Measurement of acoustic material properties of macroalgae (Ecklonia radiata)

Abstract

Temperate macroalgal forests are among the most productive ecosystems in the world. Acoustic propagation measurements have been used to monitor primary production over broad spatial scales (101–103m) in seagrass meadows, and work is in development to assess the application of acoustics for measuring aggregate production in kelp beds and forests. In addition, scientific echosounders have been used routinely for mapping these benthic habitats and, in some cases, identify dominant species. Further advances in these areas require the development of species-specific acoustic models. However there is little knowledge of the acoustic properties of macroalgae, in part because measuring sound speed in large macroalgae is challenging due to their complex morphology. In this study four different methods are developed and trialled to determine the intrinsic sound speed of Ecklonia radiata tissue based on measurement of the time of flight of an ultrasonic pulse, while compressibility is calculated from density measurements. Direct methods involved lengths of stipe and tightly packed stacks of macroalgae blade tissue. Indirect methods focused on an entire intact macroalga submerged in seawater, and a homogenate solution containing seawater and blended blade tissue. Blade tissue showed a density contrast (relative to seawater) of 1.23 and a sound speed contrast of 1.0374 for the stacks. The homogenate solution gave a sound speed contrast of 1.0424. Stipe tissue density and sound speed were lower. Density contrast was 1.04 and sound speed contrasts were 1.0179 (SD=0.0025) and 1.0064 (SD=0.0032) depending on the sample. Whole macroalgae had age- and size-dependent densities with an average density contrast of 1.11 (SD=0.09) and showed an average sound speed of 1572.8m/s (17.8°C) and contrast of 1.0404 (SD=0.0139). Compressibility was higher in stipe than blade tissue, with 3.924e-10Pa−1 and 3.982e-10Pa−1 for stipes and 3.209e-10Pa−1 and 3.180e-10Pa−1 for blade in stacks and homogenate respectively. The results show that E. radiata sound speed and density are higher, and compressibility lower, than that of seawater. This is likely related to high concentrations of alginate, and other structural and storage carbohydrates in the macroalgae, and thus may vary seasonally. The differences between tissue types found for all properties reflect the morphology and anatomy of this macroalga, with tightly condensed chloroplast cells in blade tissue and loosely packed structural cells in the stipe. This research provides essential input parameters to numerical models that will enhance acoustic habitat mapping and allow the development of acoustic inverse methods. This may enable the estimation of aggregate primary production over large spatial scales in temperate kelp habitats, thus informing their future management.