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

Abstract

SUMMARY 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(Pmax) at 17.1 mmol m–3 CO2was higher for the leaves of the freshwater macrophyte Vallisneria spiralis [Pmax=0.013±0.001 mmol m–2 s–1 (gchla+bm–2)–1 (mean ± s.e.m.)] than for the closely related species, Vallisneria americana[Pmax=0.008±0.001 mmol m–2s–1 (gchla+bm–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[Pmax=0.007±0.001 mmol m–2s–1 (gchla+bm–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 (gchla+bm–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 (δCBL) values measured directly from O2 profiles were 48±2% and 21±1% of the predicted theoretical δCBL 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.