Changes in sediment seed-bank composition of invaded macrophyte communities in a thermal river.
Thermal waters, characterised by high water temperatures throughout the year, harbour a special flora atypical of temperate climatic conditions that are adapted to the combination of high water temperatures and low oxygen and carbonate contents. However, these ecosystems are susceptible to the invasion of tropical macrophytes because conditions in thermal waters are similar to those of in warmer latitudes. We studied the vegetation and viable seed bank in an alien-dominated upstream section of a thermal river in Hungary, and a native-dominated downstream section of the same river where temperature was cooler and less stable. Our hypotheses were as follows: (1) alien and native plant species are clearly separated along the river, and this separation is driven by environmental factors (water temperature, conductivity and sediment characteristics); and (2) the species composition of seed banks reflects that of the established macrophytes, and thus, the seed-bank composition would differ in the up- and downstream reaches. We defined 20 sampling units in two sections of the Hévíz River in West Hungary. The vegetation was surveyed in every sampling unit, and environmental variables (sediment and water) were recorded. Five sediment cores were taken from each sampling unit and incubated in a greenhouse under waterlogged conditions. The mean seed-bank density was lower than any of the previously published values for aquatic plant communities. In total, fewer species germinated from the seed bank than the number of species observed in the vegetation. Moreover, the Sørensen similarity index, comparing the vegetation and seed-bank species composition, was extremely low. The lowest seed-bank density and diversity were detected in the alien-dominated upstream river section, where significantly fewer native species were present in the seed bank. Despite favourable conditions for alien macrophytes to establish in this thermal river, they did not build up considerable persistent seed banks. We conclude that the dominance of aliens modified the species composition of both the vegetation and the seed bank, mainly by depleting the seed bank of native species. Therefore, future efforts to restore native vegetation from the seed bank may require a number of different strategies.