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261.
Freshwater inputs often play a more direct role in estuarine phytoplankton biomass (chlorophyll a) accumulation than nitrogen (N) inputs, since discharge simultaneously controls both phytoplankton residence time and N loading. Understanding this link is critical, given potential changes in climate and human activities that may affect discharge and watershed N supply. Chlorophyll a (chla) relationships with hydrologic variability were examined in 3-year time series from two neighboring, shallow (<5?m), microtidal estuaries (New and Neuse River estuaries, NC, USA) influenced by the same climatic conditions and events. Under conditions ranging from drought to floods, N concentration and salinity showed direct positive and negative responses, respectively, to discharge for both estuaries. The response of chla to discharge was more complex, but was elucidated through conversion of discharge to freshwater flushing time, an estimate of transport time scale. Non-linear fits of chla to flushing time revealed non-monotonic, unimodal relationships that reflected the changing balance between intrinsic growth and losses through time and along the axis of each estuary. Maximum biomass occurred at approximately 10-day flushing times for both systems. Residual analysis of the fitted data revealed positive relationships between chla and temperature, suggesting enhanced growth rates at higher temperatures. N loading and system-wide, volume-weighted chla were positively correlated, and biomass yields per N load were greater than other marine systems. When combined with information on loss processes, these results on the hydrologic control of phytoplankton biomass will help formulate mechanistic models necessary to predict ecosystem responses to future climate and anthropogenic changes. 相似文献
262.
Dendroclimatological sampling of Scots pine (Pinus sylvestris L.) has been made in the province of J?mtland, in the west-central Scandinavian mountains, since the 1970s. The tree-ring width (TRW) chronology spans several thousand years and has been used to reconstruct June?CAugust temperatures back to 1632 bc. A maximum latewood density (MXD) dataset, covering the period ad 1107?C1827 (with gap 1292?C1315) was presented in the 1980s by Fritz Schweingruber. Here we combine these historical MXD data with recently collected MXD data covering ad 1292?C2006 into a single reconstruction of April?CSeptember temperatures for the period ad 1107?C2006. Regional curve standardization (RCS) provides more low-frequency variability than ??non-RCS?? and stronger correlation with local seasonal temperatures (51% variance explained). The MXD chronology shows a stronger relationship with temperatures than the TRW data, but the two chronologies show similar multi-decadal variations back to ad 1500. According to the MXD chronology, the period since ad 1930 and around ad 1150?C1200 were the warmest during the last 900?years. Due to large uncertainties in the early part of the combined MXD chronology, it is not possible to conclude which period was the warmest. More sampling of trees growing near the tree-line is needed to further improve the MXD chronology. 相似文献
263.
A field experiment was conducted to investigate how water-based drill cuttings and sediment type influence colonization of soft bottom communities. Bottom frames with trays containing defaunated sediments were placed at the seabed for 6 months to study colonization of macrofauna. Two different sediments (coarse and fine) were used, and 6 or 24 mm layer of water-based drill cuttings were added on top of these sediments. Some of the sediments were controls with no additions. In the end of the experiment, the oxygen availability in sediment porewater and macrofaunal abundance were reduced in treatments with 24 mm drill cuttings compared to controls. Tube-building annelids were particularly sensitive to drill cuttings. However, these responses were only minor, and notably, the drill cuttings initiated a weaker faunal response than sediment type and site of the bottom frame. Sediments capped with water-based drill cuttings thus showed a rapid colonization of macrofaunal communities. 相似文献
264.
Sets of 20 soda ash glasses, 16 soda lime glasses and 23 wood ash glasses mainly from excavations in Europe (additional soda ash glasses from Egypt) were analysed on 61 chemical elements. Average SiO2 is about 62% in soda glasses and 50% in wood ash glasses. The three groups of glasses contain on average 13% Na2O, 18% Na2O and 13% K2O as fluxes to lower the melting temperature of quartz at their production. The starting materials beside quartz were halophytic plant ash for soda ash glass, trona (Na3H(CO3)2·2H2O) and lime (clamshells) for soda lime glass and beech ash for wood ash glass. Each of the three major glass types contains specific Rare Earth Element (REE) concentrations mainly contained in quartz and its intergrown minerals. 50 Paleozoic and Mesozoic sandstones from Central Europe represent the quartz composition. The REE pattern of these glasses apparently indicates major compositional stages of the Continental Earth's Crust. The boron to lithium and sodium to potassium ratios as in seawater suggest reactions of materials for soda glass with seawater. Negative Ce anomalies in the three glasses are caused by reactions of quartz with seawater. 相似文献
265.
Hans Romang Massimiliano Zappa Nadine Hilker Matthias Gerber François Dufour Valérie Frede Dominique Bérod Matthias Oplatka Christoph Hegg Jakob Rhyner 《Natural Hazards》2011,56(2):509-527
IFKIS-Hydro is an information and warning system for hydrological hazards in small- and medium-scale catchments. The system
collects data such as weather forecasts, precipitation measurements, water level gauges, discharge simulations and local observations
of event-specific phenomena. In addition, IFKIS-Hydro incorporates a web-based information platform, which serves as a central
hub for the submission and overview of data. Special emphasis is given to local information. This is accomplished particularly
by human observers. In medium-scale catchments, discharge forecast models have an increasing importance in providing valuable
information. IFKIS-Hydro was developed in several test regions in Switzerland and the first results of its application are
available now. The system is constantly extended to additional regions and may become the standard for warning systems in
smaller catchments in Switzerland. 相似文献
266.
Irene Harder Tarpgaard Hans Røy Bo Barker Jørgensen 《Geochimica et cosmochimica acta》2011,75(11):2997-3010
Bacterial sulfate reduction in marine sediments generally occurs in the presence of high millimolar concentrations of sulfate. Published data indicate that low sulfate concentrations may limit sulfate reduction rates below 0.2-2 mM. Yet, high sulfate reduction rates occur in the 1-100 μM range in freshwater sediments and at the sulfate-methane transition in marine sediments. Through a combination of 35S-tracer experiments, including initial velocity experiments and time course experiments, we searched for different sulfate affinities in the mixed community of sulfate reducers in a marine sediment. We supported the radiotracer experiments with a highly sensitive ion chromatographic technique for sulfate with a detection limit of 0.15 μM SO42− in marine pore water. Our results showed that high and low affinities for sulfate co-occur and that the applied experimental approach may determine the observed apparent half saturation constant, Km. Our experimental and model data both show that sulfate reduction in the studied marine sediment could be explained by two dominating affinities for sulfate: a low affinity with a mean half saturation constant, Km, of 430 μM SO42− and a high affinity with a mean Km of 2.6 μM SO42−. The high-affinity sulfate reduction was thermodynamically un-constrained down to <1 μM SO42−, both in our experiments and under in situ conditions. The reduction of radio-labeled sulfate was partly reversible due to concurrent re-oxidation of sulfide by Fe(III) and possibly due to a reversibility of the enzymatic pathway of sulfate reduction. A literature survey of apparent Km values for sediments and pure cultures is presented and discussed. 相似文献
267.
Bacterioplankton productivity (BP) spatial variation was investigated in relation to potential resources, including primary productivity and dissolved organic matter, in the micro-tidal Neuse River–Pamlico Sound estuarine system, North Carolina, USA. Estuarine BP was predicted to correlate with the trophic gradient, decreasing along the salinity gradient in parallel with the decrease in organic matter and primary productivity. This prediction was tested over four years at spatial scales ranging from kilometers to meters along the riverine axis and with depth. The general pattern of BP across the salinity gradient was unimodal and matched the phytoplankton patterns in peak location and variability. Peak locations varied with discharge, especially in 2003 when above average discharge moved peaks downstream. Spatial coherence of BP with other variables was much less at short time scales. The effect of temperature, nutrients, and phytoplankton on BP varied by location, especially fresh versus brackish stations, although only temperature explained more than 20% of the BP variation. Depth variation of BP was as great as longitudinal variation and bottom samples were often higher than surface. BP was strongly correlated with particulate organic carbon at the pycnocline and bottom, highlighting the importance of particulate matter as a resource. Station-averaged BP and phytoplankton data corresponded well with two published meta-analyses, although the offset of the freshwater station suggested longitudinal differences in community composition or resource availability. 相似文献
268.
Louise Schlüter Peter HenriksenTorkel Gissel Nielsen Hans H. Jakobsen 《Deep Sea Research Part I: Oceanographic Research Papers》2011,58(5):546-556
Phytoplankton composition and biomass was investigated across the southern Indian Ocean. Phytoplankton composition was determined from pigment analysis with subsequent calculations of group contributions to total chlorophyll a (Chl a) using CHEMTAX and, in addition, by examination in the microscope. The different plankton communities detected reflected the different water masses along a transect from Cape Town, South Africa, to Broome, Australia. The first station was influenced by the Agulhas Current with a very deep mixed surface layer. Based on pigment analysis this station was dominated by haptophytes, pelagophytes, cyanobacteria, and prasinophytes. Sub-Antarctic waters of the Southern Ocean were encountered at the next station, where new nutrients were intruded to the surface layer and the total Chl a concentration reached high concentrations of 1.7 ??g Chl a L−1 with increased proportions of diatoms and dinoflagellates. The third station was also influenced by Southern Ocean waters, but located in a transition area on the boundary to subtropical water. Prochlorophytes appeared in the samples and Chl a was low, i.e., 0.3 ??g L−1 in the surface with prevalence of haptophytes, pelagophytes, and cyanobacteria. The next two stations were located in the subtropical gyre with little mixing and general oligotrophic conditions where prochlorophytes, haptophytes and pelagophytes dominated. The last two stations were located in tropical waters influenced by down-welling of the Leeuwin Current and particularly prochlorophytes dominated at these two stations, but also pelagophytes, haptophytes and cyanobacteria were abundant. Haptophytes Type 6 (sensuZapata et al., 2004), most likely Emiliania huxleyi, and pelagophytes were the dominating eucaryotes in the southern Indian Ocean. Prochlorophytes dominated in the subtrophic and oligotrophic eastern Indian Ocean where Chl a was low, i.e., 0.043-0.086 ??g total Chl a L−1 in the surface, and up to 0.4 ??g Chl a L−1 at deep Chl a maximum. From the pigment analyses it was found that the dinoflagellates of unknown trophy enumerated in the microscope at the oligotrophic stations were possibly heterotrophic or mixotrophic. Presence of zeaxanthin containing heterotrophic bacteria may have increased the abundance of cyanobacteria determined by CHEMTAX. 相似文献
269.
Varved sediments of Lake Yoa (Ounianga Kebir,Chad) reveal progressive drying of the Sahara during the last 6100 years 总被引:1,自引:0,他引:1
Pierre Francus Hans von Suchodoletz Michael Dietze Reik V. Donner Frédéric Bouchard Ann‐Julie Roy Maureen Fagot Stefan Kröpelin 《Sedimentology》2013,60(4):911-934
The sedimentological and geochemical properties of a 7·47 m long laminated sequence from hypersaline Lake Yoa in northern Chad have been investigated, representing a unique, continuous 6100 year long continental record of climate and environmental change in the eastern Central Sahara. These data were used to reconstruct the Mid to Late Holocene history of this currently hyper‐arid region, in order to address the question of whether the Mid Holocene environmental transition from a humid to a dry Sahara was progressive or abrupt. This study involved a suite of analyses, including petrographic and scanning electron microscope examination of thin sections, X‐ray diffraction, X‐radiography, granulometry, loss on ignition and magnetic susceptibility. The potential of micro‐X‐ray fluorescence core scanning was tested at very high resolution. Detailed microscopic investigation revealed the sedimentary processes responsible for the formation of the fine laminations, identified the season during which they were formed, and confirmed their annually rhythmic nature. High‐resolution X‐ray fluorescence core scanning allowed the distinction of each individual lamination over the entire record, opening new perspectives for the study of finely laminated sediment sequences. Geochemical and mineralogical data reveal that, due to decreasing monsoon rainfall combined with continuous and strong evaporation, the hydrologically open and fresh Mid Holocene Lake Yoa slowly evolved into the present‐day hypersaline brine depleted in calcium, which has existed for about the past 1050 years. During the oldest part of the investigated period, Lake Yoa probably contained a permanently stratified lower water column that was nevertheless disrupted relatively frequently by mixing events. Deep‐water anoxia became more stable because of increased salinity‐driven density stratification. In parallel, the sediment grain‐size proxies record a progressive increase of aeolian input in the course of the last 6100 years. Altogether, all geochemical and sedimentological indicators point to a progressive drying of the eastern Central Sahara, strengthening previous conclusions based on palaeoecological indicators. 相似文献
270.
Eric Deleersnijder Emmanuel Hanert Hans Burchard Henk A. Dijkstra 《Ocean Dynamics》2008,58(3-4):237-246
Occasionally, numerical simulations using local turbulence closure schemes to estimate vertical turbulent fluxes exhibit small-scale oscillations in space, causing the eddy coefficients to vary over several orders of magnitude on short distances. Theoretical developments suggest that these spurious oscillations are essentially due to the way the eddy coefficients depend on the vertical gradient of the model’s variables. An instability criterion is derived based on the assumptions that the artefacts under study are due to the development of small-amplitude, small time- and space-scale perturbations of a smooth solution. The relevance of this criterion is demonstrated by applying it to a series a closure schemes, ranging from the Pacanowski–Philander formulas to the Mellor–Yamada level 2.5 model. 相似文献