Morphological variations do not effectively reduce drag forces at high wave-exposure for the macroalgal species, Hedophyllum sessile (Laminariales, Phaeophyta)

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Abstract
It is generally accepted that variations in algal size and shape modify flow regimes and can effectively minimize hydrodynamic stresses. Correlation of thallus morphological variations with wave exposure has been well documented. Hydrodynamic performance of various morphologies of the intertidal kelp species Hedophyllum sessile was studied to evaluate if the more streamlined morphology effectively reduces drag forces at sites of high wave-exposure. Morphologies were collected from sheltered and exposed sites. Morphological changes that occurred during the winter storm season were simulated using thalli from the highly wave-exposed site; such thalli were split lengthwise (to increase blade number) and some of these individuals were then shortened. Drag forces directly measured on these morphologies ranged from 0.5 to 4.0 m s−1. Contrary to original predictions, drag coefficients, drag per projected area, and E-values were not statistically different between morphologies within experimental velocities. However, on average and per surface area, the sheltered and combined split and shortened morphologies generated more drag forces than the nonsplit and lengthwise split exposed morphologies. E-values ranged from −1.45 to −1.67 indicating that all morphologies streamline quickly in flow. The combined split and shortened morphology did not significantly reduce drag coefficients but most effectively reduced drag force by size reduction, illustrating that the most important determinant of drag force is thallus size not shape.

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