Biogeochemical Causes of Sixty-Year Trends and Seasonal Variations of River Water Properties in a Large European Basin

We evaluated long-term trends and seasonal variations in the major physical–chemical properties of the circum-neutral Slapy reservoir (Vltava, Czech Republic) from 1960 to 2019. Mean annual water temperature increased by 2.1 °C, flow maxima shifted by ~ 13 days from the early April to mid-March, and the onset of thermal stratification of water column and spring algal peaks advanced by 19 and 21 days, respectively, due to climate warming. Concentrations of major ions, phosphorus (P), and chlorophyll increased from the 1960s to the 1990–2000s, then decreased due to changing agricultural practices and legislation, intensified wastewater treatment, and decreasing atmospheric pollution. Concentrations of dissolved organic carbon (DOC) decreased from 1960 to the 1990s due to improved wastewater treatment, then began to increase in response to climate change and reduced acidic deposition. Concentrations of water constituents exhibited varying individual long-term and seasonal patterns due to the differing effects of following major processes on their production/removal in the catchment-river system: (1) applications of synthetic fertilizers, liming and farmland draining (NO3−, SO42−, Cl−, Ca2+, Mg2+, K+, and HCO3−), (2) wastewater production and treatment (DOC, P, N forms), (3) road de-icing (Cl− and Na+), (4) atmospheric pollution (SO42−), (5) climate change (DOC), and (6) the aging of reservoirs (NH4+). The water pH increased until the early 1990s, then decreased and exhibited pronounced seasonal variations, integrating the effects of changing external acidity sources and in-lake H+ sources and sinks (i.e., microbial CO2 production/consumption and availability and transformations of inorganic N), and changes in water buffering capacity. Anthropogenic and climatic effects, reservoir aging, and changes in water eutrophication thus may significantly affect water pH also in circum-neutral systems.