Beneath the waters off many of the world’s coasts lie meadows of photosynthetic seagrass. Like a prairie, savanna, or steppe, seagrass meadows support rich ecosystems. They also sequester about 10% of the carbon buried in ocean sediment. Monitoring the meadows’ health is important, especially now that they appear to be shrinking worldwide.
Seagrass leaves are threaded with air-filled channels known as aerenchyma. In 2009 Preston Wilson and Kenneth Dunton of the University of Texas at Austin hypothesized that the leaves’ air phase, not their solid phase, dominated their acoustic response. With laboratory experiments, they demonstrated that the speed of sound through a Gulf of Mexico seagrass meadow was indeed dependent on the biomass—that is, on the leaves’ mean density.
But Wilson and Dunton also showed that a simple two-phase model failed to predict the biomass dependence. A successful model, they argued, would require knowledge of the elastic properties of the 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 that earlier work. As was done in the 2009 study, they placed leaves of seagrass 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 drove sound waves through the water. Sweeping the piston’s frequency from low to high established a succession of resonant standing waves of increasing mode number. Knowing the cylinder’s length and presuming the modes went from 1 to 2 to 3 to 4 yielded the sound speed.
The new set of measurements examined samples from two different species, Posidonia oceanica and Cymodocea nodosa, taken from sites off the Mediterranean islands of Sicily and Crete. Across all the 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. oceanica from Crete, the sound speed difference was m/s/g.
Johnson, Hermand, and Wilson also looked at the anatomy of seagrass leaves under a microscope. They showed that the volume of aerenchyma in P. oceanica leaves varies more strongly from top to bottom than in C. nodosa leaves. That difference was manifested in measurements made on two samples of P. oceanica leaves, one consisting of only their top halves; the other, only their bottom halves.
Given the wide overall range in sound speed, the consistency of measurements from the same species and from the same site suggests that sound speed could serve as a potent diagnostic of the health of seagrass meadows. (J. R. Johnson, P. S. Wilson, J.-P. Hermand, JASA Express Lett. 1, 080801, 2021.)