978-1-5386-7114-6/18/$31.00 ©2018 IEEE Implementation of Acoustic Methodology for Investigation of the Ecology of Gas-Containing Toxic Cyanobacterium Microcystis sp. Ilia Ostrovsky Israel Oceanographic and Limnological Research Kinneret Limnological Laboratory Migdal, Israel ostrovsky@ocean.org.il Ernst Uzhanskii University of Haifa Leon H. Charney School of Marine Sciences Haifa, Israel Semion Kaganovsky Israel Oceanographic and Limnological Research Kinneret Limnological Laboratory Migdal, Israel Boris Katsnelson University of Haifa Leon H. Charney School of Marine Sciences Haifa, Israel 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. Keywords—hydroacoustics Microcystis, bloom formation, scum I. INTRODUCTION Cyanobacteria, a diverse group of photosynthetic oxygenic microorganisms that possess the superior acclimation capacity to develop dense blooms in freshwater lakes, ponds and reservoirs. Toxins produced during cyanobacteria blooming disrupt the functioning of aquatic ecosystems and affect the water use [1]. Large spatiotemporal variability of cyanobacteria presents a major obstacle in understanding the dynamics of their blooms. These organisms can occupy different strata, whereas laterally, floating colonies are often accumulated near the beaches due to wind and water motions [2]. Microcystis sp. is one of the most common and ubiquitous cyanobacterial genera [3]. Gas vesicles (hollow cellular structures) allow colonies to be positively or neutrally buoyant [3]. Laboratory experiments showed that Microcystis colonies can regulate their buoyancy by photosynthetic production or decomposition under dark conditions of heavy carbohydrates, and thus made diurnal vertical migrations depending on light availability [4]. A thin surface layer of cyanobacteria, a surface scum, is created and may persist for several days during the time of massive bloom of the cyanobacteria. Water motions also affect the distribution of the Microcystis colonies. Low turbulence allows buoyant cells to migrate up, while high turbulence causes uniform distribution of cells in the upper mixed water layer or through the entire water column during the holomixis time [5]. Colonies play an important role in the concentration of Microcystis near the water surface, since larger colonies have a higher rise velocity and largely contribute to scum formation [6]. Quantification of patchy Microcystis populations by using traditional sampling devices (water samplers, nets) is a methodologically complex issue due to the variable spatial distribution, formation of thin surface scum, and disruption of large colonies through physical contact with samplers. This problem can be resolved using remote sensing techniques. The presence of echo-contrasting gas bubbles in small planktonic organisms allows the use of ultrasound acoustic methods to study their spatial distribution and ertical migration [7-8]. The main aims of this work were to show 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. II. MATERIALS AND METHODS The portable scientific Simrad EY60 echo sounder operated at 120 kHz (7° opening angle) and the five-beam Teledyne RD Acoustic Doppler Current Profilers (ADCPs) – Sentinel V50 (500 kHz) were used to study the acoustic backscatter from gas-containing cyanobacterium Microcystis in Lake Kinneret (the Sea of Galilee, Israel) during its intensive development in February-March 2017 and 2018. Special attention was paid to study the distribution of Microcystis in the upper water stratum, where wind stress 236