Hydrodynamic consequences of maintaining an upright posture by different magnitudes of stiffness and buoyancy in the tropical alga Turbinaria ornata

Abstract

An upright posture in the water column exposes benthic marine organisms to faster flow than near the substratum, potentially increasing rates of mass exchange while also exposing the organisms to higher hydrodynamic forces. Benthic organisms maintain upright postures in the water column by two mechanisms, being stiff or being buoyant. This study compared how different relative magnitudes of buoyancy and flexural stiffness (EI) affect hydrodynamic forces on, and flow velocity relative to the surfaces of the tropical alga, Turbinaria ornata. Thalli of T. ornata from wave-exposed forereef environments lack pneumatocysts (air bladders) and are negatively buoyant, but have higher EI than buoyant thalli with pneumatocysts from calm lagoon environments. Simultaneous measurements of water velocity, horizontal force and algal motion were recorded for pairs of thalli of the same height-positioned side by side on the reef at a site exposed to moderate wave action. To examine the effect of buoyancy alone, lagoon thalli with intact air-filled pneumatocysts were compared with lagoon algae that were made non-buoyant by replacing the air in their pneumatocysts with water. The non-buoyant thalli experienced higher water velocities and hydrodynamic forces than lagoon thalli with bladders. To test the effect of EI alone, flexible lagoon algae with water-filled pneumatocysts were compared to non-buoyant, stiff forereef algae and there was no difference in the relative water velocity or force they experienced. To compare thalli held upright by possessing a high EI to those held upright by buoyancy, lagoon algae with pneumatocysts were compared to forereef algae. Stiff forereef algae deflected very little in moving water. Water velocities relative to their surfaces and the peak horizontal forces they experienced were higher than those on more flexible lagoon algae, which bent easily and moved with the flow. Thus, buoyant, flexible thalli of T. ornata that move with the flow experience lower relative velocities and lower forces than stiff, non-buoyant thalli that do not move with the flow.