Geometrical and mechanical properties of four species of northern European brown macroalgae

•Brown algae stem and blade tapering makes a single value insufficient for description.•Flexural rigidity is similar for all parts of brown algae.•Stiffness may be better described by flexural rigidity rather than bending modulus.•Flexural rigidity is similar for three different species in the same habitat.

Aquatic vegetation interacts with the flow by posing an obstruction and in return experiences drag and is reconfigured by acting forces. It is suggested that plant buoyancy and stiffness affect these bio-physical interactions and hence should be considered when including vegetation in physical and numerical hydrodynamic models. However, data on these parameters is either lacking for many species or existing information is insufficient to model flexible aquatic vegetation correctly. Previous studies have focused on plant stems, but did not take plant foliage into account. The present study extends the existing knowledge base by providing data for four northern European brown macroalgae and also provides a comparison between the mechanical properties of stem and blade tissue for these species. Specimens of Alaria esculenta, Laminaria digitata, Fucus serratus and Fucus vesiculosus were collected from a small tidal inlet in Norway. Other than F. serratus, the macroalgae were positively buoyant and all stems showed bending moduli in agreement with previous studies with respect to their order of magnitude. Only L. digitata exhibited a significant difference between bending moduli for stems and blades and also between the tip and the base of the blades. However, there were differences in flexural rigidities (product of bending modulus and second moment of cross-sectional area) of both stems and blades and the tip and the base of blades of all but F. serratus. This highlights the important influence of cross-sectional area and cross-sectional shape on stiffness.