Overview of the ecological situation of open pit lakes of the lignite mining in Germany

The EU Water Framework Directive (WFD) demands a good ecological status or ecological potential for natural and artificial lakes larger than 50 ha until 2027. More than 100 artificial mining lakes originate from lignite mining activities in Germany. The specific chemistry of mining lakes with their high phosphorus retention capacity is a chance for the development of clear lakes that are poor in nutrients. For neutral mining lakes, the existing typology for natural lakes which is based on the biological quality elements was adapted. For acidic lakes, the phytoplankton assessment was modified by including the diversity of the phytoplankton. In total, 36 of the largest existing mining lakes in Germany were assessed and description of the limnological lake characteristics were elaborated. Some of the relevant lakes could not be sampled because they are still in filling process and under surveillance of the mining authorities or safety demands for sampling are not given. So far, 53 % of lakes considered in this study reach the good or better ecological potential. Three lakes fail this state because environmental quality standards for river basin-specific chemical pollutants are not complied. Only one lake shows too high trophic conditions. The currently applied assessment tools especially those for macrophytes and microphytobenthos do not meet the requirements for the specific ecological situation in acidic mining lakes. Therefore, these results can only be regarded as an intermediate status of assessment for mining lakes. A revision of the assessment process of macrophytes and microphytobenthos in acidic lakes is recommended.