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

The effects of flow velocities on dark respiration and net photosynthe sis of eight submerged stream macrophytes were examined in a laborator y oxygen chamber. The shoots/leaves were exposed to saturating free-CO 2 concentrations and were attached basally so that they could move in the flowing water. Net photosynthesis declined by 34-61 % as flow velo city 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 o nly account for between 19 and 67% of the decrease in net photosynthes is. 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 s ensitive to flow. In contrast, net photosynthesis of plants firmly att ached to a supporting frame was not significantly affected by increasi ng flow velocity. This result indicates that the physical stress impos ed on the plants by agitation or stretching in the flowing water is a key factor for the observed response.