River temperature behaviour in changing environments: trends, patterns at different spatial and temporal scales and role as a stressor

River/stream water temperature is one of the master water quality parameters as it controls several key iogeochemical, physical and ecological processes and river ecosystem functioning. Thermal regimes of several rivers have been substantially altered by climate change and other anthropogenic impacts resulting in deleterious impacts on river health. Given its importance, several studies have been conducted to understand the key processes defining water temperature, its controls and drivers of change. Temporal and spatial river temperature changes are a result of complex interactions between climate, hydrology and landscape/basin properties, making it difficult to identify and quantify the effect of individual controls. There is a need to further improve our understanding of the causes of spatiotemporal heterogeneity in river temperatures and the governing processes altering river temperatures. Furthermore, to assess the impacts of changing river temperatures on the river ecosystem, it is crucial to better understand the responses of freshwater biota to simultaneously acting stressors such as changing river temperatures, hydrology and river quality aspects (e.g. dissolved oxygen levels). So far, only a handful of studies have explored the impacts of multiple stressors, including changing river temperature, on river biota and, thus, are not well known. This thesis, thus, analysed the changes in river temperature behaviour at different scales and its effects on freshwater organisms. Firstly, at a regional scale, temporal changes in river temperature within long (25 years) and short time periods (10 years) were quantified and the roles of climatic, hydrological and landscape factors were identified for North German rivers. Secondly, at a reach scale, spatial temperature heterogeneity in a sixth-order lowland river (River Spree) was quantified and the role of landscape factors in inducing such heterogeneity was elucidated. Thirdly, at a site scale, short-term behavioural responses (namely drift) of three benthic invertebrate species to varying levels of water temperature, flow, and dissolved oxygen, and to combinations of those factors were experimentally investigated. Results from this thesis showed that, at a regional scale, the majority of investigated rivers in Germany have undergone significant annual and seasonal warming in the past decades. Air temperature change was found to be the major control of increasing river temperatures and of its temporal variability, with increasing influence for increasing catchment area and lower altitudes (lowland rivers). Strongest river temperature increase was observed in areas with low water availability. Other hydro-climatological variables such as flow, baseflow, NAO, had significant contributions in river temperature variability. Spatial variability in river temperature trend rates was mainly governed by ecoregion, altitude and catchment area via affecting the sensitivity of river temperature to its local climate. At a reach scale as well, air temperature was the major control of the temporal variability in river temperature over a period of nine months within a 200 km lowland river reach. The spatial heterogeneity of river temperature in this reach was most apparent during warm months and was mainly a result of the local landscape settings namely, urban areas and lakes. The influence of urban areas was independent of its distance from the river edge, at least when present within 1 km. Heat advected from upstream reaches determined the base river temperature while climatological controls induced river temperature variations around that base temperature, especially below lakes. Riparian buffers were not found to be effective in substantially moderating river temperature in reaches affected by lake warming due to the dominant advected heat from the upstream lake. Experimental investigation indicated that increasing water temperature had a stronger short-term effect on behavioural responses of benthic invertebrates, than simultaneous changes in flow or dissolved oxygen. Also, increases in water temperature was shown to affect benthic invertebrates more severely if accompanied by concomitant low dissolved oxygen and flow levels, while interactive effects among variables vary much among taxa. These results support findings of other studies that river warming, similar to climate change, might be a global phenomenon. Within Germany, lowland rivers are the most vulnerable to future warming, with reaches affected by urbanization and shallow lentic structures being more vulnerable and, therefore, requiring urgent attention. Furthermore, river biota in lowland rivers is particularly susceptible to short-term increases in river temperature such as heat waves. Plantation of riparian buffers, a widely recognized practice to manage climate change effects, in the headwater reaches can be suggested to mitigate and prevent future warming of lowland rivers in general and also throughout river basins, as river temperature response in lowland catchments is a culmination of local and upstream conditions. However, further river temperature increase in lowland river reaches within or close to urban areas and shallow lentic structures will be more difficult to mitigate only via riparian shading and would require additional measures