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Concentration Variability (concentration + variability)
Selected AbstractsReflux and pH: ,alkaline' components are not neutralized by gastric pH variationsDISEASES OF THE ESOPHAGUS, Issue 1 2000P. Bechi The ability of the ,alkaline' components of reflux to cause harm in vivo is still open to debate, although these components have been shown in vitro to be capable of damaging the mucosa. The precipitation of bile acids and lysolecithin that occurs at low pH values is the main reason for questioning in vivo mucosal damage. This study was undertaken to determine the composition of gastric aspirates at different original pH values and the degree of solubility of the alkaline components when pH modifications are artificially induced. The samples for chemical analysis were collected from indwelling nasogastric tubes after surgical procedures that did not involve the upper gastrointestinal tract. Bile acid and lysolecithin concentrations were assessed by means of dedicated methods. Thirty-five samples were available for bile acid evaluation and 27 for lysolecithin evaluation. Bile acid and lysolecithin assessments were repeated after pH adjustment at 2, 3.5, 5.5 and 7. For easier assessment of the results, three ranges of the original pH were selected (pH,<,2, 2 , pH < 5, pH , 5). For each pH range, results were pooled together and compared with those in the other pH ranges. Bile acid concentrations were 113 ± 48, 339 ± 90 and 900 ± 303 (mean ± s.e.m. ,mol/L), respectively, in the three groups selected on account of the different original pH values. Differences were significant (p < 0.001). Both taurine- and glycine-conjugated bile acids were represented even at pH < 2. No major differences were observed in bile acid concentration with the artificially induced pH variations. Lysolecithin concentrations were 5.99 ± 3.27, 30.80 ± 8.43 and 108.37 ± 22.17 (mean ± SEM ,g/ml), respectively, in the three groups selected on account of the different original pH ranges. Differences were significant (p < 0.001). No significant differences in lysolecithin concentration were detected with the artificially induced pH variations. In conclusion, both bile acids and lysolecithin are naturally represented in the gastric environment even at very low pH values, although their concentrations decrease on lowering of the naturally occurring pH. Given the concentration variability of bile acids and lysolecithin, further studies are needed to assess the minimal concentration capable of mucosal damage in vivo. [source] Assessing the sources and magnitude of diurnal nitrate variability in the San Joaquin River (California) with an in situ optical nitrate sensor and dual nitrate isotopesFRESHWATER BIOLOGY, Issue 2 2009BRIAN A. PELLERIN Summary 1.,We investigated diurnal nitrate (NO3,) concentration variability in the San Joaquin River using an in situ optical NO3, sensor and discrete sampling during a 5-day summer period characterized by high algal productivity. Dual NO3, isotopes (,15NNO3 and ,18ONO3) and dissolved oxygen isotopes (,18ODO) were measured over 2 days to assess NO3, sources and biogeochemical controls over diurnal time-scales. 2.,Concerted temporal patterns of dissolved oxygen (DO) concentrations and ,18ODO were consistent with photosynthesis, respiration and atmospheric O2 exchange, providing evidence of diurnal biological processes independent of river discharge. 3.,Surface water NO3, concentrations varied by up to 22% over a single diurnal cycle and up to 31% over the 5-day study, but did not reveal concerted diurnal patterns at a frequency comparable to DO concentrations. The decoupling of ,15NNO3 and ,18ONO3 isotopes suggests that algal assimilation and denitrification are not major processes controlling diurnal NO3, variability in the San Joaquin River during the study. The lack of a clear explanation for NO3, variability likely reflects a combination of riverine biological processes and time-varying physical transport of NO3, from upstream agricultural drains to the mainstem San Joaquin River. 4.,The application of an in situ optical NO3, sensor along with discrete samples provides a view into the fine temporal structure of hydrochemical data and may allow for greater accuracy in pollution assessment. [source] Are diatoms good integrators of temporal variability in stream water quality?FRESHWATER BIOLOGY, Issue 4 2008ISABELLE LAVOIE Summary 1. Although diatoms have been used for many decades for river monitoring around the world, studies showing evidence that diatoms integrate temporal variability in water chemistry are scarce. 2. The purpose of this study was to evaluate the response of the Eastern Canadian Diatom Index (IDEC: Indice Diatomées de l'Est du Canada) with respect to temporal water chemistry variability using three different spatio-temporal data sets. 3. Along a large phosphorus gradient, the IDEC was highly correlated with averaged water chemistry data. Along within-stream phosphorus gradients, the IDEC integrated phosphorus over various periods of time, depending on the trophic status of the site studied (Boyer, Nicolet or Ste. Anne river) and variability in nutrient concentration. 4. In the Ste. Anne River, where nutrient concentrations were low and generally stable, an input of phosphorus induced a rapid change in diatom community structure and IDEC value within the following week. In the mesotrophic Nicolet River, the observed integration period was approximately 2 weeks. Diatom communities in the eutrophic Boyer River appeared to be adapted to frequent and significant fluctuations in nutrient concentrations. In this system, the IDEC therefore showed a slower response to short term fluctuations and integrated nutrient concentrations over a period of 5 weeks. 5. Our results suggest that the integration period varies as a function of trophic status and nutrient concentration variability in the streams. Oligotrophic streams are more sensitive to nutrient variations and their diatom communities are directly altered by nutrient increase, while diatom communities of eutrophic rivers are less sensitive to nutrient fluctuations and major variations take a longer time to be integrated into index values. 6. The longer integration period in the eutrophic environment may be attributed to the complexity of the diatom community. The results from this study showed that the diversity and evenness of the communities increased with trophic status. [source] Factors affecting pollination ecology of Quercus anemophilous species in north-west SpainBOTANICAL JOURNAL OF THE LINNEAN SOCIETY, Issue 3 2005F. J. RODRÍGUEZ-RAJO Pollination ecology of Quercus is influenced by meteorological, biotic and genetic factors. This study was undertaken to ascertain the effect induced by these factors on pollen production, release and dispersion. Aerobiological data have been used in recent years as phenological information, because the presence of pollen in the air is the result of flowering across a wide area. The onset of the Quercus pollen season and the atmospheric pollen concentrations during the pollination period in two localities of north-west Spain (Ourense and Santiago) were determined from 1993 to 2001. There were important variations in total annual pollen as a result of meteorological conditions, lenticular galls produced by Neuropterus on catkins and biennial genetic rhythms of pollen production. In order to determine the beginning of flowering, a thermal time model has been used. Chill requirements were around 800 chilling hours (CH) and heat requirements were 953 growth degree days (GDD in °C) in Santiago and 586 GDD in Ourense. Pollen in the air show positive correlation (99% significance) with daily thermal oscillation, maximum and minimum temperatures, and hours of sunshine. Regression analysis with previous days' pollen concentrations explained the high percentage of pollen concentration variability, as meteorological variables do not, on their own, explain pollen production and release. © 2005 The Linnean Society of London, Botanical Journal of the Linnean Society, 2005, 149, 283,297. [source] | ||||||||||