Determining the Relationship Between Observed Algal Activity and Ultrasonic Attenuation Measurements

A Single Frequency Acoustic Attenuation Surrogate (SFAAS) was developed by NCPA to measure suspended sediment concentrations using 20 MHz acoustic signals. Previous work hypothesized that biological factors, such as population variation or algal movements, could also be determining factors in increasing acoustic signal attenuation when there is an absence of suspended sediment transport. A change in attenuation may occur during the day in dense colonies because increasing light intensity leads to high photosynthetic rates, which can produce O₂ bubbles within colonies that increase buoyancy and cause algae to rise in the water column. Respiration at night consumes O₂ bubbles, resulting in sinking algal colonies.

Semi-controlled underwater acoustic experiments in still water with large algae populations were initiated in conjunction with USDA-ARS-NSL. Ancillary measurements were also monitored including light intensity, dissolved oxygen, and water temperature in an environment without sediment transport. Cyanobacteria was the dominant algal group in the limnocorral, and vertical movement of cyanobacteria is modulated by changes in cell density, gas vesicles inside of cells, and gas bubbles between cells in dense colonies.