Water quality management requires monitoring of natural waters condition by both biological and chemical indicators thus measuring the ecological health of a catchment. General water quality characteristics are extracted through low-frequency dataset, which allows analysis of long-term trends and seasonality. Plitvice Lakes National Park (PLNP) freshwater ecosystem includes springs, rivers, streams and lakes with its specific process of tufa formation. The aim of this study was to examine monitoring data of water quality parameters based on the performed long-term monitoring in the PLNP. This period referred to 2006 until 2017, from April – October per each year and for thirteen sites including karst springs, streams and lotic biotopes. Physico-chemical parameters were measured monthly according to standard methods. Cluster analysis grouped the sites into three types (springs, streams and lotic biotopes). Increased trends were observed for dissolved oxygen, conductivity, hardness, alkalinity, chemical oxygen demand (COD) and nitrites. Decreased trends were observed for temperature, pH and other nutrients (nitrates, ammonia and orthophosphates). Temperature values showed different trend slopes among types. Negative correlations between water level and/or discharge with conductivity and alkalinity probably indicate the effect of dilution. Conversely, nutrients and COD had positive correlation with discharge, possibly indicating resuspension from sediment during high flows. The analysis of spatial and temporal variations in hydrochemistry through longer period implied annual alterations, correlations between hydrological components and water chemistry and significant differences among monitored types. Long-term monitoring and continuous data analysis can help in future planning of monitoring program and better understanding of climate variations.
