A perennial bloom of Planktothrix agardhii (Cyanobacteria) in a shallow eutrophic French lake: limnological and microcystin production studies

A perennial bloom of the hepatotoxic filamentous cyanobacterium Planktothrix agardhii was investigated from January 1999 to February 2000 in a shallow eutrophic lake in the south of Paris (Viry-Châtillon, France). The lake is linked to the Seine River by an artificial channel and used for both recreation and fishing. Little data on cyanobacterial blooms is available in France. The aim of this preliminary study was to characterise the cyanobacterial bloom limnologically as well as for toxicity levels at one site in the lake and another in the channel. A relatively high biomass of P. agardhii was maintained throughout the year with maximum density in winter (7.9 ×10 4 to 4.5×10 6 trichomes/L). The only other significant phytoplankton species in the water bodies was also a cyanobacterium: Limnothrix redekei. The nutrient levels in the lake, although still relatively high (SRP varied in the range between 0.13 and 2 μM but NO 3 - stayed under the detection limit), were generally lower than in the channel which reflected the hydraulic influence of exchanges with the eutrophic Seine River. The concentration of microcystins (hepatotoxic cyclic heptapeptides) was measured in bloom extracts throughout the year using two different techniques: protein phosphatase 2A inhibition assay and high performance liquid chromatography coupled to diode array detection (HPLC-DAD). High microcystin concentrations, up to 5.2 μg/L MCYST-LR eq., were found at the two sampling sites, with good correlation between the two methods. The presence of five different microcystins was confirmed by further analysis of samples by LC-MS. Factors affecting seasonal population bloom dynamics, toxicity and the possible public health significance of these toxic cyanobacterial populations in the lake are discussed.