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A specific quantitative real-time PCR (qPCR) method was developed for the quantification of hepatotoxin nodularin-producing Nodularia , one of the main bloom-forming cyanobacteria in the Baltic Sea. Specific PCR primers were designed for subunit F of the nodularin synthetase gene ( ndaF ), which encodes the NdaF subunit of the nodularin synthetase gene complex needed for nodularin production. The qPCR method was applied to water samples (a total of 120 samples) collected from the Baltic Sea in July 2004. As few as 30 ndaF gene copies ml −1 of seawater could be detected, and thus, the method was very sensitive. The ndaF gene copy numbers and nodularin concentrations were shown to correlate in the Baltic seawater, indicating the constant production of nodularin by Nodularia . This qPCR method for the ndaF gene can be used for detailed studies of Nodularia blooms and their formation. ndaF gene copies and nodularin were detected mostly in the surface water but also in deeper water layers (down to 30 m). Toxic Nodularia blooms are not only horizontally but also vertically widely distributed, and thus, the Baltic fauna is extensively exposed to nodularin.

We studied the frequency and composition of potential microcystin (MC) producers in 70 Finnish lakes with general and genus-specific microcystin synthetase gene E (mcyE) PCR. Potential MC-producingMicrocystis,PlanktothrixandAnabaenaspp. existed in 70%, 63%, and 37% of the lake samples, respectively. Approximately two-thirds of the lake samples contained one or two potential MC producers, while all three genera existed in 24% of the samples. In oligotrophic lakes, the occurrence of only one MC producer was most common. The combination ofMicrocystisandPlanktothrixwas slightly more prevalent than others in mesotrophic lakes, and the cooccurrence of all three MC producers was most widespread in both eutrophic and hypertrophic lakes. The proportion of the three-producer lakes increased with the trophic status of the lakes. In correlation analysis, the presence of multiple MC-producing genera was associated with higher cyanobacterial and phytoplankton biomass, pH, chlorophylla, total nitrogen, and MC concentrations. Total nitrogen, pH, and the surface area of the lake predicted the occurrence probability ofmcyEgenes, whereas total phosphorus alone accounted for MC concentrations in the samples by logistic and linear regression analyses. In conclusion, the results suggested that eutrophication increased the cooccurrence of potentially MC-producing cyanobacterial genera, raising the risk of toxic-bloom formation.