomach contents. The stable C isotope values of bivalves indicated that river-derived inputs were, at most, a minor component of diet. These values suggested the potential importance of benthic microalgae to bivalve diet much of the year and the incorporation of ¹⁵N label applied to the sediment surface into stomach contents and muscle tissue in a field experiment supported this conclusion. Other labeled components of the microbenthos such as bacteria could also have been ingested. ¹³C-depleted values of bivalves in May coincided with elevated offshore chlorophyll a concentrations, suggesting the increased use of a phytoplankton source during the spring. We propose that intertidal suspension-feeding bivalves in this estuary use primarily resuspended microbenthos during periods of low phytoplankton concentration, but that phytoplankton increases in relative dietary importance during bloom events.Communicated by P.W. Sammarco, Chauvin     

The impact of unconfined mine tailings and anthropogenic pollution on a semi-arid environment – an initial study of the Rodalquilar mining district, south east Spain

A geochemical study of stream sediments near Rodalquilar (South-east Spain) was undertaken to investigate the impact of processing activities associated with nearby gold mining. Despite the semi-arid nature of the area, visual inspection of tailings indicates that considerable fluvial erosion occurs during rainfall events. Geochemical analysis demonstrates that Mo, Sb, Te, Bi and As are all found at elevated levels in stream sediments downstream of the tailings site. Elevated levels of Pb, Cu and Zn were also noted in stream sediments. Statistical analysis demonstrates that four factors control trace-element distribution. Factor 1 displays a strong association of Mo, Sb, Te and Bi and As and is attributed to the mine tailings. Factor 2 displays an association of Pb and Cu, and is most easily attributed to anthropogenic pollution. Factor 3 distinguishes Au and reflects the selective nature of ore-processing methods employed at the site. Factor 4 only highlights Zn, reflecting that it is derived from several, probably unrelated, sources.     

Reef coral reproduction in the eastern Pacific: Costa Rica, Panamá and Galápagos Islands (Ecuador). IV. Agariciidae, recruitment and recovery of Pavona varians and Pavona sp.a

The reproductive ecology of two eastern Pacific zooxanthellate coral species was examined as part of a continuing series of studies relating bleaching/mortality events caused by the El Niño–Southern Oscillation disturbance, and is described for study sites in Costa Rica, Panamá, and the Galápagos Islands (Ecuador). This study deals with the sibling agariciid species Pavona varians and Pavona sp.a over a 13?yr period (1985 to 1997). Both Pavona species are broadcast-spawners with some gonochoric, but mostly sequential hermaphroditic colonies. Minimum colony sizes (and ages) at first reproduction were 5?cm (5?yr) and 3?cm (2 to 3?yr), respectively, in P. varians and Pavona sp.a. In the Panamá and Galápagos populations, gonochoric colonies spawn eggs or sperm at least monthly. Six fecundity attributes were not significantly different in the two species, but the eggs of P. varians are white to beige and positively buoyant, and those of Pavona sp.a are dark green and neutrally to negatively buoyant. Eggs of both species lack zooxanthellae. Both species are reproductively active year-round, with maximum activity in the dry season in the nonupwelling Gulf of Chiriquí, and in the wet season in the upwelling Gulf of Panamá. Spawning is predominantly during full moon, and possibly also at new moon at most study sites. Spawning in P. varians and Pavona sp.a is 12?h out of phase, with the former species spawning ～1?h before sunrise and the latter about 1?h after sunset. The fecundity of Pavona spp. at Caño and the Galápagos Islands was much greater (19?900 to 27?900 eggs cm^?2?yr^?1) than at all Panamá sites (14?800 to 19?800 eggs cm^?2?yr^?1). Intraspecific crosses in both species resulted in swimming planula larvae after 25 to 36?h. Recruitment of P. varians was highest in Panamá, moderate in Costa Rica, and nil in the Galápagos Islands, matching, respectively, the contributions of P. varians to the pre-1982/1983 El Niño coral-population abundances in these areas. Recruitment success of P. varians at Uva Island was significantly related to maximum monthly positive sea surface-temperature (SST) anomalies that occurred in the year preceding recruitment over the period 1982 to 1996; recruitment failed when SST anomalies exceeded 1.6 to 1.9?C° during the severe ENSO events of 1982/1983 and 1997/1998.