Evidence for double diffusion in temperate meromictic lakes

Abstract

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 N 2 in the monimolimnia of 10 −4 −10 −2 s −2 , the stability ratio R ρ 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 CTDprofiles. 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.

A practical approach to lake water density from electrical conductivity and temperature

“…This case presented a meromictic open pit lake Dietz et al, 2008Dietz et al, , 2012von Rohden et al, 2010;Moreira et al, 2011) of moderate salinity (0.22 psu in the mixolimnion and 0.6 psu in the monimolimnion, Moreira et al, 2011), but its composition differed from the usual carbonate or chloride waters. Composition was obtained from Dietz et al (2008Dietz et al ( , 2012.…”

“…In order to include also water with a rather unusual composition, we chose two water samples from a meromictic open pit lake, the Waldsee, which contains large amounts of sulfate and dissolved iron (e.g. Dietz et al, 2008Dietz et al, , 2012Boehrer et al, 2009;von Rohden et al, 2010;Moreira et al, 2011). Finally, we used seawater, of which the composition is known at high accuracy, as a reference water for a standard comparison.…”

“…In the case of meromictic lakes, strong differences in the composition of the mixolimnion and monimolimnion must be reflected in the density equations in order to sustain the permanent stratification in the density calculations. Boehrer et al (2009) andvon Rohden et al (2010) used an equation based on density measurements of the monimolimnion and mixolimnion of the Waldsee. Boehrer et al (2010) evaluated the contribution of the different cations and anions separately in terms of partial molal volumes and implemented an algorithm, RHOMV (http: //www.ufz.de/webax), to calculate density with a secondorder approximation for temperature dependence and ionic strength dependence.…”

Ecology of Meromictic Lakes

“…Groundwater may enter the monimolimnion and contribute to its volume, and hence the chemocline slowly rises (von Rohden et al 2010). In addition, diffusion and turbulent transport of solutes from the monimolimnion can raise the density gradient locally and lead to oxygen demand in the lowest zone of the hypolimnion.…”

“…Consequently, deep waters stratified by dissolved substances can show double diffusion, if temperature gradients inflict a reduction of the density gradient (see also Brandt and Fernando 1996;Boehrer 2012). In initially stable zones with disappearing turbulence, the greatly higher 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. 2.8).…”