鄱阳湖水环境演变特征研究

鄱阳湖水质安全是鄱阳湖生态经济区规划实施的重要物质基础,也是长江流域生态安全的重要组成。本文通过收集湖区主要水质监测断面2003-2008年逐月监测资料,采用单因子评价、湖泊富营养化评价、季节性肯达尔检验等分析主要河流、湖泊水环境演变特征。研究表明,鄱阳湖区现状水质较好,全年、汛期、非汛期的I-Ⅲ类水面积比例分别为63．9％、99．3％、40．8％,水体处于中营养一轻度富营养化状态。赣江、抚河、修河等人湖控制断面I一Ⅲ类水比例高于90％,乐安河、长江等控制断面I-Ⅲ类水质比例高于70％,湖口断面现状水质较差,Ⅳ类及以下水质比例高于30％。随着鄱阳湖流域社会、经济的发展,部分水质断面污染物浓度增加,湖泊I-Ⅱ类水面积比例呈下降趋势,需要采取有效措施,减少流域污染物排放量和人湖量,实现一湖清水的目标,为水利枢纽工程实施提供良好的水环境条件。     

Water exchange in a large river–lake system: Modeling,characteristics and causes

River–lake water exchange reflects hydrological connectivity and the dynamic relationship between the river and the lake. The water exchange is crucial for lake level variation, downstream river discharge and the ecosystem. To figure out the water exchange between the Yangtze River and Poyang Lake, a data‐driven model was established based on the support vector regression and genetic algorithm technique. Nine scenarios were set with different river–lake hydrological conditions, divided into two categories: single‐element change scenarios, where only the river conditions or only the lake conditions changed, and combined scenarios, where both elements changed simultaneously. The model could accurately simulate the river–lake water exchange variations. Scenario simulation results show that increasing the river flow or lowering the lake level could cause a decrease in the lake outflow. Conversely, decreasing river flow or raising the lake level could cause an increase in lake outflow. Changing lake conditions have a stronger impact on the water exchange variation than changing river conditions if the change percentages of the situation indicator values are the same. Similarly, lake level increase has a stronger impact on the water exchange variation than lake level decrease. The combined scenarios indicate the additive effect of the corresponding single‐element change scenarios, with a clear linear relationship between their lake outflow changes. This study provides an efficient model for simulating complex hydrological flow relationships in river–lake systems, and supports the management of the Yangtze River and Poyang Lake by providing the characteristics and causes of the river–lake water exchange.     

Effect of depth on food‐web interactions in a thermally stratified floodplain lake following inundation

We investigated the controlling mechanisms of microbial and metazooplankton food‐web components at the surface‐ and bottom‐water layers of a thermally stratified floodplain lake (Lake Sakada?, Kopa?ki Rit floodplain, Croatia) during its connection with the river. Throughout the study, the lake was stratified continuously. The permutational multivariate analysis of variance provided evidence that only the surface‐water layer was impacted on by the flow and flood pulses, whereas the bottom‐water layer represented a zone of stable environmental conditions. The water depth was by one order of magnitude more important in determining environmental variables and plankton abundance than the hydrological phases. Under such circumstances, path analysis provided evidence that the planktonic food‐web interactions were strongly influenced by the sample layer, regardless of the flooding conditions. The transfer of organic matter to higher trophic levels at the surface was dominated by herbivory and, to a lesser extent, by predation, whereas bottom‐water layer was represented by additional routes (bacterivory and omnivory). The transfer of organic matter was different through the investigated water layers, of which the surface‐water layer was additionally structured by the floodplain hydrology.     

Hydrological alterations as the major driver on environmental change in a floodplain Lake Poyang (China): Evidence from monitoring and sediment records

Due to the lack of long-term records on shallow lake environmental change, knowledge of the processes and mechanisms behind the limnological response of many shallow floodplain lakes to hydrological alterations and nutrient loading is often limited. We examined seasonal monitoring data and a dated sediment core from Lake Poyang, a large floodplain lake located on the Yangtze floodplain in the SE China. Multivariate analysis based on contemporary data (diatoms and water quality) revealed that the seasonal changes in the diatom assemblage of the lake were correlated with water temperature and Secchi depth (SD), although the weak spatial effect was not negligible. During the dry winter season, low water temperature, low SD, and high nutrient levels, were accompanied by high abundances of planktonic Aulacoseira species along with Stephanodiscus hantzschii, a species well adapted to cold and eutrophic waters. During the summer wet season, however, when water temperature and SD were high and nutrient levels low, benthic and epiphytic diatoms, such as the genus Achnanthes, dominated. Sediment records of diatoms and geochemistry were used to estimate long-term variation in the ecological condition of the lake. During the past ~60 years, the lake has shifted from a natural hydrologically connected, oligotrophic lake dominated by benthic and epiphytic diatoms to a poorly hydrologically connected, eutrophic state driven by nutrient-tolerant planktonic and eutrophic diatoms. Furthermore, our results indicate that the proposed Poyang dam may severely affect the water quality and ecosystem of the lake by altering its seasonal hydrology.     

Quantifying the Human Induced Water Level Decline of China’s Largest Freshwater Lake from the Changing Underlying Surface in the Lake Region

In recent years, dramatic decline in China’s largest freshwater lake, Poyang Lake, has raised wide concerns about water supply and ecological crises in the middle–lower Yangtze River reaches. To assist in resolving the debates regarding the low water regime of the lake, the current study quantitatively assessed the enhanced water level decline from the changing underlying surface in the Poyang Lake region. It is the first time that the magnitude, temporal–spatial difference, trend development and background mechanism of lake water level variation and its causes are studied comprehensively. The results revealed that the changing underlying surface in the lake region has caused an average decline of annual water level of 0.26 m ~ 0.75 m across the lake during 2000–2014, which shows great seasonal and spatial differences. The enlarged outflow cross–section due to extensive sand mining was the major reason for the effect on water level decline in the northern lake. While, increased water surface gradient should be attributed to water level decline in the southern lake. The long–term increasing trend of annual lake water level decline reflects the cumulative effects of lake bottom topography change caused by the continuous south movement of sand mining activities.     

Satellite remote sensing reveals impacts from dam‐associated hydrological changes on chlorophyll‐a in the world's largest desert lake

We present an approach that uses satellite products to derive models for predicting lake chlorophyll from environmental variables, and for investigating impacts of changing environmental flows. Lake Turkana, Kenya, is the world's largest desert lake, and environmental flows from the Omo River have been modified since 2015 by the Gibe III dam in Ethiopia. Using satellite remote sensing, we have evaluated the influence of these altered hydrological patterns on large‐scale lake phytoplankton concentrations for the first time. Prior to dam completion, strong seasonal cycles and large spatial gradients in chlorophyll have been observed, related to natural fluctuations in the Omo River's seasonal discharge. During this period, mean lake chlorophyll showed a strong relationship with both river inflows and lake levels. Empirical models were derived which considered multiple hydro‐climatic drivers, but the best model for predicting chlorophyll‐a was a simple model based on Omo River discharge. Application of this model to data for 2015–2016 estimated that during the filling of Gibe III annual mean Lake Turkana chlorophyll declined by 30%. Future water management scenarios based on Gibe III operations predict reduced seasonal chlorophyll‐a variability, while irrigation scenarios showed marked declines in chlorophyll‐a depending on the level of abstraction. These changes demonstrate how infrastructure developments such as dams can significantly alter lake primary production. Our remote sensing approach is easy to adapt to other lakes to understand how their phytoplankton dynamics may be affected by water management scenarios.     

鄱阳湖水龄时空特征和影响因素分析

换水能力是表征湖泊水动力和污染物输移扩散的关键指标,鄱阳湖水文情势波动显著,换水能力时空特征和影响因素复杂。本文选取水龄描述鄱阳湖换水能力,建立鄱阳湖水龄模型,研究了鄱阳湖水龄的时空特征和影响因素,分析了水龄对鄱阳湖江湖关系演变的响应规律。结果表明:(1)鄱阳湖水龄具有显著的时空异质性。夏季和秋季水龄明显大于冬季和春季,碟形湖和湖湾区域水龄明显大于主河道和滩地。(2)湖口水位和流域入流是影响鄱阳湖水龄的主要因素。湖口水位对鄱阳湖水龄的影响大于流域入流;湖口水位越高则鄱阳湖水龄越大,流域入流与之相反;鄱阳湖水龄年内显著波动的主要原因是湖口水位的年内显著变化。(3)2003年后,随着江湖关系的演变,鄱阳湖水龄有所减小,其中秋季减小最为明显。研究成果能为鄱阳湖水资源保护提供科学支撑,也可为其他大型湖泊换水能力研究提供参考。     

Potential effects of a major navigation project (Paraguay–Paraná Hidrovía) on inundation in the Pantanal floodplains

The Pantanal wetland of Brazil, a vast complex of seasonally inundated floodplains along the Paraguay River, is renowned for its outstanding biological resources. A proposed navigation project known as the Paraguay–Paraná Waterway (or Hidrovía) would deepen the Paraguay River channel to facilitate year‐round navigation through the Pantanal. The possibility of decreases in river levels (stage) has aroused concerns in relation to the potential environmental impacts, however the poor understanding of the hydrological relationships between rivers and floodplains has hampered evaluation of these impacts. The present study evaluates the potential impact of river modifications on adjacent floodplains by examining the relationship between the Paraguay River stage and the extent of floodplain inundation. Satellite observations of flooded area (from passive microwave emission; monthly data for 1979–1987) are plotted against river stage from several stations throughout the region to show the stage–inundation relationships for eight subregions along the Paraguay River. Scenarios in which the Paraguay River stage is decreased from the 20th and 80th percentile values reveal large potential impacts on inundation. For stage decreases of 0.10 and 0.25 m, the total flooded area is reduced by 1430 and 3830 km² at low‐water, and by 2410 and 5790 km² at high‐water, respectively. The floodplains of the two northernmost subregions appear to be most susceptible to reductions in flooding, losing more than half of their flooded area with a 0.25‐m decrease in the low‐water stage. The ecological impacts of these reductions in flooded area may be particularly severe at low water, when the few areas that typically remain flooded throughout the dry season serve as important refuges for aquatic animals. These results underscore the need for better understanding of the hydrology of the integrated river floodplain systems in the Pantanal before river channel modifications are carried out. Copyright © 1999 John Wiley & Sons, Ltd.     

近 30 年鄱阳湖与洞庭湖水文变化与归因

为揭示鄱阳湖和洞庭湖水文特征并综合分析其影响因素对湖泊水资源管理及江湖关系, 以两湖水系统为研究对象, 采用非参数秩次相关检验法( Mann2Kendall M2K) 分析方法和近 30 年两湖2流域2长江水位和流量长序列数据集, 解析湖泊水文变化特征、趋势和程度, 并通过两湖水文的对比来系统分析长江中游大型通江湖泊水文变化规律及其影响因素, 旨在对两湖流域水资源的整体认识和把控, 丰富对江-河-湖复杂水系统作用机制与内涵的深入理解。结果表明: 两湖水位在近 30 年总体上呈“上升-非稳定状态-下降” 3 个变化阶段。2003 年前, 鄱阳湖和洞庭湖水位总体呈上升趋势, 且在 1980- 2000 年间基本处于不稳定的波动状态, 2003 年后两湖水位呈明显的下降趋势。 Mann-Kendall 分析得出: 两湖水位可能在 2005 年左右发生明显突变, 且鄱阳湖和洞庭湖水位与之前相比, 最大降 幅分别可达 9. 4% 和 3. 4% , 表明两湖水位变化的趋势程度存在差异, 鄱阳湖水情变化的敏感程度要强于洞庭湖。 对长江干流的补水期洞庭湖为 4- 10 月, 鄱阳湖为 3- 8 月, 表明两湖对长江干流水文的不同调节补偿作用。     

鄱阳湖与长江关系及三峡蓄水的影响

通过机理研究,结合实测资料分析,说明了鄱阳湖河相与湖相的分界水位,阐明了长江干流来流对湖口出流的顶托作用,并给出了湖口出现倒流的条件,得到了鄱阳湖对洪水的调节作用系数,初步理清了鄱阳湖与长江干流江湖相互影响规律。分析了三峡水库蓄水期的影响及鄱阳湖的枯水特性。三峡水库运用初期,其蓄水的前半段时间,鄱阳湖向长江干流多补水量23亿m3,后半段少补水量11亿m3。三峡水库运用30年后,在河道冲刷、可补水量减少和蓄水的共同作用下,鄱阳湖的枯水季节相当于提前了1个月左右。本研究对认识江湖关系及鄱阳湖治理具有重要意义。     

Environmental characteristics and phytoplankton productivity of a shallow Ramsar‐site floodplain in the western Himalaya

Hokersar wetland (altitude of 1584 masl), a shallow (0.5 m) floodplain waterfowl habitat in Kashmir, India, gained international importance in 2005 with its declaration as a Ramsar site. Although isolated in the western Himalayan mountains, it is being impacted by a silt‐laden river (Doodhganga). Human activities, not the least of which is affected by the closeness of the wetland to suburban areas, have gradually altered its trophic state. Its alkaline‐ and calcium‐rich waters contain many planktonic diatoms, green algae and cyanobacteria. Nitrate–nitrogen and total phosphorus concentrations are high (370–4750 and 101–968 μg L^?1, respectively). A pronounced seasonal cycle in phytoplankton production (g C m^?2 day^?1) was evident in a minimum value of 0.38 (Jan–Feb, 2002) and a maximum value of 4.02 (July, 2002), closely paralleling the seasonal cycles of temperature and light. The photosynthetic efficiency of the wetland was highest (1.97%) during the summer. The annual phytoplankton production of this wetland ecosystem was 210 × 10² KJ m^?2, suggestive of a meso‐eutrophic status.     

长江干流九江段与鄱阳湖不同季节的同位素特征

为了分析不同时期鄱阳湖与长江及其他各条河流的关系,初步探讨三峡水库运行对鄱阳湖水量的影响,对九江段长江干流、鄱阳湖地区河水、湖水进行了采样和氘氧稳定同位素分析。结果表明:水体同位素存在明显的时间和空间变化,可用于揭示湖泊与河流之间的关系。三峡补水调度过程能有效减少鄱阳湖春季的水量流失,4月份鄱阳湖流域已进入雨季,大量内河来水导致鄱阳湖水以较大的流量向长江补给;调洪削峰调度阶段,较高的长江水位对鄱阳湖存在一定的顶托作用,导致湖水难以排泄;三峡蓄水过程引起的长江水位降低加速了枯水期鄱阳湖水的流失。三峡水库的调度需要考虑到鄱阳湖流域水情,以免加重该流域丰水期的洪涝灾害和枯水期的水资源短缺。     

DISCONNECTING THE FLOODPLAIN: EARTHWORKS AND THEIR ECOLOGICAL EFFECT ON A DRYLAND FLOODPLAIN IN THE MURRAY–DARLING BASIN,AUSTRALIA

Globally, dams and water extractions are well‐recognised disruptors of flow regimes in floodplain wetlands, but little is known of the hydrological and ecological impacts of floodplain earthworks constructed for irrigation, flood mitigation and erosion control. We mapped the distribution of earthworks with high‐resolution SPOT (Système Probatoire d'Observation de la Terre) imagery in an internationally recognised Ramsar wetland, the Macquarie Marshes of the Murray–Darling Basin, Australia. There were 339 km levees, 1648 km channels, 54 off‐river storages and 664 tanks (0.5–5 m high), detected within the 4793 km² floodplain study area. Earthworks reduced localised flooding compared with undeveloped sites. The most pronounced disconnection of the original floodplain (73.0%) occurred where earthworks were most concentrated compared with areas with few earthworks (53.2%). We investigated relationships between hydrological connectivity and mortality of the perennial flood‐dependent river red gum Eucalyptus camaldulensis at 55 floodplain sites (225 × 150 m). Over half of the river red gums were dead at 21.8% of the sites. Earthworks blocked surface flows to flood‐dependent vegetation and drowned vegetation in artificially inundated off‐river storages. Mortality was due to impacts of earthworks and potentially exacerbated by effects of river regulation, water extraction and climate. River red gums were healthiest in narrow river corridors where earthworks confined flows and flows could recede freely. Rehabilitation of flood‐dependent ecosystems should focus on reinstating lateral connectivity and protecting environmental flows. Copyright © 2011 John Wiley & Sons, Ltd.     

森林植被改善对鄱阳湖流域径流和输沙过程的影响

以河流基流为切入点,研究流域植被调节径流、水土保持等微观作用影响大中型河流湖泊径流过程和水沙过程宏观效应的机理。鄱阳湖流域60年来天然降水没有发生趋势性变化。流域森林覆盖率由34.73%上升到63.00%,植被质量改善,赣江等入湖河流基流增加83 m~3/s,河流输沙量减少。2000年以后和2000年以前相比较,枯水期降水径流系数增大,年流量过程平坦化,一定程度上减小洪灾风险,有利于水资源利用和生态环境保护。2001年以后进入鄱阳湖泥沙平均每年减少1 007×10~4t;出湖泥沙增加314×10~4t。因此,鄱阳湖入江水道由淤积转变为冲刷,但出湖流量过程没有趋势性变化。     

近60年来长江对鄱阳湖倒灌水量的变化特征

鄱阳湖是长江中下游典型的通江湖泊,其出流特征及水位涨落同时受五河及长江来水的双重影响,进而形成较为复杂的江湖水文情势关系。为研究长江对鄱阳湖作用的变化规律,依据鄱阳湖五河以及出湖控制水文站1956—2016年水文数据,探讨长江对鄱阳湖倒灌水量的变化规律。研究结果表明:鄱阳湖五河七口控制站2003—2016年年平均径流量相比1956—2002年有所降低。鄱阳湖倒灌水量、倒灌天数均与汉口站来水和鄱阳湖入湖水量之差呈较好的正相关关系。2003年后,三峡水库的蓄水或放水在一定程度上影响了江湖作用的季节变化和鄱阳湖流域的旱涝机遇,一定程度上减少了长江对鄱阳湖的倒灌频次。通过对倒灌水量和倒灌天数的分析可知,在1960年代、1980年代和21世纪初,长江上中游来水对鄱阳湖的作用相对强烈;而在1970年代、1990年代和2010年以后,长江上中游来水对鄱阳湖作用较弱。研究成果对于认识变化条件下长江与鄱阳湖江湖关系变化规律具有重要意义。     

The natural history and fisheries ecology of Lake Chilwa,southern Malawi

Lake Chilwa produces between zero and 24,000 metric tons of fish per year, making it one of the most productive but variable lakes in Africa. The size of the lake varies seasonally and among years, sometimes drying completely. Its surrounding wetland and floodplain provide habitat for a diversity of birds and economically valuable grasses and reeds. When the lake has water, there is considerable activity on its shores and temporary fishing villages spring up. People move in and out of the lake basin in concert with these seasonal and longer term changes. This paper examines the environmental dynamics of Lake Chilwa and its surrounding wetlands, presents an overview of the socio-economic context of the area and discusses threats to this resilient system that might occur as a result of climate change. We conclude that management of Lake Chilwa must place the lake in the wider economic and ecological system in which it is situated. Ultimately, land-use practices within the basin present more of a threat to the resilience of the fishery and people's livelihoods than overfishing or a strict focus on the lake's resources. These perspectives present significant challenges to conventional fisheries governance.     

The effects of flow regulation and climatic variability on obstructed drainage and reverse flow contribution in a Northern river–lake–Delta complex,Mackenzie basin headwaters

A distinctive hydrological feature of the Lake Athabasca–Peace–Athabasca Delta (LA‐PAD) complex is that flow in channels that drain the system reverses direction when stage on the Peace River exceeds that for the central lakes. This river's hydrology has experienced natural and human induced changes since 1968. This study investigates the importance of spring break‐up and open‐water induced outflow obstruction and reverse flow contributions to annual lake level maxima under natural (1960–1967), regulated (1976–2004) and naturalized (1976–1996) flow regimes. Obstructed and reverse flow events during spring break‐up were common prior to and following flow regulation, suggesting that natural climatic variability in source areas below the W.A.C. Bennett Dam exerted a strong influence on their occurrence. Antecedent hydrological conditions, such as fall freeze‐up lake level, break‐up magnitude, peak spring flow and initial open‐water lake level were significantly associated with annual lake level maxima. During the summer period, lake level was linked to sustained high flows on the Peace River. The river obstructed outflow and contributed reverse flow to the LA‐PAD in each year prior to 1968. Following regulation, however, more than half the years did not experience any open‐water obstruction and/or reversal, and those that did were characterized by smaller events. The average estimated duration of obstruction was more than two weeks shorter and reverse flow volume was reduced by ～90% under a regulated regime compared to a simulated naturalized flow regime. This implied a lowered potential for lateral lake expansion into the delta floodplain in some years. The regulated hydrology could produce large stormflow and high lake levels, but only under extreme climatic events in areas below the dam and/or human‐induced alterations to normal reservoir operation. Copyright © 2009 Crown in the right of Canada and John Wiley & Sons, Ltd.     

Flooding and Surface Connectivity of Taxodium‐Nyssa Stands in a Southern Floodplain Forest Ecosystem

An understanding of the factors controlling the permanent and episodic links between the main stem of a river and the ecosystems of its alluvial floodplain is necessary for evaluating the influence of modern river processes on floodplain ecology and habitat diversity and for the successful implementation of flow regimes that meet human needs for water in a manner that sustains the ecological integrity of affected systems. In this study, we examined relationships between river hydrology and lateral hydrological connectivity, which is crucial to directing fluxes of water, material, and organisms into and across a floodplain. We did so by translating measures of river discharge for the Congaree River into high resolution maps of flood conditions for the floodplain at Congaree National Park using a 2D flood inundation model. Utilizing a graph network approach, we then analyzed the connectivity of a key wetland ecosystem, Taxodium‐Nyssa forested swamps, to the main stem river and to each other under different flows. Our results underscore that floodplain connectivity is initiated at sub‐bankfull discharges and does not depend on levee overtopping, while also clarifying that various sources of connectivity are triggered at different flow levels in specific reaches. Further, our findings demonstrate the sensitive and non‐linear response of floodplain connectivity to river flows and provide useful information to facilitate the management of flood processes in the Congaree River watershed. Copyright © 2014 John Wiley & Sons, Ltd.     

Impoundment Effects of the Three-Gorges-Dam on Flow Regimes in Two China’s Largest Freshwater Lakes

The impoundment of the Three-Gorges-Dam (TGD) significantly alters the flow regimes in Poyang and Dongting Lakes, the two largest freshwater lakes in China by reducing river discharge. This work aims to investigate the impoundment effects of the TGD on lake outflow and water level and clarify dynamic mechanism for the hydrological responses of both lakes to the TGD impoundment based on the typical case study. A hydrodynamic model is set up to simultaneously simulate hydrodynamics in the two lakes and the middle Yangtze River. Results show that TGD induces profound changes of the flow regimes in both lakes. The effects dynamically change with the TGD-induced Yangtze River discharge alteration, and are highly heterogeneous in space due to the complicated interactions between the lakes and the Yangtze River. The impoundment of the TGD accelerates Poyang Lake drainage. But that is not all for Dongting Lake, the TGD impoundment also reduces its water supply from the Yangtze River. The results from this work improve the understanding of the TGD impacts on the downstream river-lake system and provide scientific evidences for formulating better scheme for water resources management in this region.     

Impact of short‐term regulation on hyporheic water quality in a boreal river

Water regulation may alter hydraulic head gradients with consequences for the exchange of water between the river and the hyporheic zone. The objective of this study was to investigate the effect of discharge on hyporheic water quality in a regulated Swedish boreal river during a 10‐day experimental period with a sequence of alternating high‐ and low‐flow episodes. A 250 m reach was instrumented with 28 piezometers placed at 150 and 300 mm below the river bed or below the mean groundwater level in the floodplain, and these piezometers were used to measure temperature, oxygen, electric conductivity and pH. High daily variation in air temperature during the first 3 days was transmitted vertically through the stream water into the hyporheic zone within hours. An oxygen saturation of 100% in the river water corresponded to 60–70% saturation at 150 mm depth and 30% at 300 mm depth. The hyporheic oxygen concentration at 150 mm depth decreased during the experimental period, falling into a range that is potentially harmful to incubating salmonid eggs. This was interpreted as a long‐term response to the overall regulation regime, rather than a response to short‐term water regulation during the experiment. Even though the effect of short‐term regulation on the quality of hyporheic water in the river bed was limited, there was a more pronounced effect on the quality of floodplain hyporheic water. Most of the driving forces for temporal variation of water quality in the river bed came vertically from the river water, rather than from the lateral exchange. Copyright © 2008 John Wiley & Sons, Ltd.