The water bathing postlarval abalone often lies within the diffusive boundary layer (DBL) so its chemistry is greatly influenced by the metabolism of the biofilm on which the abalone feed. This study used microelectrodes to investigate the influence of water velocity and diatom morphology on dissolved oxygen and pH in the DBL. Decreasing water velocity increased the thickness of the DBL, thereby increasing the amplitude of changes in oxygen concentration. Over a film of the prostrate diatom Nitzschia ovalis Arnot, DBL thickness averaged 71, 139, 177, and 406 μm at water velocities of 78, 15, 7, and 1 mm s⁻¹ respectively. Corresponding oxygen concentrations at the biofilm surface under moderate light (75 μE m⁻² s⁻¹) and temperature (15°C) averaged 111%, 120%, 125%, and 151% of air saturation respectively, at the four velocities. The presence of a 1-mm tall diatom canopy (Achnanthes longipes Agardh) over a Nitzschia ovalis film thickened the DBL by 3-fold at 1 mm s⁻¹ and 6-fold at ∼80 mm s⁻¹. The thickened DBL and higher diatom biomass generated extreme conditions at the biofilm surface. Dissolved oxygen concentrations as high as 440% of air saturation, and pH as high as 9.8 were recorded beneath the canopy in moderate light (105 μE m⁻² s⁻¹) and temperature (15°C) at a water velocity of 1 mm s⁻¹. Changes during darkness were less extreme, with 53% oxygen saturation and pH 7.7 the minima recorded. These measurements demonstrate the extreme water chemistry that can develop in the microhabitat of postlarval abalone. The changes will be amplified by the presence of filamentous diatoms, by increased light intensity, and by lack of water movement. Standard aeration will greatly reduce the extremes experienced by postlarvae by generating water movement sufficient to thin the DBL.