A Lagrangian analysis was applied to the outputs of a coupled physical-biogeochemical model to describe the redistribution of nitrate-rich and nitrate-poor surface water masses in the tropical Pacific throughout the major 1997 El Niño. The same tool was used to analyze the causes of nitrate changes along trajectories and to investigate the consequences of the slow nitrate uptake in the high nutrient low chlorophyll (HNLC) region during the growth phase of the event. Three patterns were identified during the drift of water masses. The first mechanism is well known along the equator: oligotrophic waters from the western Pacific are advected eastward and retain their oligotrophic properties along their drift. The second concerns the persistent upwelling in the eastern basin. Water parcels have complex trajectories within this retention zone and remain mesotrophic. This study draws attention to the third process which is very specific to the HNLC region and to the El Niño period. During the 1997 El Niño, horizontal and vertical inputs of nitrate decreased so dramatically that nitrate uptake by phytoplankton became the only mechanism driving nitrate changes along pathways. The study shows that because of the slow nitrate uptake characteristic of the tropical Pacific HNLC system, nitrate in the pre-El Niño photic layer can support biological production for a period of several months. As a consequence, the slow nitrate uptake delays the gradual onset of oligotrophic conditions over nearly all the area usually occupied by upwelled waters. Owing to this process, mesotrophic conditions persist in the tropical Pacific during El Niño events. 相似文献
Geochemical signals of bulk sedimentary organic matter from three cores from Lake Tanganyika provided information about both internal processes and terrestrial inputs to the lake. Indications of land use change were detected in the geochemical records of the watersheds, and the timing of these changes was consistent with historical records of population demographics. While C:N ratios suggested that the distance from shore might be important in influencing the relative amount of allochthonous vs. autochthonous material, all cores were dominated by autochthonous organic matter. In general, nitrogen isotopes were more positive at disturbed sites, indicating inputs of enriched soil nitrate that was subsequently taken up by phytoplankton. In contrast, carbon isotopes did not reflect land use patterns, and a post-1950s decline in carbon isotope ratios found in all cores may indicate a lake-wide decrease in productivity. These interpretations were consistent with pollen and climate records. 相似文献
Our understanding on how ash particles in volcanic plumes react with coexisting gases and aerosols is still rudimentary, despite the importance of these reactions in influencing the chemistry and dynamics of a plume. In this study, six samples of fine ash (<100 m) from different volcanoes were measured for their specific surface area, as, porosity and water adsorption properties with the aim to provide insights into the capacity of silicate ash particles to react with gases, including water vapour. To do so, we performed high-resolution nitrogen and water vapour adsorption/desorption experiments at 77 K and 303 K, respectively. The nitrogen data indicated as values in the range 1.1–2.1 m2/g, except in one case where a as of 10 m2/g was measured. This high value is attributed to incorporation of hydrothermal phases, such as clay minerals, in the ash surface composition. The data also revealed that the ash samples are essentially non-porous, or have a porosity dominated by macropores with widths >500 Å. All the specimens had similar pore size distributions, with a small peak centered around 50 Å. These findings suggest that fine ash particles have relatively undifferentiated surface textures, irrespective of the chemical composition and eruption type. Adsorption isotherms for water vapour revealed that the capacity of the ash samples for water adsorption is systematically larger than predicted from the nitrogen adsorption as values. Enhanced reactivity of the ash surface towards water may result from (i) hydration of bulk ash constituents; (ii) hydration of surface compounds; and/or (iii) hydroxylation of the surface of the ash. The later mechanism may lead to irreversible retention of water. Based on these experiments, we predict that volcanic ash is covered by a complete monolayer of water under ambient atmospheric conditions. In addition, capillary condensation within ash pores should allow for deposition of condensed water on to ash particles before water reaches saturation in the plume. The total mass of water vapour retained by 1 g of fine ash at 0.95 relative water vapour pressure is calculated to be ~10–2 g. Some volcanic implications of this study are discussed.Editorial responsibility: J. Gilbert 相似文献
Global distributions of carbonyl sulfide and carbon disulfide have been calculated with a three-dimensional global model of the atmospheric general circulation (ECHAM). The model calculates a global sink strength for carbonyl sulfide of 0.3 Tg S yr-1, with vegetation uptake being the largest sink. With this sink strength, the sources have to be close to the lower limit of the present estimate in the literature. The calculated mixing ratios are higher in the Southern Hemisphere than in the Northern Hemisphere. This interhemispheric gradient is the opposite of what is observed demonstrating that the present knowledge of the distribution of sinks and sources is not fully adequate. The model calculations support the idea that the open oceans could act as a net sink of carbonyl sulfide. The calculated stratospheric photolysis of carbonyl sulfide constitutes about 4% of the total sink of carbonyl sulfide. A stratospheric production of sulfate from carbonyl sulfide of 0.013 Tg S yr-1 is obtained, which is 3 to 12 times less than what is needed to maintain the stratospheric sulfate aerosol layer. Although these results are associated with uncertainties, due to the low upper boundary and coarse vertical resolution of the model, they support recent findings of a low stratospheric production of sulfate from carbonyl sulfide. Instead, sulfur dioxide transported from the troposphere is calculated to be the most important precursor for the stratospheric sulfate aerosol layer. 相似文献
The catchment of the river Adour (SW France) has been examined in order to analyse spatio-temporal variations in a number of key variables (flow, suspended matter, nitrate and dissolved orthophosphate concentrations) over a 25-year period (1972–1996).
Within the catchment area, it has been possible to discern how hydroclimatic fluctuations have affected the watershed, with dry periods in 1972–1976 and 1983–1993 alternating with wetter phases in 1977–1982 and 1994–1995. The anthropogenic activity, primarily, involving the use of water for agricultural purposes, has also had a major impact during this period, particularly in the downstream areas of the catchment.
Suspended matter fluxes display regular downstream increases with significant erosion being evident in the mountainous region contrasting with retention in the floodplains area downstream. These fluxes exhibit temporal and spatial variations with peaks occurring every 3–5 years, 1975–1977, 1979, 1982, 1985, 1987 and 1992. Some of these peaks are suggested to be related to anthropogenic activity involving river management, including the cutting of meanders and the construction of dykes for flood prevention.
Nitrate concentrations evince a similar pattern to the suspended matter fluxes with enhanced levels of downstream. The confluence of the Adour with the Midouze appears not to have any major impact on the nitrate concentration. In the downstream areas, an uptake of nitrate is registered indicating the activity of the riparian vegetation. For the entire catchment, maximal nitrate concentrations are observed in 1979, 1982, 1987, 1991–1992 and 1995. 相似文献
This study describes the biogeochemical cycling of seston in Grand Traverse Bay, Lake Michigan. Seston was characterized by carbon and nitrogen elemental and isotopic abundances. Fluorescence, temperature, light transmittance, and concentrations of dissolved inorganic nitrogen were also determined. PCBs were analyzed from surface (10 m) seston and ΣPCB was calculated by summing all of the congeners quantified in each sample. The vertical and seasonal trends in the δ13C values of seston exhibited a broad range from −30.7 to −23.9‰. Low δ13C values that occur concurrently with a peak in fluorescence below the thermocline reflect uptake of 13C depleted respiratory CO2 and/or the accumulation of 13C depleted lipids by phytoplankton. High δ13C values late in the season likely result from a reduction in photosynthetic fractionation associated with a decrease in the CO2 pool. Seasonal δ15N values of seston were high in the spring and declined through August. The δ15N values of seston reflect a balance between fractionation during assimilation of NH4+ or NO3− and degradative processes. The seston ΣPCB and fluorescence were both high in the spring and subsequently declined, suggesting that the concentrations of PCBs in seston were associated with labile material derived from primary productivity. The strong seasonal trends in the organic geochemical characteristics of seston and concentrations of PCBs emphasize the complex nature of particle cycling in aquatic environments. 相似文献
The gel-permeation chromatography system with multicomponent detection of organic carbon, organic nitrogen, organic halogen, and UV/vis absorption measurement is shown to be a useful tool for characterization of industrial wastewaters and wastewater treatment processes. The proposed system was used to investigate biologically treated wastewater from chemical industry, whereby one branch stream was identified to be the main source of persistent halogenated organics. Various treatment processes of pulp mill wastewater were also evaluated revealing that precipitation with aluminium is very effective for the removal of the high molecular fraction whereas the biological treatment is generally less selective. In the biological treatment of tannery wastewaters, it has been shown that the aerobic treatment, in contrast to anaerobic step, causes evident alterations of the composition of the polar DOC fraction. The nitrogen containing compounds are also better decomposed in the aerobic step compared to the anaerobic one. 相似文献
Recent information on some consequences of the acute mid-water oxygen deficiency in the Arabian Sea, especially on carbon-nitrogen
cycling, is reviewed. An evaluation of published estimates of water column denitrification rate suggests an overall rate in
the vicinity of 30Tg Ny-1, but the extent of benthic contribution remains unknown. A decoupling of denitrification from primary production, unique
to the Arabian Sea, is revealed by nitrite, electron transport system (ETS) activity and bacterial production data. Results
of both enzymatic and microbiological investigations strongly point to a major role of organic carbon other than that sinking
from surface layers in supporting denitrification. Although denitrification is associated with an intermediate nepheloid layer,
it seems unlikely that the excess carbon comes with particles re-suspended along the continental margins and transported quasi-horizontally
into the ocean interior; instead, the particle maximum may directly reflect a higher bacterial abundance. It is proposed that
denitrification may be predominantly fuelled by the dissolved organic matter. 相似文献
Nitrate pollution due to deep leaching from pit latrines has caused water supply wells in eastern Botswana to exceed health limits concerning nitrate. It was deduced from the estimated intake of salt and protein by the population that, as an average, about 10 percent of the human nitrogen excretion is leached to the groundwater. This fraction was also found in southern India, where on-the-ground excretion is customary. The nitrogen circulation in general in the savanna ecosystem is not appreciably affected in spite of a large livestock density. Overall nitrate leaching is in the order of 1.5 kg N/ha/y, similar to that in another semiarid area in southern India. However, in India, there seems to be a more diffuse areal leaching from agriculture as well as from villages.Measures to minimize the nitrate leaching could be to plant deep-rooted trees adjacent to pit latrines or to use latrines that separate the urine from the faces for a more near-surface infiltration facilitating plant uptake. Measures to minimize leaching will also lessen the rick for bacterial pollution of the groundwater. 相似文献