Ultrasonic scattering responses from phytoplankton: Measurements and modelling

Phytoplankton plays a major role in the ocean's ecosystem. Phythoplanktonic organisms are the most important primary producers in the aquatic systems as well as they account for more than half of the photosynthesis on earth, providing oxygen to the atmosphere and regulating the carbon dioxide that controls the earth's climate. Therefore, developing non-invasive efficient methods to estimate algae abundance and distribution is an important issue. In that sense, a long-term multidisciplinary research programme to investigate the high-frequency acoustic response of phytoplankton has been developed. While acoustic scattering techniques have been widely used to investigate zooplankton with frequencies below 1 MHz, few reports for phythoplanktonic organisms can be found in the literature. This partial lack of publications may be related to the challenge that arises from the experimental viewpoint mainly due to the microscopic scatterers size and the small sound speed and density contrasts of the microscopic algae relative to seawater. This work attempts to provide a general review of how ultrasound scattering techniques used during at-lab and at-sea measurements evolved along the last years leading to the current state of art in this field of acoustical oceanography. The feasibility of acoustically monitoring phythoplanktonic scatterers, living either within aqueous culture media or in seawater samples, is assessed. Further purposes of contributing to phytoplankton biomass estimation and harmful algae blooms detection through ultrasound scattering methodologies have been considered here. Results from insonification of several single-species algae aqueous culture media during laboratory experiments as well as examination of nearly real-time acoustic responses of seawater samples collected with a vertical trawl phytoplankton net or Niskin bottles are presented. At-lab measurements were performed by insonification of Skeletonema pseudocostatum, Chlamydomonas reinhardtii and Euglena gracilis cultures using 2.25, 3.5 and 5 MHz narrowband transducers driven by a pulse-receiver system. Simultaneous optical counting and culture media image analysis were carried out. Backscattering cross-sections at 5 MHz of Skele-tonema pseudocostatum were computed using theoretical models for further signals simulation. Good agreement was obtained when comparing simulation results with acoustic measurements. Recent measurements at 5 MHz in waters off the southeast Argentinian coast at thirteen CTD stations within the San Jorge Gulf are also presented. Additional analysis, signal processing, simulation and modelling of the scattered signals have been conducted with the aim of correlating measured backscattered power with scatterers type and concentrations. For that purpose, optical cells observation and counting, culture media image analysis and taxonomic classification, were also accomplished. Python scripts were developed for real-time acquisition and processing of backscattered acoustic signals while unmasking of undesirable acoustic responses from spurious scattering was performed applying innovative signal processing techniques. The power backscattered by the at-sea samples from the whole water column collected with the phytoplankton net showed an increment of about 20 dB-25 dB referred to surface samples at the stations with high fluorescence values. A detailed analysis of the time dependence of the backscattered signals is fulfilled. As a summary, it can be pointed out that current results encourage further work on acoustic detection of Harmful Algae Blooms.