Toxic cyanobacteria in the lakes located in RĪga (the capital of Latvia) and its surroundings: Present state of knowledge

The intensity and toxicity of cyanobacterial blooms in lakes located near RĪga (the capital of Latvia) were investigated between 1998 and 2004. These lakes are shallow eutrophic water bodies with a high surface to depth ratio and are widely used for recreational and fishing activities. Intense cyanobacterial blooms were associated with calm weather, high water temperatures (>20°C), and low oxygen saturation in surface (40-70%) and bottom (1-30%) waters. The maximum biomass of harmful cyanobacteria occurred during August-September. Cyanobacterial blooms were dominated by Anabaena spp., Aphanizomenon flos-aquae and Microcystis spp. A significant decrease of the Shannon-Wiener species diversity index was observed during bloom periods. Hepatotoxins (microcystins) were detected in 50% of the surveyed lakes. High-performance liquid chromatography analysis of algal biomass indicated the presence of microcystins MC-LR, MC-YR, MC-RR and MC-LA. The most abundant was MC-LR, contributing on average 43% of the total microcystins. Microcystin concentrations showed a significant interannual variation. The highest concentration of microcystins was observed in Lake Lielais Baltezers.