EFFECTS OF WATER VELOCITY ON PHOTOSYNTHESIS AND DARK RESPIRATION IN SUBMERGED STREAM MACROPHYTES

The effects of flow velocities on dark respiration and net photosynthesis of eight submerged stream macrophytes were examined in a laboratory oxygen chamber. The shoots/leaves were exposed to saturating free-CO2 concentrations and were attached basally so that they could move in the flowing water. Net photosynthesis declined by 34-61 % as flow velocity increased from 1 to 8-6 cm s-1, while dark respiration increased 2-4-fold over the same range. The increase in dark respiration could only account for between 19 and 67% of the decrease in net photosynthesis. The relationship between flow velocity (U) and net photosynthesis (P) was described by: P = b x U(a). The exponent, a, varied from -0.20 to -0.48 and showed a negative correlation to the surface: volume (SA:V) ratio of the plants, i.e. species with high SA:V ratio were more sensitive to flow. In contrast, net photosynthesis of plants firmly attached to a supporting frame was not significantly affected by increasing flow velocity. This result indicates that the physical stress imposed on the plants by agitation or stretching in the flowing water is a key factor for the observed response.