Phytoplankton-based recovery requirement for urban lakes in the implementation of the Water Framework Directive’s ecological targets

This study focused on the determination of the phytoplankton-based recovery requirement, including bloom intensity thresholds in human-affected lakes, helpful in the restoration. The phytoplankton and physicochemical analyses were carried out on seven urban lakes in Kartuzy and Skępe. The Phytoplankton Metric for Polish Lakes was used to assess the ecological status and the Trophic Level Index was used to determine the trophic state. Only one lake had a good ecological status (meeting the WFD requirements for at least good status), whereas other lakes had poor or bad ecological status. All of them were eutrophic or hyper-eutrophic and the summer phytoplankton assemblages were dominated mainly by bloomforming cyanobacteria. The lowest phytoplankton bloom threshold of 2.6 mm³ l^-1 (seasonal maximum) and the following thresholds of 5 mm³ l^-1 for classes I/II; 8 mm³ l^-1 for classes II/III; 21 mm³ l^-1 for classes III/IV; 100 mm³ l^-1 for classes IV/V were proposed for urban lakes. This could directly refer to the bloom intensity for high, good, moderate, poor and bad ecological status. This all indicates a huge recovery requirement in the majority of urban lakes, and the proposed bloom intensity classification may be helpful in fulfilling the WFD targets for the ecologicallyrelevant lake status.