Evidence for double diffusion in temperate meromictic lakes
TL;DR: In this article, the authors present CTD-measurements from two shallow meromictic mining lakes, which differ in size and depth, show completely different seasonal mixing patterns in their mixolimnia.
Abstract: . We present CTD-measurements from two shallow meromictic mining lakes. The lakes, which differ in size and depth, show completely different seasonal mixing patterns in their mixolimnia. However, the measurements document the occurrence of similar seasonal convective mixing in discrete layers within their monimolimnia. This mixing is induced by double diffusion and can be identified by the characteristic step-like structure of the temperature and electrical conductivity profiles. The steps develop in the upper part of the monimolimnion, when in autumn cooling mixolimnion temperatures have dropped below temperatures of the underlying monimolimnion. The density gradient across the chemocline due to solutes overcompensates the destabilizing temperature gradient, and moreover, keeps the vertical transport close to molecular level. In conclusion, preconditions for double diffusive effects are given on a seasonal basis. At in general high local stabilities N2 in the monimolimnia of 10−4–10−2s−2, the stability ratio Rρ was in the range of 1–20. This quantitatively indicates that double diffusion can become visible. Between 1 and 6 sequent steps, with sizes between 1 dm and 1 m, were visually identified in the CTD-profiles. In the lower monimolimnion of the deeper lake, the steps systematically emerge at a time delay of more than half a year, which matches with the progression of the mixolimnetic temperature changes into the monimolimnion. In none of the lakes, the chemocline interface is degraded by these processes. However, double diffusive convection is essential for the redistribution of solutes in the inner parts of the monimolimnion at longer time scales, which is crucial for the assessment of the ecologic development of such lakes.
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01 Jan 2012
TL;DR: In this paper, the authors summarized the knowledge on mixing and transport processes in Lake Kivu, and showed that the lake is permanently stratified, with density increasing stepwise from ∼998 kg m−3 at the surface to ∼1,002 kg m − 3 at the maximum depth of 485 m.
Abstract: This chapter summarizes the knowledge on mixing and transport processes in Lake Kivu. Seasonal mixing, which varies in intensity from year to year, influences the top ∼65 m. Below, the lake is permanently stratified, with density increasing stepwise from ∼998 kg m−3 at the surface to ∼1,002 kg m−3 at the maximum depth of 485 m. The permanently stratified deep water is divided into two distinctly different zones by a main gradient layer. This gradient is maintained by a strong inflow of relatively fresh and cool water entering at ∼250 m depth which is the most important of several subaquatic springs affecting the density stratification. The springs drive a slow upwelling of the whole water column with a depth-dependent rate of 0.15–0.9 m year−1. This upwelling is the main driver of internal nutrient recycling and upward transport of dissolved gases. Diffusive transport in the deep water is dominated by double-diffusive convection, which manifests in a spectacular staircase of more than 300 steps and mixed layers. Double diffusion allows heat to be removed from the deep zone faster than dissolved substances, supporting the stable stratification and the accumulation of nutrients and gases over hundreds of years. The stratification in the lake seems to be near steady-state conditions, except for a warming trend of ∼0.01°C year−1.
45 citations
01 Jan 2017
Abstract: Lakes turn meromictic, when mixing and deep recirculation are insufficient to homogenize the water body and remove chemical gradients. A deepwater layer “the monimolimnion ” is excluded from the deep recirculation and hence develops a pronounced different chemical milieu. It persists through all seasons due to its high density. A limited number of processes are known to accomplish such a density increase of the deep water to create meromixis , such as salty inflows and partial deepwater renewal . However, also geochemical processes, such as decomposition of organic material, iron oxidation, and redissolution and calcite precipitation, can be responsible for meromixis. Other than the overlying water layer “the mixolimnion, ” the monimolimnion does not get into direct contact with the atmosphere and hence is not directly supplied with oxygen. Other substances can be enriched by precipitation and flocculation from the mixolimnion until the solubility product is reached or gas pressure grows beyond absolute pressure. As a consequence, the composition of solutes deviates clearly from usual water composition, and quantitative approaches for density must implement appropriate numerical approaches. The permanent density stratification limits the vertical transport of water and solutes. In several lakes, double-diffusive convection has been reported to significantly enhance the vertical solute transport.
42 citations
Cites background from "Evidence for double diffusion in te..."
...Groundwater may enter the monimolimnion and contribute to its volume, and hence the chemocline slowly rises (von Rohden et al. 2010)....
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...2.4; Kongressvatn, Norway: Bøyum and Kjensmo 1970; Wallendorfer See and Rassnitzer See, Germany: B€ohrer et al. 1998; Heidenreich et al. 1999; Waldsee, Germany: von Rohden et al. 2009, 2010; see also Chaps....
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...…diffusivity of heat than that of solutes can impose locally unstable conditions, which form thin convection cells (decimeters) of large horizontal extension (kilometers) (Newman 1976; Schmid et al. 2004b; von Rohden et al. 2010), separated by even thinner layers of strong density gradients (Fig....
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38 citations
Cites background from "Evidence for double diffusion in te..."
...Most observations of DD are in meromictic lakes connected with geothermal heating from the sediments and/or salty hot springs at the floor of the lake (W€uest et al. 1992; Stevens and Lawrence 1998; S anchez and Roget 2007; Rohden et al. 2010)....
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TL;DR: In this paper, changes in the water properties and biological characteristics of the highly acidic Hromnice Lake (Western Bohemia) were investigated and the absence of spring mixolimnetic turnover due to ice melting and very slow heat propagation through the chemocline with a 6-month delay were observed.
26 citations
TL;DR: In this article, the authors measured water chemistry and temperature from 2005 to 2010 in Kongressvatn, a crenogenic meromictic lake in Spitsbergen (Svalbard).
Abstract: High Arctic lakes are among the most sensitive ecosystems and climate change strongly affects their physical properties, especially water temperature, and mixing processes. To study the effect of recent climate change on such a lake in the Arctic environment, we measured water chemistry and temperature from 2005 to 2010 in Kongressvatn, a crenogenic meromictic lake in Spitsbergen (Svalbard). In addition, we monitored water column temperatures during two consecutive years and compared them to regional air temperature data and physicochemical lake data from 1962 and 1968, two relatively cold years. Summer surface water temperature was highly correlated to air temperature, and both have increased by approximately 2°C since 1962. Temperature monitoring during 2 years showed that the warm summer of 2007 resulted in increased water temperatures even in the stratified, denser hypolimnion. Our water chemistry measurements showed that the chemocline position in 2005–2010 was ca 12 m deeper than in 1962–1968, and a second, weaker, chemocline appeared at metalimnetic depths of 7–15 m. During the study period, the water level decreased by 4 m, and this change accelerated between 2008 and 2010. Our data support the hypothesis that water temperatures and stratification patterns are changing rapidly with air temperature, but changes in the catchment, such as glacial retreat and permafrost melting, may have an even stronger impact on lake properties.
25 citations
References
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TL;DR: A marked vertical trend of increasing temperature and dissolved metal concentrations is observed in the monimolimnia of some meromictic pit lakes of the Iberian Pyrite Belt (IPB) in SW Spain this article.
Abstract: A marked vertical trend of increasing temperature and dissolved metal concentrations is observed in the monimolimnia of some meromictic pit lakes of the Iberian Pyrite Belt (IPB) in SW Spain. Temperature differences between the chemocline and the pit lake bottom can be as high as 15°C (e.g. Herrerias), and the respective concentration of some metals (e.g. Fe) and metalloids (e.g. As) can increase by several orders of magnitude (e.g. Cueva de la Mora). The redox conditions also change drastically from the upper and oxygenated mixolimnion (strongly oxidizing) to the lower and anoxic monimolimnion (moderately reducing). Processes such as the inflow of metal–sulphate laden ground water from flooded shafts and galleries, and other factors such as the pit geometry or the relative depth of the lakes, must be considered to account for the observed stratification pattern. The vertical profiles of physico-chemical parameters and water chemistry obtained in Cueva de la Mora and other meromictic pit lakes of the IPB are also compatible with a reactive bottom in which several geochemical and microbial reactions (including reductive dissolution of Fe3+ minerals, bacterial reduction of Fe3+ and SO4
2− in pore waters within the sediments, and decomposition of organic matter) could be taking place.
46 citations
TL;DR: For example, it is known from laboratory experiments that such small density gradient ratios are likely to give rise to doublediffusive instabilities as mentioned in this paper, and 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.
Abstract: 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 HCO3- 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.
45 citations
TL;DR: In this article, the authors present observations of a circulation pattern that has not been documented in the limnological literature before, where surface cooling drives a vertical circulation of the upper water layer (mixolimnion), the deeper water layer is not included because of its higher salt concentration.
Abstract: [1] From a small meromictic lake, we present observations of a circulation pattern that has not been documented in the limnological literature before. While surface cooling drives a vertical circulation of the upper water layer (mixolimnion), the deeper water layer (monimolimnion) is not included because of its higher salt concentration. However, double diffusion (higher diffusivity of heat than of dissolved substances) facilitates the faster escape of heat from the monimolimnion compared to dissolved substances during cold periods. As a consequence, interfacial cooling drives a vertical circulation within the monimolimnion without breaking the stratification toward the mixolimnion. In the presented case, the geochemical setting does not permit dissolved substances to accumulate in the mixolimnion. As a consequence, the system approaches the case of two immiscible layers in thermal contact. Below the interface, a convection layer is formed that exceeds the staircase layer thickness of double diffusion when conservative salts are involved. Finally, the entire lake circulates as two separate convection layers. This has a decisive impact on the formation of gradients and the redistribution of dissolved substances in lakes.
38 citations
TL;DR: In this article, the authors describe a modeling application to a deep and steep-sided chemically stratified lake using an extended version of the lake and reservoir water quality model, DYRESM, incorporating algorithms for detailed ice cover, heat fluxes, and also internal wave-driven boundary mixing.
Abstract: Subaqueous disposal is a technique that can, under suitable circumstances, delay or mitigate the release of material containing high levels of dissolved compounds, for example, acid rock drainage, into the surrounding environment. The technique places the material in question under a relatively inert cap of lighter fluid in a deep basin, such as that left after mining. In many situations, because of low diffusion rates, the material may be considered as being isolated from the environment. However, there are a number of naturally occurring physical mechanisms that can quite efficiently bring this material to the surface, and hence, to the surrounding environment. We describe a modeling application to a deep and steep-sided chemically stratified lake using an extended version of the lake and reservoir water quality model, DYRESM, incorporating algorithms for detailed ice cover, heat fluxes, and also internal wave-driven boundary mixing. Sheltering and shading of the meteorological forcing is taken into acc...
33 citations
"Evidence for double diffusion in te..." refers background or methods in this paper
...We therefore developed specific formulas to calculate water density from measured electrical conductivity and temperature, since standard formulas (e.g. Chen and Millero, 1986) did not apply (e.g., Hamblin et al., 1999)....
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..., Hamblin et al., 1999). For Lake Waldsee the procedure was explained in detail in Boehrer et al. (2009). There, also chemical data at different depths were presented....
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TL;DR: In this paper, microstructure measurements of a triple-diffusive staircase with a stability ratio of 1.1 were presented at Lake Banyoles, a small lake in Catalonia, Spain, with a warm, salty and turbid underground inflow.
Abstract: [1] Microstructure measurements of a triple-diffusive staircase with a stability ratio of 1.1 are presented. Data were recorded at Lake Banyoles, a small lake in Catalonia, Spain, with a warm, salty, and turbid underground inflow. Turbulent scales are well resolved in the two observed convective layers and allow determination of the dissipation rates of the turbulent kinetic energy, ɛ, and of the turbulent temperature fluctuations, χ, which are found to be 3.3 × 10−7 C2/s and 2.7 × 10−9 W/kg for one of the layers and 5.9 × 10−7 C2/s and 3.8 × 10−9 W/kg for the other. Thermal spectra for the convective layers look universal in viscous-diffusive and viscous-convective subranges. Characteristic vertical displacements within convective layers could also be obtained on the basis of Thorpe scales and were found to be 0.3 times the layer thickness. Vertical convective fluctuations were estimated and found to be of the order of 10−4 m/s. When turbulent fluxes were determined within the convective layers on the basis of microstructure data and compared to those at the diffusive interfaces, they showed a stationary state with a mean thermal flux of 3.5 × 10−6 C m/s. A comparison of experimental heat fluxes to different models favors the scaling model of Grossman and Lohse (2000) for Rayleigh-Benard convection and the double-diffusive convection model of Fernando (1979a, 1979b) for low stability. If the 4/3 power law is assumed, then the parameterization proposed by Taylor (1988) for diffusive interfaces at low values of the density ratio is also in accordance with our data.
28 citations