Water quality control in reservoirs

Project No. S6017004

Since 2000 to 2004, 200 samples were taken from 50 eutrophic Czech reservoirs. Quantitative and qualitative analyses of the phytoplankton were performed to determine the amount of cyanobacteria in reservoirs studied. Microcystins (MCs) estimation in cyanobacterial biomass was made in 80 samples. The main cyanobacterial strains were successfully isolated and cultivated.

Ten of the reservoirs sampled are drinking water supplies. The other reservoirs serve mainly for recreational activities. In 2000, the phytoplankton was dominated by cyanobacteria in 47 % of all reservoirs. In 2003, cyanobacterial bloom was recorded in 70 % localities.

In August 2001, cyanobacterial bloom occurred in 6 of the 15 reservoirs sampled. Aphanizomenon flos-aquae, Anabaena planctonica, Microcystis aeruginosa and Woronichinia naegeliana were the most commonly found species.

In July 2002, cyanobacteria occurred in all reservoirs studied. However, heavy cyanobacterial bloom was recorded in 55 % of 29 reservoirs only. The most common species were Microcystis aeruginosa and Woronichinia naegeliana.

In mid July and August 2003 and 2004, 18 reservoirs in the Czech Republic were sampled for phytoplankton species composition and concentration of intracellular microcystins. As a consequence of high nutrient loading, most of the reservoirs experienced cyanobacterial blooms of various intensities with the prevalence of cyanobacteria increasing markedly in August, along with a conspicuous shift in species composition towards the dominance of Microcystis spp. The cyanobacterial blooms were primarily dominated by Microcystis aeruginosa while other cyanobacterial taxa occurred subdominantly and prevailed in only a few of the reservoirs surveyed. Microcystins were detected in 90 % of the samples and their amount also increased considerably in August, reflecting the cyanobacterial biomass.
However, MCs concentrations were lower in 2004 (median 0.8 µg L^-1) than during the dry, warm season of 2003 (median 2.8 µg L^-1). In Microcystis dominated samples, significantly higher amount of MCs (p<0.001) occurred than in samples where other taxa prevailed. Microcystins were positively correlated with chlorophyll a and cyanobacterial biovolume (p<0.05, r=0.61 and 0.66, respectively) with the strongest correlation found for Microcystis spp. biovolume (p<0.001, r=0.87), this taxon was the most important producer of MCs in Czech reservoirs. The main structural variants of MCs were MC-LR, MC-RR and MC-YR. This study’s data also indicates that the relative share of MC variants (MC-LR and MC-RR) vary considerably with time, most likely as a consequence of different species and strain compositions during the summer. This study clearly demonstrates a high prevalence of MC–producing cyanobacteria in Czech reservoirs.