Net, actual, and potential sediment-water interface NH4 fluxes in the northern Gulf of Mexico (NGOMEX): Evidence for NH4 limitation of microbial dynamics

A new approach, combining ¹⁵NH₄⁺ isotope dilution and continuous-flow techniques, provided estimates of “actual” and “net” NH₄⁺ flux and sediment NH₄⁺ demand (SAD) at the sediment-water interface (SWI) of sites in the northern Gulf of Mexico (NGOMEX). The sites included a hypoxic site (C6), two sites with intermediate oxygen levels (B7 and F5), and a normoxic site (CT). Control cores without isotope addition and other cores from the same site treated with ¹⁵NH₄⁺ labeled overflowing water differentiated between net and actual regeneration flux and actual vs. potential uptake flux of NH₄⁺. Experiments were conducted in 2008 before (July) and after (September) two successive hurricanes (Gustav and Ike) and in January and August, 2009. Actual regeneration was significantly higher than net flux at most sites. Net flux did not differ significantly in most sites/dates, but the actual regeneration, and the actual and potential uptake, showed temporal and spatial variation; the flux at the hypoxic site was more active than non-hypoxic sites. SAD, the difference between potential and actual NH₄⁺ uptake flux, was higher at the hypoxic site than at non-hypoxic sites before and after the hurricanes in 2008 and during the hypoxia season in 2009. SAD related negatively to bottom water DO values. Conclusions: (1) net flux often underestimated actual regeneration, (2) hurricane activity decreased N dynamics, and (3) microbial N limitation status at the hypoxic site related to NH₄⁺ removal processes that were independent of oxygen (e.g., anaerobic heterotrophic uptake or anammox). These results indicate a rather consistent NH₄⁺ demand at the SWI during the hypoxic season and suggest that reduced nitrogen may limit microbial dynamics in the region.     

Earth observations during space shuttle flight STS‐35: Columbia's mission to planet earth,December 2–10, 1990

The article describes methodology of space monitoring of the existing landscapes as it is practised in the CIS (former USSR). Different points of mew are presented concerning environmental monitoring as well as the aspects of its practical organization. The objectives of automation of the ecological data acquisition, storage and processing are also dealt with; necessity of geographic information systems creation is substantiated. Experience gained in the operation of geoinformation systems is considered with plans on geo‐information base development for the needs of geographical and ecological studies of the CIS landscapes. Practical experience with case studies on identifying and mapping the ecological disaster zones is described in the Aral Sea basin and the Kyzyl Kum desert.     

Coupled three-dimensional discrete element-lattice Boltzmann methods for fluid-solid interaction with polyhedral particles

Interaction between solid particles and fluid is of fundamental interest to scientists and engineers in many different applications—cardiopulmonary flows, aircraft and automobile aerodynamics, and wind loading on buildings to name a few. In geomechanics, particle shape significantly affects both particle-particle and particle-fluid interaction. Herein, we present a generalized method for modeling the interaction of arbitrarily shaped polyhedral particles and particle assemblages with fluid using a coupled discrete element method (DEM) and lattice Boltzmann method (LBM) formulation. The coupling between DEM and LBM is achieved through a new algorithm based on a volume-fraction approach to consider three-dimensional convex polyhedral particles moving through fluid. The algorithm establishes the interaction using linear programming and simplex integration and is validated against experimental data. This approach to modeling the interaction between complex polyhedral particles and fluid is shown to be accurate for directly simulating hydrodynamic forces on the particles.     

Benthic community structure and water column characteristics at two sites in the Kermadec Islands Marine Reserve,New Zealand

The Kermadec Islands Marine Reserve (KIMR), which is located at 30°S, is New Zealand's largest marine reserve at 748 000 ha, and its biota is composed of a mix of warm temperate, subtropical, and tropical species. A depth‐stratified ecological survey was conducted of the abundance and percentage cover of macrobenthic species and of the water column at two sites (Meyer Island and West Chanter Island), 2km apart. Significant differences in benthic community structure and in water column turbidity and chlorophyll concentrations were observed between the two sites despite their proximity and physical similarity. Compared with other “snapshot” surveys of benthic community structure at sites within the KIMR there was a high degree of similarity among the species observed, but often a low degree of similarity in species abundance or percentage cover as a function of depth. We suggest that despite its isolation and the degree of difficulty of working at this location, a full‐scale ecological survey of the coastal marine biota of the KIMR is warranted to better understand New Zealand's subtropical marine biota and its affinities with other marine biotas of the South Pacific.     

Tracking Groundwater Discharge to a Large River using Tracers and Geophysics

Few studies have investigated large reaches of rivers in which multiple sources of groundwater are responsible for maintaining baseflow. This paper builds upon previous work undertaken along the Fitzroy River, one of the largest perennial river systems in north‐western Australia. Synoptic regional‐scale sampling of both river water and groundwater for a suite of environmental tracers (⁴He, ⁸⁷Sr/⁸⁶Sr, ²²²Rn and major ions), and subsequent modeling of tracer behavior in the river, has enabled definition and quantification of groundwater input from at least three different sources. We show unambiguous evidence of both shallow “local” groundwater, possibly recharged to alluvial aquifers beneath the adjacent floodplain during recent high‐flow events, and old “regional” groundwater introduced via artesian flow from deep confined aquifers. We also invoke hyporheic exchange and either bank return flow or parafluvial flow to account for background ²²²Rn activities and anomalous chloride trends along river reaches where there is no evidence of the local or regional groundwater inputs. Vertical conductivity sections acquired through an airborne electromagnetic (AEM) survey provide insights to the architecture of the aquifers associated with these sources and general groundwater quality characteristics. These data indicate fresh groundwater from about 300 m below ground preferentially discharging to the river, at locations consistent with those inferred from tracer data. The results demonstrate how sampling rivers for multiple environmental tracers of different types—including stable and radioactive isotopes, dissolved gases and major ions—can significantly improve conceptualization of groundwater—surface water interaction processes, particularly when coupled with geophysical techniques in complex hydrogeological settings.     

Composition of Coronal Streamers from the SOHO Ultraviolet Coronagraph Spectrometer

The Ultraviolet Coronagraph Spectrometer on the SOHO satellite covers the 940–1350 Å range as well as the 470–630 Å range in second order. It has detected coronal emission lines of H, N, O, Mg, Al, Si, S, Ar, Ca, Fe, and Ni, particularly in coronal streamers. Resonance scattering of emission lines from the solar disk dominates the intensities of a few lines, but electron collisional excitation produces most of the lines observed. Resonance, intercombination and forbidden lines are seen, and their relative line intensities are diagnostics for the ionization state and elemental abundances of the coronal gas. The elemental composition of the solar corona and solar wind vary, with the abundance of each element related to the ionization potential of its neutral atom (First Ionization Potential–FIP). It is often difficult to obtain absolute abundances, rather than abundances relative to O or Si. In this paper, we study the ionization state of the gas in two coronal streamers, and we determine the absolute abundances of oxygen and other elements in the streamers. The ionization state is close to that of a log T = 6.2 plasma. The abundances vary among, and even within, streamers. The helium abundance is lower than photospheric, and the FIP effect is present. In the core of a quiescent equatorial streamer, oxygen and other high-FIP elements are depleted by an order of magnitude compared with photospheric abundances, while they are depleted by only a factor of 3 along the edges of the streamer. The abundances along the edges of the streamer (‘legs’) resemble elemental abundances measured in the slow solar wind, supporting the identification of streamers as the source of that wind component.     

Sensitive fluorometric procedure to determine individual amino acids in marine waters

Improved methodology for amino acid analysis coupled with semi-micro isolation techniques make feasible the measurement of individual amino acids from marine waters where sample volumes are limited (e.g., sediment pore water or culture filtrates). If a water sample contains more than 20 nmole 1^?1 of the respective compounds, dissolved free amino acids (DFAA) can be isolated from 5 ml of filtrate by cation exchange and measured on a sensitive amino-acid analyzer equipped with a fluorometric detector. Two ml generally provides sufficient sample to determine total dissolved amino acids. Lower concentrations of amino acids can be measured after concentration from larger samples (250 ml) by ligand exchange.     

Seasonal Dynamics in Dissolved Organic Carbon Concentrations in a Coastal Water-Table Aquifer at the Forest-Marsh Interface

The seasonal dynamics of dissolved organic carbon (DOC) in a subterranean estuary were examined in a coastal water-table aquifer extending across a forest-marsh interface into an adjacent tidal creek that leads to North Inlet (SC). The aquifer is characterized by groundwater flow from the forest recharge area towards the creek. DOC concentrations range from 50 to 140 mg L^-1 in the shallow portions of the aquifer below the forest and undergo seasonal changes that are inversely related to temperature and precipitation conditions. Markedly lower DOC concentrations (<10 mg L^-1) in the deep portion of the aquifer are consistent with the loss of a large fraction of the original DOC along the groundwater flow paths. Mass balance estimates indicate that over 60% of the DOC losses are due to sorption reactions whereas the rest appear to be caused by heterotrophic decay. Groundwater DOC discharge from the forest, which occurs in a restricted zone of the high marsh, is 5.5 mg carbon m^-2 d^-1 and accounts for a minor component of the annual carbon export from North Inlet. In contrast, moderately saline (2–12 ppt) ground waters below the marsh display elevated DOC concentrations (20 mg L^-1) that appear to be the result of mixing of fresh ground waters and surface seawater during tidal seepage and concentration during evapotranspiration. The flux of DOC associated with the discharge of these saline ground waters is 600 mg carbon m^-2 d^-1, which represents a significant fraction of the annual DOC budget for North Inlet.     

Fragmentation of magma during Plinian volcanic eruptions

 The ratio of the volume of vesicles (gas) to that of glass (liquid) in pumice clasts (V _G /V _L ) reflects the degassing and dynamic history experienced by a magma during an explosive eruption. V _G /V _L in pumices from a large number of Plinian eruption deposits is shown here to vary by two orders of magnitude, even between pumices at a given level in a deposit. These variations in V _G /V _L do not correlate with crystallinity or initial water content of the magmas or their eruptive intensities, despite large ranges in these variables. Gas volume ratios of pumices do, however, vary systematically with magma viscosity estimated at the point of fragmentation, and we infer that pumices do not quench at the level of fragmentation but undergo some post-fragmentary evolution. On the timescale of Plinian eruptions, pumices with viscosities <10⁹ Pa s can expand after fragmentation, as long as their bubbles retain gas, at a rate inversely proportional to their viscosity. Once the bubbles connect to form a permeable network and lose their gas, expansion halts and pumices with viscosities <10⁵ Pa s can collapse under the action of surface tension. Textural evidence from bubble sizes and shapes in pumices indicates that both expansion and collapse have taken place. The magnitudes of expansion and collapse, therefore, depend critically on the timing of bubble connectivity relative to the final moment of quenching. We propose that bubbles in different pumices become connected at different times throughout the time span between fragmentation and quenching. After accounting for these effects, we derive new information on the fragmentation process from two characteristics of pumices. The most important is a relatively constant minimum value of V _G /V _L of ∼1.78 (64 vol.% vesicularity) in all samples with viscosities >10⁵ Pa s. This value is independent of magma composition and thus reflects a property of the eruptive mechanism. The other characteristic is that highly expanded pumices (>85 vol.% vesicularities) are common, which argues against overpressure in bubbles as a mechanism for fragmenting magma. We suggest that magma fragments when it reaches a vesicularity of ∼64 vol.%, but only if sheared sufficiently strongly. The intensity of shear varies as a function of velocity in the conduit, which is related to overpressure in the chamber, so that changes in overpressure with time are important in controlling the common progression from explosive to effusive activity at volcanoes. Received: 19 April 1995 / Accepted: 3 April 1996     

The effect of salinity on ammonium sorption in aquatic sediments: Implications for benthic nutrient recycling

Ambient exchangeable ammonium concentrations in freshwater sediments are generally considerably greater than those reported for marine sediments. Laboratory measurements indicate that competition for cation exchange sites by ions in seawater is a factor responsible for the lower exchangeable ammonium concentrations in marine sediments. Exchangeable ammonium concentrations were 3- to 6-fold higher when river and estuarine sediments were incubated with fresh water relative to the same sediments incubated with salt water (%.-23). A model was developed to explore the implications for benthic nitrogen cycling of this salinity effect on exchangeable ammonium concentrations. Ammonium diffusion, exchangeable and dissolved ammonium concentrations, and nitrification rates were components of the model formulation. The model output suggests that higher exchangeable ammonium concentrations predicted in fresh water relative to marine sediments can markedly increase the fraction of the ammonium produced in sediments that is nitrified (and subsequently denitrified). These results are consistent with field and experimental laboratory data which indicate that a larger percentage of net ammonium production in aerobic freshwater sediments is nitrified and denitrified (80–100%) relative to marine sediments (40–60%).