by Christoph Netz
With another early start, it was lucky that the memorable apricot jam, a local specialty, provided a good incentive to get up in time for breakfast. Only five minutes behind schedule, we departed from Krems, with its beautiful historic center and surrounding vineyards, for Vienna.
Our first stop was the Nussdorfer Schleuse, a lock and weir where the Danube canal branches off from the Danube in the northern outskirts of Vienna. Between the canal and the main river lies an outdoor research channel that is used to investigate the flow dynamics of different riverbeds and flood plains. Here, we met with Christine Sindelar of the University of Natural Resources and Life Sciences in Vienna (BOKU), who conducts her research at the outdoor research channel. The building of the research channel was part of the DREAM project (Danube River Research and Management), which was cofinanced by the City of Vienna and the European Union. The slope between the Danube and the canal allows the scientists to experiment with flow rates of 10 cubic meters per second, and hence provides unique opportunities to make 1:1 scale simulations of river flow and sediment dynamics.
The Danube transports significant amounts of sediment, but its natural flow regime has been altered. Sediment is now largely trapped behind hydroelectric power plants, and dams and jetties have increased the speed of the river’s current. This leads to riverbed erosion, which causes the groundwater level to drop and bridges to become unstable as their bases erode. Sediment accumulates in front of hydroelectric power plants, threatening their operation. The sediment is often contaminated, making removal and processing very costly. The state and involved companies have strong interests in researching ways to stabilize the riverbeds and create a more functional sediment regime; they therefore support the research that Sindelar and her colleagues are conducting.
While we were visiting, a couple of young willow saplings were being planted into the channel bed. It was amazing to see how, as the water flow increased, the willows bent and were submerged into the rapidly flowing water. Just 10 minutes later, the saplings were standing tall and upright again as if nothing had happened. The willows’ flexibility and streamlined shape allows for high resilience to flooding, even in high-speed currents.
Next to the canal, several other plants stood ready for testing, among them young willows of different kinds and, most notably, a large amount of giant knotweed (Fallopia sachalinensis). The knotweed, along with its closely related sister species, the Japanese knotweed (Fallopia japonica), is a common invasive species along river shores and disturbed habitats such as highway verges or industrial wastelands. Due to its prevalence along river shores, its role in soil erosion during flooding is especially interesting. While we were there, a great green bush cricket was chewing on the leaves of the giant knotweed, proving that some of the local wildlife has already come to terms with the intruder, which is probably here to stay.
After our visit to the research channel, we went on a bicycle tour of the Danube Island. On the way, we stopped at the LIFE Sterlet breeding station. Sterlets (Acipenser ruthenus) are a rare fish species that we had already heard about on our journey, and now we were finally able to see them for ourselves. The sterlet is a small species of sturgeon and the only one still inhabiting the waters of the upper Danube. Hydroelectric power stations are a particular threat—they fragment the sterlets’ habitat and cut off age-old migration routes—as are fisheries and general habitat degradation. The LIFE Sterlet project has made its mission the protection and restocking of the remaining sterlet populations of the Danube. A breeding station is maintained, so that juvenile sterlets can frequently be released into the wild. We were shown around the facilities by Nina Florian, a master’s student currently writing her thesis on the sterlet-rearing program at BOKU. Next to an exhibition, there were two outdoor pools used for rearing and demonstration purposes, where we could observe larger sterlets and other sturgeons.
The heart of the facility is the breeding station. Nina showed us the pumps, the filters, and the sterlet food, and introduced us to many important details of the breeding station. Each pond is equipped with a faucet, a drain, and air pipes that bubble air into the water to enrich it with oxygen. There were thousands of sterlets in every pond, all stemming from one generation and breed, making them siblings. The sterlets are successively released into the wild as they age, and the number released surpasses the wild population by a few hundred breeding individuals. Larger individuals are equipped with remote sensors before being released so that information about the behavior and migration patterns of these rare fish can be collected. The resulting information is used to manage and direct conservation efforts.
Whether using captive and/or wild populations, a major challenge of any breeding project is maintaining genetic diversity. The breeding station monitors the genetic composition of wild and breed-stock sterlet populations to maintain a natural state and prevent inbreeding in captivity. It cannot, however, prevent inbreeding in wild populations, and the population bottleneck produced by human activities over the last hundred years has surely had detrimental impacts on genetic diversity in the wild. In the rearing ponds, we saw a large number of albino offspring, which may be an indicator of decreased genetic diversity; alternatively, the species might simply have a greater predisposition to albinism.
After eating a packed lunch at the station, we truly began our tour of the Danube Island. Herbert Weidinger, who works as a forest ranger there, introduced us to the many different perspectives, uses, and competing interests concerning the island. The artificial island, built between 1972 and 1988 as a flood protection measure, serves today as a local recreation area for those who wish to go swimming or barbecue without having to leave the city. Right next to a smaller hydroelectric power plant there is even a wakeboard park and a professional water sports center. It is also home to a thriving beaver population and contains many surprisingly pristine natural areas.
Cycling through the Danube Island, you would never guess that you were surrounded by a city of over one million inhabitants. Due to its large size—over 20 kilometers in length and around two hundred meters wide—there are plenty of spots in which to find solitude. The bike ride was a nice change from sitting in the bus or walking around, and enabled us to cover a larger distance, changing from park landscape to wilderness and back.
Later, on the bus ride to our accommodation, Gudrun Pollack from the Alpen Adria University provided us with information about the history of Vienna and its two rivers (the eponymous Vienna River and the Danube), and about the relationship of today’s Viennese citizens with the water. Compared to the Danube, the Vienna River made a rather poor impression—it was merely a sad trickle of water. That this rivulet shared its name with a metropolis like Vienna was very surprising.
We made one last, short stop to an open area in the northern city center. Whether because it hosts a precious habitat filled with dry vegetation, or because the grounds are heavily contaminated from an industry building formerly located there, this ground has been spared from development. It remained uncertain which explanation gives a better account of the truth—maybe the reality lies somewhere in between.