Long-term water temperature trends of the Warta River in the years 1960-2009

In the context of climate changes, detailed information concerning the response of different components of the environment to the observed situation is of high importance. Due to strong correlations of air and water temperatures, water ecosystems have been subject to a considerable transformation for several decades. The paper presents the analysis of long-term (1960-2009) water temperature fluctuations in the Warta River, third longest river in Poland. Based on six stations located along the bank of the Warta River, its thermal parameters and tendencies and rate of changes were determined. Moreover, based on data concerning air temperature, correlations between those two media were analysed. Water temperature in the Warta River over the last fifty years was characterised by low variability, ranging in the analysed stations from 9.5 degrees C in Sieradz to 10.3 degrees C in Poznan and Skwierzyna. The Mann-Kendall trend tests, Sen's slope estimator and the Pettitt test were applied to long-term time series of air and water temperature analysis. Long-term fluctuations of mean annual water temperatures were characterised by an increase (statistically significant, depending on the station at a level of p=0.05 and p=0.001) from 0.096 to 0.281 degrees C.dec-1. Water temperature in the case of the analysed stations appears to be particularly determined by climatic factors, as suggested by very strong correlations with air temperature (from 0.82 to 0.94). Evident changes in the thermal regime of the Warta River in the context of functioning of the entire ecosystem should be considered unfavourable. The observed as well as future increase in water temperature will modify current processes occurring in the river, referring to among others water quality, composition of ichthyofauna, or hydrological conditions. Further temperature changes should be mitigated, by among others appropriate management of the riverbank zone (planting trees) and by increasing the share of forest areas in the catchment. (C) 2019 European Regional Centre for Ecohydrology of the Polish Academy of Sciences. Published by Elsevier B.V. All rights reserved.