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 O(2) flux (P(max)) at 17.1 mmol m(-3) CO(2) 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 O(2) 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 O(2) flux and spatial location along the leaf surface, and reduced the O(2) flux of V. spiralis to values similar to V. americana. The O(2) 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 CO(2) 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 O(2) profiles were not predicted by theoretical values based on hydrodynamic measurements. Moreover, the concentration boundary layer thickness (delta(CBL)) values measured directly from O(2) profiles were 48+/-2% and 21+/-1% of the predicted theoretical delta(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.