Paleolimnology of mid-elevation lakes in the Kenya Rift Valley

Three small lakes of the Kenya Highlands have been subject to intermittent paleoenvironmental investigation for more than 50 years. In this report, the stratigraphic, core-based history of Lake Naivasha (elev. 1 890 m) is extended backward to 20 000 B.P. and is related to less complete 30 000-year core stratigraphies from nearby lakes Nakuru and Elmenteita. The paleolimnological reconstructions, primarily diatom-based, include geochemical parameters and the first ostracode stratigraphies from tropical Africa. The analyses from all three lakes converge on a history of low and fluctuating water levels over approximately the past 4 500 years, preceded by a major Holocene high stand (ca. 10 000-6 000 B.P.) and a brief earlier rise centered on 12 500 B.P. The lakes were mostly very low during the late Pleistocene (13 000–30 000 B.P.), though modestly elevated levels may have occurred early in this time period. The ostracode record, comprising at least 15 species, was relatively continuous in the Nakuru core but was restricted to sharply demarcated regions in cores from the other lakes.     

Preservation and Paleoenvironmental Significance of a Footprinted Surface on the Sandai Plain,Lake Bogoria,Kenya Rift Valley

An exhumed late Pleistocene land surface on the deltaic Sandai Plain north of Lake Bogoria, Kenya, preserves traces of bovids, suids, birds, and at least one hominid. The host sediments (Loboi Silts) are reddish brown, poorly bedded siltstones, mudstones and silty sandstones that were probably deposited in a shallow closed-basin lake. Most of the prints were impressed on exposed, moist lake-marginal mudflats. Print distribution is patchy due to a complex interaction between biogenic and sedimentological factors. The preservation of a single hominid track provides a fortuitous addition to the sparse hominid track record in East Africa. Field, petrographic, and mineralogical analyses of the fossil substrate were undertaken to determine how the footprinted surface was preserved. Comparison with modern lake-marginal processes suggests that the prints were initially stabilized by desiccation, soil-crusting, and organic films, followed by cementation of the surface sediments by calcite and analcime, with minor authigenic clay minerals and Fe-Mn-oxihydroxides. The zeolites formed by reaction of detrital silicates with saline, alkaline groundwater; calcite was precipitated from dilute runoff and fresher groundwaters. Cementation likely occurred during a prolonged period of relatively low, stable lake level. Following cementation, the surface was buried by Holocene lake sediments, then recently exhumed.     

Leaf litter transport and retention in a tropical Rift Valley stream: an experimental approach

The impact of a drought on freshwater snail and trematode communities was investigated in a lake. Before the drought, 15 gastropod species (Valvatidae, Planorbidae, Lymnaeidae, Ancylidae, Physidae) and 10 trematode species (cercariaeum, xiphidiocercariae, echinostome, furcocercariae, notocotyle, lophocercous) were recorded. The rate of parasitism was 5.13% and there were 11 host species. The 2 major consequences of desiccation were the disappearance of snails, except Valvata piscinalis and Lymnaea peregra, and the absence of trematodes infecting the surviving snails. As soon as favourable conditions were restored, the littoral area was recolonized, first by hygrophilic and amphibious species, second by aquatic species. Nine months after the drought, the gastropod community was restored. Recolonization by the trematodes was delayed compared with that of gastropods. During the study, the overall prevalence was equal to 0.36% and only 4 trematode species and 5 host species were recorded. Because of the great variability of freshwater ecosystems, long-term studies are necessary to understand the dynamics of snail and trematode populations and determine the regulatory effect of parasitism in the field.     

Disturbance of a Kenya Rift Valley stream by the daily activities of local people and their livestock

Human and domestic animal activities and visits to an impacted site in the Njoro River, Kenya, were recorded from 1994 to 1995. The activities of people in the humid and wet zones of the stream included linen washing, water abstraction, excretion, bathing and swimming. Human and animal visits and activities along the Njoro River were on a daily basis and were patterned according to the time of the day, weather and seasons. Women formed the first group to visit the stream at dawn, followed by men and lastly, children. The diurnal pattern of visits was bimodal, with major peaks between 0600 and 1100 h and from 1600 h until dusk. The intensity of all major activities peaked at around midday and donkeys and cattle formed the largest proportion of the domestic animals that visited the stream. Much of the small-scale water abstraction occurred between 0700 and 1100 h, coinciding with the time when most people visited the stream. Water abstraction was most intense during the dry season. A mean discharge of 1.7 litres per second was measured whilst 0.3 litre of water per second was abstracted at the impacted site, implying that about 20% of the flow volume of the Njoro River was abstracted at a single site in one day.The effects of the human and animal activities on the structure of macrozoobenthos in the wet zone of the Njoro River were studied on the impacted site in relation to an upstream reference site and a downstream reference site. Oligochaetes and chironomids dominated the fauna in all three sites. The impacted site had low patchiness and mean crowding, with the taxa distribution tending toward a randomly dispersed spatial pattern. The mean turnover (±SD) of the macroinvertebrates was 47±18, 48±26 and 36±22 in the impacted, downstream and upstream sites, respectively. The trampling of the streambed by humans and livestock could, therefore, alter the benthos structure through redistribution and reduction of faunal patchiness. The cumulative effect of small-scale, but widespread and frequent disturbances might have large impacts on whole river systems. This study demonstrated that, in the tropics, quantification of the daily activities of people and domestic animals is important for future management of the Njoro River and consequently, Lake Nakuru.     

Hydrology and history: land use changes and ecological responses in an urban wetland

The impacts of changing land use on hydrology and dominant plant species from 1850–1990 were investigated in a palustrine wetland in southern Wisconsin, USA. Aerial photographs, historic maps and water levels of the area were used to determine changes in land use, wetland vegetation, and groundwater and surface flows over time. Piezometers and water table wells were monitored weekly for two years. Vegetation was quantified in four one-square meter quadrats at each water level measurement site. Linear regression models and multivariate ordinations were used to relate wetland plant species to hydrologic, chemical and spatial variables. The current hydrologic budget of the wetland was dominated by precipitation and evapotranspiration, although overland flow into the wetland from the subwatershed has increased twenty-fold since 1850. Water level stabilization in the adjacent Yahara River, creek channelization, and groundwater pumping have decreased inputs of groundwater and spring-fed surface water, and increased retention of precipitation. Typha spp. and Phalaris arundinacea L. have increased in the wetland, while Carex spp. have decreased. Phalaris arundinacea was found most often in the driest sites, and the sites with the greatest range of water levels. Typha spp. dominated in several hydrologic settings, indicating that water depth was not the only factor controlling its distribution. The distributions of dominant plant species in the wetland were most closely correlated with site elevation and average water levels, with some weaker correlations with vertical groundwater inflows and specific conductance.     

Species composition,distribution and seasonal dynamics of Rotifera in a Rift Valley lake in Ethiopia (Lake Awasa)

The species composition of rotifers in Lake Awasa was studied and 40 species recorded. Seven species appeared in large numbers in the plankton seasonally. Brachionus and Keratella species made up more than 50% of the rotifer community by numbers during the observation period (1983–1987). The standing stock numbers of rotifer species are low with a maximum of 50 individuals L^–1, and some possible reasons for this observation are discussed. Most rotifer species are distributed randomly in the lake and show 3-fold fluctuations between consecutive days.The seasonal dynamics of most rotifer species are correlated with mixing periods in the lake, and the amplitude of seasonal fluctuation may be as high as 50-fold. Clear-cut seasonal succession of rotifer species was observed during the study period, but no consistent seasonal pattern for individual species was obvious. Also, observations indicate that rotifer biomass is partly sustained by availability of carbon through the bacterial pathway and that competitive exclusion for food by nauplii and ciliates probably keeps rotifer abundance low.     

Ecohydrology and the Partitioning AET Between Transpiration and Evaporation in a Semiarid Steppe

Water availability defines and is the most frequent control on processes in arid and semiarid ecosystems. Despite widespread recognition of the importance of water in dry areas, knowledge about key processes in the water balance is surprisingly limited. How water is partitioned between evaporation and transpiration is an area about which ecosystem ecologists have almost no information. We used a daily time step soil water model and 39 years of data to describe the ecohydrology of a shortgrass steppe and investigate how manipulation of soil and vegetation variables influenced the partitioning of water loss between evaporation and transpiration. Our results emphasize the overwhelming importance of two environmental factors in influencing water balance processes in the semiarid shortgrass steppe; high and relatively constant evaporative demand of the atmosphere and a low and highly variable precipitation regime. These factors explain the temporal dominance of dry soil. Annually and during the growing season 60–80% of the days have soil water potentials less than or equal to −1.5 MPa. In the 0–15 cm layer, evaporation accounts for half of total water loss and at 15–30 cm it accounts for one third of the loss. Annual transpiration/actual evapotranspiration (T/AET) ranged from 0.4–0.75 with a mean of 0.51. The key controls on both T/AET and evaporation/actual evapotranspiration in order of their importance were aboveground biomass, seasonality of biomass, soil texture, and precipitation. High amounts of biomass and late timing of the peak resulted in the highest values of T/AET.     

New studies of post-Pleistocene human skeletal remains from the Rift Valley,Kenya

Prehistoric human crania from Bromhead's Site, Willey's Kopje, Makalia Burial Site, Nakuru, and other localities in the Eastern Rift Valley of Kenya are reassessed using measurements and a multivariate statistical approach. Materials available for comparison include series of Bushman and Hottentot crania, South and East African Negroes, and Egyptians. Up to 34 cranial measurements taken on these series are utilized to construct three multiple discriminant frameworks, each of which can assign modern individuals to a correct group with considerable accuracy. When the prehistoric crania are classified with the help of these discriminants, results indicate that several of the skulls are best grouped with modern Negroes. This is especially clear in the case of individuals from Bromhead's Site, Willey's Kopje, and Nakuru, and the evidence hardly suggests post-Pleistocene domination of the Rift and surrounding territory by “Mediterranean” Caucasoids, as has been claimed. Recent linguistic and archaeological findings are also reviewed, and these seem to support application of the term Nilotic Negro to the early Rift populations.     

土地利用/覆被变化扎龙湿地蒸散发量及生态需水量的遥感估算

地表蒸散发量是影响湿地水热平衡的主要因素,也是水分损失的主要途径,对湿地生态需水量的合理确定和水资源的有效管理具有重要意义。借助遥感和GIS技术,利用2002、2010、2016年34景Landsat影像,基于时序NDVI数据对湿地下垫面物候特征的定量表征,准确获取了扎龙湿地保护区3个时期土地利用/覆被的动态变化信息。选用物理基础较好且应用广泛的SEBAL模型,估算了湿地的瞬时蒸散发量,并结合站点气象数据,实现了湿地蒸散发量在时间尺度上的扩展,分别得到日、月、年尺度的湿地蒸散发量,深入探究了扎龙湿地蒸散发的时空分布特征;最后从湿地湖泡需水量、植物需水量及生物栖息地需水量3个方面,定量估算出扎龙湿地3个时期的现状生态需水量。研究发现：扎龙湿地的土地覆被类型主要以芦苇沼泽、草地和耕地为主,其中芦苇沼泽分布占绝对优势,且2002-2016年持续增加了205.82 km²;草地、耕地呈持续减少态势,分别减少了119.35 km²和95.96km²,表明2002-2016年扎龙湿地生态系统呈恢复态势;湿地保护区蒸散发量年内均大致呈单峰型分布,符合夏季 > 春季 > 秋季 > 冬季的规律,在年际上呈现明显的递增趋势,年蒸散发量由2002年的518.87mm增加到2016年的625.98mm,增加了20.64%。为满足湿地内的生态消耗,总体上2002-2016年湿地的生态需水量也相应的增加,保护区适宜生态需水量的变动范围为5.40亿-7.08亿m³,可以维持湿地湖泊、植被、动植物栖息地的健康态势。维持核心区健康状态的最小生态需水量变化范围为2.71亿-3.32亿m³。随着遥感数据时空分辨率的提高,基于蒸散发量反演的湿地生态需水量估算将更加实用和准确,为湿地保护区制定科学合理的补水方案提供有效的技术支撑。     

鄱阳湖典型湿地土壤微生物活性对季节性水位变化的响应

为探究湿地土壤微生物对季节性水位变化的响应关系,以鄱阳湖湿地土壤为研究对象,在2014年3、6、10及2015年1月4个季节采集了3个不同高程样带的土壤样品,对土壤微生物基础呼吸、生物量及胞外酶等活性进行了测定。研究结果表明:(1)季节性水位变化不仅显著改变了土壤有机碳、溶解性有机碳、有效磷等含量,也使得微生物量碳和4种水解酶β-葡萄糖苷酶、β-木糖苷酶、N-乙酰氨基葡萄糖苷酶、磷酸酶活性表现出夏冬季较高、秋季最低的动态变化,而2种氧化酶酚氧化酶和过氧化氢酶的表现正好相反。(2)水位高程和地上植被类型同样对土壤微生物产生了显著影响,表现为南荻样带有较高营养元素含量和微生物活性。(3)一些土壤理化指标(含水量、铵态氮、有机碳、有效磷等)与微生物活性(微生物量、基础呼吸、酶活)显著相关;季节水位变化对微生物活性的影响大于高程差异。研究结果表明水位波动对湿地土壤微生物活性产生了重要影响,鄱阳湖水文节律的改变将影响到湿地土壤的正常生态功能。     

千岛湖生态保护与建设对景观格局的影响研究

生态保护和生态建设过程对景观格局产生影响并使之发生变化,这种变化又会影响景观的生态过程。应用3s技术和景观指数分析了39年(1964-2003年)内千岛湖库区土地利用和土地覆盖格局的变化。结果显示,斑块总数和景观破碎度基本保持不变,但是各斑块类型的斑块数量、面积和优势度变化非常显著。最优势斑块类型由荒山(占陆地面积的47．44％)演变为马尾松( Pinus massoniana)林(占陆地面积的52．8l％),研究区域内森林植被覆盖率增加。研究还发现景观多样性指数略微下降。景观多样性是对土地利用和覆盖类型丰富度和均匀度的概要度量,并不一定总是和景观功能呈正相关关系。上述变化归因于多项生态保护措施的实施使得自然演替得以实现。景观格局的优化使生态系统的生态功能得到提高,包括生境恢复、生物多样性增加、水土流失减少。森林植被恢复以后,马尾松林斑块的单一化趋势应在今后的生物多样性保护、生态规划和可持续发展中得到重视。     

Temporal distribution of phytoplankton in Lake Nyamusingiri in the Albertine Rift Valley,Uganda

A study aimed at investigating the temporal variation of phytoplankton assemblages in Lake Nyamusingiri was carried out during the period of December 1997–May 1998. Uganda’s freshwaters are ecologically diverse but a few are intensively studied. Research on phytoplankton has been restricted to large water bodies. There is little information on phytoplankton of the western Uganda crater lakes, which are important water and biodiversity resources. This study provided baseline data on phytoplankton, which will serve as a basis for monitoring the effects of human activities on the lake that might result in ecological transformations like loss of biodiversity because of overexploitation. A laboratory thermometer and Winker’s method were used to determine temperature and dissolved oxygen concentration, respectively. Lake transparency was measured by using the Secchi disc. A Van Dorn sampler was used to collect water samples. Nutrient and chlorophyll a concentrations were determined by using facilities at the Fisheries Resources Research Institute (FIRRI), Jinja. The Sedgwick‐Rafter counting chamber was used to analyse phytoplankton. Variation in temperature was small (25.4–26.2°C). Stable thermal stratification was not evident. The Secchi disc transparency was less than unity. The chlorophyll a value was high. Biomass was found to be light‐limited by nonalgal materials. Dissolved oxygen concentration was more than 100% in the surface waters but declined to <20% at the bottom, which reflected the eutrophic nature of the lake. Diversity indices were low. Eighteen species and five classes of phytoplankton were revealed by this study. The phytoplankton flora was dominated by chlorococcal green algae characteristic of the large eutrophic East African lakes.     

Predicted future distribution of the African skimmer in response to a changing climate,land cover and distance from water in the mid-Zambezi Valley

Climate models project a hot and dry future for Southern Africa. In this research, Maximum Entropy was used to model the extent to which climate change, land cover and distance from water edges may influence current and future distribution of the African skimmer in the mid-Zambezi Valley. Global Biodiversity Information Facility data collected between the years 2000–2019 were used to develop the models. Three models were built: one for current distribution and two for future distribution under Representative Concentration Pathways (RCPs) 2.6 and 6.0. Results revealed that annual precipitation and distance from water edges were the most important predictors of habitat suitability for the African skimmer under current and future climate. Temperature and land cover were least important in explaining current and future distribution of the species. The RCP 2.6 predicted future decrease in suitable habitat for the African skimmer in the mid-Zambezi Valley, while RCP 6.0 predicted future increase in suitable habitat for the species. This research conclusively revealed that precipitation and distance from water edges were consistently key predictors of suitable habitat for the African skimmer.     

基于遥感生态指数的新疆玛纳斯湖湿地生态变化评价

以2000、2006和2016年3期的Landsat遥感影像为基础数据源,结合前人研究成果和实地考察,借助RSEI指数,对玛纳斯湖湿地生态环境进行监测和评价。结果表明:利用主成分分析技术集成植被指数、湿度分量、地表温度和土壤指数建立的RSEI指数具有一定的适用性,可较好的对玛纳斯湖湿地生态环境质量状况及其时空变化进行监测和评价。2000、2006和2016年RSEI指数均值分别为0.227、0.183、0.234,对RSEI指数进行分级处理后,发现流域生态环境质量"较差"等级居于主导地位,"优"等级有所增加,湿地生态环境质量向好的方向发展。气候变化和人类活动共同作用于玛纳斯湖湿地产生的生态环境效应,日益增强的人类活动是湿地退化的主要原因,多年来粗放型的农业发展使玛纳斯河流域人口、经济与生态环境之间的关系严重失调。从优化水资源配置角度提出对主要源流流域水土资源大规模开发的同时,应重视尾闾湖泊湿地的生态价值与可持续发展。     

A new relictual buthid scorpion from the region of the Great Rift Valley in Kenya (Scorpiones,Buthidae)

A new genus and species of peri-Saharian buthid scorpion is described on the basis of single specimen collected in the Great Rift Valley, North of Kenya. This new scorpion taxon represents yet another endemic relict element for the faunas of the peri-Saharian regions. Comments are also included on the evolution of the Sahara and peri-Saharian regions and its possible consequences on the distribution of the extant scorpion fauna.     

近40年来洞庭湖流域土地利用及生态风险时空演变分析

以洞庭湖流域为研究对象,利用1980年、1990年、2000年、2010年和2018年5个时期的土地利用数据,在定量分析近40年流域土地利用动态特征的基础上,从景观生态学视角构建生态风险评价模型,对1980-2018年洞庭湖流域生态风险进行评价,并进一步揭示其时空格局演变特征。结果表明：（1）1980年以来,洞庭湖流域土地利用类型以林地和耕地为主。洞庭湖流域土地利用格局发生较大变化,其显著特征为建设用地不断扩张、林地基本稳定、耕地日益萎缩和水域呈扩大趋势。（2）近40年来,洞庭湖流域生态环境较为良好,以较低和中等生态风险区为主导类型。在1980-1990年、1990-2010年和2010-2018年3个时段内,生态风险呈现增长、缓和、加剧的变化过程,高风险区以洞庭湖湖盆向环洞庭湖平原扩张,洞庭湖区、湘江流域和资水流域生态风险等级明显高于其他地区,而沅水流域、澧水流域和湘江流域的东南部生态风险相对较小。     

Browsing herbivores improve the state and functioning of savannas: A model assessment of alternative land‐use strategies

Changing climatic conditions and unsustainable land use are major threats to savannas worldwide. Historically, many African savannas were used intensively for livestock grazing, which contributed to widespread patterns of bush encroachment across savanna systems. To reverse bush encroachment, it has been proposed to change the cattle‐dominated land use to one dominated by comparatively specialized browsers and usually native herbivores. However, the consequences for ecosystem properties and processes remain largely unclear. We used the ecohydrological, spatially explicit model EcoHyD to assess the impacts of two contrasting, herbivore land‐use strategies on a Namibian savanna: grazer‐ versus browser‐dominated herbivore communities. We varied the densities of grazers and browsers and determined the resulting composition and diversity of the plant community, total vegetation cover, soil moisture, and water use by plants. Our results showed that plant types that are less palatable to herbivores were best adapted to grazing or browsing animals in all simulated densities. Also, plant types that had a competitive advantage under limited water availability were among the dominant ones irrespective of land‐use scenario. Overall, the results were in line with our expectations: under high grazer densities, we found heavy bush encroachment and the loss of the perennial grass matrix. Importantly, regardless of the density of browsers, grass cover and plant functional diversity were significantly higher in browsing scenarios. Browsing herbivores increased grass cover, and the higher total cover in turn improved water uptake by plants overall. We concluded that, in contrast to grazing‐dominated land‐use strategies, land‐use strategies dominated by browsing herbivores, even at high herbivore densities, sustain diverse vegetation communities with high cover of perennial grasses, resulting in lower erosion risk and bolstering ecosystem services.     

近20年疏勒河流域生态承载力和生态需水研究

水资源是一切生物赖以生存和不可替代的基本自然资源,生态需水在维持流域生态系统平衡和生态承载力可持续性方面扮演着极其重要的角色,干旱区内陆河流域尤为突出。以疏勒河流域和其所辖县区为不同尺度区域,利用LandsatTM/ETM+/OLI遥感数据(30 m分辨率),解译该流域近20年5期土地利用数据,同时在收集和整理流域多年水文水资源基础数据的基础上,以流域生态需水为研究主线,运用多学科方法和原理,结合遥感技术、GIS技术,通过现场调查和观测,计算了流域及其所辖县区近20年生态承载力和天然植被生态需水量。结果表明:近20年来,伴随流域生态承载力的增加,生态需水量也呈增加趋势,两者呈非常明显的正相关关系,相关系数达0.6076;县域尺度上,生态需水与生态承载力正相关关系也较高,其中林、草地的生态需水与生态承载力拟合优度R~2分别达0.8519、0.7235,说明林、草地生态承载力的变化对生态需水变化的解释能力更强,二者之间的关系更为紧密;基于空间热点分析,该流域生态承载力和生态需水的热点和冷点区域均呈现相似的空间格局,说明二者之间在空间尺度上也呈正相关关系。研究结论可为疏勒河流域生态水资源量的科学配置和调控提供重要的决策依据。     

Land-use change and water losses: the case of grassland afforestation across a soil textural gradient in central Argentina

Vegetation changes, particularly those involving transitions between tree‐ and grass‐dominated covers, often modify evaporative water losses as a result of plant‐mediated shifts in moisture access and demand. Massive afforestation of native grasslands, particularly important in the Southern Hemisphere, may have strong yet poorly quantified effects on the hydrological cycle. We explored water use patterns in Eucalyptus grandis plantations and the native humid grasslands that they replace in Central Argentina. In order to uncover the interactive effects that land cover type, soil texture and climate variability may have on evaporative water losses and water use efficiency, we estimated daily evapotranspiration (ET) in 117 tree plantations and grasslands plots across a soil textural gradient (clay‐textured Vertisols to sandy‐textured Entisols) using radiometric information from seven Landsat scenes, existing timber productions records, and ¹³C measurements in tree stems. Tree plantations had cooler surface temperatures (?5°C on average) and evaporated more water (+80% on average) than grasslands at all times and across all sites. Absolute ET differences between grasslands and plantations ranged from ～0.6 to 2 mm day^?1 and annual up‐scaling suggested values of ～630 and ～1150 mm yr^?1 for each vegetation type, respectively. The temporal variability of ET was significantly lower in plantations compared with grasslands (coefficient of variation 36% vs. 49%). Daily ET increased as the water balance became more positive (accumulated balance for previous 18 days) with a saturation response in grassland vs. a continuous linear increase in plantations, suggesting lower ecophysiological limits to water loss in tree canopies compared with the native vegetation. Plantation ET was more strongly affected by soil texture than grassland ET and peaked in coarse textured sites followed by medium and fine textured sites. Timber productivity as well as ¹³C concentration in stems peaked in medium textured sites, indicating lower water use efficiency on extreme textures and suggesting that water limitation was not responsible for productivity declines towards finer and coarser soils. Our study highlighted the key role that vegetation type plays on evapotranspiration and, therefore, in the hydrological cycle. Considering that tree plantations may continue their expansion over grasslands, problematic changes in water management and, perhaps, in local climate can develop from the higher evaporative water losses of tree plantations.