Bryophyte communities' responses to environmental factors in highly seasonal rivers

Historically, in river ecosystems close attention has been paid to aquatic vascular macrophytes, but recent literature shows the important role of bryophytes, especially in undisturbed conditions. For this reason, we explore whether aquatic bryophyte patterns change in altered physical habitats and in eutrophication along highly seasonal streams in Bulgaria. Specifically, we hypothesize that: (i) bryophyte assemblage patterns change in conditions of hydromorphological degradation and eutrophication; and (ii) bryophyte assemblage patterns can serve as a signal in assessing effects of nutrient enrichment and hydromorphological degradation. Biological data were collected for 30 river sites together with catchment land use, hydromorphological alterations (e.g. channelization, alteration of habitat and riparian vegetation, dykes), main abiotic characteristics of each site (flow velocity, substrate type, shading and mean depth) and water chemistry parameters. The data set mentioned above was used to describe the responses of bryophyte communities to two major pressures in rivers resulting in habitat destruction: hydromorphological alterations and eutrophication. The results clearly reflected that habitat degradation led to particular changes in the species-abundance patterns of bryophyte communities. Both studied pressure types resulted in loss of species and decreased bryophyte abundance. Hydromorphological degradation could be distinguished by morphological growth forms: presence of mosses of a more plastic shape (creeping pleurocarps), whereas eutrophication favours only the mesotrophic preferential Leptodictyum riparium. This moss should be applied as a conceptual signal species, the abundance of which indicates ranges of hydromorphological and nutrient pressure in seasonal rivers.