The meromictic Lake La Cruz (Central Spain). Patterns of stratification

The main limnological characteristics of the biogenic meromictic Lake La Cruz, a small karstic lake located on a dolomitic substrate, are outlined below. The lake owes its chemical stratification mainly to high concentrations of bicarbonate and calcium in the permanent monimolimnion, located below a depth of 19 m. A massive precipitation of CaCO3 crystals ("whiting"), occurring in the euphotic zone every summer, leads to further sedimentation and redissolution in deep waters with enhanced CO2 concentrations. Other ions, like magnesium, ammonium, reduced iron, silicate, etc. also contribute to the chemically-induced stability of the monimolimnion. The patterns of stratification during two consecutive thermal stagnation periods (1987-1988) were influenced by different climatological conditions. Depending of the year, the location of the pycnoclines varied; the rainier spring of 1988 favoured a much shallower location of the oxic-anoxic boundary and a much higher primary production reflected by larger oxygen peaks. This greater phytoplanktonic development in upper waters occasioned a shadowing effect on purple and green phototrophic bacterial populations growing in anoxic waters poor in sulphide.