Contribution of solutes to density stratification in a meromictic lake (Waldsee/Germany)

Density differences are the key parameter for stratification stability. We used data from the iron-meromictic Waldsee, Germany, a lignite mine pit lake, to quantify the contribution of single solutes to water density and analyzed the density gradient. Iron meromictic lakes maintain their density gradient through chemical reactions. Hence, quantifying the contributions of separate solutes is essential for understanding the entire process. Based on solute concentrations and literature values of partial molal volumes, substance specific density contributions were quantitatively evaluated. Then, by direct measurements of the density of IHSS Waskish peat fulvic acid, we quantified the density contribution of dissolved organic carbon (DOC). While several solutes contributed to the density throughout the water column, only those substances that occurred at higher concentrations in the anoxic monimolimnion than in the oxic mixolimnion were crucial to sustaining the density difference between the two layers. In Waldsee, the density difference between monimolimnion and mixolimnion was attributed to dissolved Fe2+ (0.23 g/L, resulting in a 45 % of the density difference due to solutes) and to the carbonate system (HCO3 −, about 0.16 g/L and CO2, 0.03 g/L) while Ca2+ and DOC delivered only a small contribution. In summer, total density differences were dominated by temperature differences; during winter, solutes sustained meromixis. Finally, we present a complete list of specific density fractions for basically all of the density-relevant substances in fresh waters. Dichteunterschiede sind der wichtigste Faktor für die Stabilität von Schichtungen in Seen. Wir verwendeten Daten des eisenmeromiktischen Waldsees, eines Braunkohletagebausees, um den Beitrag einzelner gelöster Stoffe zur Dichte des Wassers sowie deren Beitrag zum Dichtegradienten zu bestimmen. In eisenmeromiktischen Seen wird der Dichtegradient durch interne chemische Reaktionen a