Drag and dislodgment of an intertidal macroalga: consequences of morphological variation in Mastocarpus papillatus Kützing

Abstract

Survival of intertidal macroalgae depends on their ability to withstand the large hydrodynamic forces generated by breaking waves, an ability that may be a function of both the morphology and size of the plant. Mastocarpus papillatus Kützing is an intertidal red alga that exhibits a broad range of morphologies varying in papillar length and density, thallus thickness, and degree of branching. Drag was measured in a flow tank on single female gametophyte thalli spanning a broad range of morphologies and sizes. The drag coefficients of M. papillatus are highly variable, but this variation decreases at high water velocities (3–4 m · s −1). Other intertidal macroalgae have similar ranges of drag coefficients at high water velocities. Regression analysis indicates that drag force is primarily determined by the planform area of the thallus and is not strongly influenced by morphology. The diameter of stipes (where mechanical failure most often occurs) of mature thalli do not increase in proportion to the area supported, suggesting that thalli in habitats with heavy wave action may be size-limited regardless of their morphology. Calculations predict that maximum thallus size may be limited by water velocities that occur on exposed coasts.