Measuring the complex shear modulus of seagrass with an iPhone: A COVID-19 experiment

The shear modulus of seagrass is an important, yet rarely reported, parameter used to study the resilience of seagrass communities to hydrodynamic disturbances, and to understand the propagation of acoustic waves through seagrass meadows. Two intact Thalassia testudinum seagrass bundles were sampled from Redfish Bay (RFB) and Lower Laguna Madre (LLM) in South Texas, transported under oxygenated salt water to Austin, Texas, where measurements were performed at the primary author's residence during the COVID-19 shelter-in-place order in May 2020. A subsample of ten seagrass blades from each bundle was chosen, and the average thickness and width of each blade were recorded. Next, an iPhone was suspended from a ledge by each blade and manually subjected to an initial angular displacement with the axis of rotation colinear to the blade length. Once released, the angular velocity of the underdamped system was measured by the iPhone's gyroscope. Finally, the angular frequency and decay rate were used to infer the complex shear modulus. The complex shear moduli reported for RFB and LLM blades are 71 + 3i MPa and 145 + 5i MPa, respectively. Results suggest epiphytic growth could both increase the loss factor and decrease the shear modulus of seagrass. [Work supported by ARL:UT IR&D.]