Hydrobiologia 346: 119–128, 1997.
119
c
1997 Kluwer Academic Publishers. Printed in Belgium.
Boundary-layers around bladed aquatic macrophytes
CraigL.Stevens
1
& Catriona L. Hurd
2
1
Environmental Fluid Mechanics Group, Dept. of Civil Engineering, The University of British Columbia, V6T
1Z4, B.C., Canada (address for correspondence: National Institute of Water and Atmospheric Research, 301
Evans Bay Parade, Greta Point, P.O. Box 14, 901 Kilbirnie, New Zealand; e-mail: c.stevens@niwa.cri.nz)
2
Dept. of Oceanography, The University of British Columbia, V6T 1Z4, B.C., Canada (present address: Dept.
Botany, University of Otago, Box 56, Dunedin, New Zealand)
Bamfield Marine Station, Bamfield, V0R 1B0, B.C., Canada
Received March 1996; in revised form 19 February 1997; accepted 26 February 1997
Key words: boundary-layers, nutrient uptake, diffusion, macrophyte, kelp
Abstract
This paper describes a model of the diffusive boundary-layer around an aquatic macrophyte blade. Nutrient uptake
at the base of the modelled boundary-layerwas examined using previously published laboratory data. The analysis
showed that, over the outer velocity range 0.01–0.16 ms
1
, nutrient uptake varied between that predicted by flat-
plate boundary-layer theory and that expected with no mean advection. A ratio of theoretical and actual nutrient
uptake rates was defined as a means of categorising the transition between the two transport models. The modelling
was extended to show how fluctuating boundary conditions may enhance nutrient uptake by the macrophyte; the
scenario examined here suggested that at otherwise low outer velocities, periodic stripping of the boundary-layer
by passing waves increased nutrient uptake by a factor of 10.
Introduction
Diffusiveboundary-layersaroundaquaticmacrophytes
are thought to limit the availability of essential inor-
ganic nutrients in areas of low water motion (Wheeler,
1980; Chambers et al., 1991; Koch, 1993; Hurd et al.,
1996). Efforts to model diffusion rates of nutrients
to and from aquatic macrophytes have relied on flat
plate boundary-layer theory (hereinafter FP-BLT) to
describe the flow kinematics (Wheeler, 1980; Koch,
1993). However, observations reveal that this is often
an inappropriate assumption (Hurd et al., 1997). The
existenceof differentvelocityand timescales forwater
motion, inconjunctionwithshape irregularitiessuchas
corrugations, undulations, hairs, spines, holes, stipes
and bulbs (illustrated in Figure 1) leads to a boundary-
layerthatis noteasily quantifiedwith analyticorexper-
imental models.
This paper describes an analysis of the fundamen-
tal length and time scales of diffusion and veloci-
ty boundary-layers around aquatic macrophytes. The
objective was to identify how the interaction between
thebackgroundflow anda macrophyteaffectedthe dif-
fusion of nutrients. The present study was motivated
by laboratory studies of nutrient uptake by, and visu-
alisation of boundary- layers around, different blade
morphologiesofthegiantkelpMacrocystisintegrifolia
(Hurd et al., 1996, 1997). These studies clearly illus-
trated that, while large-scale variations in blade mor-
phologyaffectedthevelocityboundary-layerstructure,
it did not affect nutrient uptake rates. The implica-
tion was that FP-BLT does not correctly predict nutri-
ent uptake. Departures from this model indicate that
transport mechanisms across velocity and diffusion
boundary-layers of different blade morphologies are
indistinguishable at scales relevant to nutrient uptake.
As water motion in coastal seas exhibits a wide
range of velocity scales, and aquatic macrophyte mor-
phologies are often geometrically complex (e.g. Fig-
ure 1), a simple model of flow and nutrient uptake is
required to retain generality. The analysis described
here considers nutrient transport across boundary lay-
Article: hydr 3918 Pips nr 136031 BIO2KAP
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