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First published online January 17, 2007
Journal of Experimental Biology 210, 522-532 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.02679

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The interaction of CO₂ concentration and spatial location on O₂ flux and mass transport in the freshwater macrophytes Vallisneria spiralis and V. americana

Gregory N. Nishihara¹ and Josef D. Ackerman^1,2,*

¹ Department of Integrative Biology, University of Guelph, Guelph, Ontario, N1G 2W1, Canada
² Faculty of Environmental Sciences, University of Guelph, Guelph, Ontario, N1G 2W1, Canada

^* Author for correspondence (e-mail: ackerman{at}uoguelph.ca)

Accepted 5 December 2006

The biology of aquatic organisms determines the maximum rates of physiological processes, but the mass transport of nutrients determines the nominal rates at which these processes occur. Maximum O₂ flux (P_max) at 17.1 mmol m^–3 CO₂ was higher for the leaves of the freshwater macrophyte Vallisneria spiralis [P_max=0.013±0.001 mmol m^–2 s^–1 (g_chla+b m^–2)^–1 (mean ± s.e.m.)] than for the closely related species, Vallisneria americana [P_max=0.008±0.001 mmol m^–2 s^–1 (g_chla+b m^–2)^–1]. The O₂ flux saturated at freestream velocities >4.5±1.2 cm s^–1 and was spatially invariant for both species. However, a tenfold decrease in CO concentration to 1.71 mmol m^–3 changed the nature of the relationship between O₂ flux and spatial location along the leaf surface, and reduced the O₂ flux of V. spiralis to values similar to V. americana. The O₂ flux [P_max=0.007±0.001 mmol m^–2 s^–1 (g_chla+b m^–2)^–1] saturated at the upstream location (i.e. 1 cm from the leading edge of the leaf) but was found to increase linearly with freestream velocity [slope=0.057±0.011 mmol m^–2 s^–1 (g_chla+b m^–2)^–1 (m s^–1)^–1] at the downstream location (i.e. 7 cm from the leading edge) at freestream velocities >1.8±0.9 cm s^–1. Conversely, mass transfer rates did not vary with CO₂ concentration, and were characteristic of a laminar concentration boundary layer at the upstream location and a turbulent concentration boundary layer at the downstream location. Rates of mass transfer measured directly from O₂ profiles were not predicted by theoretical values based on hydrodynamic measurements. Moreover, the concentration boundary layer thickness (_CBL) values measured directly from O₂ profiles were 48±2% and 21±1% of the predicted theoretical _CBL values at the upstream and downstream locations, respectively. It is evident that physiological processes involving mass transport are coupled and vary in space. Mass transport investigations of biological systems based solely on hydrodynamic measurements need to be interpreted with caution.

Key words: hydrodynamics, morphology, photosynthesis, kinetic limitation, mass transfer limitation, DIC, carbon uptake, concentration boundary layer, momentum boundary layer

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