UACE2025: Hydroacoustic assessment of spatiotemporal dynamics of toxic cyanobacterium Microcystis: the role of physical factors in bloom formation

• Session:
  Acoustic Monitoring of Marine Ecosystem
• Paper:
  Hydroacoustic assessment of spatiotemporal dynamics of toxic cyanobacterium Microcystis: the role of physical factors in bloom formation
• Author(s):
  Ilia Ostrovsky, Assaf Sukenik, Yosef Z. Yacobi, Wesley J. Moses, Boris Katsnelson, Ernst Uzhanskii
• Abstract:
  Toxic cyanobacterial blooms become of common occurrence all around the world. They disrupt the functioning of aquatic ecosystems and water use. The main difficulty in monitoring stems from the heterogenic spatial distribution of cyanobacteria. We suggest an acoustic remote sensing approach to investigate the cyanobacterium Microcystis in nature and to examine the role of physical processes affecting bloom development. We detected that gas-containing Microcystis colonies are strong acoustic backscatterers at ultrasound frequencies and can be quantified with echo sounder and with Acoustic Doppler Current Profiler (ADCP). Volume backscattering strength (Sv) was measured in Lake Kinneret (the Sea of Galilee) with Simrad EK60 echo sounder at 120-kHz, and two Teledyne ADCPs - Sentinel V20 (1000 kHz) and Sentinel V50 (500 kHz). The Sv measured with EK60 was calibrated versus fluorometrically assessed chl-a concentration, as proxy for biomass. The obtained calibration was used to convert the acoustic signal to biomass and to study vertical and horizontal organization of the Microcystis population. Profiling CTDs, an upward- or downward-looking sonar and ADCPs allowed us to estimate the evolution of surface scum in association with changes of lake stratification. We showed that the cyanobacterium tend to increase on biomass when the stability of stratification increases. During hot windless spring days develops a thin surface stratified layer where buoyant cyanobacteria largely concentrate, absorb solar radiation, and thus increase surface temperature. This generates optimal conditions for acceleration of Microcystis biomass growth. Different hydrodynamic processes on various spatial scales play an important role in formation of surface cyanobacteria patches seen on satellite images. We argue that the formation of surface patches are essential for production of high Microcystis biomass in water bodies.
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Contact details

• Contact person:
  Prof Ilia Ostrovsky
• e-mail:
  
• Affiliation:
  Israel Oceanographic and Limnological Research
• Country:
  Israel