The complexity of water bodies in the eupotamal river corridor and the main delta channels of the Romanian Danube is exemplified by the macrophyte vegetation. Two hypotheses provided the background for our study: (a) is the macrophyte vegetation of large, permanently connected branches significantly separated from that of the main river channel; (b) is the macrophyte composition of the Danube main stem significantly altered when the river divides into the three large navigable Delta channels. Water bodies considered were two contiguous sections of the main river channel, two large branches remaining from the historical floodplain, and the three main Delta channels. We quantified macrophyte diversity and floristic variation. Our data set was prepared from the MIDCC-project data base, in which macrophyte occurrence, abundance and habitat parameters are stored for contiguous survey units of the whole Danube river corridor. Field survey method followed that of Kohler and the European Standard EN14184. Results confirmed our first hypothesis: permanently connected side branches still support significantly different macrophyte assemblages, making them important indicators of floodplain connectivity. The diversion of the Danube into its three large navigable delta channels significantly alters the macrophyte vegetation from the c. 300 km of main stem up-river, substantially supporting our second hypothesis. Our results largely enhance the knowledge on aquatic plant biodiversity in the eu-potamal Danube, forming a solid base for long-term studies. We also discuss the relevance of our results regarding the ecological, as well as the conservational, quality of rivers and their floodplains.

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