Patterns of aquatic macrophyte species composition and distribution in Bulgarian rivers

The composition of aquatic bryophyte and vascular plant assemblages, and 4 environmental variables (water velocity, shading, mean depth, and substrate type), together with altitude, ecological status, and river type were monitored at 223 sites along 204 rivers during 2009 in the Pontic Province and Eastern Balkan Ecoregions, Bulgaria. The relationships between macrophytes themselves and the parameters studied were tested. Forty-nine bryophytes and 86 vascular plants were identified. Canonical correspondence analysis (CCA) showed that aquatic bryophytes and vascular plants are in inverse correlation. Bryophytes occurred mainly in conditions of higher altitude and ecological status; at the same time vascular plants were found in lowland river types defined by lower status and altitude. CCA also established that flow velocity was of prime importance for bryophyte distribution, while the substrate type had no significant correlation. In contrast, most vascular plants depended considerably on substrate and prefer finer types. The data also showed that shading explained the major part of vascular species variance. This study represents an important contribution to environmental monitoring and biodiversity conservation programmes. For the first time relationships between 135 macrophyte taxa and 7 environmental variables were tested. Abiotic factors were highlighted with major influence on river bryophyte and vascular plant communities.