Currents and turbulence within a kelp forest (Macrocystis pyrifera): Insights from a dynamically scaled laboratory model

Rosman, Johanna H., Stephen G. Monismith, Mark W. Denny, and Jeffrey R. Koseff

Limnol. Oceanogr., 55(3), 2010, 1145-1158 | DOI: 10.4319/lo.2010.55.3.1145

ABSTRACT: The effects of a Macrocystis pyrifera forest on currents and turbulence were investigated in a controlled laboratory setting using a dynamically matched 1/25-scale model. Two kelp configurations with surface canopies and one without a surface canopy were considered. Profiles of mean velocities and turbulence statistics were measured using acoustic Doppler velocimeters. Since flow within the model kelp forest was very heterogeneous, spatially averaged forms of the governing equations were used for the analysis. Stress gradients were small compared with pressure gradient, drag, and acceleration terms of the momentum budget. A good model for kelp drag is therefore required for simulating flow through a kelp forest, while the model for Reynolds and dispersive stresses is less critical. The bulk drag coefficient is highest at the up-current end of a kelp forest and decays with down-current distance as the velocity profile adjusts to the drag profile. Modeling a kelp forest as an array of vertical cylinders underestimates the net drag by a factor of 1.5 to 3 if a substantial surface canopy is present. Turbulence is generated predominantly by small-scale shear in kelp wakes. Vertical mixing of scalars is expected to be significantly smaller than in the surrounding coastal ocean because of the combination of smaller turbulent eddies and reduced currents. The decrease in horizontal transport and vertical mixing within kelp forests may have important implications for nutrient availability to kelp forest organisms and may affect the dispersal or retention of their larvae and spores.

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