The sound of the seagrass

A meadow of Oceanic posidonia. Credit: Alberto Roméo /CC BY-SA 2.5

Under the waters off many coasts of the world are meadows of photosynthetic seagrass. Like a meadow, a savannah or a steppe, seagrass beds support rich ecosystems. They also sequester about 10% of the carbon buried in ocean sediments. It is important to monitor the health of grasslands, especially now that they appear to be shrinking globally.

The seagrass leaves are threaded with air-filled channels called aerenchyma. In 2009, Preston Wilson and Kenneth Dunton of the University of Texas at Austin Assumed that the aerial phase of the leaves, and not their solid phase, dominated their acoustic response. With laboratory experiments, they demonstrated that the speed of sound through a seagrass prairie in the Gulf of Mexico indeed depended on biomass, i.e. the average density of leaves.

But Wilson and Dunton also showed that a simple two-phase model failed to predict dependence on biomass. A successful model, they argued, would require knowledge of the elastic properties of seagrass tissue and its structure.

Wilson and his collaborators, Jay Johnson of the University of Michigan and the late Jean-Pierre Hermand of the Free University of Brussels, have now extended this earlier work. As was done in the 2009 study, they placed seagrass leaves inside a glass cylinder filled with artificial seawater. The cylinder acted as a one-dimensional resonator. A vibrating piston attached to one end of the cylinder propelled sound waves through the water. Sweeping the frequency of the piston from low to high established a succession of resonant standing waves of increasing number of modes. Knowing the length of the cylinder and assuming that the modes changed from 1 to 2 to 3 to 4 gave the speed of sound.

The new series of measurements examined samples of two different species, Oceanic posidonia and Knotty Cymodocea, taken from sites off the Mediterranean islands of Sicily and Crete. In all samples, the difference in sound speed between only seawater and seawater containing seagrass ranged from -1.5 m / s per gram of biomass to -54 m / s per gram of biomass . However, the range within each sample was much narrower. For example, in the 73 samples of P. oceanic from Crete, the difference in sound speed was


m / s / g.

Johnson, Hermand, and Wilson also examined the anatomy of seagrass leaves under a microscope. They showed that the volume of aerenchyma in P. oceanic leaves vary more strongly from top to bottom than in C. nodosa sheets. This difference was manifested by measurements made on two samples of P. oceanic leaves, one consisting only of their upper halves; the other, only their lower halves.

Given the overall wide range of speed of sound, consistency of measurements from the same species and site suggests that speed of sound could serve as a powerful diagnostic of seagrass health. (JR Johnson, PS Wilson, J.-P. Hermand, JASA Express Lett. 1, 080801, 2021.)

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