Concentration changes of some elements during a year cycle in Lake Vechten

During summer-stratification in Lake Vechten (May–October) a great number of chemical parameters show variations. The concentration patterns of calcium, manganese and iron have been studied in some detail.The calcium concentration in the epilimnion is controlled by carbonate-formation; in the hypolimnion however, no saturation with respect to calcium-carbonate is reached, due to the fact, that carbondioxide equilibrium with the atmosphere no longer exists. The concentration pattern in the hypolimnion (max. concentration about 80 mg.l^–1 near the sediment, 40 mg.l^–1 in epilimnion) is controlled by the velocity of calcium transport from the sediment and perhaps by dissolution of precipitating carbonates from the epilimnion.The maximum manganese concentration in the hypolimnion (about 5 mg.l^–1) is controlled by the solubility of manganese carbonate, causing, together with manganese transport from the sediment, a uniform concentration up to two meters above the bottom. In the epi- and meta-limnion the presence of oxygen is limiting the concentration of manganese.The iron concentration in the hypolimnion shows a strong gradient existing until the overturn (max. concentration near the sediment about 20 mg.l^–1) and is controlled by a number of mechanisms: 1^o. Oxidation by oxygen and manganese oxides near the epilimnion. 2^o. Precipitation by sulphide formation; a strong correlation exists at different depths between sulphate disappearance and a slower increase of iron concentration. 3^o. Transport from the sediment.Control of iron concentration by carbonate formation is unlikely, as the solubility product of iron carbonate is greatly exceeded (10–100 fold). The relative importance of the other processes is not yet known.To describe iron- and manganese dynamics in relation to the cycle of reducing power, generated in primary production we need data about precipitating quantities of iron and manganese compounds, for which suitable sediment-traps must be developed.