Research

Instrumental water level and discharge measurements in Lower Austria for medium-sized and smaller rivers only date back a little over 100 years. Historical flood statistics are therefore limited in time, which seems insufficient, especially in times of climate change with predicted more frequent and larger extreme floods. For individual rivers, historical floods have been documented and analyzed on the basis of local chronicles and flood marks (see, for example, the work of Heinz Wiesbauer). However, it can be assumed that many documents on historical floods prior to instrumental measurements have not yet been collected and consistently evaluated. The aim of the project is to collect and process information on historical floods in Lower Austria prior to the start of instrumental hydrological recordings. The surveys focus on flood marks, but written sources and historical literature will also be collected and evaluated.

The grayling (Thymallus thymallus) and rainbow trout (Onchorhynchus mykiss), which are key species for the fishing industry, show strong population fluctuations and sometimes unsatisfactorily low stocks in large parts of the Ybbs. In particular, a decline in the grayling population in the middle reaches of the river documented in 2024 and the noticeable decline in rainbow trout raise questions about the causes responsible. The proposed project aims to explain population demographic developments using abiotic descriptors – specifically flow and water temperature. The focus here is on the critical phase of incubation and emergence, a period that has been shown to be of great relevance for the further population development of salmonids (Unfer et al., 2011, Pinter et al., 2025). An additional focus is on the development of cohorts (year classes), i.e. the survival of individual age stages from year to year. The data available for the Ybbs is ideal for these analyses, as in addition to continuous hydrological data, there is a complete seven-year data series (2019–2025) of fish stock data from a district upstream of Amstetten – a unique data basis in the hyporhithral zone that enables well-founded and detailed analyses of reproductive success and cohort development. As part of the interpretation, influencing factors such as water morphology and potential stressors such as predation pressure and interspecies competition should not be ignored. By considering seamless population development, a new basis for future fisheries management steps and considerations is created. This redefines the prerequisites for sustainable and forward-looking management and lays the foundation for further considerations on current problems in fisheries management (status of the huchen population, consequences of climate change, etc.). Such research results are of great importance for this key species in the grayling region, especially in view of the unstable stocks of grayling, which is classified as vulnerable (Wolfram & Mikschi, 2007).

Rivers are impacted by various pressures, including changing temperatures driven by climate change and other factors. Many rivers crossing lowland landscapes show changes in their thermal regime and are intensively overgrown with aquatic plants, macrophytes. Thus, the macrophytes’ impact on the microclimate of rivers, metabolism and biodiversity, changes in channel hydraulics and the effect on biogeochemistry need to be better understood. An increase in macrophyte biomass, for example, can lead to a decrease in surface water temperature and a decrease in solar radiation reaching the water, buffering temperature fluctuations. There is a need to analyse the role of aquatic macrophytes on water temperature dynamics and their critical importance for local biodiversity of primary producers and the controlling environmental factors of rivers. The project addresses this knowledge gap via an in-depth study of the influence of aquatic macrophytes on water temperature and aquatic metabolism in small lowland rivers by integrating monitoring approaches, targeted experiments and integrated modelling. The project team uses statistical models incorporating key independent variables from meteorological, hydro-morphological, and aquatic vegetation factors to provide accurate predictions of water temperature and its variability. River hydraulics and aquatic macrophyte characteristics are expected to jointly influence seasonal water temperatures by reducing the amplitude of temperature fluctuations in macrophyte-dominated stretches. Macrophyte patches in lowland rivers can buffer temperature increases and daily fluctuations, thereby mitigating shifts in algal diversity toward more temperature-tolerant species and cascading food web effects. Spatial thermal heterogeneity increases in river sections with macrophyte patches, enhancing diversity and ecosystem functions like photosynthesis among primary producers. To address these scientific questions, we investigate the temporal and spatial heterogeneity of water temperature through automated monitoring approaches and employ different statistical approaches to identify key variables. Controlled flume experiments complement the field studies in rivers to analyse causal relationships, and the gained data will be used to set up enhanced models to predict future changes due to climate change. Based on advanced modelling approaches, new insights will be gained into temperature-vegetation feedback processes and multitrophic interactions, to derive improved management strategies for an enhanced river ecosystem resilience, considering ecological interactions. Thus, this study advances our understanding of primary production, ecosystem respiration, and community dynamics in rivers in a warming world. The project team consists of scientists from Poland and Austria, teaming up expertise in hydrology, modelling and ecosystem research to address climate change effects in rivers in Poland and Austria.