Double diffusion in meromictic lakes of the temperate climate zone

Abstract

Abstract. Meromictic lakes are characterized by strong stable density stratification in and below the chemocline, which separates the oxic mixolimnion from the mostly anoxic monimolimnion. Stable density gradients involve slow vertical exchange, especially in the chemocline, where vertical transport can be as low as molecular. Typically, destabilizing temperature gradients establish in the monimolimnion as a consequence of seasonal changing heat fluxes. At the same time, gradients of solutes extending to the lake bottom stabilize the overall stratification. Double diffusive processes may create local instabilities and subsequently cause convective mixing when the destabilization due to heat gradients exceeds the stabilization by solutes (diffusive regime). This configuration can annually occur in the upper part of the monimolimnion, if seasonal temperature changes in the mixolimnion reach the top of the monimolimnion. We present CTD-measurements from two meromictic mining lakes in Germany, which document the seasonal occurrence of convective mixing in discrete horizontal homogeneous layers within the monimolimnion which can be identified by the characteristic step-like structure. In the deeper layers, the steps emerge with a time delay which is determined by the progression of the mixolimnetic temperature changes into the monimolimnion. Interestingly, the chemocline interface is not degraded by these processes. However, double diffusive convection is essential for the redistribution in the inner parts of the monimolimnion at longer time scales, which is crucial for the assessment of the ecologic development of such lakes.