The Impact of Climate Change on Physical Instream Habitats and Its Response to River Morphology

The impact of climate-induced run-off change based on 1951 – 2008 time series was investigated within the crystalline catchment of the Grosse Mühl River in Northern Austria. A significant trend change of aerial temperature, based on Mann-Whitney statistical testing, was recorded for spring 1989 (P = 98.9 %) and summer 1990 (P = 99.9 %). This led to a pronounced increase in summer drought periods. Hydrodynamic-numerical (one-dimensional / two-dimensional) modelling was applied to simulate the changing habitat characteristics due to decreasing run-off in relation to various morphological patterns (riffle-pool / plane-bed reaches). Using bathymetric data, which were sampled on cross sectional measurements, we clearly determined that plane-bed reaches (featureless bed forms) are sensitive to climate-related, reduced run-off, whereas riffle-pool reaches continued to exhibit suitable physical fish habitats even under extreme low-flow conditions. The impact of the decreased summer run-off on instream habitats was especially strong for subadult and adult grayling (target fish species). In situ measurements in microhabitats (velocity / depth) revealed habitat suitabilities. These values were taken as biotic input for habitat evaluation on the micro scale. The findings clearly show that river morphology is a decisive parameter in terms of habitat preservation and restoration in the context of the future impacts of climate change (decreased run-off).