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1.
Yuji Ito  Kazuro Momii 《水文研究》2015,29(9):2232-2242
Although few reports have described long‐term continuous anoxia in aquatic systems, Lake Ikeda in Japan experienced such conditions in the hypolimnion from 1990 to 2010. The present study aimed to assess temporal fluctuations in the lake's thermal stability from 1978 to 2011 to understand the influence of regional climate change on hypolimnetic anoxia in this lake. Because complete vertical mixing, which supplies dissolved oxygen (DO) to the hypolimnion, potentially occurs on February, we calculated the Schmidt stability index (S) in February and compared it with hypolimnetic DO dynamics. Vertical water temperature profiles were calculated using a one‐dimensional model, and calculated temperatures and meteorological data were used to analyse annual fluctuations in water temperatures, thermocline depth, meteorological variables and S. We estimated that mean annual air and volume‐weighted water temperatures increased by 0.028 and 0.033 °C year?1, respectively, from 1978 to 2011. Between 1986 and 1990, S and water temperature increased abruptly, probably due to a large upwards trend in air temperature (+0.239 °C year?1). We hypothesize that a mixing regime that lacked overturn took effect at this time and that this regime lasted until 2011, when S was particularly small. These results demonstrate that abrupt climate warming in the late 1980s likely triggered the termination of complete mixing and caused the 21‐year period of successive anoxia in Lake Ikeda. We conclude that the lake response to a rapid shift in regional climate conditions was a key factor in changing the hypolimnetic water environment and that thermal stability in winter is a critical environmental factor controlling the mixing regime and anoxic conditions in deep lakes. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

2.
Paleomagnetic measurements have been made on a continuously sampled, 5-m section of a core from Clear Lake, California. The sediments studied span an 8000-year interval centered at 25,000 years B.P., the approximate date of the large-scale, counterclockwise loop of the magnetic vector recorded in sediments from Mono Lake, California. The data from Mono Lake have been interpreted as a geomagnetic excursion with a duration of 600–1000 years. Because Clear Lake is only 320 km from Mono Lake and since each sample from Clear Lake represents 26 years of sedimentation, the magnetic signature of the Mono Lake geomagnetic excursion should be recorded in detail in the Clear Lake samples. Aside from a generally uniform shallowing of inclinations due to compaction, the paleomagnetic record from Clear Lake contains no anomalous features which would correspond to the Mono Lake excursion. Thus it has yet to be shown that the Mono Lake excursion was recorded anywhere besides Mono Lake. Even if the existence of the excursion is ultimately confirmed, its usefulness as a magnetostratigraphic horizon is limited.  相似文献   

3.
Tommeliten is a prominent methane seep area in the Central North Sea. Previous surveys revealed shallow gas-bearing sediments and methane gas ebullition into the water column. In this study, the in situ methane flux at Tommeliten is re-assessed and the potential methane transport to the atmosphere is discussed, with regards to the hydrographic setting and gas bubble modeling. We have compiled previous data, acquired new video and acoustic evidence of gas bubble release, and have measured the methane concentration, and its C-isotopic composition in the water column. Parametric subbottom sonar data reveal the three-dimensional extent of shallow gas and morphologic features relevant for gas migration. Five methane ebullition areas are identified and the main seepage area appears to be 21 times larger than previously estimated. Our video, hydroacoustic, subbottom, and chemical data suggest that ～1.5×106 mol CH4/yr (～26 tons CH4/yr) of methane gas is being released from the seepage area of Tommeliten. Methane concentration profiles in the vicinity of the gas seeps show values of up to 268 nM (～100 times background) close to the seafloor. A decrease in δ13C-CH4 values at 40 m water depth indicates an unknown additional biogenic methane source within the well oxygenated thermocline between 30 and 40 m water depth. Numerical modeling of the methane bubbles due to their migration and dissolution was performed to estimate the bubble-derived vertical methane transport, the fate of this methane in the water column, and finally the flux to the atmosphere. Modeling indicates that less than ～4% of the gas initially released at the seafloor is transported via bubbles into the mixed layer and, ultimately, to the atmosphere. However, because of the strong seasonality of mixing in the North Sea, this flux is expected to increase as mixing increases, and almost all of the methane released at the seafloor could be transferred into the atmosphere in the stormy fall and winter time.  相似文献   

4.
Atmospheric exchanges largely dominate the heat budget of deep lakes in temperate regions. Heat import and export by through-flows is of much lower entity and has been neglected or simplified in many numerical thermodynamic models of lakes. This is often due either to the unavailability of data for inflows and outflows, or to the difficulties in forecasting the evolution of their discharge and temperature in climate change studies. While disregarding through-flows may seem correct, riverine intrusions can bring warmer water than the deep hypolimnetic one to the lower metalimnion and upper hypolimnion, where sunlight does not penetrate and mixing is poor. For holomictic lakes with significant inflow contributions, this can affect the thermal structure at intermediate depths, hampering any numerical model which neglects through-flows. This study focuses on a relevant basin under such aspect, Lake Maggiore (Northern Italy/Southern Switzerland), which drains the rainiest watershed of the Southern Alps. First, we quantify to what extent a one-dimensional fixed-level model ignoring through-flows is able to predict the observed evolution of the thermal structure of the lake and the improvements resulting from reproducing the main inflows and outflows. Then, we directly discuss the influence of through-flows on the thermodynamic structure of Lake Maggiore. The General Lake Model (GLM) was here adopted, reproducing the 1998–2014 period, spanning years with different meteorological and hydrological features. Results show that a calibrated enclosed-lake model can give satisfactory results only if it employs an unrealistically low light extinction coefficient to allow heating of the deep metalimnion and hypolimnion, whose real warming strongly depends on interflows.  相似文献   

5.
Topographic data from the Shuttle Radar Topography Mission (SRTM) captures the permanent deformation of a prominent highstand of Mono Lake, California USA. Deformation of the Dechambeau Ranch highstand shoreline was measured using the elevation of the beach berm—shoreline bluff break-in-slope. Point source models and a boundary element dike model were used to approximate the source of deformation underneath the northern end of the Mono Craters. The point source model could not adequately explain the observed deformation. The dike model yielded the best results for a NW trending dike dipping 60° NE and inflated to widths greater than 60 m. The results suggest that the geometry of the source is more complex than a simple vertical dike and that the deformation is better explained with a dipping dike following a normal fault, or an elongated cryptodome.  相似文献   

6.
Decomposition of methane hydrates on the continental margins is a potentially significant source of atmospheric methane, but the input depends upon the poorly understood fate of the hydrocarbon bubbles rising from the sea floor. During a field trip to the Gulf of Mexico, three different seepages were imaged and analyzed. Three different imaging techniques were tried (side, front, and back illumination), of which back illumination produced the best results. The images were analyzed and the size-dependent bubble distribution, mass flux, and rise speeds determined. The total observed gas flux was 62.3×10−3 mol s−1, primarily methane, of which a single vent produced seven times the next largest vent. Of this major vent, 50% of the bubble mass was contained in the largest bubbles, r>5500 μm. The vertical velocities demonstrated that these bubbles were heavily contaminated with oil, which was also corroborated by bubble shape and oscillation observations.  相似文献   

7.
Data on the cycling of Cu and Zn in two eutrophic lakes are presented: Lake Greifen that becomes seasonally anoxic in its hypolimnion and Lake Sempach that is aerated during winter and oxygenated during summer. They suggest that hypolimnetic oxygenation 1) enhances the release of copper from the sediment but 2) also accelerates the entrapment and deposition of Cu and Zn by freshly formed Mn- and Fe-oxides.  相似文献   

8.
Long-term stratification of the deep hypolimnetic waters of the northern basin of Lake Lugano (Lago di Lugano) has resulted in a lack of deep-water renewal which has persisted for decades. Tritium-helium age measurements reveal that deep water has not been in contact with the atmosphere since the 1960s. Higher primary production associated with the significant increase in phosphorus concentration which occurred at this time resulted in greater autochthonous gross sedimentation rates, increasing the rate of mineralization and, consequently, the rate of release of dissolved solids (mainly HCO 3 - and Ca2+) into the deep hypolimnion. This gave rise to an intensification of the stratification and to a consequent reduction in the vertical exchange of hypolimnetic water layers. Today, the density stabilizing effect of ion release due to mineralization in the deep water is four to five times greater than the destabilizing effect of the geothermal heat flux from the earth's interior. It is known from laboratory experiments that such small density gradient ratios are likely to give rise to double-diffusive instabilities. However, even rudimentary mass balance calculations of biogeochemical components indicate that shear-induced turbulence, most likely generated by bottom currents, mixes far more efficiently than double diffusion. In the future, the biogenic density stratification is likely to persist in the deep water, unless the upward ion flux, driven by primary production, decreases by a factor of four to five.  相似文献   

9.
Organic matter mineralisation in the hypolimnion of an eutrophic Maar lake   总被引：2，自引：0，他引：2  
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10.
A well-dated, 3.3-m section of deep-lake clays from pluvial Lake Lahontan, now exposed along the shore of Pyramid Lake, Nevada, has provided a paleomagnetic record of secular variation during the time interval 25,000–36,000 years B.P. The measured ranges of inclination and declination are 40° and 75°, respectively, and represent values which are comparable to observed secular variation at northern mid-latitudes. These results and those from a previously reported study from Clear Lake, California, together provide strong evidence for the conclusion that northern California and western Nevada were not affected by any geomagnetic excursion during the time interval 21,000–36,000 years B.P. Such a conclusion puts severe constraints on the nature and timing of the proposed Mono Lake, Laschamp and Lake Mungo geomagnetic excursions.  相似文献   

11.
There were two phytoplankton blooms captured by remote sensing in Lake Michigan in 1998, one from March to May, and one during June. In this paper, those phytoplankton blooms were simulated by a coupled physical–biological model, driven by observed meteorological forcing in 1998. The model reasonably reproduced the lake currents. The biological model results, with and without riverine nutrient loading, were compared with the remote sensing data. A 3-month-long donut-like phytoplankton bloom that appeared in southern Lake Michigan was reasonably well simulated only when riverine input was included, indicating the importance of riverine nutrient input for supporting the growth of phytoplankton in Lake Michigan. The model with riverine input also captured a second event-driven phytoplankton bloom during June with weaker magnitude that occurred in mid-south Lake Michigan, which lasted for about 20 days. The major reason for the weaker bloom in June was that vertical mixing in the hydrodynamic model was too weak (leading to a mixed-layer depth of 20 m) to bring the bottom nutrient-rich water up to the epilimnion. High chlorophyll concentration that persisted in Green Bay for almost a year was simulated with less intensity.  相似文献   

12.
In Lake Baldegg, Switzerland (surface area 5.3 km2, maximum depth 66 m) the analysis of data from moored instrument systems (atmospheric boundary layer, lake temperature distribution, bottom currents) was correlated to the long-term development of vertical mixing as seen from profiles of natural isotopes (radon-222, tritium and helium-3) and chemical species. The investigation shows: 1. Vertical mixing coefficients below 25 m are small. Consequently the vertical concentration distribution of sediment emanating species in the deep hypolimnion is controlled by the bottom topography. 2. Renewal of deep hypolimnic water is significant even during stratification. 3. Weakly damped internal waves characterize the internal dynamics during stratification. 4. Horizontal bottom currents play an important role in the hypolimnion mixing and can be correlated to internal waves during stratification.  相似文献   

13.
Recent field investigations have shown that differential mixing causes a significant density driven exchange between the two eastern basins of Lake Lucerne, Gersauersee and Urnersee. The long term vertical mixing characteristics during the winter mixing period and the influence of the resulting exchange flow on water quality are examined through the analysis of ten years (1964 to 1974) of monthly limnological data. Vertical homogenization of the state variables (temperature, electric conductivity and density) occurred on average once every two years in Urnersee, but not once in Gersauersee where the ten year average winter mixed-layer depth extended to 120 m. In Urnersee intense mixing was evident over the whole water column for every year. In contrast intense mixing in the deeper layers of Gersauersee occurred approximately once every four years. A three layer model shows that theT/ 20 signature of the Gersauersee intermediate water correlates best with the Urnersee deep water confirming the regular occurrence of a density driven exchange. For the lower layers the oxygen consumption rate in Urnersee was approximately twice that for Gersauersee. It appears the exchange flow can act either as a source or sink for the oxygen balance of Urnersee deep water. The larger rate in Urnersee hypolimnion might also be associated with the larger sediment input to this system. Density driven exchange resulting from differential mixing is probably an important contribution to the vertical water replenishment in many lakes.  相似文献   

14.
Vertical distributions of the cyanobacterium Microcystis aeruginosa are examined in a shallow lake in relation to mixing and thermal stratification over three days. A model of buoyancy regulation by Microcystis aeruginosa, applicable for turbulent environments, is coupled with a one-dimensional hydrodynamic model. The coupled model is applied to Thomsons Lake in Western Australia to examine the relationship between buoyancy regulation and the daily stratification/destratification cycle. The vertical distribution of Microcystis aeruginosa in Thomsons Lake depends on the carbohydrate ballast dynamics and the colony size. When thermal stratification occurs, all the simulations show a similar general pattern of diurnal vertical migration of the Microcystis aeruginosa colonies. The colonies accumulate at the surface during the night and in the morning the colonies lose buoyancy, which leads to a reduction by ~ \sim 50% in colony concentration in the top 0.2--0.3 m of the water column. Afternoon winds redistribute the population over the entire water column. When the lake is fully mixed, the vertical migration pattern of the Microcystis aeruginosa colonies may be affected, depending on the colony size and the intensity of the mixing.  相似文献   

15.
Large lakes enclosures were used to examine the influence of nutrient (P, N) enrichment and planktivorous fish (1 + yellow perch) predation on hypolimnetic oxygen depletion. Results were compared to similar data for lakes with high (Lake St. George) and low (Haynes Lake) abundances of planktivorous fish. In both the unfertilized and fertilized enclosures, fish predation on large cladocerans increased the biomasses of pico- and nanoplankton (0.2–20 µm), phytoplankton (chlorophyll a) and total phosphorus (TP), reduced sedimentation, water clarity, and hypolimnetic oxygen concentrations (AHO). Fertilized enclosures without fish had highest TP and sedimentation rates, but the AHO were low. The high planktivore lake had higher pico- and nanoplankton, higher chlorophyll a, reduced water clarity, and lower AHO than the low planktivore lake. Areal hypolimnetic oxygen depletion (AHOD) rates were strongly related with Secchi depth and plankton size-distribution (r 2 = 0.77, and 0.79, respectively), but not as strongly with TP, chlorophyll a, and sedimentation rates (r 2 = 0.25, 0.53, and 0.02, respectively). Such observations are useful in forming a generalized hypothesis that lakes with low planktivory and high water clarity have lower oxygen depletion because 1) plankton that are settling are larger and spend less time in the hypolimnetic water column before reaching the sediment, and therefore undergo less decomposition, and 2) the euphotic depth extends into the hypolimnion and production of oxygen can take place.  相似文献   

16.
In this study, a three‐dimensional (3D) non‐hydrostatic circulation model was applied to study the thermal structure, its evolution and water circulation of Yachiyo Lake in Hiroshima, Japan. The simulations were conducted for 1 month during July 2006. The meteorological forcing variables such as wind stress, surface atmospheric pressure and heat flux transfer through the lake surface were provided by an atmospheric mesoscale model run. The vertical mixing process of the lake was calculated using the Mellor‐Yamada turbulence model. The 1‐month numerical simulation revealed the wind‐induced currents of the lake, two gyres in the mid‐layer, and depth‐averaged monthly mean currents. Further numerical experiments studying the mechanism of the two gyres in the lake showed the important role of topography in gyre formation. The thermal structure of the lake and its evolution both in space and in time as predicted by the model showed very good agreement with the observed values and characteristics of Yachiyo Lake. The internal gravity waves, which are crucial for mixing in the stratified lake, are depicted by the vertical fluctuation of isotherms. Using the non‐dimensional gradient Richardson number, Yachiyo Lake was determined to be stable under strong stratification during the study period, and therefore very sensitive to wind stress. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

17.
Depth variable vertical eddy diffusion coefficients for heat (K z) were calculated from continuously measured temperature profiles in Überlinger See (western part of Lake Constance). The temperatures were averaged over vertical intervals of 10 m yielding 14 discrete values (maximum depth of Überlinger See: 147 m). A linear fit from 10 June to 29 September 1987 was used to smooth the significant temperature fluctuations caused by internal seiches of Lake Constance.Assuming horizontal homogeneity for the smoothed data the Gradient-Flux-Method was applied to compute vertical diffusion coefficientsK z at different depths using the depth variable volumes and surfaces of the 14 layers. The resulting mean diffusion coefficients for the period from June to September are 0.04 cm2/s near the thermocline and up to 0.8 cm2/s in deeper strata (accuracy: ± 50%). It is shown that horizontal mixing between Überlinger See and Obersee (main lake) alters the computation ofK z by less than 50%.A relationship betweenK z and stability (Brunt-Väisälä) frequencyN is found which corresponds well to the theory of internal wave induced turbulence.Combining the diffusion coefficients with measured phosphorus profiles, a phosphorus flux from the hypolimnion to the epilimnion of (0.7 ± 0.4) mg P m–2 d–1 was calculated, corresponding to about 20% of the average external loading per area of Lake Constance in 1986.  相似文献   

18.
Warneck  P.  Junge  C. E.  Seiler  W. 《Pure and Applied Geophysics》1973,106(1):1417-1430
The consumption of both methane and carbon monoxide in the lower stratosphere is due predominantly to reaction with OH radicals. The possibility of deriving OH concentration from measurements of the decrease of CH4 and CO mixing ratios above the tropopause is explored. The observations and the basic chemistry are briefly summarized. Simple one-dimensional diffusion models are employed to derive expressions for the decrease of CH4 and CO mixing ratios with altitude above the tropopause, and the influence of important parameters is discussed. Vertical air velocities resulting from large-scale organized mean motion and from synoptic variations are shown to distort the concentration altitude profiles of methane and carbon monoxide, respectively. Suitable averaging of observational data is required to eliminate the effects due to vertical motion. Then a reliable value for the effective OH number density should be obtainable. At present an estimate of 4×106 molecules/cm3 is derived.  相似文献   

19.
The catastrophic release of carbon dioxide gas from Lake Nyos on 21 August 1986 is discussed in the context of the buoyancy reversal instability. Originally proposed by Randall (1980) [Randall, D.S., 1980. Conditional instability of the first kind upside-down. Journal of Atmospheric Sciences 37: 125–130.] and Deardoff (1980) [Deardoff, J.W., 1980. Cloud-top entrainment instability. Journal of Atmospheric Sciences 37: 131–147.] for the `cloud-top entrainment instability' of stratocumulus clouds, the buoyancy reversal instability has been studied experimentally in water tank experiments by Shy and Breidenthal (1990) [Shy, S.S., Breidenthal, R.E., 1990. Laboratory experiments on the cloud-top entrainment instability. Journal of Fluid Mechanics 214: 1–15.], who identified three criteria for instability. The initial disturbance must be sufficiently large, so that its Reynolds number is above the mixing transition, its Richardson number must be less than one to achieve overturning and mixing, and the buoyancy reversal parameter must be greater than a critical value, of order one. The implications and applicability of these criteria to Lake Nyos are discussed. The criterion for the Reynolds number is easily satisfied for typical wind velocities in the Lake Nyos region. Similarly, the Richardson number based on incident turbulence is easily less than unity, and therefore satisfy the second criterion for instability. In the case of Lake Nyos, the continuous release of carbon dioxide at the bottom of the lake increases the value of the buoyancy reversal parameter until it reaches its critical value, at which point an explosion occurs. This instability provides a plausible trigger for the 1986 explosion. After each explosion, the buoyancy reversal parameter returns to below its critical value, only to slowly rise again over time, as CO2 continues to enter the lake, setting the stage for the next explosion. Future explosions may be avoided if the value of the buoyancy reversal parameter is prevented from approaching its critical value by artificial mixing at the thermocline, such as with a bubble plume.  相似文献   

20.
Nicolet  M.  Peetermans  W. 《Pure and Applied Geophysics》1973,106(1):1400-1416
The vertical distribution of the methane concentration in the stratosphere is related to its dissociation by two simultaneous daytime reactions with excited oxygen atoms O(1D) and with OH radicals and depends on the stratospheric eddy diffusion coefficient.Dissociation of CH4 in the lower stratosphere leads to the production of CO molecules while in the upper stratosphere thepphotodissociation of CO2 molecules is an additional process to the CO production.In the upper stratosphere (40±10 km) there is an equilibrium between the formation and destruction processes of carbon monoxide which leads to a minimum of its mixing ratio. There is an increase of the CO mixing ratio in the troposphere and mesosphere compared with that of the stratosphere.The vertical distribution of the CO mixing ratio is closely related to the eddy diffusion coefficient in the whole stratosphere but the absolute values of the hydroxyl radical concentration also determine the values of the CO mixing ratio.  相似文献   

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