Effects of climate change on nitrate loads in a large river:: the Austrian Danube as example

Using long-term data from 1951 to 2006, a considerable increase in water temperature was observed in the Austrian Danube (approximately 0.1 degrees C per decade). The mean annual discharge rates remained fairly constant over the years, but the seasonal pattern changed: in the summer, discharge decreased whereas the opposite trend was observed during the winter months. Data from routine water samples taken in approximately monthly intervals (1991-2006) revealed high temperature dependence of nitrate and dissolved inorganic nitrogen (DIN), intermediate temperature effect on nitrite and low temperature dependence of ammonia. The temperature dependence of nitrate differed between three discharge groups (low, middle and high discharge). It was nearly linear in the low-discharge group; in the other two discharge groups the decrease of nitrate with increasing temperature was more pronounced, but levelled off at approximately 15 to 20 degrees C in the middle and high discharge groups. Scenarios assuming increasing mean air temperature of 0.5 and 1.0 degrees C were calculated for water temperature and air temperature, discharge rates and nitrate loads based on trends detected in the air temperature and discharge data from 1951 to 2006. According to these scenarios, temperatures (both water and air) will continue to increase. The discharge regime of the Danube will be affected by significantly increasing discharge rates in March and significantly decreasing rates for July. The annual mean discharge rate will not be affected. Trends for nitrate loads showed similar tendencies: while annual mean nitrate load does not change with time, the seasonal pattern of nitrate loads is expected to change. Larger amounts of nitrate will be transported downstream in winter, whereas the loads during summer will decrease. The ecological consequences of such shifts are not fully understood. Copyright (C) 2008 John Wiley & Sons, Ltd.