Application of a submersible spectrofluorometer for rapid monitoring of freshwater cyanobacterial blooms: a case study

A recently available submersible fluorescent probe was configured and used to survey the vertical distribution of the deep-living toxic and filamentous cyanobacterium Planktothrix (Oscillatoria) rubescens among the autochthonous algal communities in Lake Bourget, France. This in situ measuring spectrofluorometer, which can be used to perform chlorophyll analysis and integrated algal class determination, provides a realistic estimation of the abundance and dynamics of the cyanobacterial population that is known to produce the hepatotoxic heptapeptides microcystin RR and LR. Data provided from in-line measurements using the probe and from R rubescens cell counts obtained by discrete sampling were closely correlated (r = 0.899, p < 0.01), as were in-line data and spectrophotometric total chlorophyll a measurements (r = 0.775, p < 0.01). A survey conducted from December 1999 to May 2001 revealed that P. rubescens exhibits a deep maximum level (typically between 10 and 15 m) in spring and summer (reaching concentrations of up to 20 mug equivalent chl a 1(-1), i.e. 27 000 cells ml(-1)), whereas it spreads from the surface either to the top of the thermocline or to the bottom of the lake, in autumn and winter respectively. We propose that the probe could be used as a powerful tool for assaying the occurrence and dynamics of microalgal blooms, typically toxic cyanobacteria, that call for accurate and rapid monitoring to assess the health of the ecosystem and to alert the authorities about potential risks regarding pumping and use of the lake water for drinking-water production purposes.