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Journal ArticleDOI

Evidence for double diffusion in temperate meromictic lakes

13 Apr 2010-Hydrology and Earth System Sciences (Copernicus GmbH)-Vol. 14, Iss: 4, pp 667-674

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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Citations
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Book ChapterDOI
01 Jan 2012
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.

44 citations


Book ChapterDOI
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.

31 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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Journal ArticleDOI

27 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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Journal ArticleDOI
Abstract: Changes in the water properties and biological characteristics of the highly acidic Hromnice Lake (Western Bohemia) were investigated. This 110-year-old lake, formed as a consequence of the mining of pyritic shales, is permanently meromictic. Two chemoclines separate an extremely acidic (pH ∼ 2.6) mixolimnion from a metal-rich anoxic monimolimnion. 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. Extreme mixolimnetic oxygen maxima (up to 31 mg l −1 ) in phosphorus-rich lake (PO 4 3− up to 1.6 mg l −1 ) well correlated with outbursts of phytoplankton. Phytoplankton consist of several acido-tolerant species of the genera Coccomyxa , Lepocinclis , Chlamydomonas and Chromulina . Surface phytoplankton biomass expressed as chlorophyll- a varies from 2 to 140 μg l −1 . Multicellular zooplankton are almost absent with the exception of Cephalodella acidophila , a small rotifer occurring in low numbers. Large red larvae of the midge Chironomus gr. plumosus were found at the bottom close to the shore, with larvulae in the open water. Developmental stages (protonemata) of a moss, resembling filamentous algae, dwell in the otherwise plant-free littoral zone.

23 citations


Journal ArticleDOI
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.

20 citations


References
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Book
23 Feb 1973
Abstract: Preface 1. Introduction and preliminaries 2. Linear internal waves 3. Finite amplitude motions in stably stratified fluids 4. Instability and the production of turbulence 5. Turbulent shear flows in a stratified fluid 6. Buoyant convection from isolated sources 7. Convection from heated surfaces 8. Double-diffusive convection 9. Mixing across density interfaces 10. Internal mixing processes Bibliography and author index Recent publications Subject index.

2,717 citations


"Evidence for double diffusion in te..." refers background in this paper

  • ...The phenomenon of double diffusive convection has been discussed in detail in numerous observational, laboratory, and theoretical studies (e.g. Turner, 1973; Kelley, 2003; Schmitt, 1994)....

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  • ...Von Rohden et al. (2009) discuss this as a result of convective mixing in the mixolimnion. Convective mixing of the upper layer is strongest during summer when nocturnal cooling removes the heat of the preceding daytime through the surface. This drives a convection which reaches the (shallow) chemocline. Therefore the depth of the chemocline gradually increases during summer (chemocline erosion). In the cool season, mixing in the surface layer is weaker (wind speed 2 m above the lake surface∼0.5 m/s on average, virtually not exceeding 2 m/s), and the chemocline gradually moves upwards, presumably due to groundwater inflow. Figure 2b illustrates the situation in the larger Lake Moritzteich. The upper ∼11 m of the water column undergo the “usual” thermal cycle of temperate lakes with a warm epilimnion and a cooler hypolimnion in summer and a homothermal mixolimnion during the cool season. During the warm season starting in April, heat slowly enters the upper monimolimnion (similar to Lake Waldsee). This heating still continues while autumnal cooling already affects the mixolimnion. After the formation of local temperature maxima (e.g., on 19 November at ∼12 m), an inverse temperature profile establishes with gradients of 2–3 C/m between the mixed∼4C mixolimnion and the warmer monimolimnion. Heat diffuses out of the monimolimnion along these gradients. In general, the seasonal temperature signal at the top of the chemocline intrudes to a depth of about 16 m, i.e. at least 4 m into the monimolimnion. The monimolimnion temperatures follow the mixolimnion signal with a depth dependent delay. For example in the profile of 16 December 2008, the temperature has a local maximum at ∼ 14 m while the mixolimnion is cooler, close to 4 C. Towards the bottom, the temperature remains inversely stratified with much less variation, ending at a virtually constant value of ∼7.3C. This indicates a continuous heat flux from the sediments. With respect to electrical conductivity, we find a subdivision of the monimolimnion into two layers: The “upper” monimolimnion extends from the chemocline to ∼14 m depth. It is separated by a density step from the “lower” monimolimnion extending from∼15.8 m to the bottom. This structure as a whole has persisted for several years and has shown only little variation. While the seasonal mixing in the water bodies above the chemoclines is quite different between the lakes, their behaviour regarding the meromixis is similar. Seasonal temperature changes at the top of the chemocline proceed faster and deeper into the monimolimnion than variations of conductivity. This indicates that the effective diffusivities of heat and solutes are different. Hence, vertical transport must to a large extent be at a level close to the molecular diffusion, especially within the chemocline. The vertical density gradients are mainly caused by gradients of dissolved iron and the carbonate system. Ferrous iron (FeII ) which is transported out of the anoxic monimolimnia by diffusion is oxidized in the lower hypolimnion. As particulate ferric iron (Fe III ) it settles back into the monimolimnion, where it eventually redissolves. This chemical cycle results in the conservation of distinct chemoclines and implies an evanescent effective transport of the density regulating iron, sustaining the chemical and therefore the stable density stratification across the chemocline. These geochemical processes are discussed by means of lake Waldsee in Boehrer et al., (2009). The temporal variation of the overall electrical conductivity ( κ25) in the mixolimnion and monimolimnion of Lake Waldsee by about 10% (Fig....

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  • ...Von Rohden et al. (2009) discuss this as a result of convective mixing in the mixolimnion....

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Journal ArticleDOI
Abstract: [1] Many lakes show vertical stratification of their water masses, at least for some extended time periods. Density differences in water bodies facilitate an evolution of chemical differences with many consequences for living organisms in lakes. Temperature and dissolved substances contribute to density differences in water. The atmosphere imposes a temperature signal on the lake surface. As a result, thermal stratification can be established during the warm season if a lake is sufficiently deep. On the contrary, during the cold period, surface cooling forces vertical circulation of water masses and removal of gradients of water properties. However, gradients of dissolved substances may be sustained for periods much longer than one annual cycle. Such lakes do not experience full overturns. Gradients may be a consequence of external inflows or groundwater seepage. In addition, photosynthesis at the lake surface and subsequent decomposition of organic material in the deeper layers of a lake can sustain a gradient of dissolved substances. Three more geochemical cycles, namely, calcite precipitation, iron cycle, and manganese cycle, are known for sustaining meromixis. A limited number of lakes do not experience a complete overturn because of pressure dependence of temperature of maximum density. Such lakes must be sufficiently deep and lie in the appropriate climate zone. Although these lakes are permanently stratified, deep waters are well ventilated, and chemical differences are small. Turbulent mixing and convective deep water renewal must be very effective. As a consequence, these lakes usually are not termed meromictic. Permanent stratification may also be created by episodic partial recharging of the deep water layer. This mechanism resembles the cycling of the ocean: horizontal gradients result from gradients at the surface, such as differential cooling or enhanced evaporation in adjacent shallow side bays. Dense water parcels can be formed which intrude the deep water layer. In the final section, stratification relevant physical properties, such as sound speed, hydrostatic pressure, electrical conductivity, and density, are discussed. The assumptions behind salinity, electrical conductance, potential density, and potential temperature are introduced. Finally, empirical and theoretical approaches for quantitative evaluation from easy to measure properties conclude this contribution.

451 citations


"Evidence for double diffusion in te..." refers background or methods in this paper

  • ...where α25 = 1/(25+ n/m), and n, m are the respective regression coefficients (Boehrer and Schultze, 2008)....

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  • ...From the linear regression to the data from 1.5◦C to 30◦C the electrical conductivity for the chosen reference temperature of 25◦C could be evaluated: κ25= C(T ) α25(T −25◦C)+1 , (1) where α25 = 1/(25+ n/m), and n, m are the respective regression coefficients (Boehrer and Schultze, 2008)....

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  • ...Meromixis is a well known phenomenon, also occurring in lakes located in regions in a temperate climate (Boehrer and Schultze, 2008)....

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Journal ArticleDOI
Abstract: The modern study of double-diffusive convection began with Melvin Stern's article on "The Salt Fountain and Thermohaline Convection" in 1960. In that paper, he showed how opposing stratifications of two component species could drive convection if their diffusivities differed. Stommel ct al (1956) had earlier noted that there was significant potential energy available in the decrease of salinity with depth found in much of the tropical and subtropical ocean. While they suggested that a flow (the salt fountain) would be driven in a thermally-conducting pipe, it was Stern who realized that the two orders of magnitude difference in heat and salt diffusivities allowed the ocean to form its own pipes. These later came to be known as "salt fingers." Stern also identified the potential for the oscillatory instability when cold, fresh water overlies warm, salty water in the 1960 paper, though only in a footnote. Turner & Stommel (1964) demonstrated the "diffusive-convection" process a few years later. From these beginnings in oceanography over three decades ago, double diffusion has come to be recognized as an important convection process in a wide variety of fluid media, including magmas, metals, and stellar interiors (Schmitt 1983, Turner 1985). However, it is interesting to note that about one hundred years before Stern's paper, W. S. Jevons (1857) reported on the observation of long, narrow convection cells formed when warm, salty water was introduced over cold, fresh water. He correctly attributed the phenomenon to a difference in the diffusivities for heat and

442 citations


"Evidence for double diffusion in te..." refers background in this paper

  • ...The phenomenon of double diffusive convection has been discussed in detail in numerous observational, laboratory, and theoretical studies (e.g. Turner, 1973; Kelley, 2003; Schmitt, 1994)....

    [...]


Journal ArticleDOI
Abstract: Dissolved salts affect the thermodynamic properties of lake waters. Equations are given to calculate the following properties over the range of 0-0.6 salinity, 0/sup 0/-30/sup 0/C, and 0-180 bars: density, thermal expansibility, temperature of maximum density, maximum density and minimum specific volume, isothermal compressibility, specific heat at constant pressure, specific heat at constant volume, sound speed, adiabatic compressibility, freezing point, adiabatic temperature gradient, and static stability.

271 citations


"Evidence for double diffusion in te..." refers 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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Journal ArticleDOI
Abstract: The diffusive regime of double-diffusive convection is discussed, with a particular focus on unresolved issues that are holding up the development of large-scale parameterizations. Some of these issues, such as interfacial transports and layer-interface interactions, may be studied in isolation. Laboratory work should help with these. However, we must also face more difficult matters that relate to oceanic phenomena that are not represented easily in the laboratory. These lie beneath some fundamental questions about how double-diffusive structures are formed in the ocean, and how they evolve in the competitive ocean environment.

154 citations