Local distribution of macrophytes and consequences for sampling methods in large rivers

During the years 2003 and 2004, the distribution of macrophytes was studied at eight sites in two large rivers in the southwest of France (the Dordogne and Garonne). Using a pinpoint method, we obtained 7,083 observations and recorded physical parameters such as depth, distance from bank, water velocity, and substrate type. By using logistic regressions, we showed that a combination of these factors significantly influences the distribution of macrophytes. This enabled us to highlight transversal distribution patterns. Helophyte spermatophytes, free-floating taxa, and most of the nonanchored algae were characteristic of the shallow areas located near the banks. The deepest areas were characterized by a reduced number of species with a majority of submerged spermatophytes which are resistant to high velocity, anchored algae, and bryophytes. These results can be used to implement the Macrophyte Biological Index for Rivers (IBMR) for large rivers