Effect of water temperature on the signal of a phycocyanin probe and correction factor applied to three morphotypes to improve field monitoring of cyanobacteria

Abstract

In a context of global warming and heavy anthropization of freshwater systems, cyanobacteria monitoring is becoming an essential priority. In recent years, submersible fluorescence sensors capable of measuring pigments and estimating cyanobacterial biomass in near-real time have been deployed on a large-scale. However, these tools have several limitations, such as the detection limit of pigments like phycocyanin or the impact of temperature on the measurements. In an attempt to overcome these shortcomings, we compared the effectiveness of a phycocyanin probe at different temperatures, first on three cyanobacteria cultures (Microcystis, Dolichospermum and Synechocystis) and then on a small eutrophic lake in continuous recording for three years.In vitro results showed a strong correlation between cyanobacterial biomasses and phycocyanin probe measurements, with R2 higher than 0.96, even for biovolumes lower than hundreds of mm3.L−1. A saturation effect was demonstrated between 350 and 550 mm3.L−1, depending on the strain. In addition, laboratory tests on temperature highlighted the sensor's limitations, with a strong over- and underestimation at low (< 15 °C) and high (> 25 °C) temperatures, respectively. Given this discrepancy, a water temperature correction factor was determined for each strain and applied according to the predominant cyanobacteria group identified in the field during the three years monitoring. Signal quality improved considerably, and the stronger relationship found between the corrected signal and the biovolume of cyanobacteria obtained by microscopic enumeration enabled us to validate this temperature correction factor, offering the possibility of using it to better monitor cyanobacterial dynamics.