Abstract
Abstract. Global warming has already affected European rivers and their aquatic biota, and climate models predict an increase of temperature in central Europe over all seasons. We simulated the influence of expected changes in heat wave intensity during the 21st century on water temperatures of a heavily impacted pre-alpine Austrian river and analysed future mitigating effects of riparian vegetation shade on radiant and turbulent energy fluxes using the deterministic Heat Source model. Modelled stream water temperature increased less than 1.5 ∘C within the first half of the century. Until 2100, a more significant increase of around 3 ∘C in minimum, maximum and mean stream temperatures was predicted for a 20-year return period heat event. The result showed clearly that in a highly altered river system riparian vegetation was not able to fully mitigate the predicted temperature rise caused by climate change but would be able to reduce water temperature by 1 to 2 ∘C. The removal of riparian vegetation amplified stream temperature increases. Maximum stream temperatures could increase by more than 4 ∘C even in annual heat events. Such a dramatic water temperature shift of some degrees, especially in summer, would indicate a total shift of aquatic biodiversity. The results demonstrate that effective river restoration and mitigation require re-establishing riparian vegetation and emphasize the importance of land–water interfaces and their ecological functioning in aquatic environments.
Highlights
Stream temperature is an important factor influencing the physical, chemical and biological properties of rivers and the habitat use of aquatic organisms (Davies-Colley and Quinn, 1998; Heino et al, 2009; Magnuson et al, 1979). Heino et al (2009) suggest that freshwater biodiversity is highly vulnerable to climate change with extinction rates exceeding those of terrestrial taxa
The aims of the present study are (1) to estimate the magnitude of stream temperature rise during extreme heat events caused by the expected rise in air temperature by the end of this century and (2) to investigate the ability of riparian vegetation to mitigate the expected water temperature rise within the habitat optimum of the site-specific aquatic fauna and (3) to analyse the possible variation of vegetation and potential interaction of vegetation and discharge with respect to climate change and their impact on water temperature
In order to interpret the influence of vegetation shade on future water temperature, it is important to understand the influence of vegetation shade on the present conditions first
Summary
Stream temperature is an important factor influencing the physical, chemical and biological properties of rivers and the habitat use of aquatic organisms (Davies-Colley and Quinn, 1998; Heino et al, 2009; Magnuson et al, 1979). Heino et al (2009) suggest that freshwater biodiversity is highly vulnerable to climate change with extinction rates exceeding those of terrestrial taxa. Stream temperature is highly correlated with the assemblages of fish and benthic invertebrates along the river course (Dossi et al, 2015; Melcher et al, 2015). The duration and magnitude of the maximum summer stream temperatures in particular are limiting factors for the occurrence of many fish species. High temperatures may produce high physiological demands and stress while reducing the oxygen saturation in the water column. The increased metabolic requirements together with the decreased oxygen availability can prove to be a limiting factor or even be lethal in combination; the average optimum temperature for cold water species is below 16 ◦C (Matulla et al, 2007; Pletterbauer et al, 2015)
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