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2 2015

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

Napiórkowska-Krzebietke A., Dunalska J.

Oceanological and Hydrobiological Studies

2015

Vol. 44, No. 1

109--119

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.

Cyanobacterial bloom intensity in the ecologically relevant state of lakes : an approach to Water Framework Directive implementation

Napiórkowska-Krzebietke A.

Oceanological and Hydrobiological Studies

2015

Vol. 44, No. 1

97--108

This study focused on the determination of the cyanobacteria development in terms of the probabilistic recognition of the bloom intensity level in the ecologically relevant state of lakes. Assuming the possibility of using once-a-year sampling frequency, the ecological status or potential was assessed based on the modified Polish phytoplankton-based method (PMPLMOD) in 23 Masurian lakes. The summer cyanobacteria biomass reached 52.3 mg l-1. The biomass did not exceed the WHO-defined low risk threshold of 2 mg l-1, and was usually observed in lakes with at least good ecological status/potential. This threshold may be sufficient to propose it as a good/moderate cyanobacterial bloom intensity threshold. The average values of 3.3, 9.9 and 22.4 mg l-1 were recorded in the lakes with a moderate, poor and bad ecological status/ potential, respectively. The WHO-defined moderate risk threshold of 10 mg l-1 was exceeded only in lakes with a poor or bad ecological status/potential. The structure of cyanobacteria assemblages changed along with the deterioration of the ecological status/ potential and a decrease in Chroococcales biomass and an increase in Oscillatoriales and Nostocales biomass. Nevertheless, Planktolyngbya limnetica, Pseudanabaena limnetica and Aphanizomenon gracile had a significant presence in the total cyanobacterial biomass in all surveyed lakes.