共查询到20条相似文献，搜索用时 25 毫秒
1.
Periodic summer hypoxia occurring in the Northern Gulf of Mexico has been attributed to large nutrient inputs, especially nitrate‐nitrogen, from the Mississippi–Atchafalaya River system. The 2008 Gulf Hypoxia Action Plan calls for river corridor wetland restoration to reduce nitrate loads, but it is largely unknown how effective riverine wetland systems in the lower Mississippi River are for nitrate removal. We carried out an intensive isotope study to address this question by comparing nitrate isotopic signatures of the well‐channelized Mississippi River with those of the Atchafalaya River, which has extensive floodplains and backwater swamps. We investigated changes in δ15NNO3 and δ18ONO3 for water samples collected biweekly to monthly over a 2‐year period at the Atchafalaya River outlets (Morgan City and Wax Lake) and on the Mississippi River at Baton Rouge. In addition, in situ water quality parameters including temperature, dissolved oxygen and pH were recorded for each sampling date. Waters from both rivers showed moderately high nitrate concentration (>1 mg l?1) and undetectable (< 0.01 mg l?1) nitrite throughout the study period. The Mississippi River had slightly, but significantly higher (p=0.01) mean nitrate concentrations (1.5 mg l?1) and higher δ15NNO3 (7.7‰) than the Atchafalaya (1.1 mg l?1, 7.0‰); while no difference in δ18ONO3 (4.6‰) was found between the rivers. Flux‐weighted mean isotope values were overall lower than mean values for both the Mississippi and Atchafalaya Rivers, with a greater difference between the two rivers (7.4‰ versus 6.5‰, respectively). River flooding and hurricane storm surge also appeared to affect nitrate isotopic values. The lack of large difference in isotopic values between the Atchafalaya and Mississippi Rivers suggests that the majority of nitrate is transported through the Atchafalaya River with relatively little processing, and that riverine floodplains and wetlands are not effective sinks for nitrate, as previously assumed, because of insufficient residence time and well‐oxygenated river waters. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
2.
Before 1900, the Missouri–Mississippi River system transported an estimated 400 million metric tons per year of sediment from the interior of the United States to coastal Louisiana. During the last two decades (1987–2006), this transport has averaged 145 million metric tons per year. The cause for this substantial decrease in sediment has been attributed to the trapping characteristics of dams constructed on the muddy part of the Missouri River during the 1950s. However, reexamination of more than 60 years of water‐ and sediment‐discharge data indicates that the dams alone are not the sole cause. These dams trap about 100–150 million metric tons per year, which represent about half the decrease in sediment discharge near the mouth of the Mississippi. Changes in relations between water discharge and suspended‐sediment concentration suggest that the Missouri–Mississippi has been transformed from a transport‐limited to a supply‐limited system. Thus, other engineering activities such as meander cutoffs, river‐training structures, and bank revetments as well as soil erosion controls have trapped sediment, eliminated sediment sources, or protected sediment that was once available for transport episodically throughout the year. Removing major engineering structures such as dams probably would not restore sediment discharges to pre‐1900 state, mainly because of the numerous smaller engineering structures and other soil‐retention works throughout the Missouri–Mississippi system. Published in 2009 by John Wiley & Sons, Ltd. 相似文献
3.
《水文科学杂志》2013,58(3):531-542
Abstract River corridor wetland restoration and freshwater diversion from the lower Mississippi River are being considered as two major options to reduce nitrogen input to the Gulf of Mexico. However, it is largely uncertain how much nitrogen can actually be retained from the overflowing waters by these wetland systems. This study quantified the nitrogen inflow and outflow for the largest distributary basin of the Mississippi River, the Atchafalaya River Swamp basin. The goal of the study was to seek answers to three critical questions: (a) Does the Atchafalaya River Swamp remove a significant amount of nitrogen from the overflowing water, or is it releasing more nitrogen into the Gulf? (b) How do the nitrogen removal or release rates fluctuate seasonally and annually? (c) What are the relationships between the nitrogen removal capacity and the hydrological conditions in the basin such as river stage and discharge? By utilizing the long-term (1978–2002) river discharge and water quality data, monthly and annual nitrogen fluxes were quantified, and their relationships with the basin hydrological conditions investigated. A total nitrogen—sum of the total Kjeldahl nitrogen (TKN) and nitrate plus nitrite nitrogen (NO3+NO2)—mass input—output balance between the upstream (Simmesport) and downstream (Morgan City and Wax Lake Outlet) locations was established to examine the nitrogen removal potential for this, the largest freshwater swamp basin in North America. The results from this study showed that, over the past 25 years, the Atchafalaya River Swamp basin acted as a source for NO3+NO2 nitrogen, although the average annual output of NO3+NO2 nitrogen (174 584 Mg) was only slightly higher (2.3%) than the average annual input of NO3+NO2 nitrogen (170 721 Mg). The higher NO3+NO2 mass outflow occurred throughout summer and autumn, indicating an active role of biological processes on nitrogen in the overflowing waters of the Atchafalaya. However, this swamp basin has served as a major sink for organic nitrogen: the annual averages of TKN mass input and output were 200 323 and 145 917 Mg year?1, respectively, presenting a 27.2% removal rate by the basin. This large TKN reduction appeared high during springs and low during late summers, corresponding with the fluctuation of the hydrological conditions of the river. 相似文献
4.
Increases in nitrate loading to the Mississippi River watershed during the last 50 years are considered responsible for the increase in hypoxic zone size in Louisiana-Texas shelf bottom waters. There is currently a national mandate to decrease the size of the hypoxic zone to 5000 km2 by 2015, mostly by a 30% reduction in annual nitrogen discharge into the Gulf of Mexico. We developed an ecosystem model for the Mississippi River plume to investigate the response of organic matter production and sedimentation to variable nitrate loading. The nitrogen-based model consisted of nine compartments (nitrate, ammonium, labile dissolved organic nitrogen, bacteria, small phytoplankton, diatoms, micro- and mesozooplankton, and detritus), and was developed for the spring season, when sedimentation of organic matter from plume surface waters is considered important in the development of shelf hypoxia. The model was forced by physical parameters specified along the river-ocean salinity gradient, including residence time, light attenuation by dissolved and particulate matter, mixed layer depth, and dilution. The model was developed using measurements of biological biomasses and nutrient concentrations across the salinity gradient, and model validation was performed with an independent dataset of primary production measurements for different riverine NO3 loads. Based on simulations over the range of observed springtime NO3 loads, small phytoplankton contributed on average 80% to primary production for intermediate to high salinities (>15), and the main contributors to modeled sedimentation at these salinities were diatom sinking, microzooplankton egestion, and small phytoplankton mortality. We investigated the impact of limiting factors on the relationship between NO3 loading and ecosystem rates. Model results showed that primary production was primarily limited by physical dilution of NO3, followed by abiotic light attenuation, light attenuation due to mixing, and diatom sinking. Sedimentation was mainly limited by the first three of these factors. Neither zooplankton grazing or plume residence times acted as limiting factors of ecosystem rates. Regarding nutrient reductions to the watershed, simulations showed that about half of the percent decrease in NO3 load was reflected in decreased plume sedimentation. For example, a 30% decrease in NO3 load resulted in a 19% decrease in average plume primary production and a 14% decrease in sedimentation. Finally, our model results indicated that the fraction of primary production exported from surface waters is highly variable with salinity (7–87%), a finding which has important implications for predictive models of hypoxic zone size that assume a constant value for this ratio. 相似文献
5.
To improve quantitative understanding of mixed‐land‐use impacts on nutrient yields, a nested‐scale experimental watershed study design (n = 5) was applied in a 303(d), clean water act impaired urbanizing watershed of the lower Missouri River Basin, USA. From 2010 to 2013, water samples (n = 858 sample days per site) were analysed for total inorganic nitrogen (TIN‐N), nitrite (NO2–N) nitrate (NO3–N), ammonia (NH3–N), and total phosphorus (TP‐P). Annual, seasonal, and monthly flow‐weighted concentrations (FWCs) and nutrient yields were estimated. Mean nutrient concentrations were highest where agricultural land use comprised 58% of the drainage area (NH3 = 0.111 mg/l; NO2 = 0.045 mg/l; NO3 = 0.684 mg/l, TIN = 0.840 mg/l; TP = 0.127 mg/l). Average TP‐P increased by 15% with 20% increased urban land use area. Highly variable annual precipitation was observed during the study with highest nutrient yields during 2010 (record setting wet year) and lowest nutrient yields during 2012 (extreme drought year). Annual TIN‐N and TP‐P yields exceeded 10.3 and 2.04 kg ha?1 yr?1 from the agricultural dominated headwaters. Mean annual NH3–N, NO2–N, NO3–N, TIN‐N, and TP‐P yields were 0.742, 0.400, 4.24, 5.38, and 0.979 kg ha?1 yr?1, respectively near the watershed outlet. Precipitation accounted for the majority of the explained variance in nutrient yields (R2 values from 0.68 to 0.85). Nutrient yields were also dependent on annual precipitation of the preceding year (R2 values from 0.87 to 0.91) thus enforcing the great complexity of variable mixed‐land‐use mediated source‐sink nutrient yield relationships. Study results better inform land managers and best management practices designed to mitigate nutrient pollution issues in mixed‐land‐use freshwater ecosystems. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
6.
A refined specific‐gauge approach was developed to quantify changes over time in hydrological response on 3260 km of the Mississippi River system using long‐term data observed at 67 hydrologic measurement stations. Of these stations, 49 were unrated (stage‐only) stations, for which over 2 000 000 ‘synthetic discharges’ were generated based on measured discharge values at nearby rated stations. The addition of these synthetic discharges nearly tripled the number of stations in the study area for which specific‐gauge analysis could be performed. In order to maintain spatial homogeneity across such a broad study area, discharges were normalized to multiples of mean daily flow (MDF). Specific‐gauge analysis calculates stage changes over time for invariant discharge conditions. Two discharges were analysed: low‐flow and flood conditions at each station. In order to avoid the large errors associated with extrapolation of annual rating curves, a new ‘enhanced interpolation’ technique was developed that calculates continuous specific‐stage time series, even for rare discharges. Thus enhanced, specific‐gauge analysis is a useful reconnaissance tool for detecting geomorphic and hydrologic trends over time. Results show that on the Middle Mississippi River and Lower Missouri River, flood stages increased at all stations in spite of widespread incision of the river bed. On the Lower Mississippi River, both low‐flow and flood stages decreased, mainly the result of artificial meander cutoffs in the late 1920s and 1930s, except downstream of Natchez, MS, where net aggradation was observed. On the Upper Mississippi River, the specific‐gauge trends were dominated by emplacement of navigational dams and impoundment of slackwater pools. On all four river reaches, these results document hydrologic responses to the different engineering toolkits used on the different portions of the Mississippi River system during the past 75–150 years. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
7.
Robert L. Michel 《水文研究》2004,18(7):1255-1269
In the early 1960s, the US Geological Survey began routinely analysing river water samples for tritium concentrations at locations within the Mississippi River basin. The sites included the main stem of the Mississippi River (at Luling Ferry, Louisiana), and three of its major tributaries, the Ohio River (at Markland Dam, Kentucky), the upper Missouri River (at Nebraska City, Nebraska) and the Arkansas River (near Van Buren, Arkansas). The measurements cover the period during the peak of the bomb‐produced tritium transient when tritium concentrations in precipitation rose above natural levels by two to three orders of magnitude. Using measurements of tritium concentrations in precipitation, a tritium input function was established for the river basins above the Ohio River, Missouri River and Arkansas River sampling locations. Owing to the extent of the basin above the Luling Ferry site, no input function was developed for that location. The input functions for the Ohio and Missouri Rivers were then used in a two‐component mixing model to estimate residence times of water within these two basins. (The Arkansas River was not modelled because of extremely large yearly variations in flow during the peak of the tritium transient.) The two components used were: (i) recent precipitation (prompt outflow) and (ii) waters derived from the long‐term groundwater reservoir of the basin. The tritium concentration of the second component is a function of the atmospheric input and the residence times of the groundwaters within the basin. Using yearly time periods, the parameters of the model were varied until a best fit was obtained between modelled and measured tritium data. The results from the model indicate that about 40% of the flow in the Ohio River was from prompt outflow, as compared with 10% for the Missouri River. Mean residence times of 10 years were calculated for the groundwater component of the Ohio River versus 4 years for the Missouri River. The mass flux of tritium through the Mississippi Basin and its tributaries was calculated during the years that tritium measurements were made. The cumulative fluxes, calculated in grams of 3H were: (i) 160 g for the Ohio (1961–1986), (ii) 98 g for the upper Missouri (1963–1997), (iii) 30 g for the Arkansas (1961–1997) and (iv) 780 g for the Mississippi (1961–1997). Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
8.
水库建设改变了河流水文情势及物质迁移转化过程，从而影响水环境质量。为探究梯级筑坝影响下河流氮、磷的空间分布特征及其形成机制，以澜沧江为研究对象，于2016年和2021年分别开展了沿程水环境监测，对比分析水体中氮、磷及其形态浓度在水库建成前后的变化及沿程分布特征，探究氮、磷变化及其沿程分布的主控因子和影响机制。结果表明：由于河流建库蓄水淹没的土地释放大量土壤有机氮，新建水库段(2021年)水体总氮(TN)浓度相比于建库前(2016年)显著上升；由于建库后水流流速减缓而促进颗粒态磷沉降，水体总磷(TP)浓度显著下降。此外，河流建库蓄水后原自然河道的水环境特征改变且利于沉积物磷的释放，筑坝后水体磷酸盐(PO43--P)占生物可利用磷(Bio-P)的比例显著上升。受沿程土地利用的影响，从上游到下游水体TN浓度总体上逐渐升高，而水体TP浓度由于水库的截留效应逐渐降低。筑坝增加的水力停留时间为水库氮、磷转化提供了有利条件，主要表现为溶解性无机氮以硝态氮为主转变为以氨氮为主；同时，Bio-P中PO43--P的占比... 相似文献
9.
In 1937, the US Army Corps of Engineers cut through the “neck” of a large meander on the lower Mississippi River (below the confluence with the Ohio River) forming the Caulk Neck cutoff and creating Lake Whittington, a 26‐km long oxbow lake, in northern Mississippi. Since 1938, seasonal flooding and a boat channel connecting the lake with the Mississippi River have led to sediment accumulation in the lake, resulting in an 80‐year record of sediment quality in the river. On the basis of an age‐dated sediment core from the lake, trends in trace metals and hydrophobic organic compounds (except polycyclic aromatic hydrocarbons) follow well‐known patterns with upward trends from the 1930s to the ca 1970s, followed by downward trends to the present. Two factors contribute to these patterns: reservoir construction and changes in emissions. The construction of seven large reservoirs on the Missouri River, in particular the closure of the Fort Randall (1953) and Gavins Point (1955) Dams, greatly reduced the load of relatively clean sediment to the Mississippi River, likely contributing to downstream increases in contaminant concentrations in the Mississippi River. Increasing anthropogenic emissions also contributed to upward trends until ca 1970 when major environmental policy actions began resulting in broad decreases in emissions and downward trends in the concentrations of most of the contaminants monitored. Polycyclic aromatic hydrocarbons and phosphorus are partial exceptions to this pattern, with increases to the 1960s and variable concentrations showing no clear trend since. Published 2012. This article is a U.S. Government work and is in the public domain in the USA. 相似文献
10.
Large volumes of new accommodation have formed within the Mississippi Delta plain since the mid‐1950s in association with rapid conversion of coastal wetlands to open water. The three‐dimensional aspects and processes responsible for accommodation formation were quantified by comparing surface elevations, water depths, and vertical displacements of stratigraphic contacts that were correlated between short sediment cores. Integration of data from remotely sensed images, sediment cores, and water‐depth surveys at 10 geologically diverse areas in the delta plain provided a basis for estimating the total volume of accommodation formed by interior‐wetland subsidence and subsequent erosion. Results indicate that at most of the study areas subsidence was a greater contributor than erosion to the formation of accommodation associated with wetland loss. Tens of millions of cubic meters of accommodation formed rapidly at each of the large open‐water bodies that were formerly continuous interior delta‐plain marsh. Together the individual study areas account for more than 440 × 106 × m3 of new accommodation that formed as holes in the Mississippi River delta‐plain fabric between 1956 and 2004. This large volume provides an estimate of the new sediment that would be needed just at the study areas to restore the delta‐plain wetlands to their pre‐1956 areal extent and elevations. Published 2010. This article is a US Government work and is in the public domain in the USA. 相似文献
11.
Nitrogen (N) and phosphorus (P) dynamics in the Kuparuk River in arctic Alaska were characterized in a 3‐year study using routine samples near the mouth of the river at the Arctic Ocean, synoptic whole‐river surveys, and temporally intense sampling during storms in three headwater basins. The Lower Kuparuk River has low nitrate concentrations (mean [NO3]‐N] = 17 µg l?1 ± 1·6 SE) and dissolved inorganic N (DIN, mean [N] = 31 µg l?1 ± 1·2 SE) compared with rivers in more temperate environments. Organic forms constituted on average 90% of the N exported to the Arctic Ocean, and high ratios of dissolved organic N (DON) to total dissolved N (TDN) concentrations (mean 0·92) likely result from waterlogged soils formed by reduced infiltration due to permafrost and low hydrologic gradients. Annual export of TDN, DON, and particulate N averaged 52 kg km?2, 48 kg km?2, and 4·1 kg km?2 respectively. During snowmelt, the high volume of runoff typically results in the highest nutrient loads of the year, although high discharge during summer storms can result in substantial nutrient loading over short periods of time. Differences in seasonal flow regime (snowmelt versus rain) and storm‐driven variation in discharge appear to be more important for determining nutrient concentrations than is the spatial variation in processes along the transect from headwaters towards the ocean. Both the temporal variation in nitrate:DIN ratios of headwater streams and the spatial variation in nitrate:DIN between larger sub‐basins and smaller headwater catchments is likely controlled by shifts in nitrification and soil anoxia. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
12.
Philippe Vervier Sonia Bonvallet‐Garay Sabine Sauvage H. Maurice Valett José‐Miguel Sanchez‐Perez 《水文研究》2009,23(12):1801-1812
Phosphorus (P) concentrations in sediments and in surface and interstitial water from three gravel bars in a large river (Garonne River, southern France) were measured daily, downstream of a wastewater treatment plant for a city of 740 000 inhabitants (Toulouse). Measurements were made of vertical hydraulic gradient (VHG), total dissolved phosphorus (TDP), soluble reactive phosphorus (SRP) and total phosphorus (TP) in water and of three extractable forms of phosphorus (water extractable, NaOH extractable and H2SO4 extractable) in hyporheic sediments from the gravel bars. Dissolved phosphorus was the major contributor to TP (74–79%) in both interstitial and surface waters on all sampling dates, and in most cases surface water P concentrations were significantly higher than interstitial concentrations. Hyporheic sediment TP concentrations ranged between 269 and 465 µg g?1 and were highest in fine sediment fractions. Acid‐extractable P, a non‐bioavailable form, represented at least 95% of sediment TP. A positive relationship was observed between VHG and TP in two of the gravel bars, with wells that were strongly downwelling having lower TP concentrations. These results suggest that in downwelling zones, hyporheic sediments can trap surface‐derived dissolved P, and that much of this P becomes stored in refractory particulate forms. Bioavailable P is mainly present in dissolved form and only occupies a small fraction of total P, with particulate P comprising the majority of total P. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
13.
Particulate carbon and nitrogen dynamics in a headwater catchment in Northern Thailand: hysteresis,high yields,and hot spots 
下载免费PDF全文
下载免费PDF全文 Alan D. Ziegler Shawn G. Benner Melvin L. Kunkel Valerie X.H. Phang Massimo Lupascu Chatchai Tantasirin 《水文研究》2016,30(19):3339-3360
Rivers of South and Southeast Asia disgorge large suspended sediment loads, reflecting exceptionally high rates of erosion promoted by natural processes (tectonic and climatic) and anthropogenic (land‐use change) activities that are characteristic of the region. While particulate carbon and nitrogen fluxes have been characterized in some large Asian rivers, less is known about the headwater systems where much sediment and organic material are initially mobilized. This study, conducted in the 74‐km2 Mae Sa Experimental Catchment in northern Thailand, shows that the Sa River is an important source for particulate organic carbon (POC) and particulate organic nitrogen (PON) transported to larger river systems and downstream reservoirs. However, the yields during three years of investigation varied greatly: 5.0–22.3 Mg POC km?2 y?1 and 0.48–2.02 Mg PON km?2 y?1. The 22.3 Mg POC km?2 y?1 yield is the highest reported for any river on the Asian continent. Stream samples collected during 12 storms showed that almost 3% of the total suspended solid load is POC 0.7 µm to 2.0 mm in size. This percentage is higher than other values for most large rivers on the continent. Further, we documented a strong pulse hysteretic behaviour in the stream, whereby peak fluxes of POC and PON are often delayed (anticlockwise hysteresis) or accelerated (clockwise hysteresis) relative to stream flow peaks (or are complex), complicating the prediction of storm‐based or annual particulate carbon and nitrogen fluxes. Stream turbidity and total suspended sediment are reasonable proxies for POC and PON concentrations, while stream discharge is not a good predictor variable. Observed C:N ratios for measured particulate samples range from 3 to 83, with the high‐end values likely associated with fresh (non‐decomposed) vegetative material greater than 2 mm in diameter. The C:N ratio (weighted based on three sediment sizes) for 12 events ranges from 7.5 to 15.3. These modest values reflect the relatively low C:N ratios for small size fractions (0.7–0.63 µm) that comprise 50–90% of the TSS load in the events. Overall, organic material <0.63 µm contribute about 75% of the total POC load and 80% of the PON load. The annual C:N ratio for the river is approximately 10–11. Collectively, our findings indicate the occasionally high yields make the Sa River—and potentially other similar headwater rivers—a hot spot for POC and PON transported to downstream water bodies. Complex hysteresis patterns and high year‐to‐year variability hinders our ability to calculate and predict these yields without continuous, automated monitoring of discharge and turbidity. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
14.
Evaluating relative sensitivity of SWAT‐simulated nitrogen discharge to projected climate and land cover changes for two watersheds in North Carolina,USA 下载免费PDF全文
下载免费PDF全文 We investigated how projected changes in land cover and climate affected simulated nitrate (NO3?) and organic nitrogen discharge for two watersheds within the Neuse River Basin, North Carolina, USA, for years 2010–2070. We applied the Soil and Water Assessment Tool watershed model to predict nitrogen discharge using (1) atmospheric carbon dioxide (CO2) concentrations predicted by the Intergovernmental Panel on Climate Change, (2) land cover change predicted by the Integrated Climate and Land Use Change project and (3) precipitation and temperature simulated by two statistically downscaled and bias‐corrected Global Circulation Models. We determined the sensitivity of simulated nitrogen discharge to separate changes in each treatment [(1) CO2, (2) land cover and (3) precipitation and temperature (PT)] by comparing each treatment to a reference condition. Results showed that nitrogen discharges were most sensitive to changes in PT over the 60‐year simulation. Nitrogen discharges had similar sensitivities to the CO2 and land cover treatments, which were only one‐tenth the influence of the PT treatment. Under the CO2 treatment, nitrogen discharges increased with increasing ambient CO2. NO3? discharge decreased with increased urbanization; however, organic nitrogen had a varied response. Under the PT treatment, there was high spatial variability in nitrogen discharges. In a single year, certain sub‐basins showed an 80% increase in nitrogen discharge relative to reference, while others showed a 400% decrease. With nitrogen discharge showing high sensitivity to PT change, we suggest that more emphasis should be placed on investigating impacts of PT on nutrient transport in the Neuse River Basin. Published 2015. This article is a U.S. Government work and is in the public domain in the USA 相似文献
15.
This study addresses the spatial variations in water quality along the River Vène (France). The Vène drains a 67 km2 rural basin, with a large karstic area, located in a Mediterranean context. A 1 day sampling campaign was conducted along the river, in winter low‐flow conditions (February 2003). Physico‐chemical parameters and water flow discharge were measured in situ during the sampling campaign. Water quality was evaluated by determining the concentrations of nitrogen and phosphorus in water and bed‐sediment samples. Nitrogen and phosphorus loads were evaluated taking into account the measured concentrations and discharge. The campaign included 18 sampling points and concerned the whole river from the spring to the outlet, plus the main inputs, i.e. sewage treatment works, main tributaries and karstic springs. The spatial evolution of nitrogen and phosphorus loads along the river allowed the significant role of point‐source inputs to be demonstrated. The decrease in nutrient loads along the river occurred mainly in specific reaches where fine sediments had accumulated. In these zones, phosphorus is trapped in the bed sediments in calcium‐bound phosphates due to precipitation processes. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
16.
金沙江下游梯级水库对氮、磷营养盐的滞留效应 总被引：1，自引：0，他引：1
氮、磷是水域重要的营养或污染物质，大型水库修建将对江河氮、磷物质的输运产生重要影响.以金沙江华弹、向家坝水文站2006-2016年实测水质资料为依据，通过建立污染物浓度与流量比值（TN/Q、TP/Q）与含沙量（S）的关系式，对金沙江下游溪洛渡、向家坝梯级水库蓄水前后进出库总氮（TN）、总磷（TP）浓度及通量的变化特征进行研究.结果表明：（1）华弹站不受蓄水影响，TN和TP浓度在0.38~1.41和0.01~0.73 mg/L之间变化，向家坝站蓄水前TN和TP浓度在0.32~1.33和0.03~0.42 mg/L之间变化，蓄水后在0.35~1.29和0.01~0.05 mg/L之间变化，蓄水后TN浓度较蓄水前略有升高，但TP浓度较蓄水前约降低75%；（2）蓄水前华弹站TN浓度与向家坝站基本接近，TP浓度总体低于向家坝站，蓄水后华弹站TN浓度低于向家坝站，TP浓度明显高于向家坝站；（3）金沙江TN以硝态氮（NO3--N）为主，占TN浓度的67.3%~91.8%；（4）两站的TN浓度随流量和含沙量变化较小，TP浓度与流量和含沙量均呈正相关关系；（5）华弹站TN、TP年通量在48357~135827和4720~14163 t之间变化，年均值分别为90337和8932 t，向家坝站蓄水前后TN年通量在64232~130966和71675~149647 t之间变化，蓄水后通量总体高于蓄水前，TP年通量在8851~18624和3131~7300 t之间变化，蓄水后通量远低于蓄水前；（6）水库蓄水对出库TN浓度与通量无明显影响，但TP浓度与通量较蓄水前明显降低，其中通量年均滞留率约为67.0%. 相似文献
17.
Samuel Teissier Sabine Sauvage Philippe Vervier Frédéric Garabétian José‐Miguel Sánchez‐Pérez 《水文研究》2008,22(3):420-428
A mass‐balance approach was used to estimate in‐stream processes related to inorganic nitrogen species (NH4+, NO2? and NO3?) in a large river characterized by highly variable hydrological conditions, the Garonne River (south‐west France). Studies were conducted in two consecutive reaches of 30 km located downstream of the Toulouse agglomeration (population 760 000, seventh order), impacted by modification of discharge regime and high nitrogen concentrations. The mass‐balance was calculated by two methods: the first is based on a variable residence time (VRT) simulated by a one‐dimensional (1‐D) hydraulic model; the second is a based on a calculation using constant residence time (CRT) evaluated according to hydrographic peaks. In the context of the study, removal of dissolved inorganic nitrogen (DIN) for a reach of 30 km is underestimated by 11% with the CRT method. In sub‐reaches, the discrepancy between the two methods led to a 50% overestimation of DIN removal in the upper reach (13 km) and a 43% underestimation in the lower reach (17 km) using the CRT method. The study highlights the importance of residence time determination when using modelling approaches in the assessment of whole stream processes in short‐duration mass‐balance for a large river under variable hydrological conditions. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
18.
To investigate the origin and behaviour of nitrate in alluvial aquifers adjacent to Nakdong River, Korea, we chose two representative sites (Wolha and Yongdang) having similar land‐use characteristics but different geology. A total of 96 shallow groundwater samples were collected from irrigation and domestic wells tapping alluvial aquifers. About 63% of the samples analysed had nitrate concentrations that exceeded the Korean drinking water limit (44·3 mg l?1 NO3?), and about 35% of the samples had nitrate concentrations that exceeded the Korean groundwater quality standard for agricultural use (88·6 mg l?1 NO3?). Based on nitrogen isotope analysis, two major nitrate sources were identified: synthetic fertilizer (about 4‰ δ15N) applied to farmland, and animal manure and sewage (15–20‰ δ15N) originating from upstream residential areas. Shallow groundwater in the farmland generally had higher nitrate concentrations than those in residential areas, due to the influence of synthetic fertilizer. Nitrate concentrations at both study sites were highest near the water table and then progressively decreased with depth. Nitrate concentrations are also closely related to the geologic characteristics of the aquifer. In Yongdang, denitrification is important in regulating nitrate chemistry because of the availability of organic carbon from a silt layer (about 20 m thick) below a thin, sandy surface aquifer. In Wolha, however, conservative mixing between farmland‐recharged water and water coming from a village is suggested as the dominant process. Mixing ratios estimated based on the nitrate concentrations and the δ15N values indicate that water originating from the village affects the nitrate chemistry of the shallow groundwater underneath the farmland to a large extent. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
19.
Thi Phuong Quynh Le Viet Nga Dao Emma Rochelle‐Newall Josette Garnier XiXi Lu Gilles Billen Thi Thuy Duong Cuong Tu Ho Henri Etcheber Thi Mai Huong Nguyen Thi Bich Ngoc Nguyen Bich Thuy Nguyen Nhu Da Le Quoc Long Pham 《地球表面变化过程与地形》2017,42(9):1329-1341
Riverine transport of organic carbon from terrestrial ecosystems to the oceans plays an important role in the global carbon cycle. The Red River is located in Southeast Asia where river discharge, sediment loads and fluxes of elements (carbon, nitrogen and phosphorus) associated with suspended solids have been dramatically altered over past decades as a result of reservoir impoundment and land use, population, and climate change. Dissolved organic carbon (DOC) and particulate organic carbon (POC) concentrations were measured monthly at four stations of the Red River system from January 2008 to December 2010. The results reveal that POC changed synchronically with total suspended solids (TSS) concentration and with the river discharge, whereas no clear trend was observed for DOC concentration. The mean value of total organic carbon (TOC = DOC + POC) flux in the delta of the Red River was 31.5 × 1013 ± 4.0 × 1013 MgC.yr?1 (range 27.9–35.8 × 1013 MgC.yr?1 which leads to a specific TOC flux of 2012 ± 255 kgC.km?2.yr?1 during this 2008–2010 period. About 80% of the TOC flux was transferred to the estuary during the rainy season as a consequence of the higher river water discharge. The high mean value of the POC:Chl‐a ratio (1585 ± 870 mgC.mgChl‐a?1) and the moderate C:N ratio (7.3 ± 0.1) in the water column system suggest that organic carbon in the Red River system is mainly derived from erosion and soil leaching in the basin. The effect of two new dam impoundments in the Red River was also observable with lower TOC fluxes in 2010 compared with 2008. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
20.
Dirk Nürnberg Martin Ziegler Cyrus Karas Ralf Tiedemann Matthew W. Schmidt 《Earth and Planetary Science Letters》2008,272(1-2):278-289
The Loop Current mediating the oceanic heat and salt flux from the Caribbean Sea into the Atlantic Ocean and its interference with the Mississippi River discharge are critical for both the regional climate in the Gulf of Mexico area and the water vapor transport towards high northern latitudes. We present a 400-kyr record of sea surface temperature and local surface salinity from the northeastern Gulf of Mexico (IMAGES core MD02-2575) approximated from combined planktonic foraminiferal δ18O and Mg/Ca, which reflects the temporal dynamics of the Loop Current and its relationship to both varying Mississippi discharge and evolution of the Western Hemisphere Warm pool. The reconstructed sea surface temperature and salinity reveal glacial/interglacial amplitudes that are significantly larger than in the Western Hemisphere Warm pool. Sea surface freshening is observed during the extreme cool periods of Marine Isotope Stages 2, 8, and 10, caused by the strengthened Mississippi discharge which spread widely across the Gulf favored by the less established Loop Current. Interglacial and interstadial sea-surface conditions, instead, point to a strengthened, northward flowing Loop Current in line with the northward position of the Intertropical Convergence Zone, allowing northeastern Gulf of Mexico surface hydrographic conditions to approach those of the Caribbean. At these times, the Mississippi discharge was low and deflected westward, promoted by the extended Loop Current. Previously described deglacial megadischarge events further to the west did not affect the northeastern Gulf of Mexico hydrography, implying that meltwater routing from the Laurentide Ice Sheet via the Mississippi River is unlikely to have affected Atlantic Meridional Overturning Circulation. 相似文献