Responses in river water quality during summers with extreme weather periods in Europe

Abstract: Europe has experienced several climate extremes during the past decades. These extreme events have increased in number and intensity and are projected to further intensify. Previous studies show that heatwaves, drought, and flood can have an impact on water quality in several ways. Drought can cause reduced dilution of nutrients, limiting oxygen availability as a result of increased water temperature, and reduced primary production are all example of such impacts and which can affect many biological processes. The aim of this study was to determine the impact on water quality caused by extreme events over a wider geographical extent covering Europe and across multiple rivers. This was done by using Waterbase database, water sample data retrieved during the years 2013-2015, and climate data retrieved from Copernicus Climate Change Service. Changes in the concentration of dissolved oxygen, nitrate, phosphate and chlorophyll-a during years that experienced summer climate extremes (2013 and 2015) relative to the values of a reference year were tested statistically, and further explored through correlation and regression analyses. The nutrients and dissolved oxygen were expected to increase and decrease, respectively, with higher air temperature, especially in rivers with small catchment areas. Dissolved oxygen concentrations increased significantly during 2013 and 2015 at mid latitudes compared to concentrations in 2014. A spearman’s correlation analysis was performed to determine any driver of the oxygen concentration patterns, showing a significant correlation between temperature and concentrations and both mid and high latitudes. To visualize the results of the correlation analysis, a linear regression was performed on the relationship between temperature and concentration of dissolved oxygen and showed significant results at both mid and high latitude, though contradictory to each other. No significant differences between concentrations of neither nutrient nor chlorophyll and year were found. Therefore, results were only in part consistent with findings of previous studies, making an overall interpretation difficult and not straightforward. The question about the effect of climate extremes on water quality is complex and includes a variety of variables not accounted for, but potentially highly influential on the parameters used in this study. Additionally, a more geographical spread of sample locations, and a higher consistency in sampling, and increase in frequency, would allow for a more robust and precise analysis.