Flow-induced morphological variations affect diffusion boundary-layer thickness of macrocystis pyrifera (Heterokontophyta, Laminariales)

In slow mainstream flows (<4–6 cm Æ s)1), the transport of dissolved nutrients to seaweed blade surfaces is reduced due to the formation of thicker diffusion boundary layers (DBLs). The blade morphology of Macrocystis pyrifera (L.) C. Agardh varies with the hydrodynamic environment in which it grows; wave-exposed blades are narrow and thick with small surface corrugations (1 mm tall), whereas wave-sheltered blades are wider and thinner with large (2–5 cm) edge undulations. Within the surface corrugations of wave-exposed blades, the DBL thickness, measured using an O2 micro-optode, ranged from 0.67 to 0.80 mm and did not vary with mainstream velocities between 0.8 and 4.5 cm Æ s)1. At the corrugation apex, DBL thickness decreased with increasing seawater velocity, from 0.4 mm at 0.8 cm Æ s)1 to being undetectable at 4.5 cm Æ s)1. Results show how the wave-exposed blades trap fluid within the corrugations at their surface. For wavesheltered blades at 0.8 cm Æ s)1, a DBL thickness of 0.73 ± 0.31 mm within the edge undulation was 10- fold greater than at the undulation apex, while at 2.1 cm Æ s)1, DBL thicknesses were similar at <0.07 mm. Relative turbulence intensity was measured using an acoustic Doppler velocimeter (ADV), and overall, there was little evidence to support our hypothesis that the edge undulations of wave-sheltered blades increased turbulence intensity compared to wave-exposed blades. We discuss the positive and negative effects of thick DBLs at seaweed surfaces. Key index words: blade morphology; diffusion boundary layer; kelp; Macrocystis pyrifera; microoptode; oxygen profiles; turbulence intensity Abbreviations: ADV, acoustic Doppler velocimeter; DBL, diffusion boundary layer