Support of the Self-purification Processes in Lakes Restored in Poland

This study analyses the dynamics of organic matter and algae pigments in water of urban lakes subjected to various restoration techniques. Most of the lakes were restored with the inactivation method using coagulant PAX 18. One of them was supported with biomanipulation. Assessments of the organic carbon form content (POC, in suspension; DOC, dissolved form) and the chlorophyll and pheophytin concentrations were conducted. The ratios of individual parameters were then evaluated, and the relationships between these indicators were tested. It was found that decreasing primary production and the associated organic matter in a suspended form can improve self-purification processes in restored lakes. The decreasing POC after restoration leads to a reduction in the amount of DOC in the lake and determines the balance of production and decomposition. This improvement may initially be able to reduce the DOC content, mainly due to easily biodegradable compounds. The lower primary production supplies a lower content of hardly biodegradable compounds (when algae decay). After that, the amount of high-molecular-weight DOC can be reduced. The improved self-purification of the lakes was also reflected in a reduced chlorophyll/pheophytin ratio. There was no effective reduction of the organic matter content after the first restoration in polymictic lakes with high loaded biogens and organic matter (DOC) from the catchment.