Measurement of acoustic material properties of macroalgae (Ecklonia radiata)

Jo Randall, Jean Pierre Hermand, Marie Elise Ernould, Jeff Ross, Craig Johnson

Research output: Contribution to journalArticleResearchpeer-review

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.

Original languageEnglish
Pages (from-to)430-440
Number of pages11
JournalJournal of Experimental Marine Biology and Ecology
Volume461
Issue numberDecember
DOIs
Publication statusPublished - 1 Dec 2014
Externally publishedYes

Fingerprint

macroalgae
acoustics
compressibility
seawater
macroalga
primary production
primary productivity
habitat
habitats
acoustic properties
acoustic property
seagrass meadow
tissues
Ecklonia radiata
properties of materials
sound
tissue
speed
alginate
alginates

Cite this

Randall, Jo ; Hermand, Jean Pierre ; Ernould, Marie Elise ; Ross, Jeff ; Johnson, Craig. / Measurement of acoustic material properties of macroalgae (Ecklonia radiata). In: Journal of Experimental Marine Biology and Ecology. 2014 ; Vol. 461, No. December. pp. 430-440.
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Measurement of acoustic material properties of macroalgae (Ecklonia radiata). / Randall, Jo; Hermand, Jean Pierre; Ernould, Marie Elise; Ross, Jeff; Johnson, Craig.

In: Journal of Experimental Marine Biology and Ecology, Vol. 461, No. December, 01.12.2014, p. 430-440.

Research output: Contribution to journalArticleResearchpeer-review

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T1 - Measurement of acoustic material properties of macroalgae (Ecklonia radiata)

AU - Randall, Jo

AU - Hermand, Jean Pierre

AU - Ernould, Marie Elise

AU - Ross, Jeff

AU - Johnson, Craig

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AB - 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.

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