Double diffusion in meromictic lakes of the temperate climate zone

Abstract Meromictic lakes are characterized by strong stable density stratification in and belowthe chemocline, which separates the oxic mixolimnion from the mostly anoxic moni-molimnion. Stable density gradients involve slow vertical exchange, especially in the 5 chemocline, where vertical transport can be as low as molecular. Typically, desta-bilizing temperature gradients establish in the monimolimnion as a consequence ofseasonal changing heat fluxes. At the same time, gradients of solutes extending tothe lake bottom stabilize the overall stratification. Double diffusive processes maycreate local instabilities and subsequently cause convective mixing when the desta- 10 bilization due to heat gradients exceeds the stabilization by solutes (diffusive regime).This configuration can annually occur in the upper part of the monimolimnion, if sea-sonal temperature changes in the mixolimnion reach the top of the monimolimnion.We present CTD-measurements from two meromictic mining lakes in Germany, whichdocument the seasonal occurrence of convective mixing in discrete horizontal homo-