首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 109 毫秒
1.
The results are presented of an intensive study of phytoplankton assemblage carried out in the Berounka River above its confluence with the Vltava River (Czech Republic) in the period 2002–2007. The annual and interannual changes of phytoplankton development (based on high frequency of sampling) and their relation to hydrological conditions and concentrations of main nutrients are analysed. A marked decline of nutrient concentrations was observed during the period 1996–2007. The annual mean values of total P decreased from 0.43 mg L−1 to 0.16 mg L−1, those of N-NO3 from 4.6 mg L−1 to 1.5 mg L−1 and N-NH4 from 1.9 mg L−1 to 0.04 mg L−1. Despite this, the phytoplankton biomass remained at a high level. The seasonal mean values of chlorophyll-a ranged from 51.0 μg L−1 to 116.8 μg L−1 in the same time period. An obviously stronger relationship was found of the phytoplankton biomass and pattern of its development to the variation of flow rates than to the existing level of nutrient concentrations. A significantly decreasing relationship (R2 = 0.384, P < 0.001) of chlorophyll-a to flow rates and a significantly increasing relationship (R2 = 0.359, P < 0.001) of chlorophyll-a to water temperatures were found, based on monthly mean values for the seasonal period 2002–2007. The results obtained indicate a remarkable increase of phytoplankton biomass and its prolongated occurrence in watercourses, which can be expected due to the consequences of the predicted climate change (i.e. higher occurrence of summer droughts and low precipitation amounts accompanied by a substantial drop of flow rates, increase of air and water temperatures), as described in the respective scenarios for the territory of the Czech Republic. Simulations by the regional climate models HIRHAM and RCAO and emission scenario SRES indicated the increase of air temperature by 2.5–5 °C, decrease of precipitation amount by 6–25% and decline of flows by 14–43% in the Berounka River for the scenario period 2071–2100.  相似文献   

2.
Seasonal variations in the biomass (Chl a) and primary production (14C-method) of phytoplankton were studied during 12 months of 2005 in the three Ethiopian Rift Valley Lakes (ERVL) Ziway, Awassa and Chamo. Chl a showed an average value of 40, 20, and 30 mg m−3 for the three lakes, respectively. Integrated areal primary production for the total phytoplankton (g C m−2 d−1) varied 2-fold in the three lakes but on different levels, from 0.67–1.8 in L. Ziway, 1.8–4.6 in L. Awassa, and 1.0–2.6 in L. Chamo. The overall photosynthetic efficiency of utilizing photosynthetically active radiation by the phytoplankton on molar basis (mmol C mol of photons−1) resulted in an average value of 1.4 for L. Ziway, 3.5 for L. Awassa and 1.6 for L. Chamo. Among the different factors regulating phytoplankton primary productivity, light penetration and nutrients were the most important in the three lakes. The seasonal variations of incident radiation (most values between 5 and 7 E m−2 h−1) and water temperature (most values between 22 and 24 °C) were small and unlikely to result in the marked differences in phytoplankton primary production. Although relative increase in nutrient concentrations occurred following the rainy periods, the major algal nutrients were either consistently low (nitrate and/or silicate) or high (phosphate and/or ammonium) and remained within a narrow range for most of the study period in all the three lakes. Consequently, phytoplankton biomass and primary production seem to be maintained more by nutrient regeneration or turnover (facilitated by high temperature) than by allochthonous nutrient input. This would be coupled with wind-induced mixing that would play an important role in determining hydrographic characteristics (water column structure) and the associated redistribution of nutrients and phytoplankton, the availability of light and subsequently the spatial (vertical) and temporal patterns of phytoplankton production in these three ERVL. Phytoplankton production (PP) is regarded as a good predictor of fish yield in lakes and seasonal measurements of PP is a prerequisite for good such estimates.  相似文献   

3.
Lake Atitlan, one of the most important lakes not only in Central America but in the whole world, is facing serious problems with increasing water pollution. Over the last several decades, the uncontrolled nutrient input into the lake has lead to high P levels and low N:P ratios, initiating cyanobacterial blooms. The first bloom occurred in December of 2008, followed by more extensive bloom in October 2009. The blooms are formed by cyanobacteria from the rare planktic Lyngbya hieronymusii/birgei/robusta complex. Based on the species morphology, the Atitlan population corresponds to L. robusta and this is the first case of reported bloom of this species worldwide. Remote sensing images documented that at the maximum bloom development, 40% of the 137 km2 of the lake area were covered by dense patches of Lyngbya, with the chlorophyll a concentration reaching over 100 μg L−1. The only toxins detected in the 2009 bloom were trace levels of cylindrospermopsin and saxitoxin with 12 and 58 ng g−1, respectively. The nitrogen fixation followed a pattern expected in non-heterocytous cyanobacteria, i.e., the nitrogenase activity was minimal during the day, while during the night the activity reached 2.2 nmol C2H4 μg Ch a−1 h−1. Delta 15N of −0.86‰ was well in the range given for nitrogen fixing organisms. The cell C, N and P content was 36.7%, 5.9% and 0.9%, respectively, resulting in the molar ratio of 105:14.4:1. A well designed and executed lake monitoring program, strict control of nutrient input into the lake, and public education are the necessary prerequisites for potential prevention of even more severe blooms than the one from 2009.  相似文献   

4.
The ‘Chicken Creek’ artificial catchment area, Welzow-South, E Germany, created to study processes and structures of initial ecosystem development, discharges into a small experimental lake (A=3805 m2, V=3992 m3, zmax=2.4 m). This lake was man-made in 2005 and filled by natural surface runoff until January 2006. In summer 2006 and 2008, the actual development of sediments and the evolution of the phosphorus (P) cycle were studied. 19.7% of the original lake volume was filled by sediment within the first 3 years. A fine-grained sediment representing silt (6.3-63 μm) accumulated at high accretion rates at the deepest point (200 mm a−1, 0-24 mm week−1) due to massive erosion in the catchment. The sediment is low in organic matter (2.5-5.2%) and total P (TP, 0.31-0.97 mg g−1). Low amounts of P associated with degradable organic matter and surplus of metal hydroxides (Fe:P∼40, Al:P∼20) favor an efficient P binding and low dissolved P concentrations in pore water (1-107 μg l−1). Hence, the mineral sediment quality and the low rates of P release (0.06 mg m−2 d−1) revealed that a lake at an initial stage of development has essentially no sedimentary P cycle compared to eutrophic shallow lakes. However, the increasing emersed and submersed macrophyte growth will control further lake succession by intensifying the internal nutrient cycling. The macrophytes drive the evolution of a sedimentary P cycle by mobilizing and translocating P, by accumulating carbon and thus by stimulating microbial and redox processes.  相似文献   

5.
After the phreatic eruption in 1982–83, volcanic activities at Kusatsu–Shirane volcano had been decreasing and reached a minimum in 1990, had turned to a temporal rise in activity up to 1994 and then decreased again at least up to 1997. During this low-activity period we observed a relatively short (≤ 1 y) cyclic variation in polythionates (PT) in the Yugama lake water. Spectral power density analysis of the PT time-series by an autoregressive (maximum entropy spectral estimation, MESE) method indicates that the major frequency in the PT variations is 1.0 y− 1 and the minor is 2–3 y− 1 (1.0 and 0.3–0.5 y in periodicity, respectively). Annual variations in the lake temperature are ruled out for explaining these periodicities. We attribute these cyclic variations to a cyclic magnification-reduction in meteoric-water injection into a hydrothermal regime where volcanic gases from cooling magma bodies at depth and cooler oxidized groundwater come into contact with each other. This interaction may result in a periodical change in the composition and flux of SO2 and H2S gases being discharged into the lake and forming PT. From a phase deviation (2–3 months) in the cycles between the annual precipitation, including snowmelt, and the PT time-series, we estimated the maximal depth of a hydrothermal reservoir beneath the lake assuming a vertical hydraulic conductivity (5 × 10− 3 cm/s) of the volcanic detritus around the summit hydrothermal system. Chloride in the lake water reached a maximum 1.5 years faster than PT. This is most likely due to a gradual elevation of the potentiometric front of a concentrated sublimnic solution in the hydrothermal reservoir. Variations of dissolved SO2 and H2S in the lake water were not consistent with those of the fumarolic gases on the north flank of the volcano. This fact together with additional observations strongly suggests that these fumaroles may have the same origin but are chemically modified by a subsurface aquifer. The PT monitoring at active crater lakes during a quiescent period can provide insight into the annual expansions and reductions of a volcano-hosted hydrothermal reservoir.  相似文献   

6.
Thermokarst lakes cover > 20% of the landscape throughout much of the Alaskan Arctic Coastal Plain (ACP) with shallow lakes freezing solid (grounded ice) and deeper lakes maintaining perennial liquid water (floating ice). Thus, lake depth relative to maximum ice thickness (1·5–2·0 m) represents an important threshold that impacts permafrost, aquatic habitat, and potentially geomorphic and hydrologic behaviour. We studied coupled hydrogeomorphic processes of 13 lakes representing a depth gradient across this threshold of maximum ice thickness by analysing remotely sensed, water quality, and climatic data over a 35‐year period. Shoreline erosion rates due to permafrost degradation ranged from < 0·2 m/year in very shallow lakes (0·4 m) up to 1·8 m/year in the deepest lakes (2·6 m). This pattern of thermokarst expansion masked detection of lake hydrologic change using remotely sensed imagery except for the shallowest lakes with stable shorelines. Changes in the surface area of these shallow lakes tracked interannual variation in precipitation minus evaporation (P ? EL) with periods of full and nearly dry basins. Shorter‐term (2004–2008) specific conductance data indicated a drying pattern across lakes of all depths consistent with the long‐term record for only shallow lakes. Our analysis suggests that grounded‐ice lakes are ice‐free on average 37 days longer than floating‐ice lakes resulting in a longer period of evaporative loss and more frequent negative P ? EL. These results suggest divergent hydrogeomorphic responses to a changing Arctic climate depending on the threshold created by water depth relative to maximum ice thickness in ACP lakes. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

7.
. Kaste  P. J. Dillon 《水文研究》2003,17(12):2393-2407
In‐lake retention of inorganic nitrogen species (nitrate and ammonium) was estimated from mass balances in five acid‐sensitive lakes in southern Norway and eight in southern Ontario, Canada, to evaluate an empirical in‐lake N retention (RN) model. This model is included in the First‐order Acidity Balance (FAB) model, which currently is used for calculation of critical acid loads and exceedances in many countries. To estimate in‐lake RN, the FAB model uses a recommended mass transfer coefficient (SN) of 5 m year−1, which mainly is derived from NO3− mass balances in Canadian lakes. To date, the in‐lake RN model has not been evaluated for large parts of Europe. At the Norwegian study sites receiving the highest N deposition (>120 meq m−2 year−1) the net in‐lake retention of inorganic N (TIN) exceeded the corresponding terrestrial retention by a factor of 1·1–2·6. Despite differences in N loading and hydrology at the Norwegian and Canadian sites, both the mean mass transfer coefficients for NO3− (SNO3; 6·5 versus 5·6 m year−1) and TIN (STIN; 7·9 versus 7·0 m year−1) were of comparable magnitude. Both mean values and ranges of SNO3 suggest that the default SN value presently recommended for FAB model applications seems valid over a large range in N inputs and areal water loads (qs). However, owing to the relatively few data available for lakes with high qs values (15–150 m year−1), it is recommended that more lakes within this range be included in future studies to obtain a more precise prediction of in‐lake N retention over a wide qs gradient. Also, when considering that the FAB model treats all inorganic N leaching from a catchment as NO3−, it seems reasonable to use a default STIN value instead of just SNO3 when estimating in‐lake RN. In that case, the in‐lake RN presently calculated by the FAB model might be slightly underestimated. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

8.
Zeyong Gao  Fujun Niu  Zhanju Lin 《水文研究》2020,34(26):5659-5673
Thermokarst lakes play a key role in the hydrological and biogeochemical cycles of permafrost regions. Current knowledge regarding the changes caused by permafrost degradation to the hydrochemistry of lakes in the Qinghai-Tibet Plateau (QTP) is limited. To address this gap, a systematic investigation of thermokarst lake water, suprapermafrost water, ground ice, and precipitation was conducted in the hinterland of the QTP. The thermokarst lake water in the QTP was identified to be of the Na-HCO3-Cl type. The mean concentrations of HCO3− and Na+ were 281.8 mg L−1 (146.0–546.2 mg L−1) and 73.3 mg L−1 (9.2–345.8 mg L−1), respectively. The concentrations of Li+, NH4+, K+, F−, NO2−, and NO3− were relatively low. Freeze-out fractionation concentrated the dissolved solids within the lake water during winter, which was deeply deepened on lake depth and lake ice thickness. Owing to solute enrichment, the ground ice was characterized by high salinity. Conversely, repeated replenishment via precipitation led to lower solute concentrations in the ground ice near the permafrost table compared to that within the permafrost. Although lower solute concentration existed in precipitation, the soil leaching and saline ground ice melting processes enhanced the solute load in suprapermafrost water, which is considered an important water and solute resource in thermokarst lakes. The influencing mechanism of permafrost degradation on thermokarst lake hydrochemistry is presumably linked to: (1) the liberation of soluble materials sequestered in ground ice; (2) the increase of solutes in suprapermafrost water and soil pore water; and (3) the changes in lake morphometry. These results have major implications on the understanding of the effects of ground ice melting on ecosystem functions, biogeochemical processes, and energy balance in a rapidly changing climate.  相似文献   

9.
湖泊水面与大气之间垂直方向的动量、水汽和热量通量与风速、湿度和温度梯度之间存在比例关系,因此在湖泊水-气相互作用研究中,这比例系数(交换系数)是关键因子.在以往的研究中,交换系数通常直接采用水面梯度观测法或海洋大气近地层的参数化方案进行计算.本文采用涡度相关系统和小气候系统仪器在太湖平台上直接观测的通量和气象要素,对上述交换系数(最小均方差原则)进行优化,结果为:动量交换系数CD10N=1.52×10-3、水汽交换系数CE10N=0.82×10-3、热量交换系数CH10N=1.02×10-3,与其他内陆湖泊涡度相关观测数据的推导结果一致.本文的研究结果表明:与海洋参数化方案相比,在相同的风速条件下,湖面的空气动力学粗糙度比海洋高,这可能是由于受到水深的影响;如果采用海洋参数化方案,会导致湖泊年蒸发量的估算值偏大40%.太湖的动量、水汽和热量交换系数可以视为常数,可以不考虑稳定度和风速的影响.这是因为本文中83%的数据为近中性条件.敏感性分析表明:如果考虑稳定度的影响,LE模拟值的平均误差降低了0.5 W/m2,H的平均误差降低了0.4 W/m2,u*的计算值没有变化;如果考虑风速的影响,u*模拟值的平均误差降低了0.004 m/s,LE的平均误差升高了1.3 W/m2,H的模拟结果几乎不受影响.这一结果能为湖气相互作用研究提供参考.  相似文献   

10.
We observed a phytoplankton bloom downstream of a large estuarine plume induced by heavy precipitation during a cruise conducted in the Pearl River estuary and the northern South China Sea in May–June 2001. The plume delivered a significant amount of nutrients into the estuary and the adjacent coastal region, and enhanced stratification stimulating a phytoplankton bloom in the region near and offshore of Hong Kong. A several fold increase (0.2–1.8 μg Chl L−1) in biomass (Chl a) was observed during the bloom. During the bloom event, the surface water phytoplankton community structure significantly shifted from a pico-phytoplankton dominated community to one dominated by micro-phytoplankton (>20 μm). In addition to increased Chl a, we observed a significant drawdown of pCO2, biological uptake of dissolved inorganic carbon (DIC) and an associated enhancement of dissolved oxygen and pH, demonstrating enhanced photosynthesis during the bloom. During the bloom, we estimated a net DIC drawdown of 100–150 μmol kg−1 and a TAlk increase of 0–50 μmol kg−1. The mean sea–air CO2 flux at the peak of the bloom was estimated to be as high as ∼−18 mmol m−2 d−1. For an average surface water depth of 5 m, a very high apparent biological CO2 consumption rate of 70–110 mmol m−2 d−1 was estimated. This value is 2–6 times higher than the estimated air–sea exchange rate.  相似文献   

11.
The Feldberger Haussee, a highly eutrophic stratified hard-water lake located in the eastern part of Germany's Baltic lake district, was selected for a restoration programme combining external nutrient loading reduction and long-term biomanipulation. In 1980 the external phosphorus loading (1.8 g TP m−2 a−1) decreased by 90%, but water quality did not improve significantly within the following 6 years. In 1985 biomanipulation was initiated, with manual removal of cyprinid fish coupled with piscivore introductions as the principal measures. The expected changes in the pelagic community and improvements of water quality occurred after a delay of several years. Despite intensive manual removal of cyprinids and stocking of piscivores, standing stocks of cyprinid fish remained relatively high (130-260 kg ww ha−1) after some years of decline. Compared to the pre-biomanipulation period, mean seasonal (May-September) Daphnia spp. biomass roughly doubled (0.037 g C m−3vs. 0.084 g C m−3). However, the predominance of small (<1 mm) D. cucullata throughout the whole investigation period indicated that planktivory was still substantial. Paired observations between edible phytoplankton biomass and Daphnia spp. indicated that a significant decline in algal stocks would only occur if herbivorous biomass was above a certain threshold (0.2 g C m−3). Reduced external and internal loading in concert with pelagic calcite precipitation were most likely responsible for the decline in lake phosphorus concentrations, thereby substantially improving the water quality of Feldberger Haussee. Although this is not supported by quantitative evidence, we hypothesize that resource-related water quality improvements were caused by changes in the structure of the pelagic community leading to increased calcite precipitation. In agreement with the results of other investigations, we conclude that because stabilising mechanisms such as macrophyte growth were lacking in Feldberger Haussee, biomanipulation in stratified lakes may not be as successful as has been observed in shallow lakes. However, in hard-water lakes, calcite precipitation may act as another stabilising resource-related mechanism. Phosphorus associated with sedimenting calcite particles is insensitive to redox-conditions and may therefore not be re-mobilised from lake sediments even if hypolimnetic oxygen is depleted.  相似文献   

12.
This paper presents data on the first identification, characterization and quantification of hepatotoxic microcystins and neurotoxic anatoxin-a in water samples of Lake Baringo, Kenya. The shallow turbid Lake Baringo was investigated five times between June 2001 and May 2002. The phytoplankton community was mainly dominated by the cyanobacterium Microcystis aeruginosa. Due to the high turbidity the phytoplankton biomass was low, ranging between 1.5 and 8.2 mg L−1. High mean total phosphorus concentration (1.0 mg L−1) and mean total nitrogen concentration (2.8 mg L−1) typical for hypertrophic lakes were found. Using HPLC technique the hepatotoxins microcystin-LR, -RR and -YR and the neurotoxin anatoxin-a were detected in the water samples. The microcystin concentrations varied from 310 to 19800 μg microcystin-LR equivalents g−1 DW and the anatoxin-a concentration ranged from 270 to 1260 μg g−1 DW. To our knowledge this is the first evidence of cyanobacterial toxins in Lake Baringo.  相似文献   

13.
Our investigation on macrophytes in Lake Qattieneh, a large, shallow reservoir in western Syria, is a first assessment of ecological status of this lake. We studied spatial distribution patterns of aquatic macrophyte vegetation and heavy metal concentrations to determine if they can be used as indicators of point sources of pollution. Industrial and municipal point sources at the lake shore increase nutrient load and contamination by heavy metals. Water analyses revealed high concentrations of some heavy metals at some littoral sites: Ni 88.7 μg L−1, Cr 49.99 μg L−1, Co 14.38 μg L−1, and Cu 11.65 μg L−1. Despite hypertrophic conditions and high heavy metal contamination, we recorded several submerged macrophyte species with heterogeneous spatial distribution patterns. Whereas Potamogeton pectinatus L. dominates in the eastern part of the lake, near industrial point sources, both Myriophyllum spicatum L. and Potamogeton lucens L. form extended patches in the western part of the lake. The shallow, littoral areas near villages are dominated by Ranunculus trichophyllus Chaix in Villars and Ceratophyllum demersum L. The west-east gradient in nutrient and heavy metal concentrations in waters and sediments are reflected by the spatial distribution of submersed species. While the heavy metal concentrations of the water body vary considerably in different seasons, the contents in submersed macrophytes integrate seasonal variations of longer time periods. Spatial distribution and tissue accumulation of littoral macrophyte species reflect the environmental conditions at respective sites such as heavy metal contents in water and sediment.  相似文献   

14.
Abiotic effects of UV on planktonic P kinetics   总被引：1，自引：0，他引：1  
  相似文献   

15.
In inland waters, transparent exopolymer particles (TEP) can affect carbon export and sequestration in sediments with consequences for lake C budgets. We measured TEP concentration in 32 lakes from two contrasting lake districts covering wide ranges in biological and chemical characteristics. North temperate lakes, located in a wet region, have low to moderate ionic strength and low to high dissolved organic carbon with corresponding variation in color (light absorbance). Mediterranean lakes located in a semiarid region were characterized by high ionic strength and high concentrations of dissolved organic carbon but low color. TEP concentrations were large relative to the living portion of the particulate organic carbon pool in both Mediterranean (36%) and north temperate (33%) lakes. TEP concentrations ranged from 36 to 1,462 μg [as Gum Xanthan equivalents (GX eq)] L−1 in north temperate lakes. In the Mediterranean lakes, concentrations were higher that previously reported for other systems and ranged from 66 to 9,038 μg GX eq L−1. TEP concentration was positive and significantly related to chlorophyll a (chl a) in north temperate lakes and in the entire data set. Although a significant and positive relationship between TEP and chl a was also detected in the Mediterranean lakes, bacterial abundance was most strongly related to TEP. In contrast with the positive influence of phytoplankton and bacteria on TEP, there were weaker relationships between TEP and the chemical variables tested. We observed a significant and positive relationship between pH and TEP (for all lakes) but this relationship was indirectly driven by a co-variation of pH with phytoplankton biomass based on multiple regression analysis. For the Mediterranean lakes, the negative (but not significant) trends between TEP and both conductivity and divalent cations suggest thresholds above which TEP will likely be destabilized. Under these conditions, TEP may flocculate or disperse in the water column.  相似文献   

16.
An enzyme assay was developed for studies on peroxidase activities in humic lake water. 3,4-Dimethoxybenzyl alcohol (veratryl alcohol, VeraOH) was used as tracer substrate, and peroxidase (EC 1.11.1.7) activity was measured by high-performance liquid chromatography. The chemical stability of VeraOH and its application as peroxidase substrate was tested under light and dark conditions, different hydrogen peroxide (H2O2) concentrations and humic matter contents. VeraOH was stable under low UV radiation at in situ conditions in lake water (<0.010...0.25 kJ m–2 d–1), laboratory conditions (<0.05...0.30 kJ m–2 d–1), and low (1...100 μM) H2O2 concentrations. However, peroxides oxidized VeraOH above 1...10 mM H2O2 concentration in sterile Millipore-Q and humic lake water. Dark incubations showed little VeraOH oxidation products. The developed peroxidase assay was tested in the growth medium of Phanerochaete chrysosporium and a bacteria isolate (P.M.D. 20.4.3.1) from mesohumic lake Pääjärvi. Peroxidase activities were also measured in natural microbial communities under standard laboratory and under in situ conditions in humic lake water. Incubation times of about 5 to 12 days were usually needed to record significant (P < 0.05) peroxidase activities, in lake waters. In situ peroxidase activities varied in pelagial surface water (0...0.5 m) on a seasonal scale between 74 nmol L–1 h–1 and 273 nmol L–1 (mean: 176 nmol L–1 h–1) and within the water column between 110 nmol L–1 h–1 and 800 nmol L–1 h–1 (mean: 500 nmol L–1 h–1) in polyhumic lake Mekkojärvi.  相似文献   

17.
Environmental determinants of littoral paludification in boreal lakes   总被引：1，自引：0，他引：1  
Numerical studies of the relationships between littoral processes and environmental variables are important because they provide understanding of the impacts of natural and human factors on lake systems. In this study, littoral paludification, its occurrence, types and main environmental determinants were studied in boreal lakes with varying size (41–560 km2) and total phosphorus content (4.5–35.5 μg L−1) in Finland. The relative importance of catchment characteristics, water quality and water level regulation on paludification was analysed at the lake level (n=20), whereas the importance of site morphology, soil quality and aquatic vegetation was analysed at the site level (n=289). This study utilised two novel statistical methods in numerical analyses: generalised linear mixed models and variation partitioning (VP). Bottom ward overgrowth was the main paludification type in the studied lakes. At the lake level, the magnitude of spring flooding and the altitude of the lake had a negative response to paludification, whereas the existence of water level regulation, cover of clay soil and total phosphorus content correlated positively to paludification. At the site level, the paludification was negatively related to slope angle, and positively to the occurrence of clay soil, Glyceria maxima (reed sweet-grass) and the abundance of eutrophy indicator species. However, a considerable amount of variation in littoral paludification both at lake and site levels was accounted for by the joint effects of predictors and may thus be causally related to two or all three groups of predictors.  相似文献   

18.
Mount Erebus, a large intraplate stratovolcano dominating Ross Island, Antarctica, hosts the world's only active phonolite lava lakes. The main manifestation of activity at Erebus volcano in December 2004 was as the presence of two convecting lava lakes within an inner crater. The long-lived Ray Lake, ~ 1400 m2 in area, was the site of up to 10 small Strombolian eruptions per day. A new but short-lived, ~ 1000–1200 m2 lake formed at Werner vent in December 2004 sourced by lava flowing from a crater formed in 1993 by a phreatic eruption. We measured the radiative heat flux from the two lakes in December 2004 using a compact infrared (IR) imaging camera. Daily thermal IR surveys from the Main Crater rim provide images of the lava lake surface temperatures and identify sites of upwelling and downwelling. The radiative heat outputs calculated for the Ray and Werner Lakes are 30–35 MW and 20 MW, respectively. We estimate that the magma flux needed to sustain the combined heat loss is ~ 250–710 kg s− 1, that the minimum volume of the magma reservoir is 2 km3, and that the radius of the conduit feeding the Ray lake is ~ 2 m.  相似文献   

19.
  相似文献   

20.
The aim of this paper is to illustrate the effects of spatial organization of lake chains and associated storage thresholds upon lake-overflow behaviour, and specifically their impact upon large scale flow connectivity and the flood frequency of lake overflows. The analysis was carried out with the use of a multiple bucket model of the lake chain system, consisting of a network of both lakes and associated catchment areas, which explicitly incorporated within it three storage thresholds: a catchment field capacity threshold that governs catchment subsurface stormflow, a total storage capacity threshold that governs catchment surface runoff, and a lake storage capacity threshold that determines lake overflow. The model is driven by rainfall inputs generated by a stochastic rainfall model that is able to capture rainfall variability at a wide range of time scales. The study is used to gain insights into the process controls of lake-overflow generation, and in particular, to explore the crucial role of factors relating to lake organization, such as the average catchment area to lake area (AC/AL) ratio and the distribution of AC/AL with distance in the downstream direction (increasing or decreasing). The study showed that the average AC/AL value was the most important factor determining the frequency of occurrence and magnitude of floods from a landscape consisting of lake chains. The larger the average AC/AL value the more runoff is generated from catchments thus increasing both the occurrence and magnitude of lake overflows. In this case the flood frequency curve reflects that of the catchment area, and lake organization does not play an important role. When AC/AL is small the landscape is lake dominated, the spatial organization of lakes has a significant impact on lake connectivity, and consequently on flood frequency. One of the aspects of lake organization that may have a significant influence on lake connectivity is the spatial distribution of AC/AL from upstream to downstream (increasing or decreasing). In a landscape in which AC/AL increases downstream, lake overflow will occur more frequently relative to a similar landscape (i.e. identical AC/AL) with a constant value of AC/AL. When AC/AL decreases downstream, however, runoff inputs from the upstream parts will trigger lake overflow in the downstream parts, and consequently, full connectivity may be achieved leading to increased flood frequencies.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号