Implementation of Acoustic Methodology for Investigation of the Ecology of Gas-Containing Toxic Cyanobacterium Microcystis sp

Abstract

Toxic cyanobacterium Microcystis affects the functioning of aquatic ecosystems and water quality. New remote sensing approaches need to be developed for in situ study of Microcystis bloom development. The aims of our work were to display the advantages of hydroacoustic techniques for in situ studying the gas-containing cyanobacterium Microcystis sp. and to examine the role of physical factors in Microcystis bloom and surface scum formation. We found that gas-containing Microcystis colonies are strong acoustic backscatterers at ultrasound frequencies. This allowed developing a novel approach for in situ quantification and of Microcystis populations. Dynamics of the echo-reflecting layer monitored with a bottom-mounted 500 kHz ADCP showed distinct near-diurnal vertical fluctuations caused by vertical dispersion of the cyanobacterium in response to development or fading of daily thermocline in late winter - early spring. During the Microcystis bloom season the volume backscattering strength in the water column was also quantified with a 120 kHz echosounder EY60, which was calibrated versus particle volume concentrations acquired with a Submersible Particle Size Analyzer LISST −100x and fluorometrically measured chl-a concentrations (a proxy of Microcystis biomass during the bloom development). Our study of the formation of Microcystis surface scum in relation to physical forces and water motions suggest that creation of surface scum plays an important role in the fast increase of Microcystis biomass in Lake Kinneret (Israel). Hydroacoustic methods can be widely used for in situ investigations of the spatiotemporal variability of gas-containing cyanobacteria.