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The Danube is the second-longest river in Europe that is subjected to various man-made alterations, including those related to hydro-power plants. We surveyed and analyzed the presence and abundance of macrophytes in the main channel from 2582 river kilometers (rkm) to 171 rkm. We also assessed selected habitat parameters in the sampled river stretches. Sixty-eight different plant species were recorded along the entire course. Among neophytes, we found Elodea nuttallii, E. canadensis, Vallisneriaspiralis and Azolla filiculoides. Based on similarity analysis, we distinguished 15 plant communities, most of which were defined as associations, which were classified into 5 alliances and represented three vegetation classes, namely vegetation of rooted hydrophytes Potamogetonetea, the vegetation of pleustophytes Lemnetea and vegetation of marshes Phragmitetea. The number and abundance of plant species, as well as plant communities recorded in single stretches, varied along the course. Canonical correspondence analysis (CCA) revealed that environmental parameters explained 21% of plant species composition. CCA runs with neophytes explained 41% of the variance, and current velocity, water transparency, species number and bank structure were significant variables. The present study revealed that the free-running sections of the river are poor in number and abundance of plant species, whereas impounded reaches mainly show an opposite result.

The Lobau wetland and oxbow ensemble is part of the large Danube River floodplain area between Vienna (Austria) and Bratislava (Slovakia). The Danube River regulation scheme implemented in the late 19th century considerably affected the Lobau area, which is part of the Danube National Park today. Terrestrialization processes in the floodplain started with the river regulation and pose an increasing threat to the aquatic and wetland vegetation of this ecosystem. Former vegetation studies were either directed toward phytosociological description of associations or to a general vegetation inventory and mapping, where transitions between vegetation types are not accounted for. Our purpose is to provide a detailed insight into the vegetation zonation pattern along the aquatic-terrestrial inundation gradient in the Lobau floodplain area. We sampled a total of 49 belt transects. Samples in each transect were classified into groups using two-way indicator species analysis (TWINSPAN). Resulting groups of all transects were subjected to fuzzy c-mean clustering (FCM), defining ‘community types’ by means of indicator species analysis (ISA). We found 36 significant indicator species typifying 14 community types. Nine types belong to the shoreline vegetation and five types characterize the aquatic habitats. In addition to these community types, two transitional zones were found, one defined as open water/wetland transition and one as wetland/upland transition. Along the aquatic-terrestrial environmental gradient, different floristic and quantitative relations of community types and transition zones were identified. Finally, the importance of such a fine-scale description of zonation patterns along the inundation gradient is discussed in relation to long-term monitoring programmes as the basis for rehabilitation or wetland conservation measures.

The complexity of water bodies in the eu-potamal 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.

The UNESCO Biosphere Reserve Lobau (Austria), established in 1977 within the city limits of Vienna, is part of the Danube National Park and one of the ecologically richest parts of the Danube River Corridor. Hardly noticed by diverging stakeholder interests the smaller aquatic habitats in the floodplain area are acutely threatened by wetland ageing and terrestrialisation processes. In 2007 an aquatic plant survey was carried out in the water bodies of the active floodplain on the left bank of the Danube main river channel as well as in the semi-separated water bodies of the Lobau. Results clearly show the impact of through-flow on aquatic plant species composition and abundance, as these metrics are significantly higher in the semiseparated location. Suitable water regime for all sensitive aquatic plants is an essential prerequisite for sustaining aquatic macrophyte diversity in the Lobau Biosphere Reserve, and could provide an ecohydrological basis for rehabilitation activities in other regulated river floodplains located in cultivated landscapes.

Ecological compensation structures in the reservoir of the Freudenau hydro-electric power plant (River Danube, Vienna, Austria) developed into habitats dominated by aquatic macrophytes. Ecological functioning of the macrophytes is documented by their attractiveness to several fish guilds, including the rheophilic group and by their ability to restrict phytoplankton growth. However, in comparison with different types of natural floodplain water bodies in the Danube River corridor, with other power plant reservoirs and with the free flowing reach of the Austrian Danube between Vienna and Bratislava the composition of the indicator species among the aquatic plants was significantly different, as proven by statistical analysis. This puts the compensation structures at an intermediate position between the macrophyte vegetation of power plant reservoirs without such constructions and certain floodplain water ensembles as well as the free flowing reach. Some similarities exist especially with respect to the distribution of growth forms of the aquatic plants. Based on these results it can be concluded that with regard to the Water Framework Directive such compensation structures may serve as a means to provide conditions to meet the “good ecological potential”.

Recording and assessment of aquatic macrophytes was a request for the Joint Danube Survey 2 (JDS2). New insight regarding occurrence, abundance and specific distribution of macrophytes was based on methodological adaptations better adjusted to the size of this large European river and permitted more appropriate statistical interpretation. Regarding the ecological status of sampling stretches, an intentional, preliminary way of interpretation is provided, respecting trendsetting new international literature. Due to longer river stretches recorded, a higher number of species was detected in JDS2. Each of the ten official river sections showed an individual character of the macrophyte vegetation. Results of JDS2 macrophyte survey are put in relation with international literature and side effects are discussed, which are of relevance when assessing macrophytes in large rivers for purposes of science, European Water Framework Directive or regarding conservation issues.

Floodplain lakes and associated wetlands in tropical dry climates are controlled by pronounced and severe seasonal hydrologic fluctuations. We examined the plant community response to a bimodal flooding pattern in the Zapatosa Floodplain Lake Complex (ZFLC), northern Colombia. We measured floristic and quantitative change in four sampling periods emphasizing seasonal differences in plant abundance and life-form structure. Of 79 species identified in the lake complex, 52 were used to characterize eight community types via classification and ordination procedures. Results show that community structure does not change significantly during the flooding/receding stages. But maximum drawdown phase significantly disrupts the aquatic community structure and the exposed shorelines become colonized by ruderal terrestrial plants. Early rainfalls at the beginning of the wet season are emphasized as an important feature of plant regeneration and community development. The general strategy of the ZFLC vegetation can be framed into the flood pulse concept of river-floodplain systems. Thus, plant communities are mainly responding to disturbances and destruction events imposed by extreme water level fluctuations.

Elatine paramoana, a new species from the high Andean region of the Cordillera Oriental in Colombia, is described, illustrated, and compared to its nearest relatives,E. ecuadoriensis andE. fassettiana. Notes on morphological variation and general ecology and a key to the central and northern Andes species are also given.