Hydrodynamic consequences of flexural stiffness and buoyancy for seaweeds: a study using physical models

An upright posture in the water column may expose benthic marine organisms to faster flow higher in the water column 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 one of two mechanisms, stiffness or buoyancy. Turbinaria ornata is a tropical macroalga that uses either buoyancy or flexural stiffness (EI), depending on its flow habitat. This study used physical models of T. ornata to compare the effect of different magnitudes of these two mechanisms on relative water velocity and hydrodynamic forces in both unidirectional and wavy flow. Models of the alga were constructed to span and exceed natural levels of T. ornata's EI and buoyancy. Models with high EI and high buoyancy maintained upright postures in both unidirectional flow and waves, and experienced higher forces than models with low EI and moderate or low buoyancy that deflected in the direction of water motion. In waves, buoyant models that were deflected by high velocity rebounded back into upright positions when the flow slowed. Non-buoyant, flexible models were also pushed over by flow but lacked the ability to rebound upright, which led to decreased force in unidirectional flow, but high force in waves.