Seasonal patterns of pollen sedimentation in Lake Montcortès (Central Pyrenees) and potential applications to high-resolution paleoecology: a 2-year pilot study

Lakes with varved sediments are especially well suited for paleoecological study, from annual to even seasonal resolution. The interpretative power of such high-resolution paleoenvironmental reconstructions relies on the availability of modern analogs with the same temporal resolution. We studied seasonal pollen sedimentation in varved Lake Montcortès, Central Pyrenees (Spain), as a modern analog for high-resolution reconstruction of Late Holocene vegetation and landscape dynamics. Seasonal samples were obtained from sediment traps that were submerged near the maximum water depth for a 2-year period (fall 2013 to fall 2015). Seasonal pollen sedimentation was compared with meteorological variables from a nearby weather station. Bulk pollen sedimentation, dominated by Pinus (pine) and Quercus (oak), followed a clear seasonal pattern that peaked during the spring/summer, coinciding with maximum temperature and precipitation, minimum relative humidity and moderate winds from the SSE. Pollen sedimentation lags (PSL) were observed for most pollen types, as substantial amounts of pollen were found in the traps outside of their respective flowering seasons. Two pollen assemblages were clearly differentiated by their taxonomic composition, corresponding to spring/summer and fall/winter. This pattern is consistent with existing interpretation of the sediment varves, specifically, that varves are formed by two-layer couplets that represent the same seasonality as pollen. We concluded that pollen sedimentation in Lake Montcortès exhibits a strong seasonal signal in the quantity of pollen, the taxonomic composition of the pollen assembalges, and relationships between the pollen and meteorological variables. Thus, varved sediments provide a potentially powerful tool for paleoecological reconstruction at seasonal resolution. This method could be used not only to identify paleoenvironmental trends, but also to identify annual layers and therefore date sediments, even in the absence of evident sediment laminations. A satisfactory explanation of PSL will require further studies that examine internal lake dynamics and pollen production/dispersal patterns.     

Comparison of anatomical structure and photosynthetic characteristics between the two photosynthetic organs of the desert plant Hedysarum scoparium

The desert plant Hedysarum scoparium uses leaflets and rachises as its photosynthetic organs. The abundance of leaflets was lower under unfavorable environmental conditions and higher with improved water conditions. To examine the characteristics associated with the adaptation of H. scoparium to its environment, we selected plants with both compound leaves and rachis without leaflets to study the anatomical structures and gas exchange characteristics of the two organs. The results show that the water storage tissues in rachises were more developed compared with the leaflets. The diurnal courses of the net photosynthetic rate for the rachis and the leaflet were both in a bimodal pattern. Meanwhile, both two peak values of the rachis were significantly higher than those of the leaflet. The daily average transpiration rate was significantly higher in the rachis than in the leaflet in order to lower the temperature of the rachises. It was concluded that under desert drought conditions, the leaflets of H. scoparium were partially or completely degraded to reduce the transpiration area as an adaptive response to water deficit, and only the rachises were retained as photosynthetic organ. The rachises were found to be better suited to a desert habitat than the leaflets.     

A temporal analysis of seasonal start dates across 25 urban environments in the eastern United States

This research examines changes to seasonal start dates (1948–2015) across urban locations in the eastern United States. For 25 cities, percentile-specific thresholds of temperature (T_a) and apparent temperature (AT) were used to define seasonal start dates. Seasonal changes were evaluated across four time periods: 0300, 0900, 1500, and 2100 UTC. Incorporating spatial and temporal variability into seasonal delineation, this research critiques the ways in which seasons are defined and builds on existing research regarding seasonal climate change. Both T_a and AT results generally indicate earlier starts to the summer and spring seasons, while the starts to winter and autumn seasons have been delayed. On average, a forward shift of 11 days was found for summer. Larger changes were found for AT thresholds when compared to air temperature alone. Temporally, the largest and most significant changes were found for the 0300 and 2100 observations. The observed changes to seasons have significant impact on a range of processes, including phenology and human health, and may be appropriate to consider further in future climatology studies.     

隔离降雨对杉木林土壤可溶性有机碳和微生物生物量碳的影响

通过在亚热带杉木林内设置不同隔离降雨强度试验:完全隔离降雨、隔离60%降雨、隔离20%降雨和对照(自然降雨),研究隔离降雨对0~20 cm土层可溶性有机碳(DOC)和微生物生物量碳(MBC)含量的影响,结果表明:0~10 cm和10~20 cm土层,除完全隔离降雨处理土壤DOC峰值出现在春季外,其他处理均出现在秋季。0~10 cm土层中完全隔离降雨和隔离60%降雨处理的土壤MBC峰值出现在春季,而隔离20%降雨和对照的则出现在夏季,10~20 cm土层各处理对应的MBC最大值分别出现在春季、夏季、夏季和秋季。随着土层加深,4种处理土壤DOC、MBC含量均显著降低。0~10 cm土层,完全隔离降雨处理的土壤DOC、隔离60%降雨土壤MBC均与土壤含水量显著正相关,杉木林土壤DOC和MBC对降水变化响应具有明显的季节性。     

Trend analysis of major hydroclimatic variables in the Langat River basin,Malaysia

Trend analyses of monthly, seasonal and annual rainfall, air temperature, and streamflow were performed using Mann‐Kendall test within the Langat River basin to identify gradual trends and abrupt shifts for 1980 − 2010. Annual rainfall showed an increasing trend in upstream flow, a combination of decreasing and increasing trends in middle stream flow, and a decreasing trend in downstream flow. Monthly rainfall in most months displayed an insignificant increasing trend upstream. Stations with significant increasing trends showed larger trends in summer than those of other seasons. However, they were similar to the trends observed in annual rainfall. Annual minimum air temperature showed a significant decreasing trend upstream and significant increasing trends in the middle stream and downstream areas. Annual maximum air temperature portrayed increasing trends in both upstream and middle stream areas, and a decreasing trend for the downstream area. Both monthly and seasonal maximum air temperatures exhibited an increasing trend midstream, whereas they demonstrated trends of both decreasing and/or increasing temperatures at upstream and downstream areas. Annual streamflow in upper, middle and lower catchment areas exhibited significant increasing trend at the rates of 0.036, 0.023 and 0.001 × 10³ m³/y at α = 0.01, respectively. Seasonal streamflow in the upstream, midstream and downstream areas displayed an increasing trend for spring (0.55, 0.33 and 0.013 m³/y respectively) and summer (0.51, 0.37, 0.018 m³/y respectively). The greatest magnitude of increased streamflow occurred in the spring (0.54 m³/y). Significant increasing trends of monthly streamflow were noticed in January and August, but insignificant trends were found in May, September and November at all stations. Annual streamflow records at the outlet of the basin were positively correlated with the annual rainfall variable. This study concludes that the climate of the Langat River basin has been getting wetter and warmer during 1980‐2010.     

The ecology of Lerista labialis (Scincidae) in the Simpson Desert: reproduction and diet

This study investigated the diet and reproductive cycle of the southern sandslider skink Lerista labialis, in the dunefields of the Simpson Desert, central Australia. We dissected preserved specimens to obtain gut contents and measure gonad volume, and also live-captured animals from 1999 to 2000 to assess the impact of rainfall on population structure and body condition. This skink breeds during the austral summer. Males start spermatogenesis in July and testes volume is largest in December. Females have enlarged ovarian follicles in November and produce oviducal eggs from November to February. One or more clutches of two eggs are produced per breeding season. L. labialis specializes on termites, which represent 78% of the diet by percentage occurrence. The remaining 22% of this skink's diet comprises Hemiptera, Neuroptera and unknown prey items. Body condition, and probably increased reproduction and juvenile survival, were higher in 2000 (wet year) than 1999 (relatively dry year); proportion of juveniles also increased in the population in autumn, winter and summer 2000 compared to the previous year. Late rainfall in autumn 2000 may have triggered these demographic changes by increasing prey accessibility. Long-term research is required into growth rates, longevity and inter-annual population dynamics of L. labialis to fully evaluate its life history strategy.     

北京地区长序列季节尺度降水研究

本文采用线性回归,11年滑动平均,Mann-Kendall,连续Morlet小波分析等方法对北京地区286年长降水序列在季节尺度上进行了趋势与周期分析。分析结果表明：（1）在过去的将近300年,北京地区四季除了冬季之外降水呈上升趋势;（2）夏季降水和年降水的上升趋势在17世纪80年代后上升的非常明显并且在很长的一段时间是显著的;（3）夏季降水和年降水的突变点发生分别发生在1764和1768年,这之后北京夏季降水和年降水从下降趋势变为上升趋势;（4）四季降水表现出不尽相同的周期特征。130–170年, 80–95年, 75–95年和 55–65年被认为是春夏秋冬四季降水的最主要周期;（5）153年,85年,83年,59年分别为北京地区春夏秋冬四季降水的第一主周期;（6）夏季降水与年降水变化阶段基本相似,夏季降水完全控制年降水;根据两者85年第一主周期判断北京地区在2009–2030年这个时段正处于一个少降水期。     

疏勒河源冻土区高寒草甸土壤碳氮含量特征

土壤碳氮是高寒植被响应多年冻土区生态环境变化的重要营养和能源物质,但对其调查仍以生长季的单次采样为主,缺乏对其他季节的研究,这对于准确把握多年冻土区土壤碳氮含量及储量评估存在明显局限性。为此,本研究以青藏高原东北缘祁连山西段疏勒河源多年冻土区高寒草甸为对象,对0—50 cm土层土壤有机碳（Soil Organic Carbon, SOC）、全氮（Total Nitrogen, TN）含量及其比值（C/N）的剖面分布和季节变化及其影响因素进行分析。结果表明：（1）SOC、TN剖面分布规律一致,0—10 cm土层均显著高于10—50 cm各层（P<0.05）,0—50 cm深度仅秋季逐渐下降,而春夏冬季0—30 cm递减。（2）SOC、TN含量存在季节变化,SOC表现为夏季>冬季>春季>秋季,TN表现为春秋冬季含量一致,夏季略低。（3）C/N季节变化显著,夏季显著最高,秋季显著最低（P<0.05）。（4）土壤含水量和生物量是影响SOC、TN及C/N剖面分布和季节变化的关键因素。（5）夏季土壤碳氮密度均高于全年平均。可见,仅单一节点（生长季为主）调查以表征全年土壤碳氮储量存在高估趋势。     

塔克拉玛干地区气候变化对全球变暖的响应

从地面水汽压(大气含水量)、平均风速、湿润指数、相对湿度和气压的角度分析在全球变暖的情况下,塔克拉玛干地区气候的年和季节变化特征,结果表明:①年和四季平均风速呈阶梯式下降趋势,具有显著减小的线性趋势,并于1973年发生了由大到小的突变。②夏、秋、冬季和年地面水汽压(大气含水量)自20世纪80年代以来呈较大幅度波动式上升,具有显著的线性增加趋势。夏、秋季地面水汽压于1969年和1973年发生了由少到多的突变。秋、冬季大气含水量的显著增加并没有导致降水量的增加,降水量的变化不能充分反映大气含水量的变化。③夏季湿润指数有显著增加趋势,春季有微弱的上升趋势,而降水量夏季和年有显著增加趋势,春季有微弱的上升趋势,说明综合反映气候干湿变化的湿润指数变化与单用降水量表示的气候干湿变化不完全一致。④夏、秋季和年相对湿度呈波动式上升趋势,夏季和年相对湿度分别于1970年和1974年发生了由低到高的突变。⑤年和四季的平均气压40a来无变化。     

重建豫西近五百年分季水旱序列的试验

本文通过对河南省历代旱涝气候史料的分析,建立了豫西区1470-1979年分季(春、夏,秋)水旱序列,并且对序列及其可靠性作了分析.     

Seasonal variation in dust events and the causes of the variation in the Tarim Basin, China

We analyzed dust event occurrence and its seasonal distribution at 16 sites in the Tarim Basin,China.Although the overall frequency of dust events was the highest in spring in this region,its variation in other seasons could be classified into three patterns:(1) frequency of dust events in autumn > that in summer > that in winter(at the Kashi and Kuche sites);(2) frequency in summer > that in winter > that in autumn(at the Ruoqiang site);and(3) frequency in summer > that in autumn > that in winter(at all other areas of the Tarim Basin).The frequency of dust events and their seasonal variations in the Tarim Basin were mainly controlled by wind speed and locally available dust sources;the former was the key control when dust sources did not differ significantly.The seasonal variation in evaporation had a smaller,but still significant effect on the frequency of dust events.     

近10年蒙古高原植被覆盖变化及其对气候的季节响应

利用2001~2010年间MODIS NDVI数据、同期气象数据和MODIS土地覆盖分类产品,探讨蒙古高原植被覆盖变化趋势及其对气温和降水量的季节响应特征。结果表明,10 a来,蒙古高原植被覆盖度呈增加趋势和呈下降趋势的面积基本持平;春季和夏季植被覆盖度呈下降趋势,而秋季呈上升趋势,降水量是最主要的影响因子;在秋季5种植被类型均呈增加趋势,而在春季和夏季不同植被类型的增减趋势因植被类型而异。     

The response of ostracod shell chemistry to seasonal change in a Mediterranean freshwater spring environment

We established the relationships between water chemistry changes in a pool fed by a permanent spring and seasonal variations in trace-element contents (Sr & Mg) in the shells of the ostracod species Herpetocypris intermedia, based on monthly collections of ostracod and water samples. The water chemistry of the investigated pool (Maïques, Valëncia, Spain) was dominated by calcium and bicarbonate, and showed marked seasonal variation in alkalinity, Ca²⁺ content, Sr/Ca and Mg/Ca ratios. Although the variability in the water chemistry was relatively low (~10% relative standard deviation over the entire period), the trace-element contents in the ostracod shells tracked the seasonal change in the water chemistry of Maïques pool. Moreover, due to the rapid renewal of H. intermedia population, this species is able torecord in its shells the evolution of the water chemistry at a monthly time scale. Our results also showed that, in the Maïques pool system, ostracod Sr/Ca and Mg/Ca ratios increased with the decrease in water salinity.To our knowledge, this is the first geochemical study of ostracods dwelling in spring environments. The results of this study may be applied to paleohydrological reconstruction using ostracods preserved in sediments deposited around springs (i.e. travertine and tufa deposits).     

Sediment Fluxes and Varve Formation in Sihailongwan,a Maar Lake from Northeastern China

Data derived from monthly sediment traps in Sihailongwan, a maar lake in northeastern China, yielded a detailed record of seasonal sediment fluxes. Sediment fluxes correspond to seasonal climatic variations. The diatom flux shows two distinct peaks in September and November, whereas the flux of chrysophyte stomatocysts shows a maximum in May. The blooms of diatoms may be related to the subsequent deepening of the thermocline in September and lake overturn in spring and November, and influx of nutrient-rich groundwater sometime after the onset of the summer monsoon. The fluxes of organic matter and siliciclastics show a distinct seasonal pattern. They are varying between 0.03 and 0.56 g m⁻² d⁻¹ and reach a maximum in May. Quartz in the trap samples indicates that the siliciclastic matter may originate from distant aeolian sources. Sediment trap data and thin section investigations confirm the seasonality of Lake Sihailongwan sediments. Dark-colored layer, which mainly consists of valves of Cyclotella comta, might be deposited during autumn, and then is followed by a light-colored mixed layer starting with siliciclastics deposited after ice-out. The varved sediments in the U-shaped Lake Sihailongwan represent a sensitive siliciclastic and geochemical archive of paleoenvironmental variability in this data-sparse area. Detailed investigations of varved sediments should provide decadal to annual records of seasonal sediment flux and its relation to climatic parameters. Especially the diatomaceous layer is regarded to indicate summer climatic fluctuations, while the thick siliciclastic layer could be an indictor of dust events.     

Paleohydrology of Lake Nhaucati (southern Mozambique), ∼400 AD to present

This paper investigates the correlations between lake level change, rainfall variability and general atmospheric forcing in southern Africa. The analysis of fossil diatom assemblages in a sediment sequence from the small, rain-fed Lake Nhaucati, southern Mozambique, is presented and discussed in relation to regional palaeoclimate data. The accumulation of organic sediments in Lake Nhaucati began 1,600 years ago when the lake level was rising. Lithology and pollen suggest a low stand at 800 AD, which correlates with other climate proxies from the summer rainfall region of southern Africa. The diatom assemblage suggests that lake levels were high between 900 and 1300 AD, with shorter low stands at c.1100 and 1200 AD. The period after 1400 AD was marked by a slow rate of accumulation and consequently a low temporal resolution. The correlation with other climate proxies in the summer rainfall region, written sources, and pollen data suggests repeated droughts corresponding to the Little Ice Age, though the driest periods may have caused complete desiccation of the lake. Higher lake levels are suggested after 1800 AD, though written sources suggest droughts in the beginning of the twentieth century. The analysis shows a good correlation with palaeoclimate data from the summer rainfall region and confirms the presence of an anti-phase relationship between the summer rainfall region of southern Africa and the bi-modal rainfall region of east tropical Africa. It also supports the general hypothesis that variation in the intensity of the Inter Tropical Convergence Zone is the main agent modulating rainfall over southern and eastern Africa on centennial timescales.     

Rainfall interception by sand-stabilizing shrubs related to crown structure

On the edge of the Tengger Desert in northern China,revegetation has changed the landscape from moving dunes to stabilized dunes covered by shrubs,which further modifies the pattern of rainfall redistribution.To study rainfall interception loss by shrubs and its relationship to rainfall properties and crown structure,throughfalls passing through crowns of Artemisia ordosica Krash.and Caragana korshinskii Kom.were measured using nine PVC cups under the canopy of each of the two shrubs during 73 rain events over a three-year period,with total rainfall of 260.9 mm.Interception losses of gross rainfall by A.ordosica and C.korshinskii account for 15% and 27% of the total on a crown area basis,and 6% and 11% on a ground area basis,respectively.Individual throughfall(T) and interception(I) were significantly related to rainfall amount(Pg),duration(D),and intensity(R).Ratios of throughfall to rainfall(T/Pg) and interception to rainfall(I/Pg) were not only significantly related to Pg,D,and R,but also to shrub species,and interactions of species with crown volume(CV) and leaf area index(LAI).Under most rain events,interceptions by C.korshinskii with greater CV and LAI were significantly higher than those by A.ordosica,and more rainfall interception occurred at locations closer to the stems of the two shrubs.For C.korshinskii,I/Pg had a significant positive linear relation with CV and LAI,while T/Pg had a significant negative linear relation with them.CV has a greater influence on T/Pg and I/Pg than does LAI.Using a regression method,canopy water storage capacities are estimated to be 0.52 and 0.68 mm,and free throughfall coefficient to be 0.62 and 0.47 for A.ordosica and C.korshinskii,respectively.     

Seasonal and spatial changes in the zooplankton community of Kongsfjorden, Svalbard

Seasonal changes in the zooplankton composition of the glacially influenced Kongsfjorden, Svalbard (79°N, 12°E), and its adjacent shelf were studied in 2002. Samples were collected in the spring, summer and autumn in stratified hauls (according to hydrographic characteristics), by means of a 0.180-mm Multi Plankton Sampler. A strong front between the open sea and the fjord waters was observed during the spring, preventing water mass exchange, but was not observed later in the season. The considerable seasonal changes in zooplankton abundance were related to the seasonal variation in hydrographical regime. The total zooplankton abundance during the spring (40–2010 individuals m⁻³) was much lower than in the summer and autumn (410–10 560 individuals m⁻³). The main factors shaping the zooplankton community in the fjord include: the presence of a local front, advection, the flow pattern and the decreasing depth of the basin in the inner fjord. Presumably these factors regulate the gross pattern of zooplankton density and distribution, and override the importance of biological processes. This study increased our understanding of seasonal processes in fjords, particularly with regard to the strong seasonal variability in the Arctic.     

Lake Redó ecosystem response to an increasing warming the Pyrenees during the twentieth century

The ecosystem response of Lake Redó (Central Pyrenees) to fluctuations in seasonal air temperature during the last two centuries was investigated by comparison of reconstructed air temperatures with the sediment record. Fine slicing allowed a resolution of 3–6 years according to the ²¹⁰Pb dating, although it was still difficult to easily investigate the response to air temperature forcing, since extreme fluctuations in temperature occur on interannual time-scales. However, the resolution was sufficient to show responses on decadal and century scales. An overall tendency to warming in mean annual temperature in the Central Pyrenees has been caused by summer and in particular by autumn increases. Many of the measured sediment variables apparently responded to these long term trends, but the significance of the relationships was highly conditioned by the structure of the data. The variables responding most on the finer time scales were the microfossils. For diatoms, chironomids and chrysophytes the main variability correlated to summer and to autumn temperatures. For two planktonic species, Fragilaria nanana and Cyclotella pseudostelligera, we found a link of their variability with temperature fluctuations in their growing months (September and October, respectively). This relationship appeared at a certain point during a general warming trend, indicating a threshold in the response. On the other hand, no significant changes in the dominant species could be linked to temperature, nor in any significant subgroup of the 180 diatom species present in the core. In contrast, for most chironomids (particularly Paratanytarsus austriacus, Heterotrissocladius marcidus and Micropsectra radialis) a negative relationship with summer temperature extended throughout the studied period. This response of the whole group gives chironomids a more robust role as indicators for recording temperature changes on long time-scales (e.g., through the Holocene) and for lake signal inter-comparison. Finally, our results indicated that, in all cases, there was a significant resilience to high frequency changes and hysteresis despite extreme fluctuations. Although we were dealing with organisms with one or many generations per year, their populations seemed to follow the decadal trends in air temperature.     

科尔沁沙地奈曼旗1970—2010年降水的多时间尺度分析

利用回归分析、小波分析和Mann-Kendall （M-K）突变分析研究了1970—2010年奈曼旗的降水变化特征。结果表明：（1） 不同雨量级中,0~5 mm降水事件最多,占全年降水事件的70%以上,其降水量也最多,占全年降水量的18%;降水间隔期以0~5 d为主,占全年无降水期的73%。（2） 1970—2010年奈曼旗年降水及季降水波动较大,基本呈相对少雨期和相对多雨期相间分布的特征;年降水及春、夏、秋和冬季降水的倾向率分别为-12.4、0.8、-3.2、-1.3 mm/10a和-0.3 mm/10a; 年降水及春、夏和秋季降水在1990年左右以正距平为主,而在1985年之前和1995年以后以负距平为主;冬季降水基本随年份增加呈减少的趋势。（3） 1月降水量最少,7月最多。7月降水量约占年降水量的30%以上,且与其他各月相比均有显著差异。（4） 通过小波分析发现,奈曼旗年降水存在6~8年的较短周期和17年左右的较长周期变化;年降水和夏、秋及冬季降水在进入21世纪后都有减少趋势。（5） M-K突变分析表明,奈曼旗年降水减少及春季降水增加的突变点分别为 2004年和2006年。     

Late Holocene paleoenvironmental change in a Great Lakes coastal wetland: integrating pollen and diatom datasets

Multiple proxy indicators are regularly used to present robust arguments for paleoenvironmental change. We use fossil pollen and diatoms from a 495-cm core taken from Cootes Paradise, a coastal wetland in the western end of Lake Ontario, to investigate ecological changes in the late Holocene. We use consensus analysis to demonstrate that pollen diagrams are best zoned after the data have been split into source area, in this case upland and wetland taxa, because each group responds differently to environmental change. We also use consensus analysis to demonstrate the sensitivity of clustering to the distance measure used. The record begins at 2400 ¹⁴C years BP, when the wetland was dominated by shallow water emergents and epiphytic diatoms. At 2100 ¹⁴C years BP, a decline in the epiphytic diatoms Epithemia spp., a rise in Poaceae (cf. Zizania aquatica) pollen and a coincident increase in pollen concentration suggest a water level rise at this time. At about 800 ¹⁴C years BP, the diatom record shows a pulse in small benthic Fragilaria species; shortly after, a shift occurs in the upland tree pollen spectra involving an increase in Pinus and a decline in Fagus. This shift in upland forest trees has been associated elsewhere with the Little Ice Age and the diatom data present some evidence for impacts of this climatic event on aquatic systems. The diatom and pollen records both indicate large changes associated with the effects of European settlement, including rises in Ambrosia as well as Typha angustifolia pollen. Planktonic diatoms dominate post-settlement assemblages indicating large-scale hydrological and ecological changes, probably associated with the introduction of carp and alterations to the Iroquois bar which separates the wetland from Lake Ontario. Our work at Cootes Paradise is important for multi-proxy coastal wetland studies in general, in addition to the late Holocene environmental history and prehistory of the Great Lakes region.