WaterSubject

The aim of the study was to evaluate pattern of the aquatic macrophyte species distribution along the Danube fluvial corridor in Slovakia, and to identify the impact of environmental abiotic parameters on macrophyte species diversity. Field sampling was performed in the period 1999–2005 from the boat. Aquatic habitats were divided into 365 survey unit (SU). The survey of aquatic macrophytes and abiotic parameters followed the European standard approach EN 144184 2003. The plant mass estimate (PME – a semi-quantitative estimation of the amount of individual species in a SU, which takes into account three-dimensional development of plant stands) was estimated according to a five-point-scale in each SU; environmental pattern, were assessed over six abiotic parameters (river km, bank type, sediment type, flow velocity class, land-use type, and heavily man-modified water bodies). Altogether, four hydrologic connectivity types of aquatic habitats were distinguished: the Danube River, Open Arms, Separated Arms, and Seepage Water-bodies.

In total, 54 aquatic macrophytes were recorded for the whole data set of the Danube fluvial corridor. The PME data of true aquatic macrophytes and the length of SUs created a basis for numerical derivates, relative plant mass (RPM), mean mass indices (MMT, MMO) and the distribution ratio (d).

The results correspond with comparable studies on this topic: the highest macrophytes species diversity occurred in Separated Arms. On the contrary, macrophytes had the lowest richness in the Danube River main channel, although their diversity was slightly higher in heavily man-modified water bodies (such as the hydropower plant’s reservoir and the abandoned main channel of the so-called Old Danube). Our results suggest that the lateral connectivity types of the river water bodies, primarily characterised by different hydrologic dynamics and human impact expressed as land-use types are responsible for the variability of aquatic macrophyte assemblages along the Danube corridor in Slovakia.

The Danube passes through Slovakia in its middle reaches. However, the length of the Danube main channel of only 172 km is characterised by high spatial heterogeneity of abiotic environment and biodiversity.

In total, 54 aquatic macrophytes across studied habitats of Slovakia’s fluvial corridor were recorded in the period 1999–2005. The species assemblage in this river reaches differs from the Upper Danube in Germany, where Ranunculus fluitans and F. antipyretica are abundant, mainly in fast-flowing channelised sections. However, the chain of reservoirs induces the succession of Nuphar lutea and narrow-leaved pondweeds. The reaches of the Middle and Lower Danube and the Danube Delta feature a similar floristic structure. Only some thermophilous species, such as Azolla filiculoides were found downstream from Slovakia to the south Hungary  occurring in water bodies across the Middle and Lower Danube. However, A. filiculoides was found in the Slovak tributary of the Danube – in the impoundment of the Váh River. Also, Vallisneria spiralis is distributed in Serbia  and Bulgaria, and Romania. It was also recorded in Austria, though as a neophyte species. The alien species Cabomba caroliniana invades canals and the main channel in Hungary. An endangered species, Marsilea quadrifolia, was found in a floodplain lake in Bulgaria. Based on the preliminary data of the MIDCC project, refer to C. demersum, P. pectinatus, and Butomus umbellatus as the most frequent species in the entire Danube River corridor. Wide distribution show both alien invaders E. canadensis and E. nuttallii, which are rapidly spreading along the whole Danube in last decades.

Recent spacing of aquatic habitats, their hydro-dynamics and physical traits had mainly developed in response to the management, e.g., the riverbed regulation for flood protection, navigation, and hydropower plants engineering. Main pattern of aquatic macrophytes is closely related to morpho-hydrology and trophic state of habitats, which change in response to the regulation of the anastomosis of rivers. The typology of the former channels based on their connectivity with the river and groundwater (permanent connection, temporal connection and connection through river flood) in relation to plant species richness was used for the Rhône River in France. Spatial connectivity along and between the river is considered as the most important factor in shaping aquatic vascular plants assemblages of lowland rivers in England and in the Danube River as well.

Land use of riparian zone and bank structure are the most important parameters explaining variance in the composition of macrophyte community in Slovenian watercourses and this corresponds with our investigation. describe the urban and industrial areas as suitable habitats especially for the submersed species along the Danube corridor. Both land use and bank structure parameters indicate human impact in the landscape surrounding the river corridor. Large anthropogenic effects on hydrology are associated with the construction of dams, and diversions for hydroelectricity and agricultural purposes. This probably results in local enrichment of macrophyte productivity through the deposition of fertile sediment and enhanced availability of dissolved nutrients. The degradation of natural habitats has been cited as a cause of loss of native species and the invasion of alien species.

The hydrological connectivity type of aquatic habitats and utilisation of surrounding country (land-use type) explain the main environmental gradient along the Danube corridor in Slovakia: studied aquatic habitats were arranged through (i) Danube River and Open Arms to Separated Arms and Seepage Waters, (ii) natural to man-modified habitats. The results correspond with four main types of connectivity, which were arbitrarily distinguished at the beginning of our study.

Danube River is characterised by the lowest values of both species diversity and mean value of MMT, due to high flow velocity, prevailing gravel sediment, and artificial bank structure. In general, high current velocity, coarse-grained material of banks and bottom make the environment unsuitable for vascular aquatic plants. Some rhizophytes, such as Z. palustris, Potamogeton species, M. spicatum, etc. sporadically occupy sheltered microhabitats of slow flow velocity with silt located near the mouths of tributaries, or the arms of the Danube main channel. In the same or similar sites, pleustophytes (free-floating species), such as duckweeds and C. demersum, were recorded only scarcely. Only the moss species C. riparius colonised washed boulders of rip–rap zone, owing to rhizoids that enable it to attach. described similar moss patches on regulated Danube banks in Austria, where, prior to the regulation, vascular plants grew on the fine-coursed substrate.

In our investigation, the highest species richness of 16 vascular hydrophytes was recorded in the Old Danube, due to the flow rate reduction after the water diversion in the hydropower plant’s bypass canal in 1992. Practically no aquatic macrophytes before this technical adjustment occurred there, only a small clump of P. pectinatus was found at 1830 river km in 1991. Similar results from the macrophyte survey were obtained along the right, Hungarian, bank of the Old Danube using the same method. Z. palustris was the most widespread aquatic plant that formed a submerged carpet on thin deposits of mud along the shallow littoral. Only low occurrence and sparse distribution of other macrophytes, such as P. pectinatus, P. perfoliatus, and E. nuttallii was generally recorded. The ?unovo Reservoir, which construction was completed in 1993, had successfully been colonised by macrophytes. However, they preferred the left-side upper reaches of the impoundment with a shallow littoral covered with deposits of fine sediments due to decreased water velocity. After the pioneering Z. palustris and E. nuttallii, P. nodosus currently becomes abundant. Similarly, favourable habitats for macrophyte colonisation have developed in the impoundments of hydropower plants in Austria, and in the Danube lower reaches in Serbia.

Open Arms followed the Danube River in species diversity and had similar species composition. Alien species E. nuttallii frequently occurred in some of them, mainly in heavily modified arms (e.g. harbours) located in urban areas. A semi-natural network of arms varied in morphological patterns and hydrodynamics. Since 1993, a special inlet structure ensures flow continuity and controls water level in the anabranch system on the left bank of the Old Danube River. A large-scale of flow velocities and water depths in river branches, with the velocity as fast as 1 m s^?1 at some places, to near stagnant water in side branches, and stagnant water in old dead branches supplied with groundwater distinguishes these arms. Spatial boundaries between both types of Open and Separated Arms have a temporal character. Species composition in Open Arms is generally similar to those in the Danube River, but plant species abundance, displayed by mean value of MMT, is rather higher. Submerged rhizophytes dominate: patches of pondweeds are common; E. nuttallii is ubiquitous (d=0.83).

Some Separated Arms are located outside the flood-protection dam. The majority, however, remained in the inundation area developing in response to fluvial deposits or decreased water level. Only during high floods, such as in 2002, they establish connectivity with the main channel. This occasional flood disturbance is nevertheless very important because it slows down the vegetation succession, which may lead to the decline of aquatic macrophyte species richness and diversity. Highest species richness and diversity is generally known from these separated habitats, what correspond with our results. Floating-leaved species, such as N. alba, N. peltata, Salvinia natans, S. aloides, and T. natans, were only recorded in lentic conditions. Eutrophic conditions also favoured filamentous algal communities. Many species have shown the clumped distribution here. As to endangered species list, this type of habitat is of special value.

Surveyed Seepage canals and pit lakes, built in 1979–1992, relate to the Gab?íkovo Hydropower Plant construction. Water supply is maintained through the infiltration of relatively cold groundwater, which is either stagnant or of low flow velocity, has good transparency and is only slightly eutrophic. This new habitat provides a niche for the plant succession. Already in 1999, 25 aquatic macrophytes were recorded in one of the canals and the number of species continued to increase: 29 macrophytes were found in the canals in 2001.The abundance of species was relatively high as it is indicated by the MMT mean value. Species assemblage in Seepage Water-bodies is different enough comparing to others habitats. Most frequent are stoneworts (Characeae), which usually occur as pioneer macro-algae in newly dug ecosystems supplied with groundwater, followed by other slightly eutrophic species such as G. densa, Hippuris vulgaris, and A. repens. The endangered G. densa, for example, originally occurred near Gab?íkovo, in a natural small stream’s upper reaches. After the hydropower plant was activated, the stream dried out and G. densa had vanished, but as soon as in 1996 it was repeatedly recorded, this time in a new seepage canal. Similarly, found G. densa and H. vulgaris in its greatest density in slightly eutrophic reaches; both, however, were absent in sections with higher nutrient supply in the Moosach river. New Seepage Water-bodies have increased the heterogeneity and biodiversity of the landscape, providing a secondary habitat for endangered and rare species. Many aquatic plant species have found a refuge in these man-made habitats; nevertheless, their occurrence might probably be a temporal phenomenon.

The Slovak reaches of the Danube River corridor is a unique floodplain ecosystem. The relevant part, “Dunajské luhy”, is listed in the Ramsar Convention as an internationally important wetland. Twenty-nine endangered species of the Slovak flora were found in this area during our study. Unfortunately, new man-modified aquatic habitats are being successfully invaded by the alien species E. nuttallii.

Our results suggest that the connectivity types of river water bodies, primarily characterised by different hydrologic dynamics and human impact expressed in land-use types, are responsible for the variability of aquatic macrophyte assemblages along the Danube corridor in Slovakia. These factors include the complex of disturbance pattern with a wide amplitude of pressure in time and space.