Long-term water temperature fluctuations in coastal rivers (southern Baltic) in Poland

Abstract The paper presents water temperature fluctuations in coastal rivers (Rega, Parsęta, Słupia, Łupawa, Łeba) located in the southern Baltic Sea area in Poland. Based on the available detailed data on daily values of the analysed characteristic in the years 1971-2015, tendencies of changes in the period were determined. It includes an analysis of temperature fluctuations in reference to mean annual and mean monthly values. In all of the five cases, the mean annual temperature showed an increasing tendency. The value of water warming in particular rivers was approximate, and ranged from 0.26 °C∙dec-1 to 0.31°C∙dec-1. At the monthly scale, the highest increase was recorded in April, and amounted to 0.46°C∙dec-1 on average. Water temperature fluctuations were particularly determined by climatic changes, and strongly correlated with air temperature. Local factors characteristic for coastal rivers, i.e. presence of infrastructure of small water power plants and predominance of groundwater alimentation, had a secondary effect on changes in the thermal regime of the analysed rivers.