CO2 degassing from Pico Island (Azores,Portugal) volcanic lakes

The CO₂ degassing from lakes on Pico Island (Azores archipelago) were characterized in order to estimate the total diffuse CO₂ output and identify the possible sources of CO₂. Two surveys have been made in each lake (Capitão, Caiado, Rosada, Peixinho, Paúl and Seca), in the winter and summer periods. These water bodies show small surface areas and are rather shallow, with depths ranging from 1.8 to 8.6 m. Water samples are cold, both in winter and summer periods, not presenting variations along the water column, with acid to neutral pH (5.26–7.06). The electrical conductivity values point out to very diluted waters (mean range between 27 and 33.4 μS cm⁻¹), of the Na-Cl type, corresponding to meteoric waters influenced by marine salts.To measure the CO₂ flux at the lakes surface the modified accumulation chamber method was used, and a total of 1632 measurements were accomplished (711 in winter surveys and 921 in summer). Two statistical analysis (GSA and sGs) were applied to the results of diffuse CO₂ flux measurements, showing that the CO₂ flux values measured in theses lakes are relatively low (0.60–20.47 g m⁻² d⁻¹), what seems to indicate a single source for CO₂ (biogenic source), also suggested by the water δ¹³C isotopic signature.CO₂ emissions range between 0.04 t d⁻¹ (Rosada_1) and 0.25 t d⁻¹ (Caiado_1) during the winter surveys, being in general similar to the values recorded during the summer surveys that vary between 0.03 t d⁻¹ (Peixinho_2 and Seca_1) and 0.30 t d⁻¹ (Caiado_2). Taken into account the surface area of the lakes, the highest values were estimated for both surveys made in Seca Lake (˜13 t km⁻² d⁻¹). The occurrence of a dense macrophyte mass in a few of the studied lakes, such as Caiado and Seca, seems to enhance the CO₂ flux from these water bodies.     

Recent pH and CO2 profiles at Lakes Nyos and Monoun, Cameroon: implications for the degassing strategy and its numerical simulation

In situ pH profiles are reported for the first time for Lakes Nyos and Monoun. The pH profiles were converted to CO₂ profiles using HCO₃⁻ profiles calculated from conductivity data. Recent observations (1993–1996) at Lake Nyos indicates that CO₂ still accumulates below 180 m depth at a rate of 125 Mmol year⁻¹. At Lake Monoun, the majority of CO₂ is present below a depth of 60 m, only 25 m below the saturation depth. Consequently, a potential danger of gas explosion is high at both lakes, and artificial degassing of the lakes should be performed as soon as possible. A system for industrial degassing of the lakes is proposed. The system, based on the self-sustained gas lift principle, consists of multiple pipes (14 cm in diameter) with different intake depths; 12 pipes for Lake Nyos (four each at 185, 195 and 205 m) and three pipes for Lake Monoun (at 70, 80 and 90 m). The stepped degassing at different depths is intended to keep the maximum stability of the lakes. The proposed degassing operation was simulated using the code for both lakes. In 5 years, approximately 50% of currently dissolved CO₂ in Lake Nyos and 90% in Lake Monoun will be removed. The expected changes in the thermal and chemical structures of the lakes as degassing proceeds will be most easily monitored with a carefully calibrated CTD equipped with a pH sensor. The simulation indicates that the discharged degassed water will sink to a level of neutral buoyancy, i.e. to a maximum of 70 m at Lake Nyos and 35 m at Lake Monoun. There would be no possibility of triggering a gas explosion by this plunge of discharged water because the water present there would have already been replaced by water at lower CO₂ concentration, during the degassing from shallower pipes.     

Application of CCA for study on modern lake diatoms and environment in the Tibetan Plateau

The relations between lake surface sediment diatoms and water environmental variables were revealed effectively by use of a new multivariate canonical correspondence analysis (CCA) based on 45 lakes in the Tibetan Plateau. Water depth, conductivity, Cl^?, Mg²⁺, K⁺ and pH, identified from 12 contemporary water environmental variables, can significantly and independently explain the diatom distributions (p<0.05). The first two axes (λ1=0.34, λ2=0.27) capture 16.1% of the variance in the species data, and account for 57.4% of the variance in diatom-environment relationship. The deletion of redundant environmental variables and unusual samples do not influence the explanation to diatom data. The final CCA result indicates that the water depth and the salinity are the two important environmental gradients and influence the diatom distribution in the plateau lakes. The water depth correlates with axis 1, while conductivity, Cl^?, Mg²⁺ and K⁺, indicating the direction of salinity changes, correlate with both of the first two axes. The definition of diatom-environment model may provide a basis for further quantitative inference on diatom-environment transfer function.     

Characteristics of δ13CDIC in lakes on the Tibetan Plateau and its implications for the carbon cycle

Dissolved inorganic carbon isotope (δ¹³C_DIC) is an important tool to reveal the carbon cycle in lake systems. However, there are only few studies focusing on the spatial variation of δ¹³C_DIC of closed lakes. Here we analyze the characteristics of δ¹³C_DIC of 24 sampled lakes (mainly closed lakes) across the Qiangtang Plateau (QTP) and identify the driving factors for its spatial variation. The δ¹³C_DIC value of these observed lakes varies in the range of ? 15·0 to 3·2‰, with an average value of ? 1·2‰. The δ¹³C_DIC value of closed lakes is close to the atmospheric isotopic equilibrium value, much higher than that in rivers and freshwater lakes reported before. The high δ¹³C_DIC value of closed lakes is mainly attributed to the significant contribution of carbonate weathering in the catchment and the evasion of dissolved CO₂ induced by the strong evaporation of lake water. The δ¹³C_DIC value of closed lakes has a logarithmic correlation with water chemistry (TDS, DIC and pCO₂), also suggesting that the evapo‐concentration of lake water can influence the δ¹³C_DIC value. The δ¹³C_DIC value shows two opposite logarithmic correlations with lake size depending on the δ¹³C_DIC range. This study suggests that the δ¹³C in carbonates in lacustrine sediments can be taken as an indicator of lake volume variation in closed lakes on QTP. Copyright © 2011 John Wiley & Sons, Ltd.     

Motile phytoplankton species such as Gonyostomum semen can significantly reduce CO2 emissions from boreal lakes

Boreal lakes typically have high levels of allochthonous dissolved organic matter (DOM), causing light limitation of photosynthetic CO₂ consumption while stimulating CO₂ production. They are therefore considered as important sources of atmospheric CO₂. However, boreal lakes are also experiencing a marked expansion of bloom-forming motile phytoplankton organisms that can circumvent the shading effect of DOM by performing phototaxis and that thus might have an impact on the lakes’ CO₂ balance. We tested this idea in a DOM-rich lake using the widespread raphidophyte flagellate Gonyostomum semen as model organism. Employing continuous field measurements, we found that G. semen can reduce the partial pressure of CO₂ even at low algal densities. Periods with high algal densities were associated with CO₂ undersaturation and invasion of atmospheric CO₂. The mean daily net losses of CO₂ to the atmosphere during and after a G. semen bloom were estimated at 12.9 and 70.4 mmol C m^–2 day^–1, respectively. G. semen caused steep pCO₂ gradients in space and time, which make it difficult to unveil the species’ impact without employing continuous pCO₂ vertical profiling. This suggests that the effect of G. semen on the CO₂ balance of boreal lakes might have been overlooked in the past. Taken together, our data suggest that G. semen can significantly reduce the CO₂ emissions from boreal lakes despite high concentrations of allochthonous DOM. G. semen and other motile phytoplankton species should therefore be considered when estimating CO₂ emissions from boreal lakes, especially if these organisms continue their expansion.     

The most acidified Austrian lake in comparison to a neutralized mining lake

This study investigated two mining lakes located in the north of Lower Austria. These lakes arose 45 years ago when open cast lignite mining ceased. The lakes are separated by a 7-m wide dam. Due to the oxidation of pyrite, both lakes have been acidified and exhibit iron, sulphate, and heavy metal concentrations several orders of magnitude higher than in circumneutral lakes. The water column of both lakes is divided into two layers by a pronounced chemocline. The smaller mining lake (AML), with pH close to of 2.6, is the most acidic lake in Austria, whereas flooding with stream water and by drainage from the surrounding fields neutralized the adjacent larger pit lake. The goal of our study was to investigate the effect of flooding on its physical, chemical and biological properties, in comparison to the pristine AML. Even relative to other extremely acidic lakes, the flora and fauna in the AML was reduced and composed of only two flagellate, one ciliate, and one rotifer species. The simplified pelagic food web in the mixolimnion consisted of heterotrophic bacteria, the mixotrophic flagellates Chlamydomonas acidophila and Ochromonas sp., the ciliate Oxytricha sp., and the rotifer Cephalodella sp. The latter two are as yet undescribed new species. The heliozoan Actinophrys sp. that may act as top predator occurred only in low abundance. The euglenid Lepocinclis buetschlii formed a stable deep chlorophyll maximum (DCM) at 7 m depth. Highest cell numbers of L. buetschlii in the DCM exceeded 10⁸ L^?1. The neutralized mining lake harboured higher plankton diversity similar to that of natural circumneutral lakes. A peak of at least 16 different phytoplankton taxa was observed during summer. The zooplankton consisted of several copepod species, daphnids and other cladocerans, and at least six different rotifer species. Several fish species occurred in the neutralized lake. Although the effect of non-permanent flooding was largely sustainable, interannual fluctuations of the pH affected the plankton community and reduced its species diversity.     

Lake morphometry and wind exposure may shape the plankton community structure in acidic mining lakes

Acidic mining lakes (pH <3) are specific habitats exhibiting particular chemical and biological characteristics. The species richness is low and mixotrophy and omnivory are common features of the plankton food web in such lakes. The plankton community structure of mining lakes of different morphometry and mixing type but similar chemical characteristics (Lake 130, Germany and Lake Langau, Austria) was investigated. The focus was laid on the species composition, the trophic relationship between the phago-mixotrophic flagellate Ochromonas sp. and bacteria and the formation of a deep chlorophyll maximum along a vertical pH-gradient. The shallow wind-exposed Lake 130 exhibited a higher species richness than Lake Langau. This increase in species richness was made up mainly by mero-planktic species, suggesting a strong benthic/littoral - pelagic coupling. Based on the field data from both lakes, a nonlinear, negative relation between bacteria and Ochromonas biomass was found, suggesting that at an Ochromonas biomass below 50 μg C L⁻¹, the grazing pressure on bacteria is low and with increasing Ochromonas biomass bacteria decline. Furthermore, in Lake Langau, a prominent deep chlorophyll maximum was found with chlorophyll concentrations ca. 50 times higher than in the epilimnion which was build up by the euglenophyte Lepocinclis sp. We conclude that lake morphometry, and specific abiotic characteristics such as mixing behaviour influence the community structure in these mining lakes.     

Phytoplankton response to pH rise in a N-limited floodplain lake: relevance of N2-fixing heterocystous cyanobacteria

Phosphorus has been traditionally regarded as the controlling nutrient for phytoplankton growth, however, N-limitation is likely to occur in several environments. For example, nitrogen is considered the main nutrient limiting phytoplankton in floodplain lakes of the Paraná River basin. However, N₂-fixing heterocystous cyanobacteria (N₂-cyano) are usually absent in these water bodies. The low pH values frequently found may limit the development of these algae. We hypothesise that long-term lake isolation allows conspicuous phytoplankton growth during summer, resulting in high photosynthetic rates and pH. This scenario combined with low DIN (dissolved inorganic nitrogen) would favour the development of N₂-cyano. Phytoplankton composition was studied during 16 months in a vegetated and isolated floodplain lake in the Paraná basin. Additionally, pH was artificially increased in in situ mesocosms to test effects on phytoplankton structure. Lake phytoplankton was dominated by flagellates (cryptophytes and euglenophytes) and small coccoid algae (chlorophytes and colonial cyanobacteria). Algal biomass was highest during warm periods. Although pH increased up to 8.8 during the second summer period, N₂-cyano remained rare, most likely because of the high DIN concentration recorded. The alkalophilic diatom Cyclotella meneghiniana dominated and was positively correlated with pH. Conversely, PO₄⁼ concentrations in the mesocosm experiment were high and DIN remained relatively low. pH enhancement in the treated mesocosms (avg. pH = 8.2) promoted the development of N₂-cyano (Anabaena spp.) and C. meneghiniana, which after 1 month of incubation accounted together for 50% of the biomass in contrast to less than 1.6% in control containers. Our results support the hypothesis that during the warm season and under low DIN concentration, high pH favours N₂-cyano growth in these lakes. We provide new evidence supporting the idea that even under optimal nutrient conditions, N₂-cyano do not thrive unless other requirements are satisfied.     

不同丰枯情景下长江三角洲非通江湖泊(滆湖、淀山湖和阳澄湖)有色可溶性有机物组成特征

长三角地区大部分湖泊为非通江湖泊,地势低平,港汊及闸坝众多,水流宣泄不畅,水力滞留时间较长,加之周边地区城镇人口稠密.因此与水滞留时间短的通江湖泊相比,非通江湖泊的有色可溶性有机物(CDOM)来源和组成具有差异性.本文选取了3个重要的中型非通江供水湖泊——滆湖、淀山湖和阳澄湖,对枯水期、平水期、丰水期3种水文情景下CDOM组成结构变化特征进行分析,从而进一步揭示该类湖泊CDOM来源和对水文情景响应的内在机理.结果表明:滆湖、淀山湖和阳澄湖通过平行因子分析法得到2种类腐殖质(C1和C4)和2种类蛋白质(C2和C3),湖泊CDOM结构受到降雨事件和人类活动的双重影响.三个湖泊类蛋白质的高值在空间上主要集中在人类活动频繁的湖区,并且类蛋白质平均荧光强度与叶绿素a浓度相关性较差,说明湖泊类蛋白质组分受到内源藻类降解、外源人类生产生活排放双重作用的影响.三个湖泊类蛋白质的平均荧光强度和总氮浓度均在枯水期显著高于丰水期,说明降雨量的增加可以稀释湖泊有机质浓度;同时,陆源类腐殖质C1与溶解性有机碳、总氮、总磷、叶绿素a浓度呈显著正相关,并且随着降雨量增加,类蛋白质的占比逐渐降低,滆湖从86.84%降低至62.49%,淀山湖从96.53%降低至90.56%,阳澄湖从98.40%降低至96.26%,说明降雨事件也可以增强径流的冲刷作用,携带更多腐殖化程度高的陆源有机质进入湖泊.本研究发现降雨过程和人类活动共同作用于滆湖、淀山湖和阳澄湖CDOM库,研究结果可以为进一步保障太湖流域人类用水安全提供参考依据.     

青藏高原陆相介形类的分布特征及其对生态环境的响应

统计分析了青藏高原不同区域水体环境表层沉积物陆相介形类的属种分布特征,探讨了介形类在不同水环境下（盐度、pH值及水深）对生态环境的响应.结果显示,青藏高原现生介形类共计21属67种,其中Candona candida、Ilyocypris bradyi、Eucypris inflata、Limnocythere dubiosa、Limnocythere inopinata、Paracypricerus angulata、Leucocytherella sinensis和Leucocythere mirabilis为青藏高原地区的常见种.湖泊、河流、洼地和湿地4类水体环境中,湖泊中介形类最为丰富,达19属62种;青藏高原东北部（祁连山和柴达木盆地）、北部（昆仑山）、西部和南部不同区域的介形类常见种存在较大差异,可能是区域海拔、pH值和盐度综合作用的结果.淡水和微咸水环境介形类属种数量较咸水及盐湖中丰富,分别有17属41种和13属42种,Limnocythere dubiosa （0.52~90.6 g/L）和Leucocythere mirabilis（0.51~174.63 g/L）在淡水、咸水及盐湖中均有出现,适应盐度范围较广;pH值在8.0~10.0范围内介形类属种多样性最丰富,表明大部分介形类具有嗜碱性的特征;青藏高原陆相介形类属种多样性随水深的增加而降低,浅湖（0~15 m）中介形类属种最为丰富,达到17属52种,其中Candona candida（0.2~80 m）和Leucocythere dorsotuberosa（0.3~110 m）从滨湖至深湖区均有分布,二者均具有较大的水深适应范围.     

Dynamics of phosphorus in sediments of a naturally acidic lake

The mechanisms which controls the fixation and/or release of P in sediment of an extremely acidic lake（pH = 2.0 to 3.0） and its response to the influence of eutrophic urban waste water were investigated.The results,in the chemical composition,in the mineralogy of the sediment and in the material as obtained from sediment traps,show that the lake sediments are mainly volcanic material reflecting volcanic features of the basin.The sedimentation rate calculated for the lake(2.5×10^-2 mg m^-2 day^-1) was higher than that observed in other similar glacial lakes in both Andean Patagonia and also elsewhere in the world.The Total Phosphorus concentration in sediments was higher than figures reported by other authors for mining acid lakes,and the main fraction of P was found associated with organic matter.There was no control by Fe or Al on P,because both are in solution at pH < 3.0.It was concluded that changes in the natural input of nutrients（derivatives of Copahue volcano fluid,the discharge of sewage,or basin run-off） are responsible for a high concentration of SRP and N-NH₄⁺ in the lake.Laboratory experiments showed that sediments have no ability to retain phosphorus and a continuous release of P from the sediments into the water column was observed.The assays where the pH was artificially increased showed that the P still remains in solution until at least pH 7.0.It was concluded that changes in the natural input of nutrients due to:1) the volcanic fluids,2) the increase in sewage charges,or 3) surface runoff upstream,maintain a high trophic state with high concentrations of dissolved P and N-NH₄⁺,although the threshold of neutral pH in the lake is exceeded.This study will enable a better understanding about of the mechanism of release/fixation of phosphorus in acidic sediments in order to assist in making decisions regarding the conservation and management of this natural environment.     

Chemical characteristics of the crater lakes of Popocatetetl, El Chichon, and Nevado de Toluca volcanoes, Mexico

Three crater lakes from Mexican volcanoes were sampled and analyzed at various dates to determine their chemical characteristics. Strong differences were observed in the chemistry among the three lakes: Nevado de Toluca, considered as dormant, El Chichón at a post-eruptive stage, and Popocatépetl at a pre-eruptive stage. Not surprisingly, no influence of volcanic activity was found at the Nevado de Toluca volcano, while the other volcanoes showed a correlation between the changing level of activity and the evolution of chemical trends. Low pHs (<3.0) were measured in the water from the active volcanoes, while a pH of 5.6 was measured at the Nevado de Toluca Sun lake. Changes with time were observed at Popocatépetl and El Chichón. Concentrations of volcanic-gas derived species like Cl⁻, SO₄²⁻ and F⁻ decreased irregularly at El Chichón from 1983 until 1997. Major cations concentrations also diminished at El Chichón. A 100% increase in the SO₄²⁻ content was measured at Popocatépetl between 1985 and 1994. An increase in the Mg/Cl ratio between 1992 (Mg/Cl=0.085) and 1994 (Mg/Cl=0.177) was observed at Popocatépetl, before the disappearance of the crater lake in 1994. It is concluded that chemical analysis of crater lakes may provide a useful additional tool for active-volcano monitoring.     

Biogeochemical processes involving dissolved CO2 and CH4 at Albano, Averno, and Monticchio meromictic volcanic lakes (Central–Southern Italy)

This paper focuses on the chemical and isotopic features of dissolved gases (CH₄ and CO₂) from four meromictic lakes hosted in volcanic systems of Central–Southern Italy: Lake Albano (Alban Hills), Lake Averno (Phlegrean Fields), and Monticchio Grande and Piccolo lakes (Mt. Vulture). Deep waters in these lakes are characterized by the presence of a significant reservoir of extra-atmospheric dissolved gases mainly consisting of CH₄ and CO₂. The δ¹³C-CH₄ and δD-CH₄ values of dissolved gas samples from the maximum depths of the investigated lakes (from ?66.8 to ?55.6?‰ V-PDB and from ?279 to ?195?‰ V-SMOW, respectively) suggest that CH₄ is mainly produced by microbial activity. The δ¹³C-CO₂ values of Lake Grande, Lake Piccolo, and Lake Albano (ranging from ?5.8 to ?0.4?‰ V-PDB) indicate a significant CO₂ contribution from sublacustrine vents originating from (1) mantle degassing and (2) thermometamorphic reactions involving limestone, i.e., the same CO₂ source feeding the regional thermal and cold CO₂-rich fluid emissions. In contrast, the relatively low δ¹³C-CO₂ values (from ?13.4 to ?8.2?‰ V-PDB) of Lake Averno indicate a prevalent organic CO₂. Chemical and isotopic compositions of dissolved CO₂ and CH₄ at different depths are mainly depending on (1) CO₂ inputs from external sources (hydrothermal and/or anthropogenic); (2) CO₂–CH₄ isotopic exchange; and (3) methanogenic and methanotrophic activity. In the epilimnion, vertical water mixing, free oxygen availability, and photosynthesis cause the dramatic decrease of both CO₂ and CH₄ concentrations. In the hypolimnion, where the δ¹³C-CO₂ values progressively increase with depth and the δ¹³C-CH₄ values show an opposite trend, biogenic CO₂ production from CH₄ using different electron donor species, such as sulfate, tend to counteract the methanogenesis process whose efficiency achieves its climax at the water–bottom sediment interface. Theoretical values, calculated on the basis of δ¹³C-CO₂ values, and measured δ¹³C_TDIC values are not consistent, indicating that CO₂ and the main carbon-bearing ion species (HCO₃ ^?) are not in isotopic equilibrium, likely due to the fast kinetics of biochemical processes involving both CO₂ and CH₄. This study demonstrates that the vertical patterns of the CO₂/CH₄ ratio and of δ¹³C-CO₂ and δ¹³C-CH₄ are to be regarded as promising tools to detect perturbations, related to different causes, such as changes in the CO₂ input from sublacustrine springs, that may affect aerobic and anaerobic layers of meromictic volcanic lakes.     

Phytoplankton abundance, biomass and diversity within and between Pantanal wetland habitats

During the high water season, the flooding reduces environmental heterogeneity in aquatic ecosystems of the Pantanal wetland. When the water level recedes, lakes and channels are formed as individual systems. Therefore, we expected the spatial heterogeneity during the low water phase resulting in changes on biological communities leading to high phytoplankton abundance, biomass and diversity within and between habitats. To test this hypothesis, we analyzed eight freshwater systems (five oxbow lakes, two channels, and the river) during the low water period. Phytoplankton biomass, abundance, diversity (alpha, beta, gamma) and diversity metrics as richness (species per sample), Shannon diversity (H′) and evenness were measured in all systems along with nutrient concentrations, zooplankton and bacteria abundances. We found 97 species as gamma diversity. The alpha diversity was unexpectedly low in comparison to most other South American floodplain systems (38 species in river, 24 in channels and 29 in lakes). Also, the systems are relatively similar in composition (beta diversity, 28%). Connectivity differences between systems highlighted differences in phytoplankton abundance and biomass (fresh weight) ranging from 1428 ind mL⁻¹ (river) to 3710 ind mL⁻¹ (lakes) and from 0.71 mg L⁻¹ (river) to 2.9 mg L⁻¹ (lakes), respectively. However, our results did not indicate significant differences in phytoplankton species richness between the systems during the low water. Our main conclusions are that local factors may be responsible for changes on phytoplankton community and the time of isolation during the low water phase was not sufficient to promote changes in phytoplankton diversity between the habitats.     

Effects of permafrost degradation on thermokarst lake hydrochemistry in the Qinghai-Tibet Plateau,China

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.     

Environmental impacts on the distribution of microbial tetraether lipids in Chinese lakes with contrasting pH: Implications for lacustrine paleoenvironmental reconstructions

Glycerol dialkyl glycerol tetraethers(GDGTs) in lake sediments are useful biomarkers for the continental paleoclimatic reconstruction. However, the environmental controls on the distribution of these compounds, in particular the 6-methyl isomers of bacterial branched GDGTs(bGDGTs), in the lakes with contrasting pH, are still unknown, hindering their application for paleo-reconstructions. Here, we investigated the environmental impacts on the distribution of GDGTs in 17 alkaline lakes and 1 acid lake in China. It was found that the dissolved oxygen content in water column may have an impact on the distribution of archaeal isoprenoid GDGTs(iGDGTs) by causing the change in archaeal communities. The ratio of GDGT-0/crenarchaeol increases with decreasing oxygen content, indicating that the relative abundance of anaerobic methanogenic archaea or Miscellaneous Crenarchaeotic Group(MCG) vs. aerobic Thaumarchaeota is controlled by the oxygen content dissolved in water of these lakes. Thaumarchaeota are likely to contribute only a small proportion of iGDGTs in the relatively oxygen-depleted lakes, and thus TEX_(86) is not suitable for the reconstruction of the surface temperature of these lakes. The abundance ratio of iGDGTs to bGDGTs(R_(i/b)) appears to show no relationship with water pH in all the lakes, but exhibits a significant positive correlation with the water depth of the acid Lake Qinghai in Tengchong. As expected, the methylation degree of bGDGTs(MBT′) was found to correlate with both mean annual air temperature(MAT) and water pH, and the cyclization degree of bGDGTs(CBT) correlates only with water pH in these lakes. However, the MBT′_(5ME), an index to measure the methylation degree of 5-methyl bGDGTs, exhibits no relationship with MAT, whereas MBT′_(6ME), the methylation degree of 6-methyl bGDGTs, was found to correlate significantly with MAT. This is opposite to the situation observed in the global soils, pointing to a different adaptation of b GDGT-producing bacteria to environmental variables or different microbial sources of bGDGTs in these lakes. The relative abundance of 6- vs. 5-methyl bGDGTs is controlled by pH in these lakes, similar to that observed in worldwide soils. Hence, the isomer ratio(IR) of 6-methyl bGDGTs or CBT′ can be used as a proxy for water pH, although they might be influenced by other environmental factors including temperature in the lakes with a narrow range of pH.     

柴达本盆地沙下盐湖的卤水化学及矿物沉积特征——以察尔汗盐湖区北部外围地带为例

对柴达木盆地察尔汗盐湖区外围沙下盐湖的卤水及沉积进行了综合研究。沙下盐湖卤水化学组成与地表径流和开放性盐湖卤水之间存在明显的差异性,具有高Na⁺+Cl^-、低Mg²⁺+Ca²⁺+SO₄^2-、贫K⁺+CO₃^2-+HCO₃^-等特征。沙下盐湖析盐层位含有新生矿物并夹带碎屑矿物,其盐类矿物组合为:石盐+羟氯镁铝石+光卤石。25℃等温蒸发相图表明,其卤水演化方向往光卤石析出区迁移,在穿越上覆盖层通道中卤水发生的物理化学反应与独特的沉积特征,可以作为继续寻找沙下盐湖的指导。     

Modern and Pleistocene boron isotope composition of the benthic foraminifer Cibicidoides wuellerstorfi

Here we present the first species-specific study of boron isotopes in the epibenthic foraminifer species Cibicidoides wuellerstorfi. Coretop samples from a water depth profile from 1000 to 4500 m on the northern flank of the Walvis Ridge are 4.4‰ lower than the values expected, based on calculations of the δ¹¹B_borate of ambient seawater. Similar values for this foraminifer species are presented from ODP site 668B at the Sierra Leone Rise, in the equatorial Atlantic. The consistency between data of the same species suggests the offsets are primary, rather than diagenetic. Glacial C. wuellerstorfi from ODP 668B and Walvis Ridge have boron isotope compositions only slightly different to interglacial samples, that is no larger than + 0.10 pH units, or + 23 µmol kg^− 1 in [CO₃²⁻] above the reconstructed glacial lysocline, and − 0.07 pH units, or − 14 µmol kg^− 1 in [CO₃²⁻] below. We use these results to suggest that glacial deep water pH in the Atlantic was similar to interglacial pH. The new data resolve the inconsistency between the previously reported high bottom water pH and the lack of significant carbonate preservation of the glacial deep ocean.     

Response of surface water chemistry to reduced levels of acid precipitation: comparison of trends in two regions of New York,USA

In light of recent reductions in sulphur (S) and nitrogen (N) emissions mandated by Title IV of the Clean Air Act Amendments of 1990, temporal trends and trend coherence in precipitation (1984–2001 and 1992–2001) and surface water chemistry (1992–2001) were determined in two of the most acid‐sensitive regions of North America, i.e. the Catskill and Adirondack Mountains of New York. Precipitation chemistry data from six sites located near these regions showed decreasing sulphate (SO₄^2?), nitrate (NO₃^?), and base cation (C_B) concentrations and increasing pH during 1984–2001, but few significant trends during 1992–2001. Data from five Catskill streams and 12 Adirondack lakes showed decreasing trends in SO₄^2? concentrations at all sites, and decreasing trends in NO₃^?, C_B, and H⁺ concentrations and increasing trends in dissolved organic carbon at most sites. In contrast, acid‐neutralizing capacity (ANC) increased significantly at only about half the Adirondack lakes and in one of the Catskill streams. Flow correction prior to trend analysis did not change any trend directions and had little effect on SO₄^2? trends, but it caused several significant non‐flow‐corrected trends in NO₃^? and ANC to become non‐significant, suggesting that trend results for flow‐sensitive constituents are affected by flow‐related climate variation. SO₄^2? concentrations showed high temporal coherence in precipitation, surface waters, and in precipitation–surface water comparisons, reflecting a strong link between S emissions, precipitation SO₄^2? concentrations, and the processes that affect S cycling within these regions. NO₃^? and H⁺ concentrations and ANC generally showed weak coherence, especially in surface waters and in precipitation–surface water comparisons, indicating that variation in local‐scale processes driven by factors such as climate are affecting trends in acid–base chemistry in these two regions. Copyright © 2005 John Wiley & Sons, Ltd.