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Aquatic Biology

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AB 14:41-56 (2011)  -  DOI:

Effects of a small-bladed macroalgal canopy on benthic boundary layer dynamics: implications for nutrient transport

Louise T. Kregting1,5,*, Craig L. Stevens2, Christopher D. Cornelisen3,6, Conrad A. Pilditch4, Catriona L. Hurd1

1Department of Botany, University of Otago, Dunedin 9016, New Zealand
2National Institute for Water and Atmospheric Research, Wellington 6021, New Zealand
3Department of Marine Science, University of Otago, Dunedin 9015, New Zealand
4Department of Biological Sciences, University of Waikato, Hamilton 3240, New Zealand
5Present address: School of Biological Sciences, Queens University of Belfast, Northern Ireland, BT7 1NN, UK
6Present address: Cawthron Institute, Nelson 7042, New Zealand

ABSTRACT: Field and laboratory velocity profiles were used to quantify boundary layer dynamics within communities of a small (<0.2 m tall), dense canopy-forming seaweed Adamsiella chauvinii (Rhodophyta) in a soft-sediment habitat and to examine the role of hydrodynamics in modulating nutrient supply. At the ‘canopy scale’ there was a mixing layer at the fluid−canopy interface where turbulent kinetic energy was greatest, potentially enhancing nutrient uptake in this region. In the lower half of the canopy, a drag-dominated area of very low water velocity (<0.01 m s−1) occurred. Spectral analysis revealed a reduction in energy within the canopy of around 1 Hz. The hydrodynamic parameters obtained from flume measurements were in good agreement with those recorded in the field. To understand the implications of the hydrodynamic environment on nutrient uptake, a flushing ratio was developed that compares the time for macroalgae to remove all nutrients from the canopy volume relative to the timescale for flushing. Results suggest that when nutrient demand is low, the canopy is well flushed and not mass-transfer limited. However when nutrient demand is high, the canopy can deplete nutrients more quickly than they can be replenished by ambient flows and are potentially mass-transfer limited.

KEY WORDS:  Macroalgae · Water motion · Boundary layers · Currents · Canopies · Turbulence · Nutrient uptake · Adamsiella chauvinii

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Cite this article as: Kregting LT, Stevens CL, Cornelisen CD, Pilditch CA, Hurd CL (2011) Effects of a small-bladed macroalgal canopy on benthic boundary layer dynamics: implications for nutrient transport. Aquat Biol 14:41-56.

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