STEWART, Hannah/L; Univ. of California, Berkeley: Hydrodynamic consequences of buoyancy vs. flexural stiffness in the tropical alga Turbinaria ornata

Benthic organisms maintain upright postures by high flexural stiffness (EI) or by buoyancy. An upright position in the water column can potentially increase mass transfer and light interception but may also expose sessile organisms to greater hydrodynamic forces. This study compared how buoyancy and EI affect hydrodynamic forces and flow velocity at the surface of the tropical alga Turbinaria ornata. Thalli of it T. ornata from wave-exposed fore reef environments (FR) lack air bladders and are negatively buoyant, but have higher EI than buoyant thalli from calm lagoon environments (LG). Simultaneous measurements of water velocity, horizontal force and algal motion were recorded for pairs of thalli positioned side by side on the reef at a site exposed to moderate wave action. For these experiments LG algae were cut to the same length as FR algae to remove the effect of size. To compare thalli held upright by EI to those held upright by buoyancy, LG algae with air bladders were run simultaneously with FR algae. Stiff FR algae did not experience substantial deflection therefore water velocities relative to their surfaces and mean peak horizontal force were higher than on more flexible LG algae, which moved with the flow. To examine the effect of buoyancy alone, LG thalli with intact bladders were run simultaneously with LG algae with water-filled bladders (filled). Filled LG thalli experienced higher surface velocities and forces than LG thalli with bladders. To test the effect of EI, filled LG algae were run with FR algae, and there was no difference in the force or surface velocity experienced. Therefore, force and water velocity at the surface of T. ornata are affected by buoyancy, and combinations of EI and buoyancy, but not by the differences in EI alone.