In situ monitoring of cyanobacteria using phycocyanin fluorescence probes in two eutrophic lakes in southwestern Finland

Abstract

Cyanobacteria blooms can complicate or even prevent the economical or recreational use of waters. Many of the bloom forming cyanobacteria species are also potential producers of harmful cyanotoxins. The most commonly used standard method for quantifying phytoplankton biomass is based on inverted microscopy. The method has relatively high accuracy and is the only one producing biomass results on taxonomic level, but it requires high expertise and is time-consuming, producing delay to the results. Since 2006, we have studied practical solutions of continuous on-line and in situ monitoring of cyanobacteria using phycocyanin fluorescence probes. In many published studies, phycocyanin concentration has been proven to be a useful proxy for the actual concentration of cyanobacteria cells or biovolume. Two eutrophic lakes were studied, Lake Littoistenjärvi (during the years 2006-2011) and Lake Kuralanjärvi (2008) in southwestern Finland, using stationary monitoring stations equipped with TriOS microflu-blue probes and a temperature sensor. To assess the cyanobacteria biomass, the fluorescence station results were compared to independent composite water samples analyzed using the standard inverted microscopy method. The fluorescence results were positively correlated to cyanobacteria biomass in both lakes (R2=0.58, p<0.001 in Lake Littoistenjärvi and R2=0.83, p<0.001 in Lake Kuralanjärvi). However, the post-calibration coefficients to convert phycocyanin fluorescence into absolute biomass varied between the two lakes probably due to the different phytoplankton taxa composition. In Lake Kuralanjärvi, also cyanotoxin (microcystine) concentration was correlated to fluorescence results (R2=0.77, p<0.001). Using local site specific biomass post-calibrations, phycocyanin fluorescence can be used to estimate the absolute biomass of cyanobacteria and can also indicate the possible occurrence of microcystins. The fluorometer probe station techniques used in these studies are an applicable and relatively low-cost method to monitor short-term changes in cyanobacteria abundance. With nearly real-time data transfer possibilities, they can be used in the management and early warning applications to minimize the possibly harmful effects of cyanobacteria blooms.