Water velocity, growth‐form and diffusion resistances to photosynthetic CO₂ uptake in aquatic bryophytes

Abstract

Abstract. Boundary‐layer resistances of aquatic bryophytes for CO 2 diffusion in water were estimated from wind tunnel measurements of evaporation of aniline in air, using the principle of dynamic similarity. The results indicated resistances at water velocities between 0.02 and 0.2 m s −1 ranging from about 35 to 5 s mm −1 and 70 to 9 s mm −1 , respectively, for the mat‐forming liverworts Nardia compressa and Scapania undulata , measured on a projected area (canopy) basis. Over a range of velocities from 0.01 to 0.2 m s −1 the estimated CO 2 boundary‐layer resistance of the streamer‐like shoots of the moss Fontinalis antipyretica is between about 180 and 15 s mm −1 . Comparison with experiments on photosynthetic 14 CO 2 ‐uptake at a range of water velocities suggests that boundary‐layer resistance limits photosynthesis at velocities below about 0.01 m s −l in Fontinalis and below about 0.1 m s −1 in the mat‐forming species. It is suggested that high leaf‐area index allows the mat growth form more effectively to exploit the low boundary‐layer resistance at high velocities while remaining relatively invulnerable to drag. By contrast, the streamer form allows Fontinalis to maximize surface area under conditions where boundary‐layer resistance is limiting.