Planktonic diatom communities in temperate South-Central Chilean lakes with a focus on Asterionella formosa and the genus Aulacoseira     

Van de Vyver  Evelien  Pinseel  Eveline  Verleyen  Elie  Vanormelingen  Pieter  Van Wichelen  Jeroen  de Jong  Rixt  Urrutia  Roberto  Vyverman  Wim 《Journal of Paleolimnology》2022,68(3):279-296

Journal of Paleolimnology - Paleolimnological records from temperate South-Central Chilean lakes revealed a recent and marked increase in the relative abundances of the key-planktonic diatom...  相似文献   

Flow separation at the inner (convex) and outer (concave) banks of constant‐width and widening open‐channel bends     

Koen Blanckaert  Maarten G. Kleinhans  Stuart J. McLelland  Wim S. J. Uijttewaal  Brendan J. Murphy  Anja van de Kruijs  Daniel R. Parsons  Qiuwen Chen 《地球表面变化过程与地形》2013,38(7):696-716

There is a paucity of data and insight in the mechanisms of, and controls on flow separation and recirculation at natural sharply‐curved river bends. Herein we report on successful laboratory experiments that elucidate flow structure in one constant‐width bend and a second bend with an outer‐bank widening. The experiments were performed with both a flat immobile gravel bed and mobile sand bed with dominant bedload sediment transport. In the constant‐width bend with immobile bed, a zone of mainly horizontal flow separation (vertical rotational axis) formed at the inner bank that did not contain detectable flow recirculation, and an outer‐bank cell of secondary flow with streamwise oriented rotational axis. Surprisingly, the bend with widening at the outer bank and immobile bed did not lead to a transverse expansion of the flow. Rather, flow in the outer‐bank widening weakly recirculated around a vertical axis and hardly interacted with the inner part of the bend, which behaved as a constant‐width bend. In the mobile bed experiment, downstream of the bend apex a pronounced depositional bar developed at the inside of the bend and pronounced scour occurred at the outside. Moreover the deformed bed promoted flow separation over the bar, including return currents. In the constant‐width bend, the topographic steering impeded the generation of an outer‐bank cell of secondary flow. In the bend with outer‐bank widening, the topographic steering induced an outward expansion of the flow, whereby the major part of the discharge was conveyed in the central part of the widening section. Flow in the outer‐bank widening was highly three dimensional and included return currents near the bottom. In conclusion, the experiments elucidated three distinct processes of flow separation common in sharp bends: flow separation at the inner bank, an outer‐bank cell of secondary flow, and flow separation and recirculation in an outer‐bank widening. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

Short-Term and Interannual Variability in Primary Production in the Low-Salinity Zone of the San Francisco Estuary     

Wim J. Kimmerer  Alexander E. Parker  Ulrika E. Lidstr?m  Edward J. Carpenter 《Estuaries and Coasts》2012,35(4):913-929

We measured primary production during spring?Csummer 2006?C2007 to determine the carbon supply to the low-salinity pelagic food web of the San Francisco Estuary (SFE). Weekly or biweekly samples were taken at three stations of fixed salinity for size-fractionated primary production and biomass, both as chlorophyll and from biovolume based on counts. Error variance in productivity estimates arose mainly from the depth integration of ¹⁴C uptake, showing the importance of productivity measurements at high light levels for estimates of depth-integrated production. Temporal and spatial variability in production were surprisingly small. Combining data from this study with long-term monitoring data, productivity and biomass were variable in time and salinity but without persistent patterns and with infrequent blooms. Production within the low-salinity zone was unresponsive to variation in freshwater flow, in contrast to findings in other estuaries where nutrient loading drives variability in production and other regions of the SFE where production responds to residence time or to stratification. Estimated annual primary production was only 25 and 31?g?C?m^?2?year^?1 during 2006 and 2007, only half of it in cells >5???m. These results imply that phytoplankton provided poor food web support for higher trophic levels, probably contributing to the long-term decline in fish abundance in the brackish to freshwater region of the estuary.  相似文献   

High-precision high field strength element partitioning between garnet, amphibole and alkaline melt from Kakanui, New Zealand   总被引：2，自引：0，他引：2  

Eric C. Fulmer  Oliver Nebel  Wim van Westrenen 《Geochimica et cosmochimica acta》2010,74(9):2741-37

The high field strength elements (HFSE: Zr, Hf, Nb, Ta, and W) are an important group of chemical tracers that are increasingly used to investigate magmatic differentiation processes. Successful modeling of these processes requires the availability of accurate mineral-melt partition coefficients (D). To date, these have largely been determined by ion microprobe or laser ablation-ICP-MS analyses of the run products of high-pressure, high-temperature experiments. Since HFSE are (highly) incompatible, relatively immobile, high-charge, and difficult to ionize, these experiments and their analysis are challenging. Here we explore whether high-precision analyses of natural mineral-melt systems can provide additional constraints on HFSE partitioning.The HFSE concentrations in natural garnet and amphibole and their alkaline host melt from Kakanui, New Zealand are determined with high precision isotope dilution on a multi-collector-ICP-MS. Major and trace element compositions combined with Lu-Hf isotopic systematics and detailed petrographic sample analysis are used to assess mineral-melt equilibrium and to provide context for the HFSE D measurements. The whole-rock nephelinite, ∼1 mm sized amphiboles in the nephelinite, and garnet megacrysts have similar initial Hf isotope ratios with a mean initial ¹⁷⁶Hf/¹⁷⁷Hf_{(34 Ma)} = 0.282900 ± 0.000026 (2σ). In contrast, the amphibole megacrysts are isotopically distinct (¹⁷⁶Hf/¹⁷⁷Hf_{(34 Ma)} = 0.282830 ± 0.000011). Rare earth element D values for garnet megacryst-nephelinite melt and ∼1 mm amphibole-nephelinite melt plotted as a function of ionic radii show classic near-parabolic trends that are in excellent agreement with crystal lattice-strain models. These observations are consistent with equilibrium between the whole-rock nephelinite, the ∼1 mm amphibole grains within the nephelinite and the garnet megacrysts.High-precision isotope dilution results for Zr and Hf in garnet (D_Zr = 0.220 ± 0.007 and D_Hf = 0.216 ± 0.005 [2σ]), and for all HFSE in amphibole are consistent with previous experimental findings. However, our measurements for Nb and Ta in garnet (D_Nb = 0.0007 ± 0.0001 and D_Ta = 0.0011 ± 0.0006 [2σ]) show that conventional methods may overestimate Nb and Ta concentrations, thereby overestimating both Nb and Ta absolute D values for garnet by up to 3 orders of magnitude and underestimating D_Nb/D_Ta by greater than a factor of 100. As a consequence, the role of residual garnet in imposing Nb/Ta fractionation may be less important than previously thought. Moreover, garnet D_Hf/D_W = 17 and D_Nb/D_Zr = 0.003 imply fractionation of Hf from W and Nb from Zr upon garnet crystallization, which may have influenced short-lived ¹⁸²Hf-¹⁸²W and ⁹²Nb-⁹²Zr isotopic systems in Hadean time.  相似文献   

Carbonate removal by acidification causes loss of nitrogenous compounds in continental margin sediments   总被引：1，自引：0，他引：1  

Lutz Lohse  Rikus T. Kloosterhuis  Henko C. de Stigter  Willem Helder  Wim van Raaphorst  Tjeerd C. E. van Weering 《Marine Chemistry》2000,69(3-4)

This study reports on measurements of organic carbon (C_org) and total nitrogen (N_tot) in surface sediments originating from 6 transects along the northwest European continental margin. After elimination of carbonates by an acidification technique using sulphurous acid, both elements were analysed in the same sediment sample using an elemental analyser. C_org and N_tot in the sample were comparatively low, ranging between 1 and 10 mg C and 0.2 to 1 mg N g⁻¹ dry sediment. In a second analysis, the samples were analysed without acid addition, resulting in N_tot concentrations of 0–50% higher compared to their acidified counterparts. As a consequence, molar C/N ratios derived from the analysis of C_org and N_tot in the acidified sample ranged between 6 to 11, while the N_tot separate analysis reduced C/N ratios to 6 to 8. It is suggested that the addition of sulphurous acid to eliminate inorganic carbon volatilises nitrogenous organic matter.  相似文献   

Distinct patterns of bank erosion in a navigable regulated river     

Gonzalo Duró  Alessandra Crosato  Maarten G. Kleinhans  Timotheus G. Winkels  Hessel A.G. Woolderink  Wim S.J. Uijttewaal 《地球表面变化过程与地形》2020,45(2):361-374

Distinct bankline patterns appeared after the removal of protection works along a navigable reach of the Meuse River. A series of oblique embayments now dominate the riverine landscape after ten years of bank erosion, but their location and asymmetry cannot be explained yet. This work analyses and integrates field measurements of flow, ship waves, bank composition, bed topography and historical maps to explain the observed patterns along two reaches of the river. An extraordinary low-water-level event generated by a ship accident provided the unique opportunity to also analyse the subaqueous bank topography. The results indicate that the formation of oblique embayments arises from the combination of floodplain heterogeneity, structured by scroll-bar deposits, and the regulation of water levels, resulting in ship-wave attack at a narrow range of bank elevation for 70% of the time. Substrate erodibility acts on the effectiveness of trees to slow down local bank erosion rates, which is possibly enhanced by a positive feedback between woody roots and cohesive soil. The strong regulation of water levels and the waves generated by the intense ship traffic produce an increasingly long mildly-sloping terrace at the bank toe and progressively dominate the bank erosion process. This study demonstrates the important role of floodplain and scroll bar formation in shaping later bank erosion, which has implications for predictive numerical models, restoration strategies, and understanding the role of vegetation in bank erosion processes. © 2019 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd  相似文献   

HEIFE非均匀陆面上区域能量平衡研究   总被引：8，自引：1，他引：7  

马耀明  王介民  Massimo Menenti  Wim Bastiaanssen 《气候与环境研究》1997,2(3):293-301

卫星遥感在研究非均匀陆面上地－气间能量和水循环过程时有其独到的作用。本文介绍了一种利用陆地资源卫星ＴＭ资料进行非均匀陆面上区域能量平衡研究的参数化方案。以两个景的ＴＭ资料（１９９１年７月９日,夏季;１９９１年１０月２９日,近冬季）为个例,结合“黑河实验”（ＨＥＩＦＥ）期间的地面观测资料分析研究了实验区非均匀陆面上地表特征参数（地表反射率、标准化差值植被指数和地表温度）及能量平衡各分量（地表净辐射通量、土壤热通量、感热和潜热通量）的区域分布及季节差异,同时将所得的结果与地面观测的“真值”作了比较。结果表明：（１）由于黑河实验区下垫面状况十分复杂,戈壁、沙漠与绿洲交错分布,故在整个实验区内各地表特征参数及能量平衡各分量的分布范围亦比较宽;（２）地表特征参数及能量平衡各分量在实验区的绿洲、戈壁及沙漠上各有其特定的代表值;（３）地表能量平衡各分量的区域平均值在整个实验区内基本平衡;（４）夏季与近冬季的地表特征参数及能量平衡各分量的分布特征存在着显著差异。所得的这些结果与地面观测的“真值”和局地研究的结论基本一致。这些分析对非均匀下垫面中尺度模式陆面过程参数化方案的建立以及模式预报效果的检验都具有不可缺少的重要意义。  相似文献