Thermobaric stratification in very deep Norwegian freshwater lakes

Thermobaric stratification was investigated in very deep lakes of elongated shape. We considered the spring time configuration when large freshwater lakes may have their deepest recirculation. In five very deep freshwater lakes in Norway, profiles were measured in spring 2006 with multiparameter probes. Temperature profiles confirmed thermobaric stratification in all of these lakes, i.e. the temperature profiles crossed the 4 °C line and thus were affected by pressure effects on the temperature of maximum density. Electrical conductance was low in all of the investigated lakes, and vertical gradients were too small to play a role in the density stratification. In all lakes, deep water was well supplied with oxygen. Three out of five lakes showed a temperature gradient close to Eklund's stability theorem. However, the remaining two lakes clearly opposed the theorem. Bottom temperatures were more closely related to lake length than their maximum depths. Oxygen saturation profiles documented recent intensive gas exchange with the atmosphere. They agree well with the possible formation of deep water from density driven flows at horizontal temperature gradients close to 4 °C.     

Review of the natural conditions and anthropogenic threats to the Ethiopian Rift Valley rivers and lakes

This review article considers the lakes found within the Ethiopian Rift Valley system and the rivers draining from the highlands towards them. The Ethiopian Rift Valley system extends in a north‐east–south‐west direction, bisecting the Ethiopian highlands. They differ in sizes and hydrological and hydrogeological settings. The lakes support a variety of flora and fauna, mainly fishes, reptiles, birds and mammals. The lakes and their feeder rivers, however, are currently being subjected to more human use pressure than previously experienced, attributable mainly to water abstractions for irrigation, household water supply and industrial activities in the catchments. Soil erosion, primarily attributable to the lack of soil conservation practices on farm lands, overgrazing and deforestation, along with unregulated irrigation practices, are generating large quantities of silt reaching the lakes. Improper utilization of water and land resources in the catchments, along with population growth and climate change impacts, is escalating the threats to the health of the lake and river ecosystems. As a result, some of the lakes exhibited reduced water levels and increased salinity. This article reviews the major environmental changes happening to these lakes and their feeder rivers. Visits were made to the area to update previous information while also consolidating the trends of change in the aquatic systems and the watersheds attributed to intense development practices.     

三峡水库蓄水期长江中游湖泊调蓄能力变化

长江中游通江湖泊的调蓄能力与整个区域的水安全密切相关。在对洞庭湖、鄱阳湖(下称“两湖”)调蓄机理探讨的基础上,揭示了两湖泊调蓄的临界状态及临界条件,分析了三峡水库蓄水期两湖泊调蓄能力的变化。研究结果表明:洞庭湖与鄱阳湖具有相同的调蓄机理,其调蓄能力变化受入湖流量和长江干流顶托的双重影响,湖泊调蓄过程表现为入湖与出湖水量的动态调整,湖泊调蓄量不断向0趋近。三峡水库运用后,两湖泊9—10月份调蓄能力下降明显,与2003—2007年相比,2008—2014年两湖汛后调蓄水量的减小更大。     

Assessing the Relationship Between River Mobility and Habitat

Human interventions that limit channel mobility such as bank stabilization are frequent in riparian zones in urban or agricultural environments. This is potentially problematic because channel mobility is an important geomorphological and ecological agent that structures natural instream and riparian ecosystems. This study aims to (1) quantify the relationship between mobility and three types of habitat‐related features, namely bars, oxbow lakes and log jams, for a 54‐km‐long reach of the Yamaska Sud‐Est River (Quebec, Canada), which runs from the forested Appalachian Mountains to the agricultural St‐Lawrence lowlands, and (2) evaluate the impact of human interventions and geomorphological characteristics on these three features. Channel mobility was measured from historical aerial photos for the period 1950–2009. A combination of high‐resolution aerial photos, LiDAR digital elevation model, and field observations was used to measure and map sediment bars, oxbow lakes and log jams, as well as several geomorphological characteristics (channel width, slope, sinuosity and floodplain width). A strong link between the mobility and the presence of habitat features is revealed, but local geomorphological contexts result in different mobility patterns responsible for specific habitats. Floodplain to channel width ratio appears as the best geomorphological factor predicting habitat diversity. Human intervention, mostly through bank stabilization, also appears to be a key factor limiting mobility and its related habitats. These results highlight the importance of defining a protected mobility corridor along rivers where geomorphic processes such as bank erosion can freely occur, as it is an essential process that should be integrated in land use planning and river management. Copyright © 2015 John Wiley & Sons, Ltd.     

我国湖泊水环境问题与水生态系统修复的探讨

随着全球气候变暖和人类对湖泊资源的不合理开发利用及污染物的大量排放,我国湖泊出现面积萎缩与干涸,富营养化加剧,淤塞严重,水生态系统退化,生物多样性受损等问题。针对湖泊水环境问题,总结了湖泊生态系统修复的实践,从截断外源污染和控制内源污染两方面提出了修复措施,并进一步给出了修复的对策与建议。     

A numerical tool to predict ice growth in lakes

Oil and gas exploration activities on the North of Slope of Alaska rely on ice roads to provide access to the exploration sites, with negligible effects on the tundra. The construction of ice roads requires a large volume of water which, in turn, poses challenges to water resource managers. A tool capable of predicting ice thickness in a given lake could certainly benefit the oil industry. This study focuses on the development and use of such a predictive tool. Using a simplified Stefan equation, the tool is based on a combination of three weather scenarios (warm, average and cold winters), current winter condition and ice thickness measurements. It was applied to four lakes located on the Arctic Coastal Plain and one lake in the foothill area. The study results indicate the model predictions are relatively accurate (7% average percentage error) for monthly time spans. The study results also reveal a temporal variation in a coefficient used in the calculations, although previous literature utilized a constant coefficient to estimate the ice thickness.     

Secondary production and biomass of Cladocera in marginal lakes after the recovery of their hydrologic connectivity in a river–reservoir transition zone

Secondary production and biomass of Cladoceran populations were studied in two marginal lakes whose connections to a tropical river were re‐established after a prolonged drought period. This study investigates the process of the recovery of the cladoceran populations after the hydrologic reconnection with the river was established. It also compares the energetic dynamic of Cladoceran populations in two lakes, one with a wide and another with a narrow connection to the river. Samples were collected every 48 h for 1 month, immediately after the river water inflow to the lakes, and 7 months after the reconnection with the river during a period of hydrologic stability of the lakes. Secondary production and biomass were compared between lakes and periods, to identify the main controlling factors for the observed variations in Cladoceran productivity. Cladocerans were more productive during the rainy period, immediately after the increased water volume, when high water temperatures were recorded in the two lacustrine systems. Secondary production and biomass values were higher in the more eutrophic lake, wherein the water chemical and physical characteristics presented slow alterations during the rainy period. Although the lake with large spatial heterogeneity and linkages with the river exhibited high Cladoceran richness, the secondary production and biomass were low. Variations in water volumes and temperatures and changes in trophic status affected the secondary production and biomass of the main Cladoceran species in the two studied lakes.     

Water use (and abuse) and its effects on the crater‐lakes of Valle de Santiago,Mexico

Most Mexicans live in the arid and semiarid regions that represent two‐thirds of the Mexican territory, where water is scarce. Natural, as well as human, causes are favouring the degradation of Mexican lakes. There is a clear need to develop and implement sustainable water‐use programmes at a catchment scale. However, the accelerated degradation rate of the Mexican lakes means that there will not be enough time to perform whole‐basin evaluations to establish sustainable water‐use programmes before the lakes dry up. The case of the Valle de Santiago crater‐lakes clearly illustrates the declining trend that Mexican inland aquatic resources follow. Vegetation clearance, overgrazing, abatement of phreatic waters and salinization have induced severe erosion and overall desertification (land degradation) in the basin for what, it seems, a long time (i.e. prehispanic times). In this way, human activities could be provoking at least the following negative consequences: a hotter and drier local climate, water scarcity, dust storms and soil salinization. The aquatic (surface and groundwater) resources of the Valle de Santiago basin have been seriously threatened. Two of the four crater‐lakes have already dried up and phreatic mantle abatement reaches up to 2.5 m per year. In spite of these facts, no sustainable water‐use programme has been established yet. The future scenery of this Mexican basin looks alarmingly like many other basins in the central and northern Mexican territories.     

Prediction of regulation effects on natural biological rhythms in south-central African freshwater fish

Fish in south-central African freshwaters are governed by biological rhythms. First, these are associated with the markedly seasonal summer annual rainfall when breeding takes place with an initially high mortality among newspawned juveniles. Second, a longer term rhythm of annual rainfall variability occurs in cycles up to or more than a decade. These cause water levels to vary from exceptionally high levels to near desiccation in endorheic systems. Studies of the shallow endorheic lakes Mweru Wa'Ntipa (Zambia), Ngami (Botswana), and Chilwa (Malawi) are cited showing that species diversity rapidly increases by colonization from adjacent ‘inocula’ (relict communities surviving in more permanent nearby water bodies, e.g. deep pools in perennial streams) when filling starts. During the drydown phase of the cycle, diversity decreases through lack of annual breeding environment and diminution of water quantity and quality. A few, usually three, species remain longer which, being physiologically hardy or showing phenotypic plasticity, are tolerant of higher salinity, temperature, etc., conditions associated with dry-out. Despite the lakes' wide geographical separation these taxa in every case are a single species of the tilapia, genus Oreochromis, the catfish Clarias gariepinus, and the minnow Barbus paludinosus, which build up large populations valuable to local fisheries. Evidence from the Pongolo River, Zululand, regulated by a large dam, shows that fish of downstream endorheic lakes react similarly to artificial water deprivation, with mainly the same taxa persisting. Managed water releases timed to coincide with breeding rhythms largely restore the conditions which previously prevailed. Data from Zambezi impoundments indicate conversely that reaction of previously riverine fish to large man-made lakes is for previously sparse lentic taxa like Oreochromis to proliferate greatly while lotic forms like Labeo are reduced in number. Since fish stocks are a valuable natural resource, such basic scientific results may be applied with great advantage. The collaboration between engineer, biologist, and management authority can minimize adverse environmental effects and predict better opportunity cost assessments of a projected river manipulation.     

近40年来中国西北内陆5个典型湖泊面积变化遥感分析

在全球气候变暖的背景下,研究中国西北内陆地区湖泊的面积变化不仅对水资源管理和可持续发展战略有着重要意义,也为评价气候变化与人类活动对湖泊的影响提供参考。以中国西北内陆区5个典型湖泊为研究对象,利用1970-2015年的Landsat MSS/TM/ETM+/OLI影像数据,利用归一化差异水体指数(Normalized Difference Water Index,NDWI)和改进型归一化差异水体指数(Modified Normalized Difference Water Index,MNDWI)分别提取5个典型湖泊边界并获取湖泊面积,同时利用5个典型湖泊周边的气象站降雨量、蒸发量等气象数据,尝试分析湖泊面积发生变化的原因。结果表明:红碱淖近30年来面积呈减小趋势,入湖河流拦蓄和煤炭地下开采等是导致面积减少的原因;博斯腾湖近40年来的面积呈先增加后减少的趋势,其面积变化主要受地表径流和积雪融水的影响;呼伦湖近40年来面积呈减少的趋势,其面积变化主要受入湖径流量的影响;贝尔湖近30年来呈相对稳定的趋势,其面积变化主要入湖径流量的影响;青海湖近40年来面积呈现先减小后增大的趋势,其面积变化主要受区域降水和入湖径流量的影响。