Dispersal of plant fragments in small streams

1. Streams are subject to frequent natural and anthropogenic disturbances that cause sediment erosion and loss of submerged vegetation. This loss makes downstream transport and retention of vegetative propagules on the streambed very important for re-establishing vegetation cover. We measured dispersal and retention of macrophyte stem fragments (15-20 cm long) along 300 m long reaches of four small to medium sized Danish lowland streams. 2. The number of drifting stem fragments declined exponentially with distance below the point of release. This finding makes the retention coefficient (k, m(-1)) in the exponential equation a suitable measure for comparisons among different macrophyte species, and between stream reaches of different hydrology and vegetation cover. 3. Buoyancy of macrophyte tissue influenced retention. Elodea canadensis stems drifted below the water surface, and were more inclined to be retained in deeper water associated with submerged plants and obstacles in the streambed. Ranunculus peltatus stems were more buoyant, drifted at the water surface, and were more inclined to be trapped in shallow water and in riparian vegetation. 4. The retention coefficient of drifting stems increased with the relative contact between the flowing water and streambed, bank and vegetation. Thus, the retention coefficients were highest (0.02-0.12 m(-1)) in shallow reaches with a narrow, vegetation-free flow channel. Here there were no significant differences between E. canadensis and R. peltatus. Retention coefficients were lowest (0.0005-0.0135 m(-1)) in deeper reaches with wider vegetation-free flow channels. Retention of E. canadensis was up to 16 times more likely than retention of R. peltatus. 5. Overall, the longitudinal position in the stream system of source populations of species capable of producing numerous stems, the species-specific retention coefficients of stems, and the retention capacity of stream reaches should be important for species distribution in perturbed stream systems. Retention of stems is probably constrained in headwaters by the small downstream flux of stem fragments because of the restricted source area, and constrained in downstream reaches by small retention coefficients. Macrophyte retention may, consequently, peak in medium-sized streams.