Plankton distribution associated with frontal zones in the vicinity of the Pribilof Islands

We studied the effect of four types of fronts, the coastal front, the middle front, the shelf partition front and the shelf break front on the quantitative distribution and the composition of plankton communities in the Pribilof area of the eastern Bering Sea shelf in late spring and summer of 1993 and 1994. The coastal fronts near St. Paul and St. George Islands and the coastal domains encircled by the fronts featured specific taxonomic composition of planktonic algae, high abundance and production of phytoplankton, as well as large numbers of heterotrophic nanoplankton. The coastal fronts also were characterized by high values of total mesozooplankton biomass, high concentrations of Calanus marshallae, as well as relatively high abundances of Parasagitta setosa and Euphausiacea compared to surrounding shelf waters. We hypothesize that wind-induced erosion of a weak thermocline in the inner part of the coastal front as well as transfrontal water exchange in subthermocline layers result in nutrient enrichment of the euphotic layer in the coastal fronts and coastal domains in summer time. This leads to prolonged high primary production and high phytoplankton biomass. In this paper a new type of front—the shelf partition front located 45–55 km to the north-east off St. Paul Island—is described, which is assumed to be formed by the flux of oceanic domain waters onto the shelf. This front features a high abundance of phytoplankton and a high level of primary production compared to the adjacent middle shelf. Near the southwestern periphery of the front a mesozooplankton peak occurred, composed of C. marshallae, with biomass in the subthermocline layer, reaching values typical for the shelf break front and the highest for the area. High abundance of phyto- and zooplankton as well as heterotrophic nanoplankton and elevated primary production were most often observed in the area adjacent to the shelf break front at its oceanic side. The phyto- and mesozooplankton peaks here were formed by oceanic community species. The summer levels of phytoplankton numbers, biomass and primary production in the shelf break frontal area were similar to those reported for the outer and middle shelf during the spring bloom and the coastal domains and coastal fronts in summer. In the environment with a narrow shelf to the south of St. George Island, the mesozooplankton peak was observed at the inner side of the shelf break front as close as 20 km from the island shore and was comprised of a “mixed” community of shelf and oceanic species. The biomass in the peak reached the highest values for the Pribilof area at 2.5 g mean wet weight m⁻³ in the 0–100 m layer. Details of the taxonomic composition and the numbers and production of phytoplankton hint at the similarity of processes that affect the phytoplankton summer community in the coastal domains of the islands, at the coastal fronts, and at the oceanic side of the shelf break front. The middle front was the only one that had no effect on plankton composition or its quantitative characteristics in June and July. Location of a variety of frontal productive areas within 100 km of the Pribilof Islands creates favorable foraging habitat for higher trophic level organisms, including sea birds and marine mammals, populating the islands.     

General patterns of circulation, sediment fluxes and ecology of the Palamós (La Fonera) submarine canyon, northwestern Mediterranean

Currents, particle fluxes and ecology were studied in the Palamós submarine canyon (also known as the Fonera canyon), located in the northwestern Mediterranean. Seven mooring arrays equipped with current meters and sediment traps were deployed along the main canyon axis, on the canyon walls and on the adjacent slope. Additionally, local and regional hydrographic cruises were carried out. Current data showed that mean near surface and mid-depth currents were oriented along the mean flow direction (NE–SW), although at 400 and 1200 m depth within the canyon current reversals were significant, indicating a more closed circulation inside the canyon. Mean near-bottom currents were constrained by the local bathymetry, especially at the canyon head. The most significant frequency at all levels was the inertial frequency. A second frequency of about three days, attributed to a topographic wave, was observed at all depths, suggesting that this wave was probably not trapped near the bottom. The current field observed during the most complete survey revealed a meandering pattern with cyclonic vorticity just upstream from and within the canyon. The associated vertical velocity ranged between 10 and 20 m/day and was constrained to the upper 300 m. This latter feature, together with other computations, suggests that during this survey the meander was not induced by the canyon but by some kind of instability of the mean flow.In the canyon, suspended sediment concentration, downward particle fluxes, chlorophyll and particulate C and N were significantly higher up-canyon from about 1200 m depth than offshore, defining, along with the different hydrodynamics, two canyon domains: one from the canyon head to about 1200 m depth more affected by the canyon confinement and the other deeper than 1200 m depth more controlled by the mean flow and the shelf-slope front. The higher near-bottom downward total mass fluxes were recorded in the canyon axis at 1200 m depth along with sharp turbidity increases and are related to sediment gravity flows. During the deployment period, the increase in downward particle fluxes occurred by mid-November, when a severe storm took place. On the canyon walls at 1200 m depth, suspended sediment concentrations, downward particle fluxes, chlorophyll and particulate C and N were higher on the southern wall than on the northern wall inversely to the current’s energy. This could be caused by an upward water supply on the southern canyon wall and/or the mean flow interacting with the canyon bathymetry. In the swimmers collected by the sediment traps, the dominant species was an elasipod holothurian, which has not been recorded in other canyons or elsewhere in the Mediterranean, indicating particular speciation.     

Inhomogeneities in the circumstellar envelope of the A0E herbig star AB aur

Simultaneous high-resolution spectroscopy in H andUBVRI polarimetric observations are proposed as an effective method for the search for circumstellar inhomogeneities in A0-type Herbig stars. The new results for AB Aur are presented as a successful example of the use of this method. The analysis of about 100 CCD H profiles (R = 30 000) and more than 150 polarimetric measurements obtained in January, 1994 allowed to discover a long-lived stream-like inhomogeneity in the circumstellar gaseous envelope.     

High-Performance Parallel Solver for Integral Equations of Electromagnetics Based on Galerkin Method

A new parallel solver for the volumetric integral equations (IE) of electrodynamics is presented. The solver is based on the Galerkin method, which ensures convergent numerical solution. The main features include: (i) memory usage eight times lower compared with analogous IE-based algorithms, without additional restrictions on the background media; (ii) accurate and stable method to compute matrix coefficients corresponding to the IE; and (iii) high degree of parallelism. The solver’s computational efficiency is demonstrated on a problem of magnetotelluric sounding of media with large conductivity contrast, revealing good agreement with results obtained using the second-order finite-element method. Due to the effective approach to parallelization and distributed data storage, the program exhibits perfect scalability on different hardware platforms.     

The Arctic Coastal Dynamics Database: A New Classification Scheme and Statistics on Arctic Permafrost Coastlines

Arctic permafrost coasts are sensitive to changing climate. The lengthening open water season and the increasing open water area are likely to induce greater erosion and threaten community and industry infrastructure as well as dramatically change nutrient pathways in the near-shore zone. The shallow, mediterranean Arctic Ocean is likely to be strongly affected by changes in currently poorly observed arctic coastal dynamics. We present a geomorphological classification scheme for the arctic coast, with 101,447?km of coastline in 1,315 segments. The average rate of erosion for the arctic coast is 0.5?m? year^?1 with high local and regional variability. Highest rates are observed in the Laptev, East Siberian, and Beaufort Seas. Strong spatial variability in associated database bluff height, ground carbon and ice content, and coastline movement highlights the need to estimate the relative importance of shifting coastal fluxes to the Arctic Ocean at multiple spatial scales.     

On the chemistry of mantle and magmatic volatiles on Mercury

The surface of Mercury contains ancient volcanic features and signs of pyroclastic activity related to unknown magmatic volatiles. Here, the nature of possible magmatic volatiles (H, S, C, Cl, and N) is discussed in the contexts of formation and evolution of the planet, composition and redox state of its mantle, solubility in silicate melts, chemical mechanisms of magma degassing, and thermochemical equilibria in magma and volcanic gases. The low-FeO content in surface materials (<6 wt%) evaluated with remote sensing methods corresponds to less than 2.3 fO₂ log units below the iron-wüstite buffer. These redox conditions imply restricted involvement of hydrous species in nebular and accretion processes, and a limited loss of S, Cl, and N during formation and evolution of the planet. Reduced conditions correspond to high solubilities of these elements in magma and do not favor degassing. Major degassing and pyroclastic activity would require oxidation of melts in near-surface conditions. Low-pressure oxidation of graphite in moderately oxidized magmas causes formation of low-solubility CO. Decompression of reduced N-saturated melts involves oxidation of nitride melt complexes and could cause N₂ degassing. Putative assimilation of oxide (FeO, TiO₂, and SiO₂) rich rocks in magma chambers could have caused major degassing through oxidation of graphite and S-, Cl- and N-bearing melt complexes. However, crustal rocks may never have been oxidized, and sulfides, graphite, chlorides, and nitrides could remain in crystallized rocks. Chemical equilibrium models show that N₂, CO, S₂, CS₂, S₂Cl, Cl, Cl₂, and COS could be among the most abundant volcanic gases on Mercury. Though, these speciation models are restricted by uncertain redox conditions, unknown volatile content in magma, and the adopted chemical degassing mechanism.     

WSO-UV progress and expectations

The World Space Observatory Ultraviolet (WSO-UV) is the space mission that will grant access to the ultraviolet (UV) range in the post Hubble epoch. WSO-UV is equipped with instrumentation for imaging and spectroscopy and it is fully devoted to UV astronomy. In this article, we outline the WSO-UV mission model and present the current status of the project.     

Similarity in microbial amino acid uptake in surface waters of the North and South Atlantic (sub-)tropical gyres

The Earth’s most extensive biomes – the oceanic subtropical gyres – are considered to be expanding with current surface ocean warming. Although it is well established that microbial communities control gyre biogeochemistry, comparisons of their metabolic activities between gyres are limited. In order to estimate metabolic activities including production of microbial communities, the uptake rates of amino acids leucine, methionine and tyrosine at ambient concentrations were estimated in surface waters of the Atlantic Ocean using radioisotopically labelled tracers. Data were acquired during six research cruises covering main oceanic provinces herein termed: North and South Atlantic Gyres, Bermuda Atlantic Time-series Study site (BATS), Equatorial region, and Mauritanian Upwelling (off Cape Blanc). Data were divided between provinces, the extents of which were identified by ocean colour data, in order to achieve provincial mean uptake rates. Leucine and methionine uptake rates did not differ between sampling periods, and were comparable between the North and South subtropical gyres. Furthermore, variation in uptake rates measured throughout the two oligotrophic gyres, where sampling covered ∼4 × 10⁶ km², was considerably lower than that measured within the Mauritanian Upwelling and Equatorial regions, and even at the BATS site. Tyrosine was generally the slowest of the amino acids to be taken up, however, it was assimilated faster than methionine within the Mauritanian Upwelling region. Thus, we propose that one value for leucine (12.6 ± 3.2 pmol L⁻¹ h⁻¹) and methionine (10.0 ± 3.3 pmol L⁻¹ h⁻¹) uptake could be applied to the oligotrophic subtropical gyres of the Atlantic Ocean. However, with the significantly lower uptake rates observed at the BATS site, we would not advise extrapolation to the Sargasso Sea.